Horner, R. 1996. Strategies for fish health management of tank-raised walleye. Pages 339-341 in 

R. C. Summerfelt, editor. Walleye culture manual. NCRAC Culture Series 101. North Central 

Regional Aquaculture Center Publications Office, Iowa State University, Ames. 

Strategies for Fish Health Management of 

Tank-Raised Walleye 

Rodney Horner, lllinois Department of Natural Resources, 25410 N Fish Hatchery Rd, Topeka, IL 61567 

Introduction 

No drugs or chemicals are registered by FDA (Food 

and Drug Administration) for use in the fish health 

management of walleye. Although salt and other lowregulatory priority drugs may be used, strategies to 

reduce stress must be employed for an effective fish 

health program. This paper will describe techniques that 

are used at Jake Wolf Fish Hatchery, Topeka, Illinois to 

minimize stress. 

Methods 

Pond culture 

At Jake Wolf Fish Hatchery, we incubate walleye eggs 

at 53°F (12°C). Ten to 14 d before stochng, plastic-lined 

ponds are fertilized with organic fertilizers to achieve a 

Secchi disk reading of about 12 in (30 cm) when the fry 

are stocked. Fry are stocked into ponds at 100,000/acre 

(247,70O/ha) when the fry are 4-5 d old. If the fry are 

stocked at that time, there will be sufficient zooplankton 

to grow them to 1.25-2 in (32-50 mm) in 30-40 d. If 

the zooplankton production increases dramatically, the 

water clears due to the grazing of zooplankton on algae, 

and Secchi disk readings increase to 7.5 ft (2.3 m), 

which is the pond bottom. At 3040 d, careful monitoring of water clarity is necessary. The Secchi Qsk 

readmg will begin to decrease as the algal populations 

recover, because the fish are cropping the zooplankton. 

When the Secchi disk reading reaches about 2 ft (0.6 

m), the fish are harvested. If this scenario occurs, a 

harvest of >70,000 fingerlingdacre (>172,970/ha) may 

be realized. 

The first stressor to intensive culture of walleye is poor 

condition of the fingerlings when they are harvested 

from the pond. If harvest is delayed, the fish may 

exhaust their food supply and stop growing. If the 

Secchi disk reahng reaches 1 ft (30 cm), which means 

that the zooplankton population is exhausted, the 

fingerlings will begin to starve. Fingerlings harvested in 

poor condition that are brought inside and trained on 

pelleted diets will quickly succumb to columnaris 

(Flexibacter columnaris) or they will not accept 

formulated feed. If the harvest is timed correctly, 70- 

75% of the fingerlings will accept the pelleted diet. 

Another stressor is pH shock. When the Secchi disc 

measurement in the ponds before harvest is <2 ft 

(30 cm), pond pH is probably high, and there is a 

possibility of pH shock while moving the fish from 

pond to tanks. At the Jake Wolf Hatchery, the pH at 

harvest may be 10 in the pond, 7.8 in the hatchery 

building, which is supplied with ground (well) water. 

Draining the pond from the bottom will draw down the 

high-pH top surface water onto the fingerlings being 

harvested. Flushing the pond kettle with fresh water 

will relieve this problem. 

Tank culture 

Timing of harvest and movement of fingerlings from 

the ponds to the hatchery is critical because of 

columnaris Qsease, a stress-mediated disease, that can 

result in mass mortality. Facilities for tank culture must 

be ready when the fingerlings are harvested. Feed must 

be presented to them immediately or they will never 

commence feeding. If there is any delay, major outbreaks of columnaris will occur. As the diet training 

period begins, columnaris will kill the non-feeders 

within 10 d, regardless of chemotherapy. 

At Jake Wolf Fish Hatchery, our best success at 

reducing stress is achieved using single-pass heated 

(6X-7O0F, 20-21°C) well water. If well water is not 

available, we use filtered pond water, but Qsease 

problems develop because of its poorer quality and 

higher temperature. 

Gas pressure, measured as AP, in the training tanks is 

another stressor. Gas supersaturation in our hatchery 

NCRAC Culture Series 101 - Walleye Culture Manual 339 
Chapter 16- Diseases and Chemotherapeutics 

was subtle and episodic. Typically, AP was low (4-5 

mm Hg) but it often spiked briefly enough to stress the 

fish. We noted a reduction in outbreaks of columnaris 

occurred at the Jake Wolf Hatchery after we installed 

packed column degassers on each culture unit. 

In our early efforts to convert pond-raised fingelings to 

formulated feed, we used 1,100-gal (4,164-L) rectangular tanks. These tanks were unsuitable because of poor 

flow characteristics; dead spots in the tank corner; and 

accumulations of uneaten food, feces, and ammonia. 

We observed the first case of gill rot fungus 

(Branchiomyces sanguinis) in walleye in North America 

due to the degraded environment. 

Initially, we fed the fish every 5 min at a total of 100% 

body w eight/d, with continuou s il lumination , After 

feedng for 3 d, the tanks contained a large mass of 

uneaten food. When we switched to circular tanks, their 

self cleaning capabilities quickly improved environmental conditions. Circular tanks seem preferable to 

rectangular tanks, because it allows the fish to swim 

continuously without encountering a barrier. The result 

is a calmer, less stressed fish. Feeder placement can be 

more easily optimized, and the circular motion of the 

water tends to keep feed particles suspended much 

longer than those in rectangular tanks, giving the fish 

several chances at striking at a food particle before it 

drops to the bottom. In our trials, with Biodiet or 

Biotrainer feeds (Bioproducts, Inc., Warrenton, OR) 

were readily accepted and trained the fish to accept 

pelleted feed. In some years, we were able to train 95% 

of the fish to accept a pelleted diet. 

Careful phasing in changes in feed size is critical. If 

particle size is changed too soon, some fish may starve 

because their mouths are too small to ingest the larger 

pellet. If the feed size is not increased soon enough, 

food may be wasted and growth rate will be reduced. 

Walleye are skittish and are spooked by movement 

above them. Walleye do not adjust as well as other 

cultured species to activities of hatchery workers. This 

problem can be reduced by lighting the interior of the 

culture tank and darkening the room or by covering the 

tank on the side where human activity occurs. 

Fish density is another factor which can contribute to 

occurrence of diseases. We keep the density (lbs/ft3 or 

kg/m3) at no more than 0.9 lb/ft3 (14 kg/m3). When 

density exceeds 0.2 lb/ft3 (3.2 kg/m3) early in the 

training period, the fish in a tank are dvided into two 

lots. Loading (lb/gpm of flow, gkpm), should not 

exceed 1.24 lb/gpm (149 g/Lpm) early in the cycle, but 

as fish increase in size, loading must be reduced to 0.5 

to 0.7 lb/gpm (60-84 g/p/m). 

Disease treatment 

Strict attention to the density and loading requirements 

has prevented most disease outbreaks in our facility. 

Inevitably however, in spite of all these efforts, 

columnaris, fungus, or parasite problems can occur. 

As a treatment for columnaris, we use a salt (NaCl) 

bath at 2% for as long as the fingerlings will tolerate it. 

This may be combined with lowering the temperature 

out of the optimal range for columnaris. If done 

promptly, these treatments have stopped columnaris 

outbreaks. 

Although it has been used effectively in the past, 

formalin is not currently labeled as a fungicide or 

parasiticide for any life stage of walleye. As a result, we 

used hydrogen peroxide (H202) as a chemotherapeutant 

for the first time in 1994. FDA has declared the use of 

H,O, as a fungicide to be of low regulatory priority. No 

Investigational New Animal Drug Application must be 

filed for its use, provided that it is used within guidelines. It is a low regulatory compound because its 

breakdown products are oxygen and water. Trials by 

Richardson and Colesante (1 995) in&cated that 15- 

minute exposure of walleye eggs to 500 ppm H,O, were 

nearly as effective as 250 ppm formalin as a fungicide 

on walleye eggs, but clumping was noted in eggs 

treated with H,O,, leadmg to some loss of otherwise 

healthy eggs. They also noted that these clumps of dead 

eggs sometimes floated and plugged the outlet of the 

jar. Small eggs included a tendency to form bubbles on 

the egg and float the entire mass out of the hatching jar. 

This may be overcome by temporarily transferring the 

eggs to a bucket for treatment. Hydrogen peroxide does 

have some promise as a fungicide for eggs, it may be 

necessary if formalin does not receive the hoped for 

label extension by FDA for use on walleye. However, 

fingerling walleye are very sensitive and will tolerate 

less H,O, than other species (Clayton and Summerfelt 

1995). At Jake Wolf Hatchery, at 118 ppm total alkalinity, pH 7.8, and 68°F (2073 healthy 3-in (76-mm) 

340 NCRAC Culture Series 101- Walleye Culture Manual 
Diseases and Chemotherapeutics - Chapter 16 

fingerlings seem to tolerate a treatment of 50-100 ppm 

for up to 1 h. 

Conclusion 

stress is a fish health factor that contributes to occurrence of &sease. Stress must minimizedd to achieve 

successful habituation of walleye to formulated feed. 

Most stress is related to techniques that are controlled 

by the aquaculturist. Our experience at Jake Wolf 

Hatchery demonstrates that successful pond culture of a 

1.5-2.0 in (3.8-5.0 crn) fingerling and subsequent 

success of training these fingerlings to formulated feed 

are strongly influenced by the degree to which we 

control the stress factors. 

References 

Clayton, R. D. and R. C. Summerfelt. 1995. The toxicity 

of hydrogen peroxide to fingerling walleyes. Page 47 

in Minutes of 1995 Coolwater Fish Culture Workshop, 

Penn State University, State College, Pennsylvania. 

Richardson, D. E. and R. T. Colesante. 1995. The 1994 

egg treatment tests: hydrogen peroxide vs formalin. 

Pages 86-91 in Minutes of 1995 Coolwater Fish 

Culture Workshop, Penn State University, State 

Co I I eg e , Pen n sylvan i a. 

NCRAC Culture Series 101 - Walleye Culture Manual 341 
Chapter 16 - Diseases and Chemotherapeutics 

342 NCRAC Culture Series 101- Walleye Culture Manual 
Kebus, M. J. 1996. Disease investigation of tank-reared triploid walleye. Pages 343-345 in R. C. 

Summerfelt, editor. Walleye culture manual. NCRAC Culture Series 101. North Central Regional 

Aquaculture Center Publications Office, Iowa State University, Ames. 

Disease Investigation of Tank-Reared 

Triploid Walleye 

Myron J. Kebus, Wisconsin Aquatic Veterinary Service, 2027 E. Mifflin St., Madison, Wl, 53704 

Introduction 

This report describes an investigation of a disease 

outbreak in an experimental group of 1,400, 11 -monthold (150-200 mm) triploid walleye. They were used in 

an experiment by the University of Wisconsin-Madison 

Aquaculture Program (UWAP) at their campus facility. 

A disease investigation was required to mitigate the 

mortalities and to investigate whether the mortalities 

were related to husbandry practices, facility design, or 

to experimental treatment. Information on the cause of 

the mortality was needed for design of future research 

and to optimize fish health conditions in the facility. 

The information is presented here as a contribution to 

commercial production of walleye. 

Methods 

The triploid walleye that were the focus of this report 

were housed in a 5,000-gal (1 8,925 L) circular fiberglass tank. The experimental tank was supplied with 

tempered municipal well water presumed to be pathogen-free. The temperature had been held at 70°F (21°C) 

for the previous 75-d, and the flow was 15-25 gpm 

(57-95 Lpm). The inflow produced a circular flow 

current in the tank. The fish originated from eggs 

collected from wild diploid walleye broodstock. The 

eggs were treated with pressure-shock to induce 

triploidy. Triploidy was determined by flow cytometry. 

Their first month post-hatch was spent in a fertilized 

0.25 acre (0.10 ha) earthen rearing pond at the Lake 

Mills aquaculture facility of the UWAP. When fish were 

2.5-3 in (40-50 mm) total length they were harvested 

using a light-harvest technique. The fingerlings were 

transferred to circular fiberglass tanks when they were 

trained to accept a commercial salmon starter feed. 

They were subsequently fed commercial dry trout feed 

of appropriate pellet size. 

When fish reached 4-inch (100-mm) they were dvided 

into two treatment groups. One group of 300 fish 

remained in the Lake Mills facility and were housed in 

60-gal (227 L) and 200-gal (757 L) circular fiberglass 

tanks. The other group consisted of 1,450 fish that were 

transported to and housed at the UWAP campus facility 

in a 5,000-gal (18,927 L) circular fiberglass tank. This 

later group was subjected to similar water quality and 

nutritional parameters as the group at the Lake Mills 

facility. 

Prior to the dsease outbreak in the UWAP site, the 

triploid walleye were growing rapidly, and were feedmg 

aggressively. Mortality in both groups of triploid 

walleye was ~0.01 %/d. Two d prior to the disease 

outbreak, the campus triploid walleye were confined, 

anesthetized (-50 mgL Finquel), captured, and 

weighed and measured. Their tank was partially drained 

and the fish were collected using a seine. The weighng 

and measuring procedure lasted 3-h. Coincidentally, 

less than 24-h after the procedure, a mechanical failure 

resulted in a drop in tank temperature from 70-59°F 

(21-15°C). The day after the temperature decline, the 

fish fed poorly, appeared lethargic, and mortality 

increased to 3% of the population on March 16, 1994. 

Thirty-six h after the procedure water temperature was 

returned to 70"F(21 "C). 

Potential sources of horizontal disease transmission 

were investigated. No new arrivals of fish to the campus 

facility had occurred in the previous 4 months. Six and 

11 months earlier, columnaris disease had occurred in 

rainbow trout, and EdwardsieEZa turdu infection in 

channel catfish held in the facility. However, these 

disease outbreaks occurred in tanks in rooms other than 

the one that housed the triploid walleye. 

The potential for transmission of infectious agents from 

contact with contaminated equipment was believed to 

be minimal due to hygiene practices. However, dip nets 

used to collect, or transfer fish were shared among 

groups of experimental species. These nets were 

disinfected in 500-ppm chloramine-T solution, and 

NCRAC Culture Series 101 - Walleye Culture Manual 343 
Chapter 16- Diseases and Chemotherapeutics 

rinsed in water, between use with different species and 

tanks. It was assumed that the chloramine-T disinfection of nets effectively destroyed most potential 

infectious agents. The net used to capture the triploid 

walleye for the weighing and measuring procedure was 

housed in dry storage for 6 months prior to its use. It 

was used to seine dqdoid and triploid walleye and 

yellow perch fingerlings from earthen ponds at the Lake 

Mills facility. The net contained dried pond mud. 

In the tank, fish were generally lethargic, swimming 

primarily in the lower one third of the tank, and they 

refused to feed. No abnormal appearance or odor of the 

water was detected. Due to the flow-through design of 

the tanks and the time elapse since the weighing and 

measuring procedure, measurements of water quality 

parameters associated with accumulation of fish 

metabolic wastes was not pursued. Additional complicated and expensive water analysis was reserved 

pendmg the results of less expensive, rapid necropsy 

results. 

Twelve moribund fish were collected for necropsy 

examination. They were not emaciated, which was 

evidence for an acute rather than a chronic disease 

problem. Gross abnormalities on external exam 

included: generalized pale appearance (1 2/12), hemorrhage at base of tail (1 2/12), anal fins (1 2/12), mandible 

and gill operculum (4/12), and diffuse yellow discoloration of ventral abdomen (1212). All fish had fraying on 

most of their fins. A gross exam of internal organs 

revealed no abnormalities, other than absence of food 

material in the digestive tract. 

Skin scrapings, fin clips, and wet mounts of gills arches 

from the sampled moribund and dead fish from the 

campus tanks were examined microscopically. The skm 

scrapings, and fin clips revealed parasitic, fungal, and 

bacterial agents. Numerous actively motile monogenetic trematode parasites with no eye spots, and two 

large hooks were evident on skin scrapings and fin clips 

of all moribund fish examined. Fungal organisms with 

non-septate filamentous hyphae were readily evident, 

particularly on the slun scrapings. Long filamentous 

gram-negative bacteria often in clusters were occasionally noted, particularly on the skin scrapings. The 

presumptive a diagnosis was severe gyrodactyliasis, 

moderate saprolegniasis, and a few fish with columnaris 

&sease. None of the dead fish exhlbited any external 

parasitic, fungal, or bacterial pathogens. A comparable 

investigation was performed on the Lake Mills fish and 

no abnormalities were found. 

Appropriate samples of fish from the campus tank were 

collected to attempt to rule-out numerous bacterial and 

viral agents as contributory causes to systemic dsease. 

Sterile hdney samples were collected for bacteriological isolation and identification. The samples were 

submitted to a private veterinary laboratory experienced 

in the isolation of fish pathogens. No bacteria were 

isolated from the samples submitted. Kidney and spleen 

samples were collected from 60 moribund and apparently healthy fish and submitted for virology testing at a 

federal fish health lab. The virology results were also 

negative. 

It was determined that the first pathogen that had to be 

controlled was the ectoparasite, because it was considered the major underlying agent initiating damage to the 

shn, and it could be treated relatively easily. A formalin 

treatment at 167 ppm for 60 min in a static bath was 

attempted. Formalin is not approved for use in walleyes, 

although it is available by Investigational New Animal 

Drug (INAD), or extra-label veterinary prescription. 

Thirteen fish were transferred from the affected tank 

into a 227-L (60-gal) trial treatment tank and provided 

with supplementary aeration with airstones throughout 

the treatment period. All 13 fish died during the 

treatment trial. Our next alternative trial treatment was a 

2.0% concentrated salt bath for 60 min. No mortalities 

resulted from the trial treatment, therefore we proceeded to treat the entire group of fish. The standpipe 

was lowered to achieve a practical volume. Supplementary aeration was provided with airstones that maintained dissolved oxygen levels of 6.6-8.6 ppm during 

treatment. The treatment was repeated the following 

day, and again 2 d later. The fish showed evidence of 

irritability and loss of equilibrium during treatment, 

however no mortalities occurred during treatment, and 

mortality rates declined with each subsequent treatment. 

A sample of the treated fish were examined 14-d after 

the last treatment. There was no microscopic evidence 

of parasitic, bacterial, or fungal organisms. The fish 

recovered, regained vigorous appetites, and grew well. 

The fish were re-examined 90- later in preparation for 

44-h truck transport to Washington state. This exam, &d 

not indicate any parasitic, bacterial, or fungal organisms. Although there is stress associated with crowding 

344 NCRAC Culture Series 101- Walleye Culture Manual 
Diseases and Chemotherapeutics - Chapter 16 

and transport, only one mortality occurred during 

transport, supporting the perceived benefit of eliminating treatable pathogen prior to exposure of the fish to 

this stress. 

Conclusion 

The finchngs of our investigation support the need to 

pursue existing principals of fish health, and employ a 

systematic, practical approach to disease diagnosis and 

treatment. The findings support the need to examine 

moribund opposed to dead fish for necropsy examination. Also, it is obviously beneficial to perform a trial 

treatment on a dispensable sample of the affected 

population before treating the entire population. 

Contrary to common assumption, ectoparasites can be a 

significant problem in tank-reared fish. The origin of 

the ectoparasites is speculative. The possibilities 

include: subclinical infestation since pond rearing stage, 

introduction by contaminated equipment, transmission 

from subclinically fish housed in other tanks in the 

campus facility. In this case, stress seemed to play a 

crucial role in precipitating clinical disease in seemingly healthy fish. Fish should be examined for fish 

pathogens prior to introduction into new rearing units, 

or exposure to stressful procedures. Fish pathogens can 

be opportunistic. There was no evidence to indicate that 

a disease outbreak of this nature is particular to tankreared triploid walleye. Rather it supports the importance of decontaminating fish prior to introduction into 

indoor tank facilities. Diseased fish can be effectively 

treated, returned to vigorous health, and subjected to 

stressful procedures, such as transport, without significant mortalities. 

NCRAC Culture Series 101 - Walleye Culture Manual 345 
Chapter 16- Diseases and Chemotherapeutics 

346 NCRAC Culture Series 101- Walleye Culture Manual 
Schnick, R. A. 1996. Chemicals and drugs. Pages 347-354 in R. C. Summerfelt, editor. Walleye 

culture manual. NCRAC Culture Series 101. North Central Regional Aquaculture Center 

Publications Office, Iowa State University, Ames. 

Chemicals and Drugs 

Rosalie A. Schnick, Michigan State University, 3033 Edgewater Lane, La Crosse, WI 54603- 1088 

Regulations and policies for walleye culture 

In the United States, the application of chemicals to 

any organism such as walleye or to their environment is 

regulated by the U.S. Food and Drug Administration’s 

Center for Veterinary Medicine (CVM), the U.S. 

Environmental Protection Agency (EPA), or the Animal 

and Plant Health Inspection Service (APHIS). CVM 

controls the use of drugs (e.g., therapeutants and 

anesthetics), EPA controls the application of chemicals 

and pesticides in the environment, and APHIS regulates 

all veterinary biologics (vaccines, non-drug biological 

therapeutants, and diagnostic test kits). In cases 

involving water treatments to control pathogens or the 

organisms that can cause disease, the jurisdiction 

becomes unclear and has changed over time. Each 

agency has developed guidelines and policies to 

implement the laws for their respective fields of 

responsibility. A recent publication by the Joint 

Subcommittee on Aquaculture’s Worhng Group on 

Quality Assurance in Aquaculture Production (Working 

Group) has summarized the federal regulations of 

aquaculture substances by CVM, EPA, and APHIS 

(Worlung Group 1992). 

The most important law enacted by the U.S. Congress 

having a major impact on walleye culture is the Federal 

Food, Drug, and Cosmetic Act. Section 5 12 of the Act 

states that a drug shall be considered unsafe and subject 

to enforcement unless it has an approved new animal 

drug application (NADA) and that the intended use of a 

drug and its labeling conform to the approved application. The Act provides exemptions for the investigational use of unapproved drugs called “investigational 

new animal drug” (INAD) exemptions. 

CVM has developed regulations and policies to assist 

with implementing the Act. These are regularly published in the Federal Register and codified in Title 21 of 

the Code of Federal Regulations. Examples of these 

rulings pertinent to walleye culture include regulations 

on applications for new animal drugs, investigational 

use of new animal drugs, minor use provisions, me&- 

cated feeds, and good laboratory practices for nonclinical laboratory studies. In addition, CVM continues 

to develop guidelines and policies that are not codified 

but are either on file at CVM or placed in the Compliance Policy Guide. Good examples of these provisions 

are “Extra-label use of new animal drugs in foodproducing animals” (Compliance Policy Guide 

7125.06), “Guidelines for the preparation of data to 

satisfy the requirements of Section 5 12 of the Act 

regarding minor use of animal drugs,” and low regulatory priority (LRP) provisions. 

Status of chemicals and drugs for walleye 

cu I t u re 

Current information on federally approved uses 

(includmg unapproved drug provisions) of drugs, 

vaccines, and pesticides in aquaculture production and 

in natural aquatic sites is available in a recent publication by the Working Group (Working Group 1994). This 

publication will be kept up-to-date electronically by the 

U.S. Department of Agriculture’s National Program 

Leader for Aquaculture with input from CVM, EPA, 

and APHIS. All aquaculture producers are responsible 

for using the products that not only are federally 

approved but that also meet state and local regulations. 

The proper use of the products listed in this publication 

is important for insuring their safety and effectiveness, 

and the reduction or prevention of drug overuse, 

possible undesirable side effects, and illegal residues. 

Not many drugs or chemicals are available to walleye 

producers, as is the case with many other fish species 

(Table 1). The only approved drug for use on walleye is 

Finquel, an anesthetic that has a 21 -d withdrawal time. 

The only vaccine commercially available for use on 

walleye is Autogenous Bacterin available from BioMed, 

Inc. and Aqua Health, Ltd. It is effective on unspecified 

bacterial diseases (Working Group 1994). 

NCRAC Culture Series 101 - Walleye Culture Manual 347 
Chapter 16 - Diseases and Chemotherapeutics 

Table 1. FDA-approved new animal drugs (Working Group 1994) 

Trade NADA Active 

Name Number Sponsor Drug Species Uses 

Finquel 42-427 Argent Tricaine Ictaluridae, 

Chemical methanesulfonate Salmon idae, 

Laboratories, Inc. Esocidae, and 

Percidae. (In other 

fish and 

cold-blooded 

animals, the 

drug should 

be limited to 

hatchery or 

laboratory use.) 

Temporary 

immobilization 

(anesthetic) 

Formalin-F 137-687 Natchez 

Animal 

Supply 

Formalin Trout, salmon, 

catfish, largemouth 

bass, and bluegill 

Control of external 

protozoa and 

monogenetic 

trematodes. 

Control of fungi 

of the family 

Saprolegniacae. 

Control of 

external protozoa, 

monogenetic 

trematodes. 

Control of fungi 

of the family 

Saprolegniacae. 

Control of 

external protozoa 

and monogenetic 

trematodes 

Control of fungi 

of the family 

Saprolegniacae 

Control of external 

protozoan parasites 

Control of enteric 

se pt i ce m i a 

Control of 

furunculosis 

Control of 

furunculosis 

Salmon, trout, 

and esocid eggs 

Trout, salmon, 

catfish , 

largemouth bass, 

and bluegill 

Salmon, trout, 

and esocid eggs 

Paracide-F 1 40-83 1 Argent 

Chemical 

Laboratories, Inc. 

Formalin 

Parasite-S 140-989 Western 

Chemical Inc. 

Formalin Trout, salmon, 

catfish , 

largemouth bass, 

and bluegill 

Salmon, trout, 

and esocid eggs 

Cultured penaeid 

shrimp 

Romet-30 Hoffman n- Sulfadimethoxine Catfish 

LaRoche, Inc. and ormetoprim 

125-933 

Salmonids 

Sulfamerazine 033-950 American Cyanamid Sulfamerazine 

in Fish Grade' Company 

Rainbow trout, 

brook trout, 

and brown trout 

Terramyci n 038-439 Pfizer, Inc. Oxytetracycline Catfish 

for Fish 

Control of bacterial 

hemorrhagic septicemia 

and pseudomonas disease 

Lobster Control of gaffkemia 

disease, furunculosis, 

bacterial hemorrhagic 

septicemia, and 

pseudomonas disease 

skeletal tissue 

Salmonids Control of ulcer 

Pacific Salmon Marking of 

'According to sponsor, this product is not presently being distributed. 

348 NCRAC Culture Series 101- Walleye Culture Manual 
Diseases and Chemotheraneutics - Chanter 16 

Chemicals that are legal to use in walleye culture under 

existing pesticide labels include certain algicides, 

herbicides, and fish toxicants. These products must be 

used according to label directions and restrictions, 

especially those that pertain to use when fish are 

present. The algicides include copper compounds and 

copper sulfate; fish toxicants include antimycin and 

rotenone; and herbicides include 2,4-D, acid blue and 

acid yellow combination, dichlobenil, diquat dibromide. 

endothall, fluridone, and glyphosate (Working Group 

1994). 

Because of the high cost, the large number of unapproved drugs, and general lack of interest by the 

pharmaceutical industry, aquaculture groups had to 

define the most needed chemicals, prioritize them, and 

eliminate the least important drugs from consideration. 

Several groups (U.S. Fish and Wildlife Service, 

National Biological Service, certain states, and Fish 

Health Section of the American Fisheries Society) 

requested and received rulings from CVM that resulted 

in the declaration of 18 unapproved drugs as being “low 

regulatory priority,” six drugs considered as not low 

regulatory priority (i.e., requiring INAD’s), 21 drugs or 

family of drugs that are considered high regulatory 

priority (HRP) for enforcement action, two drugs with 

deferred regulatory action, and nine substances that did 

not meet the definition of a drug for their intended uses 

(Tables 2 and 3). The use of drugs considered as HRP 

must be covered by either INAD’s or approved 

NADA’s; CVM also determined that any nitrofuran 

(e.g., furazolidone), malachite green, and methylene 

blue would not be granted any INAD exemptions and 

that sulfonamides would not likely be granted INAD 

exemptions until the carcinogenicity issues have been 

clarified (Working Group 1994; Geyer 1993; Homaire 

1994; CVM, personal communication). 

The FDA is unlikely to object at present to the use of 

low regulatory priority substances (Table 2) if the 

following conditions are met: 

1. The drugs are used for the prescribed inhcations, 

including species and life stage where specified. 

2. The drugs are used at the prescribed dosages. 

3. The drugs are used according to good manage4. The product is of an appropriate grade for use in 

5.An adverse effect on the environment is unlikely. 

ment practices. 

food animals. 

FDA’s enforcement position on the use of these 

substances should be considered neither an approval nor 

an affirmation of their safety and effectiveness. Based 

on information available in the future, FDA may take a 

different position on their use. 

Classification of substances as new animal drugs of low 

regulatory priority does not exempt facilities from 

complying with other federal, state, and local environmental requirements. For example, facilities using these 

substances would still be required to comply with 

National Pollutant Discharge Elimination System 

requirements. 

The LRP drugs of greatest importance to walleye 

culture include hydrogen peroxide and sodium chloride 

(salt). Hydrogen peroxide has been used experimentally 

to control fungal infections in eggs (Colesante 1995). 

Although hydrogen peroxide has the potential as a 

parasiticide, it has been found to be extremely toxic to 

fingerling walleye at concentrations greater than 50 

ppm in a one-hour static bath (R.D. Clayton and R.C. 

Summerfelt, Iowa State University, personal communication). Salt is a proven parasiticide and also useful and 

allowed as an osmoregulatory aid under LRP status. 

Copper sulfate and potassium permanganate have been 

deferred for regulatory action when they are used to 

control fungal and bacterial infections and external 

parasitic infestations on walleye. INAD’s for both these 

compounds have been developed at the encouragement 

of CVM. 

Chemicals used to regulate water pH are not considered 

drugs and therefore, CVM does not regulate them. 

These chemicals include calcium carbonate, calcium 

hydroxide, sohum hydroxide, and tris buffer. EPA or 

the states have primary jurishction over these compounds. Facilities need to comply with the rules 

developed by the agency responsible for administering 

the National Pollutant Discharge Elimination System 

(NPDES) permits. 

Dimethylpolysiloxane, when used to reduce foam or 

scum, is classified as a non-drug by CVM. It is also 

under EPA jurisdiction, and facilities are required to 

contact the NPDES permitting agency prior to discharging water containing ths chemical. 

NCRAC Culture Series 101 - Walleye Culture Manual 349 
Chapter 16 - Diseases and Chemotherapeutics 

Provisions for unapproved drug use in walleye 

culture 

currently, 18 unapproved drugs are considered to be 

LRP aquaculture drugs. CVM will not object to the use ~ ~~~~~~~ ~ 

of drugs that arc classified as LRP if they arc used 

under the conditions indicated, at the prescribed levels, 

according to good management practices, of an 

appropriate grade for use on food animals, and not 

FDA recognized that provisions were needed to allow 

the use of certain unapproved drugs in fish culture until 

data are developed for full approvals (i.e., NADAs). The 

first provision was the designation of drugs as LRP; 

Table 2. Unapproved new animal drugs of low regulatory priority (LRP) for FDA’ (Working 

Group 1994) 

Common Name Permitted Use 

Acetic Acid 

Calcium chloride 

Calcium oxide 

Carbon dioxide gas 

Fuller’s earth 

Garlic (whole) 

Hydrogen peroxide 

Ice 

Magnesium sulfate 

(Epsom salts) 

Onion (whole) 

Papain 

Potassi u m c h Io ri de 

Povidone iodine 

compounds 

Sod i um bicarbonate 

(baking soda) 

Sodium chloride (salt) 

Sodium sulfite 

Thiamine hydrochloride 

Urea and tannic acid 

Used as a dip at a concentration of 1,000-2,000 mg/L for 1-1 0 minutes as a parasiticide for fish 

Used to increase water calcium concentration to ensure proper egg hardening. Dosages used would be 

those necessary to raise calcium concentration to 10-20 mg/L calcium carbonate. Also used to increase 

water hardness up to 150 mg/L to aid in maintenance of osmotic balance in fish by preventing electrolyte 

loss. 

Used as an external protozoacide for fingerling to adult fish at a concentration of 2,000 mg/L for 5 sec. 

Used for anesthetic purposes in cold- cool- and warmwater fish. 

Used to reduce the adhesiveness of fish eggs in order to improve hatchability. 

Used for control of helminth and sea lice infestations in marine salmonids at all life stages. 

Used at 250-500 mg/L to control fungi on all species and at all life stages of fish, including eggs. 

Used to reduce metabolic rate of fish during transport. 

Used to treat external monogenetic trematode infestations and external crustacean infestations in fish at all 

life stages. Used in freshwater species. Fish are immersed in a solution of 30,000 mg/L magnesium sulfate 

and 7,000 mg/L sodium chloride for 5-10 min. 

Used to treat external crustacean parasites and to deter sea lice from infesting external surface of fish at all 

life stages. 

Used as a 0.2% solution in removing the gelatinous matrix of fish egg masses in order to improve hatchabil 

ity and decrease the incidence of disease. 

Used as an aid in osmoregulation to relieve stress and prevent shock. Dosages used would be those 

necessary to increase chloride ion concentration to 10-2,000 mg/L. 

Used as a fish egg disinfectant at rates of 50 mg/L for 30 min during water hardening and 100 mg/L solution 

for 10 min after water hardening. 

Used at 142-642 mg/L for 5 min as a means of introducing carbon dioxide into the water to anesthetize fish. 

Used as a 0.5-1% solution for an indefinite period as an osmoregulatory aid for the relief of stress and 

prevention of shock. Used as a 3% solution for 10-30 min as a parasiticide. 

Used as a 15% solution for 5-8 min to treat eggs in order to improve hatchability. 

Used to prevent or treat thiamine deficiency in salmonids. Eggs are immersed in an aqueous solution of up 

to 100 ppm for up to 4h during water hardening. Sac fry are immersed in an aqueous solution of up to 

1,000 ppm for up to 1 h. 

Used to denature the adhesive component of fish eggs at concentrations of 15 g urea and 20 g NaCII5 L of 

water for about 6 min, followed by a separate solution of 0.75 g tannic acid/5 L of water for an additional 6 

min. These amounts will treat about 400,000 eggs. 

350 NCRAC Culture Series 101- Walleye Culture Manual 
Diseases and Chemotheraneutics - Chanter 16 

likely to cause an adverse effect on the environment 

(Worlung Group 1994; Table 2). 

The second provision for the use of unapproved drugs is 

somewhat limited, but it offers some relief. That 

provision is extra-label use. CVM will allow extra-label 

drug use if the health of the animals is threatened and if 

suffering or death would result from failure to treat the 

affected animals. The major provisions of extra-label 

use includes: 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

Only applies to drugs approved for other animal 

species ; 

Only available for practicing veterinarians; 

A valid veterinarian-client-patient relationship 

exists; 

There is no approved or effective approved drug 

available for the species in question; 

The veterinarian assumes responsibility for 

efficacy, animal safety, and residue levels in 

edible tissues; 

Not for use with drugs mixed in feeds; 

Not for use with drugs that prevent disease; 

Not for use with drugs that improve growth rates 

or enhance reproduction or fertility; and 

Human drugs are eligible. 

Thus, under these criteria, only formalin could be 

prescribed for use on walleye by practicing veterinarians. Until the passage of “The Animal Medicinal Drug 

Use Clarification of 1994” (Public Law 103-396), extralabel use of medcated feeds in aquaculture was 

considered by CVM to be low regulatory priority if the 

requirements of the Extra-label Use Compliance Policy 

Guide 7125.06 were met and as long as the feeds were 

formulated and labeled properly in accordance with 

medicated feed regulations. This meant that 

oxytetracycline medcated feed approved for use in 

catfish could be prescribed extra-labelly for walleye by 

a licensed veterinarian and still be consistent with the 

extra-label use policy guidelines. When CVM promulgates extra-label regulations under the 1994 law 

(required by October 1996), The dscretionary extralabel use in feed may be eliminated or curtailed because 

of the law expressly prohbits extra-label use of feed 

use drugs (CVM, personal communication). 

All the remaining aquaculture compounds considered to 

be drugs by CVM are expected to have approvals or to 

be used under provision of an INAD exemption (Geyer 

1993). There are two types of INADs: standard and 

compassionate. The compassionate INADs are being 

granted to producer groups and agencies willing to 

accept the responsibility of administering these INAD’s. 

Under a compassionate INAD exemption, data must be 

generated to support the approval of the drug; if it is 

not, the INAD will not be renewed (Geyer 1994). All 

chemicals defined as drugs that are either currently used 

or anticipated for emergency use must be covered under 

an INAD so that the need for emergency use is eliminated or at least greatly reduced. All users of these 

identified drugs, except researchers working on 

laboratory animals, are included under tlvs INAD 

process-public and private production facilities, 

fishery management units, and participants in fishng 

tournaments, etc. (Stefan 1994). This process is not a 

panacea, but it is the only means of using the needed 

drugs legally (Geyer 1993). 

FWS recognized that, as a national resource agency, it 

needed to provide leadership in complying with CVM 

regulations. In October, 1992, FWS started the process 

by initiating a survey of the chemical and drug needs of 

national fish hatcheries and other fishery facilities. At a 

workshop on INAD’s in January, 1993, it was determined which drugs and chemicals would: 1) have 

INADs developed or renewed; 2) be presented to FDA 

for an opinion on their status; 3) not require INAD; and 

4) not be acted upon. When the chloramine-T INAD 

was completed for FWS by the newly created National 

Biological Service (NBS), a template was offered for 

others to develop an INAD on chloramine-T or other 

water treatment compounds. 

As of September, 1994, CVM had permission to 

disclose 74 INAD permits that are in place for 26 

aquaculture drugs. Twenty-two disclosable, aquaculture 

INAD’s were granted by CVM for use on walleye and 

include chloramine-T, oxytetracycline, human chorionic 

gonadotropin, copper sulfate, a copper compound, 

formalin, and diquat dibromide. Updated lists of 

disclosable INAD’s may be obtained from CVM. 

Walleye producers are encouraged to enroll in the 

quality assurance program being developed by the 

National Aquaculture Association (S hepherdstown, 

WV). This program is important because it will 

improve production efficiency and provide the consumers with farm-raised products that are healthy and free 

of any harmful chemical residues (Worlung Group 

1994). 

NCRAC Culture Series 101 - Walleye Culture Manual 351 
Chapter 16 - Diseases and Chemotherapeutics 

Table 3. Status of aquaculture chemicals not considered low regulatory priority (LRP) 

drugs. 

Su bstances-Uses Drugs not High Regulatory Regulatory Action Chemicals 

LRP' Priority' Deferred not Dr u gs2 

Acriflavine - egg disinfectant; bactericide, 

fungicide, and parasiticide 

Benzalkonium chloride - bactericide 

Benzethonium chloride - bactericide 

Benzocaine - anesthetic 

Calcium carbonate - water pH and alkalinity control 

Calcium hydroxide - water pH control 

Carp pituitary extract - hormone 

Chloramphenicol - bactericide 

Copper sulfate - parasiticide, bactericide, and fungicide 

Cytochalasin B - oyster triploidy inducer 

Dimethylpolysiloxane - foam reducer 

Epinephrine - clam and oyster metamorphosis inducer 

Erythromycin 

b-Estradiol - hormone 

Fluoroquinolones (e.g., enrofloxacin, sarafloxacin) 

- bactericides 

Human chorionic gonadotropin - hormone 

L-Dopa - clam and oyster metamorphosis inducer 

Luteinizing hormone - releasing hormone analog - hormone 

Malachite green - parasiticide and fungicide 

Methylene blue - parasiticide 

1 7-a-met hy I testoste rone - hormone 

Metomidate - anesthetic 

Nit rof u rans (e. g . , nit rof u razone, f u razol id one, n i fu rpi ri no I) - 

Oxygen - maintain saturated dissolved oxygen conditions 

Oxytetracycline - fish marker and bactericide 

Ozone - disinfectant and organic compound remover 

Penicillin - control bacteria in algae fed to oyster larvae 

Penicillin dihydrostreptomycin - bactericide 

Potassi u m per mang an a te - oxi d i ze r, d i si n fect an t , 

parasiticide, bactericide, and fungicide 

Quinaldine - anesthetic 

Quinolones (e.g., oxolinic acid, naladixic acid) 

- bactericides 

Salmon pituitary extract - hormone 

bactericides 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

Sodium chlorite - bactericide and fungicide 

Sodium hydroxide - water pH control 

Streptomycin - control bacteria in algae fed 

to oyster larvae 

X 

Sulfonamides (e.g., sulfamerazine, sulfadimethoxine) 

- bactericides 

Trichlorfon - parasiticide X 

Tris buffer - water pH buffering 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

'Compassionate INAD's or NADA's are required for the use of these drugs; however, no compassionate INAD's will be issued for 

malachite green, methylene blue, chloramphenicol, or the sulfonamides. 

*These chemicals are not considered drugs for the specific intended uses recommended. 

352 NCRAC Culture Series 101- Walleye Culture Manual 
Diseases and Chemotherapeutics - Chapter 16 

Data generation for drug approvals for walleye 

cu It ure 

The INAD process will work only if INAD’s lead to 

approved NADA’s. The INAD’s will allow the use of 

drugs on a provisional basis until all the data needed by 

FDA for new animal drug applications are generated. 

These data include target animal safety, residue, 

environmental, and mammalian safety studes. These 

data must be developed in a timely manner or the 

INAD’s will be rescinded. These data (except field 

efficacy) must be generated according to good laboratory practices (GLP) provisions. 

Data required for the approval of human chorionic 

gondatropin (HCG) as a hormone to regulate spawning 

is being funded the Striped Bass Technical Committee, 

American Fisheries Society, and implemented by 

Auburn University and Intervet, Inc. Currently, CVM 

has lsclosed that at least 13 INAD’s on HCG are in 

effect. The National Research Support Program 

Number 7 (formerly the IR-4 program) has projects 

underway for amoxicillin, chloramine-T, copper sulfate, 

formalin, and oxytetracycline. 

Fundmg to support the generation of human food 

safety, analytical methods development, and environmental fate data for certain drugs for use on freshwater 

fish was recently obtained by a partnership with the 

International Association of Fish and Wildlife Agencies 

(IAFWA), on behalf of state agencies, NBS, and FWS 

(NBS 1994). Research to gain approvals or extensions 

of approvals for eight drugs identified as high priority 

by the states and to establish the crop grouping concept 

was initiated July 1 , 1994 and will extend until June 30, 

1999. During that 5-year period, at least 37 states will 

contribute a total of $3.6 million, NBS’s Fish Farming 

Experimental Laboratory, Stuttgart, AR will provide 

some base fundmg, and the Upper Mississippi Science 

Center (formerly the National Fisheries Research 

Center, La Crosse, WI) will provide $4 million toward: 

(1) development of data under GLP provisions to gain 

CVM approval of NADA’s for use in aquaculture for 

certain essential drugs; and (2) development of research 

information to allow acceptance of a crop grouping 

concept by CVM. Data will be generated to allow CVM 

to assess whether a few selected fish species can be 

used as surrogates for all or most of the cultured 

freshwater fishes in the United States. 

Walleye culture should benefit greatly from the IAFWA 

project. Walleye are included in the development of 

data toward: (I) an extension of formalin to control 

fungi on eggs, as well as expansion to control external 

parasites and fungi on fish themselves; (2) an extension 

of oxytetracycline in the feed additive formulation to 

control furunculosis, bacterial hemorrhagic septicemia, 

and pseudomonas dlsease, to mark fish, and an expansion to control flexibacteriosis; (3) a NADA for 

chloramine-T to control flexibacteriosis; (4) classification as LRP drugs or NADAs for copper sulfate and 

potassium permanganate to control external fungal and 

bacterial infections and external parasitic infestations; 

(5) a NADA for benzocaine as an anesthetic and 

sedative with a low withdrawal time to supplement the 

use of Finquel; and (6) delineation of safe and efficacious concentrations of hydrogen peroxide as a LRP 

fungicide for walleye in all life stages, includmg eggs, 

and potential expansion of the LRP drug status to 

control external parasitic infestations and fungal and 

bacterial infections. Sarafloxacin is one of the drugs 

included in the IAFWA project and is a potential 

candidate to control bacterial diseases in walleye; 

however, research on sarafloxacin will be delayed until 

new information becomes available on the 

fluoroquinolones. The U.S. Food and Drug Administration will determine the extent of their use in animal 

husbandry “in terms of both the benefits of these drugs 

to animals and the potential risks to humans with 

respect to the use of these drugs in animals, includmg 

further antimicrobial resistance in human pathogens” 

(Anon. 1994). 

Walleye culture will benefit because walleye will be 

one of the four fish species used to test the crop 

grouping concept under the IAFWA project. Information will be generated on the pharmacohnetics, uptake, 

and elimination of drugs in walleye that will lead to 

their inclusion in NADAs, thus providing more treatment options for walleye culturists, researchers, and 

fishery managers. 

Summary 

walleye culturists have the legal use of a few chemicals and drugs at the present time. However, INAD’s in 

effect on walleye could lead to NADA approvals that 

will increase options for controlling dsease and 

improving production conditions. A major project 

through the IAFWA will provide the fundmg needed to 

NCRAC Culture Series 101 - Walleye Culture Manual 353 
Chapter 16 - Diseases and Chemotherapeutics 

complete research on hydrogen peroxide and to develop 

NADA's on seven drugs important to the culture of 

walleye. 

Ref e re n ces 

Anon. 1994. FDA hearing set for fluoroquinolone animal 

drugs. Food Chemical News 36(34):20-23. 

Colesante, R.T. 1995. Transporting and handling of 

walleye eggs, fry, fingerlings and broodstock. Pages 

79-84 in R.C. Summerfelt, editor. Walleye culture 

manual. NCRAC Culture Series 101. North Central 

Regional Aquaculture Center Publications Office, 

Iowa State University, Ames, Iowa. 

Geyer, R.E. 1993. FDA enforcement policy for aquaculture. Salmonid XVI1(1):4-10. 

Geyer, R.E. 1994. The aquaculture industry: a look 

ahead. Salmonid XVlll(1):12-14; 23. 

Homaire, S. 1994. FDA enforcement as it relates to 

aquaculture. Salmonid XVI I 1(1):7-9. 

NBS (National Biological Survey). 1994. Approval of 

drugs for public fish production. National Fisheries 

Research Center, La Crosse, WI. 

Stefan, G.E. 1994. Impact of FDA regulations on 

aquaculture animal drug use. Salmonid XVlll(1):lO12. 

Working Group (Working Group on Quality Assurance in 

Aquaculture Production, Joint Subcommittee on 

Aquaculture). 1992. Federal regulations of drugs, 

biologicals, and chemicals used in aquaculture 

production. National Agricultural Library, Beltsville, 

MD. 

Working Group (Working Group on Quality Assurance in 

Aquaculture Production, Joint Subcommittee on 

Aquaculture). 1994. Guide to drug, vaccine, and 

pesticide use in aquaculture. Texas Agricultural 

Extension Service, College Station, TX. June 1994. 

68 PP. 

354 NCRAC Culture Series 101- Walleye Culture Manual 
McAllister, P. E. 1996. Viruses associated with diseases of walleye, sauger, and walleye x sauger 

hybrids. Pages 355-367 in R. C. Summerfelt, editor. Walleye culture manual. NCRAC Culture 

Series 101. North Central Regional Aquaculture Center Publications Office, Iowa State University, Ames. 

Viruses Associated with Diseases of 

Walleye, Sauger, and Walleye x Sauger 

Hybrids 

Philip E. McAllister, U. S. Department of the Interior, National Fish Health Research Laboratory, 1700 Leetown 

Road, Kearneysville, WV 25430 

(Note: All Figures appear on pages 136-138.) 

Scientific literature concerning viral diseases of young 

walleye, sauger, and walleye x sauger hybrids is 

basically nonexistent. In contrast, virus-associated 

disease in adult walleye and sauger is well documented, 

probably because the lesions associated with these virus 

infections are visually prominent (Table 1). Although 

superficial, the lesions are temporarily chsfiguring and 

can affect the marketability of the fish. These viral 

infections are not associated with epizootic mortality; 

but, in some cases, losses can occur from secondary 

bacterial and fungal infection of abraded, dslodged, or 

sloughed lesions. 

Introduction 

The four viral diseases of walleye are: lymphocystis, 

dermal sarcoma, diffuse epidermal hyperplasia, and 

discrete epidermal hyperplasia. Infectious pancreatic 

necrosis virus has also been isolated from walleye, but 

no signs of dsease were evident. From sauger, only one 

virus-associated disease is documented, lymphocystis. 

From walleye x sauger hybrids, no virus-associated 

disease is reported. 

The occurrence of viral disease in young walleye, 

sauger, and walleye x sauger hybrids and interrelationships between viral dsease prevalence between younger 

and older fish need to be determined. Disease in adult 

fish could be a result of infection in early life, and 

conversely infections in adult fish could be transmitted 

to the young. In addition, anecdotal observations, at 

least for lymphocystis, suggest that increasing geographical dstribution of viral dnease of these fishes 

could be a consequence of culture and stocking 

programs. 

Ly rn p hocyst is 

Descriptors - disease: lymphocystis 

lymphocystis disease 

lymphocystis iridovirus 

- virus: lymphocystis virus 

Walleye and sauger are among the more than 125 

species of fish susceptible to lymphocystis, a viral 

disease characterized by profound cell enlargement 

(Amin 1979; Wolf 1988). The disease was first reported 

in the United States in walleye (Mavor and Feinberg 

191 8). Lymphocystis is recognized worldwide and 

occurs in warm, cool, and cold water species from 

freshwater, estuarine, and marine environments. 

Lymphocystis is a chronic, but seldom fatal, disease. 

The nodular lesions eventually rupture or slough and 

heal. 

Clinical signs and histopathology 

Fish with lymphocystis develop macroscopic, superficial nodules on fins (Figure 1), skin, and eyes and 

sometimes on internal organs and tissues (Templeman 

1965; Dunbar and Wolf 1966; Dukes and Lawler 1975; 

Amin 1979; Wolf 1988). Nodules are cream to pink or 

gray, dependmg on the condtion of the overlying 

epithelium and on the degree of vascularity. The lesions 

are composed of one or more cells that increase 21 05- 

fold in volume to become 0.3 to 2.0 mm in diameter 

(Figure 2). 

Lymphocystis virus infection causes a unique cellular 

hypertrophy (Nigrelli and Ruggieri 1965; Dunbar and 

Wolf 1966; Lopez et al. 1969; Yamamoto et al. 1985a). 

The cytoplasm and nucleus become greatly enlarged as 

infection progresses (Figures 3 and 4); infected cells do 

not &vide. Degenerative changes in the nucleus include 

condensation and fragmentation of chromatin and 

NCRAC Culture Series 101 - Walleye Culture Manual 355 
Chapter 16 - Diseases and Chemotherapeutics 

Figure 1. Walleye showing lymphocystis lesions on the 

caudal fin. FromYamamoto et al. (1976). Reprinted with 

permission of National Research Council of Canada. 

Figure 2. Aggregate of individual lymphocystis cells on 

ventral fin of a walleye. From Yamamoto et al. (1985a). 

Reprinted with permission of The Japanese Society of 

Fish Pathology. 

Figure 3. Light micrograph of lymphocystis cell and 

surrounding tissue showing epthielial cell layer, the 

grossly hypertrophied lymphocystis cell, and a layer of 

hyaline substance surrounding the lymphocystis cell. 

From Yamamoto et al. (1976). Reprinted with permission of 

National Research Council of Canada. 

Figure 5. Electron micrograph of a thin section of a 

lymphocystis cell showing accumulation of lymphocystis 

virus in the cell cytoplasm. Line = 1 mm. FromYamamoto 

et al. (1976). Reprinted with permission of National 

Research Council of Canada. 

Figure 4. Light micrograph of walleye dermal sarcoma 

lesion in which two lymphocystis cells are embedded. 

From Yamamoto et al. (1976). Reprinted with permission 

of National Research Council of Canada. 

Figure 6. Dermal sarcoma nodules on the operculum of a 

walleye. From Yamamoto et al. (1985a). Reprinted with 

permission of The Japanese Society of Fish Pathology. 

356 NCRAC Culture Series 101- Walleye Culture Manual 
Diseases and Chemotherapeutics - Chapter 16 

Figure 7. Fingerling walleye experimentally infected by 

intramuscular injection of cell-free filtrate of walleye 

dermal sarcoma tissue. Arrows indicate multiple dermal 

sarcomas on the skin. From Bowser et al. (1990). 

Reprinted with permission of the American Fisheries 

Society. 

Figure 9. Areas of diffuse epidermal hyperplasia (arrows) 

on the dorsal skin surface of a walleye. FrornYamamoto et 

al. (1985a). Reprinted with permission of The Japanese 

Society of Fish Pathology. 

Figure 8. Electon micrograph of a thin section of a walleye 

dermal sarcoma cell showing enveloped retrovirus 

particles. Virus particles measure about 135 nm in 

diameter; arrows indicate the electron-dense, inner core of 

the virus particle. From Yamamoto et al. (1976). Reprinted 

with permission of National Research Council of Canada. 

Figure 10. Electron micrograph of a thin section of walleye 

diffuse epidermal hyperplasia tissue showing enveloped 

herpesvirus particles in intercellular spaces. From Kelly et 

al. (1983). Reprinted with permission of Blackwell 

Scientific Publications. 

Figure 11. Areas of discrete epidermal hyperplasia (arrows) 

on the dorsal fin of a walleye. FrornYamamoto et al. 

(1 985b). Reprinted with permission of Blackwell Scientific 

Publications. 

Figure 12. Light micrograph of walleye discrete epidermal 

hyperplasia tissue (right portion of the picture). Normal 

epidermis appears in the left portion of the picture (area of 

double bar). From Walker (1969). 

NCRAC Culture Series 101 - Walleye Culture Manual 357 
Chapter 16 - Diseases and Chemotherapeutics 

Figure 13. Electron micrograph of walleye discrete 

epidermal hyperplasia showing enveloped retrovirus 

particles in intercellular spaces. From Yamamoto et al. 

(1 985b). Reprinted with permission of Blackwell Scientific 

Publications. 

enlargement, &stortion, and dmolution of the nucleoli. 

Within the expandmg cytoplasm, Feulgen-positive 

inclusions develop that appear as web-like filaments or 

as dense vacuolated bodies. Web-like, filamentous 

inclusions are common in walleye and sauger (Figure 3) 

(Walker 1966). Associated with inclusions are 

lymphocystis virus particles 

(Figure 5). As the infected cell 

develops, a thick hyaline 

capsule forms at the periphery 

(Pritchard and Malsberger 

1968; Howse and Christmas 

1970). Proliferating fibroblasts 

isolate infected cells, and 

accumulating plasma cells, 

lymphocytes, macrophages, 

and polymorphonuclear 

leukocytes are evidence of host 

inflammatory response. 

Lesions eventually heal with 

little evidence of scar tissue. 

Diagnosis and virus 

detect ion 

Lymphocystis can occur in 

walleyes that exhibit clinical 

signs of dermal sarcoma, 

diffuse epidermal hyperplasia, 

and discrete epidermal 

hyperplasia (Figure 4; Table 

1). Lymphocystis and walleye 

dermal sarcoma have similar macroscopic clinical signs 

(Figures 1 and 6). In addition, lymphocystis can be 

confused with superficial cysts of certain parasites 

(Wolf 1988). Definitive diagnosis requires histological 

examination (Figures 3 and 4). 

Lymphocystis virus can be isolated from homogenates 

of lesion tissue. Cell cultures vary in susceptibility to 

individual isolates of lymphocystis virus, suggesting 

that there are intrinsic differences among virus isolates 

from different hosts (Wharton et al. 1977; Kelly et al. 

1980a). Lymphocystis virus replication can occur in 

freshwater fish cell lines of centrarchid (BF-2, BF-W, 

and LBF-l), percid (WC-l), and sciaenid (SP-1) origin 

(Wolf et al. 1966; Midlige and Malsberger 1968; 

Wharton et al. 1977; Kelly et al. 1980a; Walker and Hill 

1980; Berthiaume et al. 1984) and in a variety of cell 

lines derived from marine fish (Middlebrooks et al. 

1979, 1980, 1981). The virus does not replicate in 

cyprinid (FHM), ictalurid (BB), or salmonid (RTG-2) 

cells (Wolf, 1988; McAllister, 1993a). 

Electron micrographs of walleye lesions show enveloped lymphocystis virus particles that are 300-380 nm 

Table 1. Clinical characteristics of virus-associated skin lesions 

of walleye and sauger. 

Skin Lesion CI i n ical Characteristics Virus 

Lym phocystis 

Walleye dermal sarcoma 

Walleye diffuse 

epidermal hyperplasia 

Walleye discrete 

epidermal hyperplasia 

Greatly enlarged, spherical cell; lridovirus 

Lesion usually aggregate of cells; 

Texture-smooth, undulating; 

Color-cream to pink or gray. 

Single or multiple nodules; 

Shape-dome to spherical; 

Texture-smooth to rough; 

Color-white to pink. 

Laterally-spreading, indistinct; 

S hape-t h in, flat, ill -def i n ed ; 

Text u re-vi sco us s I i m e, m u co i d ; 

Co lor-t rand ucent. 

Sharply delineated, firm, thick; 

Shape-dome plaque; 

Texture-smooth, undulating; 

Color-translucent, clear to gray. 

Retrovirus 

Herpesvirus 

Retrovirus 

358 NCRAC Culture Series 101- Walleye Culture Manual 
Diseases and Chemotheraneutics - Chanter 16 

in diameter (Figure 5); the icosahedral viral capsid is 

-260 nm in diameter (Yamamoto et al. 1976, 1985a). 

Lymphocystis virus, an iridovirus, is remarkably stable. 

Significant levels of infectivity were recovered from 

lyophilized lesion homogenate that had been held for 

13.5 years at 4OC, and residual infectivity was detected 

in dned lesion tissue held for 15 years at 4°C (Wolf et 

al. 1979). Lymphocystis virus is inactivated by exposure 

to lipid solvents (i.e., ether or chloroform), is progressively inactivated by sonication, but is stable to multiple 

cycles of freezing and thawing (Wolf 1988). 

Transmission and epidemiology 

Experimentally, lymphocystis can be transmitted by 

cohabitation, by exposure to water containing virus, by 

feeding excised lesion and lesion homogenate, and by 

applying lesion homogenate to gills and to scarified 

slun (Wolf 1962, 1988). The dsease can also be 

experimentally transmitted by subcutaneous and 

intraperitoneal lesion implantation and by subdermal 

and intramuscular injection of lesion homogenate and 

medium from virus-infected cell cultures (Wolf 1962; 

Dunbar and Wolf 1966; Wolf et al. 1966, 1979). 

Lymphocystis can be experimentally transmitted with 

relative ease among species of a genus, but with 

difficulty among families. Such host specificity 

suggests the existence of strains of lymphocystis virus 

(Walker 1958; Wolf 1988). 

Based on results of experimental challenges, 

lymphocystis virus is transmitted under natural conditions by exposure to waterborne virus or by ingestion of 

virus-infected cells (Stephens et al. 1970; Lawler et al. 

1974). Mechanical injury to epithelial surfaces provides 

a portal of entry for waterborne virus. The stability of 

lymphocystis virus to drying and freezing and thawing 

suggests that the virus can remain infectious for an 

extended period (Wolf et al. 1979). Templeman (1 965) 

reports that the numbers of lymphocystis lesions are 

greater in heavily parasitized fish, and Ryder (1961) 

suggests that social behavior is a factor in the transmission of lymphocystis. During spawning, behavioralassociated abrasion of the mucous layer protecting the 

skin, in conjunction with congregation of fish on 

spawning grounds, provides higher probability of 

contact with the virus. Amin (1 979) reports that the 

prevalence of lymphocystis is related to the population 

density, and when a population is composed of a higher 

percentage of larger, older fish, the prevalence of 

lymphocystis is higher. 

In naturally infected walleye and sauger populations, 

lymphocystis occurs with equal frequency in both sexes 

and is observed throughout the year (Amin 1979; 

Bowser et al. 1988). Prevalence is higher in walleye 

than in sauger (Amin 1979). Survey data show that 

prevalence of lymphocystis in walleye varies with the 

season and approaches 38% in some populations. 

Prevalence is highest in early spring (water temperature 

= 0-SOC), is lowest from late spring to early autumn 

(water temperature = 10-25"C), and then increases 

significantly in late autumn (water temperature = 5- 

10°C) (Bowser et al. 1988). Host inflammatory response to lymphocystis lesions can occur throughout 

the year (Bowser et al. 1988). Experimentally, 

lymphocystis can occur in many species of any age, but 

natural infections of walleye and sauger have been 

reported only in older fish-an observation possibly 

skewed by sampling methodology (Amin 1979; 

Yamamoto et al. 1985b; Bowser et al. 1988). 

Fishery management activities can affect the prevalence 

and geographcal distribution of lyrnphocystis. Scale 

sampling, fin clipping, tagging, and similar sampling 

and marking practices promote higher prevalence of 

lymphocystis. Clifford and Applegate (1 970) reported 

lymphocystis occurred at higher prevalence in tagged 

(35.4%) compared to untagged (2.2%) walleye and 

found lesions were concentrated at the site of tag 

attachment. The occurrence of lymphocystis in areas 

previously free from the disease has been associated 

with stocking of hatchery-raised walleye, which were 

predominantly stocked as fingerlings (Amin 1 979). 

Industrial contaminants do not appear to be a contributing factor in increasing the prevalence or severity of 

viral skin lesions in walleyes. Smith et al. (1992) found 

no evidence that pollution affected the prevalence of 

one form of skin lesion (i.e., lymphocystis, dermal 

sarcoma, or &screte or &ffuse epidermal hyperplasia) 

over another and that aromatic hydrocarbon contaminant levels were actually lower in walleye with 

lymphocy stis. 

Lymphocystis is a chronic, seldom fatal disease. 

Secondary bacterial and fungal infections can develop 

at sites of abraded or &dodged lesions, which can 

affect the health of a fish. Walleye with lymphocystis 

NCRAC Culture Series 101 - Walleye Culture Manual 359 
~ 

Chapter 16 - Diseases and Chemotherapeutics 

weighed 5.5-6.5% less than fish of the same length that 

were lymphocystis-free (Hile 1954), and Ryder (1961) 

reported that fish with lymphocystis lesions are more 

easily captured. 

Treatment and control 

Lymphocystis occurs widely in nature, and there is no 

practical treatment for the disease. 

Several approaches have been used to attempt prevention or control of lymphocystis in intensive culture 

systems. Fish should be raised in controlled water 

supplies (i.e., well water or spring water) because the 

virus can enter a production facility by way of open, 

untreated water supplies that contain infected fish. 

Further, raising fish in colder water can at least slow 

lesion development (Wolf 1988). Once lymphocystis is 

detected, removal of lesion-bearing fish is generally 

ineffective to control the infection. 

The use of drugs or chemicals to control lymphocystis 

has not been extensively studied. Lopez et al. (1 970) 

showed that the antineoplastic drug 6-mercaptopurine 

inhbited the appearance of virus-induced cytopathic 

effects in cell culture and controlled lymphocystis in 

fish. 

Currently, no drug, chemical, vaccine, or pesticide is 

approved for use to treat lymphocystis in walleye, 

sauger, or walleye x sauger hybrids (Federal Joint 

Subcommittee on Aquaculture 1994). 

Walleye dermal sarcoma 

Descriptors - &sease: walleye dermal sarcoma 

- virus: walleye dermal sarcoma 

walleye dermal sarcoma 

WDS virus 

WDS 

retrovirus 

virus 

Walleye dermal sarcoma is a slun tumor that is commonly seen in walleye from the Great Lakes regions of 

the United States and from the central and western 

regions of Canada (Walker 1958, 1961, 1969; 

Yamamoto et al. 1976; Bowser et al. 1988). Walleye 

dermal sarcoma is recognized as caused by a virus 

because the tumor can be transmitted using cell-free 

filtrates from tumors and because retrovirus-like 

particles are associated with tumor tissue. The disease is 

principally associated with adults, but has been experimentally transmitted to fingerlings. 

Clinical signs and histopathology 

walleye dermal sarcoma is a surface lesion that 

consists of single or multiple dermal nodules covered 

by a thin layer of epidermis (Figures 6 and 7). Nodules 

occur predominantly on the trunk, but also occur on the 

head and fins. They are 1-10 mm in diameter, have a 

dome to spherical shape, and are smooth to rough in 

texture and firm in composition. Nodules are pink to 

white appearance, depending on the level of vascularity 

and the density of fibrous tissue. Abrasion or detachment of a tumor leaves an open, bleedng lesion. The 

nodules appear similar in gross morphology to 

lymphocystis (Figures 1 and 6), and lymphocystis cells 

can be found embedded in the sarcoma. However, the 

highly proliferative sarcoma cells can be readily 

distinguished microscopically from the hypertrophied 

lymphocystis cells (Figure 4). 

The tumor is confined to the shn, and invasion of 

adjacent tissue or metastasis usually does not occur. 

Invasion of surrounding tissue has been seen in experimentally infected, fingerling walleye (Paul Bowser, 

personal communication). Dermal sarcoma cells are not 

enlarged, but they vary in morphology and are extensively conjoined by interdgitating cytoplasmic processes, collagen, and transverse capillaries (Yamamoto 

et al. 1985a; Martineau et al. 1990b). The cytoplasm 

varies in abundance and can be highly vesiculated. The 

size and shape of the nucleus varies, and can appear 

sharply marginated (Walker 196 1, 1969; Yamamoto et 

al. 1976, 1985a). Tumor composition varies from a 

highly proliferative, irregularly arranged sarcoma to a 

densely whorled, well organized fibroma (Martineau et 

al. 1990b). Bowser et al. (1 988) report that an inflammatory response to the tumor is temperature related, 

with significant response in spring and summer and 

little to no response in autumn. Martineau et al. (1990b) 

indicate that the density of lymphocyte infiltration 

between the tumor and adjacent dermis occurs with no 

significant seasonal variation. Dense lymphocyte 

infiltration of the tumor is associated with necrosis. 

360 NCRAC Culture Series 101- Walleye Culture Manual 
Diseases and Chemotheraneutics - Chanter 16 

Diagnosis and virus detection 

walleye dermal sarcoma often occurs in fish concurrently showing clinical signs of lymphocystis and 

diffuse and discrete epidermal hyperplasia. Walleye 

dermal sarcoma can be readily confused with 

lymphocystis because both conditions have similar 

macroscopic clinical signs (Table 1). Conclusive 

dagnosis requires histopathological examination. 

A virus that causes this disease has not been isolated in 

cell culture. Polymerase chain reaction analysis of 

infected cell cultures suggests that the virus replicates 

in cultured cells without dramatic cytopathology (Paul 

Bowser, personal communication). Electron micrographs of tumor tissue show accumulations of 

retrovirus-like particles (Walker 1961, 1969; Yamamoto 

et al. 1976, 1985a; Martineau et al. 1990a). Virus 

particles are more prevalent in tumors occurring in the 

spring of the year (Paul Bowser, personal communication). The type C retrovirus-like virions are 90-135 nm 

in diameter and contain a centrally placed, electron 

dense nucleoid =75 nm in diameter (Figure 8). Variations in the overall diameter of the virion are apparently 

due to different methods of processing and measurement. Virus particles bud from cytoplasmic membranes 

and accumulate in intercellular spaces and cytoplasmic 

vesicles. The retrovirus (RNA genome) associated with 

walleye dermal sarcoma seems to be ultrastructurally 

different from retrovirus associated with walleye 

hscrete epidermal hyperplasia (Figure 13) (Yamamoto 

et al. 1985b), and it is decidedly different from the 

iridovirus (DNA genome) that causes lymphocystis 

(Figure 5) (Yamamoto et al. 1976) and the herpesvirus 

(DNA genome) associated with walleye diffuse 

epidermal hyperplasia (Figure 10) (Kelly et al. 1983). 

No evidence of virus was seen in a cell line (WC-1) 

derived from walleye dermal sarcoma tissue (Kelly et 

al. 1980a). 

The retrovirus etiology of walleye dermal sarcoma is 

supported by several observations. Retrovirus-like 

virions were purified from tumor homogenate, and 

reverse transcriptase activity was associated with the 

purified product (Martineau et al. 1991). Molecular 

cloning of purified virus showed the genome to be 

uniquely larger (13.2 kb) than other retroviruses 

(Martineau et al. 1992). A DNA species found only in 

tumor tissue hybridzed with complementary DNA 

synthesized from retrovirus RNA purified from tumor 

homogenate (Martineau et al. 1991). Walleyes injected 

with cell-free tumor filtrates developed the tumor; 

whereas, those injected with ether-treated, cell-free 

tumor filtrate did not, which suggested that ether 

treatment disrupted the putative retrovirus lipid envelope inactivating viral infectivity (Bowser and Wooster 

1994). 

Transmission and epidemiology 

walleye dermal sarcoma was experimentally transmitted to 3-month-old fingerlings by intramuscular 

injection of cell-free tumor filtrate (Figure 7) (Bowser 

et al. 1990; Martineau et al. 1990a; Bowser and Wooster 

1994). Tumors were macroscopically evident 14 weeks 

after injection in fish held at 10°C and 8 weeks in fish 

held at 15"C, and were microscopically evident 4 weeks 

after injection in fish held at 20°C. Tumors developed 

inhscriminately on the trunk (150% of body surface), 

but also occurred on the head and fins. The tumors were 

most profound at or near the injection site. Water 

temperature affected the size and frequency of experimentally induced tumors. Tumors were larger and more 

numerous in fish held at 15°C and 20°C and were 

significantly smaller and fewer in number in fish held at 

10°C (Bowser et al. 1990). Unfortunately, no study was 

performed to determine if tumor prevalence and 

development was affected by water temperatures 

>20"C. 

Seasonal prevalence of the tumor is temperature related. 

Prevalence is high in early spring (water temperatures = 

0-5"C), decreases in late spring (water temperature = 

1 5-20°C), and is lowest in the summer (water temperatures = 20-25°C). The tumor regresses during the 

summer when water temperatures are >20"C (Bowser 

and Wooster 1991), but as water temperatures decline to 

I1 5°C in early autumn, tumor prevalence approaches 

the level seen in early spring. The temperature dependency of tumor prevalence may be related to host 

immune response (Bowser et al. 1988). 

Walleye dermal sarcoma is routinely observed in adult 

fish and the prevalence can approach 29% of the 

population (Yamamoto et al. 1976; Bowser et al. 1988; 

Smith et al. 1992). The tumor often occurs in walleye 

that also show clinical signs of lymphocystis and 

diffuse and discrete epidermal hyperplasia (Yamamoto 

et al. 1985a, 1985b; Bowser et al. 1988; Smith et al. 

1992). Dermal sarcoma is not routinely seen in young 

walleye, althoughYamamoto et al. (1976) reported a 

NCRAC Culture Series 101 - Walleye Culture Manual 361 
Chapter 16- Diseases and Chemotherapeutics 

high prevalence of dermal sarcoma and lymphocystis in 

pre-spawning walleye (250-600 g). 

Purported sex-related and age-related differences may 

reflect the seasonality of tumor prevalence, sampling 

methods, and sport fishmg pressure. Yamamoto et al. 

(1976) observed a greater prevalence of lymphocystis 

and walleye dermal sarcoma in females than in males 

and a greater prevalence in younger fish than in older 

fish. Bowser et al. (1988) reported the opposite results, 

i.e., prevalence was greater in males than females and 

greater in older than younger fish. Experimental 

transmission to fingerling walleye indcated that young 

fish were susceptible to infection and suggested that 

sexual maturity is not a prerequisite for development of 

the tumor. Extrapolation of results from experimental 

transmission suggests that young fish would be susceptible to infection under natural condtions, but no 

supporting data have been reported. 

The mechanism for transmission in nature is unknown. 

Walker (1969) suggested that the dermal tumors were 

individually infected sites, with infection initiated as the 

result of abrasion. Martineau et al. (1 990b) questioned 

the abrasion hypothesis and noted that the epithelium 

covering dermal sarcoma lesions was intact and showed 

no evidence of trauma. Furthermore, the results of 

experimental infections in which tumor development 

was remote from the site of injection suggested the 

virus was distributed systemically (Martineau et al. 

1990a). 

A growing body of evidence suggests that a variety of 

neoplasms are associated with aromatic hydrocarbon 

contaminants in water and sediments (Baumann et al. 

1991). While industrial contaminants could play a role 

in increasing the prevalence or severity of viral slun 

lesions in walleye, a comparative assessment of 

aromatic hydrocarbon contaminant levels showed that 

contaminant levels were actually lower in walleye with 

the dermal sarcoma (Smith et al. 1992). 

Thus, while the mechanism for transmission in nature 

remains undefined, susceptibility to and transmission of 

walleye dermal sarcoma could be affected by the 

interactions of a variety of factors, such as fish density 

and behavior, environmental and physiological stress 

factors, immune competence, and abundance and 

stability of the virus. 

Treatment and control 

No methods of treatment or control have been 

described for walleye dermal sarcoma. Holding fish at 

elevated water temperature (>20°C) seemingly promotes tumor regression (Bowser and Wooster 1991). 

Captive fish with abraded lesions or open wounds from 

sloughed or dislodged lesions are susceptible to 

secondary bacterial and fungal infections. 

Currently, no drug, chemical, vaccine, or pesticide is 

approved for use to treat walleye dermal sarcoma 

(Federal Joint Subcommittee on Aquaculture 1994). 

Walleye diffuse epidermal hyperplasia 

Descriptors - disease: walleye diffuse 

epiderm a1 h y perp 1 asi a 

- virus: Herpesvirus vitreum 

percid herpesvirus 1 

walleye herpesvirus 

A skin lesion described as walleye diffuse epidermal 

hyperplasia is seen in walleye populations from Oneida 

Lake, New York and from Saskatchewan, Canada (Kelly 

et al. 1983; Yamamoto et al. 1985a, 1985b; Bowser et 

al. 1988). The condition probably occurs in walleye 

populations throughout North America (Yamamoto et 

al. 1985b; Smith et al. 1992). A herpesvirus, designated 

Herpesvirus vitreum, has been observed in and isolated 

from walleye dffuse epidermal hyperplasia tissue 

(Kelly et al. 1980b, 1983). The shn lesion does not 

cause mortality. 

Clinical signs and histopathology 

walleye diffuse epidermal hyperplasia is a flat, 

laterally spreadmg, indistinct cell proliferation 

(Yamamoto et al. 1985a). The lesion develops to several 

centimeters in hameter and resembles translucent, 

thickened slime (Figure 9). The normal epidermal 

structure is disorganized, and underlying tissue often 

appears mildly edematous. Cell nuclei are slightly 

enlarged, contain granular inclusions, and occasionally 

form syncytia. Gross examination indicates no 

hyperplastic involvement beyond the slun, suggesting 

no invasion of adjacent tissue or metastasis. 

362 NCRAC Culture Series 101- Walleye Culture Manual 
Diseases and Chemotheraneutics - Chanter 16 

Diagnosis and virus detection 

The walleye diffuse epidermal hyperplasia can occur in 

fish that also show clinical signs of lymphocystis, 

walleye dermal sarcoma, and walleye discrete epidermal hyperplasia, but the lesions can be distinguished by 

dBerential clinical signs (Table 1). The lesions of 

walleye dermal sarcoma and lymphocystis are nodular 

in appearance and clearly Qstinctive from the walleye 

epidermal hyperplasias (Figures 1, 2, 6, 9, and 11). The 

lesions of walleye diffuse epidermal hyperplasia are 

thin, flat, and ill-defined, and those of walleye discrete 

epidermal hyperplasia are dome-shaped, sharply 

delineated, and raised. Histological examination should 

be used to verify presumptive visual diagnosis (Figures 

3,4, and 12). 

H. vitreum can be isolated from homogenates of diffuse 

epidermal hyperplasia tissue by using cell lines derived 

from walleye ovary (WO) and dermal sarcoma tumor 

(WC-1) and by using a cell line (We-2) derived from 

unspecified walleye tissue (Kelly et al. 1980a, 1980b, 

1983). The virus replicates with distinctive syncytiumtype cytopathology at 4°C and 15"C, but not at 20°C. 

The virus does not replicate in cell lines of cyprinid 

(FHM), ictalurid (BB), or salmonid (CHSE-214 and 

RTG-2) origin. At 15OC, syncytia began forming about 

2 weeks after infection and subsequently lysed (Kelly et 

al. 1983). Replication of H. vitreum in cell culture was 

inhibited by DNA analogues (Kelly et al. 1983). 

Electron micrographs of hyperplastic tissue and 

infected cell cultures show intracellular and extracellular accumulations of virus particles with the morphology and pattern of replication of a herpesvirus (Figure 

10). Inclusions of viral precursor material are present in 

the cell nucleus and virions are seen budding from 

nuclear membranes. Mature, enveloped virions are 190- 

230 nm in dmneter and contain a nucleocapsid about 

100 nm in diameter (Kelly et al. 1983 j. 

Transmission and epidemiology 

Although a herpesvirus can be isolated from walleye 

dffuse epidermal hyperplasia tissue, a causal relationship between virus infection and the disease has not 

been proven by experimental transmission. 

Of the four virus-associated skin lesions of walleye, 

walleye Qffuse epidermal hyperplasia occurs at the 

lowest reported prevalence. Lesions are observed in 

early spring at a prevalence 14%, but are rarely seen 

from late spring through autumn (Yamamoto et al. 

1985a, 1985b; Bowser et al. 1988). The seasonal 

appearance of the skin lesion seems to be a function of 

water temperature. Bowser et al. (1988) found that 

lesions were prevalent only when the water temperature 

was 15°C. The Qsease is observed annually in some 

populations, indxating the virus persists. Yamamoto et 

al. (1985aj suggested that winter temperature stress and 

physiological changes associated with spawning may 

activate latent infections. Smith et al. (1 992) indicated 

that industrial pollution was not a factor in the prevalence of walleye epidermal lesions. 

Walleye diffuse epidermal hyperplasia lesions are 

observed on older, sexually mature fish (Yamamoto et 

al. 1985b; Bowser et al. 1988). However, the purported 

age-related prevalence may be a function of sampling 

methodology. 

Treatment and control 

The diffuse epidermal hyperplasia occurs in feral, 

sexually-mature walleyes, and no method for treatment 

or control has been reported. The decreased prevalence 

of lesions at warmer water temperatures suggests that 

the disease be controlled in captive populations by 

raising fish at temperatures >5"C. 

Currently, no drug, chemical, vaccine, or pesticide is 

approved for use to treat walleye dffuse epidermal 

hyperplasia (Federal Joint Subcommittee on Aquaculture 1994). 

Walleye discrete epidermal hyperplasia 

Descriptors - disease: walleye discrete 

- virus: walleye discrete 

epidermal hyperplasia 

epidermal hyperplasia 

retrovirus 

hyperplasia retrovirus 

walleye epidermal 

A skin lesion described as walleye discrete epidermal 

hyperplasia occurs with some frequency in walleye 

from Alberta, Manitoba, and Saskatchewan, Canada and 

from Oneida Lake, New York (Walker 1969; Yamamoto 

et al. 1985a, 1985b; Bowser et al. 1988). The condition 

probably occurs in walleye populations throughout 

NCRAC Culture Series 101 - Walleye Culture Manual 363 
Chapter 16 - Diseases and Chemotherapeutics 

Clinical signs and histopathology 

walleye discrete epidermal hyperplasia is a firm, 

smooth lesion that appears as a translucent, clear to 

gray, sharply delineated, dome-shaped plaque (Figure 

11). The lesions are 0.25-1.5 mm thick and several 

centimeters in diameter-lesions 7 cm in diameter have 

been reported. Multiple proliferations often coalesce. 

Lesions are common on the fins, but can occur on any 

body surface. 

North America (Yamamoto et al. 1985b; Smith et al. 

1992). Walleye hscrete epidermal hyperplasia is 

thought to be caused by a virus, because retrovirus-like 

particles arc seen in electron micrographs of lesion 

tissue. 

diameter (Figure 13). The virions are enclosed in a 

peplomer laden envelope and contain an electron dense, 

centrally placed core (Walker 1969; Yamamoto et al. 

1985a, 1985b). Virions bud from cell surface membranes, but have not been seen free in the cytoplasm or 

within vesicles. No virus-like particles have been 

observed in the dermis adjacent to the lesion or in 

normal epidermis. 

Transmission and e p i demi o I og y 

Experimental transmission of walleye Qscrete 

epidermal hyperplasia has not been reported, and little 

is known about the pathobiology of the disease. 

The uniformly homogenous, undifferentiated, 

hyperplastic cells are confined to the epidermis (Figure 

12). Although roughly cuboidal in shape, affected cells 

have long cytoplasmic extensions that leave large 

spaces between individual cells. The lesion is not 

vascularized. Walleye discrete epidermal hyperplasia 

does not invade adjacent tissue or metastasize, nor is 

there evidence of necrosis of the dermal, subcutaneous, 

or muscle layers. Mucus cells are present at the 

periphery of the lesion, but arc infrequent in the body of 

the hyperplasia. 

Diagnosis and virus detection 

walleye discrete epidermal hyperplasia can occur in 

fish that also show clinical signs of lymphocystis, 

walleye dermal sarcoma, and walleye diffuse epidermal 

hyperplasia. Diagnosis is based on clinical signs 

coupled with hstopathological examination (Table 1). 

The gross appearance of dscrete epidermal hyperplasia 

is hstinct from the nodular lesions of lymphocystis 

(Figures 1 and 2) and walleye dermal sarcoma (Figures 

6 and 7). Walleye discrete epidermal hyperplasia lesions 

are sharply delineated and raised (Figure 11) compared 

to the thnner, ill-defined lesions of walleye dffuse 

epidermal hyperplasia (Figure 9). Histological examination should be used verify presumptive visual dagnosis 

(Figures 3,4, and 12). 

The prevalence of walleye discrete epidermal 

hyperplasia varies; in some populations 220% of the 

fish are affected (Walker 1969; Yamamoto et al. 1985a, 

1985b; Bowser et al. 1988). As with other walleye slun 

lesions, prevalence varies seasonally. The highest 

prevalence of walleye discrete epidermal hyperplasia 

occurs at colder water temperatures. Based on limited 

data, lesions were most prevalent in early spring (water 

temperature = 0-SOC), were not found in late spring, 

summer, or early autumn (water temperature = 15- 

25"C), but can occur at low prevalence in late autumn 

(water temperature = 5-10°C) (Yamamoto et al. 1985b; 

Bowser et al. 1988). Lesions arc found only in older 

fish (Bowser et al. 1988). No lesions have been observed in fingerlings and juveniles. There are no reports 

documenting lesion regression or recovery, nor are there 

reports of secondary bacterial or fungal infections 

associated with abraded, dislodged, sloughed lesions. 

Treatment and control 

No methods of treatment or control have been 

described. The seasonal prevalence of the lesions 

suggests that culturing fish at water temperatures 21 5°C 

would inhibit development of walleye discrete epidermal hyperplasia. 

Currently, no drug, chemical, vaccine, or pesticide is 

approved for use to treat walleye dscrete epidermal 

hyperplasia (Federal Joint Subcommittee on Aquaculture 1994). 

Attempts to isolate virus in cell culture have not been 

successful (Yamamoto et al. 1985b). However, electron 

micrographs of walleye discrete epidermal hyperplasia 

show type C retrovirus-like particles 80-120 nm in 

364 NCRAC Culture Series 101- Walleye Culture Manual 
Diseases and Chemotheraneutics - Chanter 16 

Infectious pancreatic necrosis virus 

Descriptors - disease: no pathology reported in 

walleye 

necrosis virus 

aquatic birnavirus 

- virus: infectious pancreatic 

Infectious pancreatic necrosis virus (IPNV) was 

isolated from adult walleye during a routine virological 

examination. The virus is one of the more ubiquitous of 

the fish viruses, and is known primarily for causing 

high levels of mortality in young-of-the-year salmonid 

fishes. IPNV is isolated with increasing frequency from 

non-salmonid fishes, mollusks, and crustaceans (Hill 

1982; Wolf 1988; McAllister 1993b). Some of the 

IPNV’s are initially isolated from diseased fish while 

others are recovered from apparently healthy wild and 

cultured species (McAllister and Owens 1995). 

No signs of disease accompanied isolation of the virus 

from walleye. The diseased or disease-free status of the 

host of origin does not directly correlate with virulence 

of the virus isolate in other species. An IPNV isolate 

avirulent in one host may be highly virulent in another 

(McAllister and McAllister 1988). The virulence of 

IPNV isolates ranges from highly virulent to avirulent 

regardless of whether the IPNV’s are from salmonid, 

nonsalmonid, or molluscan sources (McAllister and 

Owens 1995). The virus can be transmitted by 

waterborne exposure or by consuming virus-carrier fish. 

In general, IPNV can be isolated from homogenates of 

internal organs and from surface mucus, feces, and sex 

products, using a variety of salmonid and non-salmonid 

cell lines (McAllister 1993b). IPNV is identified by 

serological reactivity with polyclonal or monoclonal 

antisera. 

The IPNV-carrier status and variables of host specificity 

and virulence should be determined prior to the transfer 

of fish and invertebrate species from their established 

domains. Such transfers have the potential of exposing 

resident species to non-indigenous IPNV ’s, or conversely, the introduced species can be susceptible to 

strains of IPNV indigenous to the new habitat. 

Currently, no drug, chemical, vaccine, or pesticide is 

approved for use to treat infectious pancreatic necrosis 

in any species (Federal Joint Subcommittee on Aquaculture 1994). 

Acknowledgements 

1 thank Dr. Julie A. Bebak and Kathleen W. McAllister 

for their critical reviews of the manuscript and H. 

Monte Stuckey for assistance with reproduction of 

photographic materials. 

References 

Amin, 0. M. 1979. Lymphocystis disease in Wisconsin 

fishes. Journal of Fish Diseases 2207-217. 

Baumann, P. C., M. J. Mac, S. 6. Smith, and J. C. 

Harshbarger. 1991. Tumor frequencies in walleye 

(Stizostedion vitreum) and brown bullhead (Ictalurus 

nebulosus) and sediment contaminants in tributaries 

of the Larentian Great Lakes. Canadian Journal of 

Fisheries and Aquatic Sciences 48:1804-1810. 

Berthiaume, L., R. Alain, and J. Robin. 1984. Morphology and ultrastructure of lymphocystis disease virus, 

a fish iridovirus, grown in tissue culture. Virology 

135:10-19. 

Bowser, P. R., and G. A. Wooster. 1991. Regression of 

dermal sarcoma in adult walleyes. Journal of Aquatic 

Animal Health 3:147-150. 

Bowser, P. R., and G. A. Wooster. 1994. Ether sensitivity 

of walleye dermal sarcoma virus. Journal of Aquatic 

Animal Health 6:178-I 79. 

Bowser, P. R., D. Martineau, and G. A. Wooster. 1990. 

Effects of water temperature on experimental 

transmission of dermal sarcoma in fingerling walleyes. Journal of Aquatic Animal Health 2157-161. 

Bowser, P. R., M. J. WoIfe, J. L. Forney, and G. A. 

Wooster. 1988. Seasonal prevalence of skin tumors 

from walleye (Stizostedion vitreum) from Oneida 

Lake, New York. Journal of Wildlife Diseases 24:292- 

298. 

Clifford, T. J., and R. L. Applegate. 1970. Lymphocystis 

disease of tagged and untagged walleyes in a South 

Dakota lake. Progressive Fish-Culturist 32:177. 

Dukes, T. W., and A. R. Lawler. 1975. The ocular lesions 

of naturally occurring lymphocystis in fish. Canadian 

Journal of Comparative Medicine 39:406-410. 

Dunbar, C. E., and K. Wolf. 1966. The cytological course 

of experimental lymphocystis in the bluegill. Journal 

of Infectious Disease 11 6:466-472. 

NCRAC Culture Series 101 - Walleye Culture Manual 365 
Chapter 16 - Diseases and Chemotherapeutics 

Hile, R. 1954. Fluctuations in growth and year-class 

strength of the walleye in Saginaw Bay. Fishery 

Bulletin 56:7-59. 

Hill, B. J. 1982. Infectious pancreatic necrosis virus and 

its virulence. Pages 91 -1 14 in R. J. Roberts, editor. 

Microbial diseases of fish. Academic Press, London. 

Howse, H. D., and J. Y. Christmas. 1970. Lymphocystis 

tumors: histochemical identification of hyaline 

substances. Transactions of the American Microscopical Society 89:276-282. 

Federal Joint Subcommittee on Aquaculture. 1994. 

Guide to drug, vaccine, and pesticide use in aquaculture. Texas Agricultural Extension Service, Texas 

A&M University, College Station. 

Kelly, R. K., H. R. Miller, 0. Nielsen, and J. W. Clayton. 

1980a. Fish cell culture: characteristics of a continuous fibroblastic cell line from walleye (Stizostedion 

vitreum vitreum). Canadian Journal of Fisheries and 

Aquatic Sciences 37:1070-1075. 

Kelly, R. K., 0. Nielsen, and T. Yamamoto. 1980b. A new 

herpes-like virus (HLV) from fish (Stizostedion 

vitreum vitreum). In Vitro 16:255. 

Kelly, R. K., 0. Nielsen, S. C. Mitchell, and T. Yamamoto. 

1983. Characterization of Herpesvirus vitreum 

isolated from hyperplastic epidermal tissue of 

walleye, S tizostedion vitreum vitreum (M itch i I I). 

Journal of Fish Diseases 6:249-260. 

Lawler, A. R., H. D. Howse, and D. W. Cook. 1974. 

Lymphocystis infections in the silver perch, Bairdiella 

chrysura. Journal of the Mississippi Academy of 

Sciences 1 9: 183. 

Lopez, D. M., M. M. Sigel, A. R. Beasley, and L. S. 

Dietrich. 1969. Biochemical and morphologic studies 

of lymphocystis disease. National Cancer Institute 

Monograph 31 :223-236. 

Lopez, D. M., M. M. Sigel, A. R. Beasley, and L. S. 

Dietrich. 1970. DNA synthesis in lymphocystisinfected cells: effect of 6-mercaptopurine. Bacteriological Proceedings 1970: 190-1 91. 

Martineau, D., P. R. Bowser, G. Wooster, and L. D. 

Armstrong. 1990a. Experimental Transmission of a 

dermal sarcoma in fingerling walleyes, (Stizostedion 

vitreum vitreum). Veterinary Pathology 27:230-234. 

Martineau, D., I? R. Bowser, G. Wooster, and J. L. 

Forney. 1990b. Histologic and ultrastructural studies 

of dermal sarcoma of walleye, (Pisces: Stizostedion 

vitreum). Veterinary Pathology 27:340-346. 

Martineau, D., I? R. Bowser, R. R. Renshaw, and J. W. 

Casey. 1992. Molecular characterization of a unique 

retrovirus associated with a fish tumor. Journal of 

Virology 66: 596-599. 

Martineau, D., R. Renshaw, J. R. Williams, J. W. Casey, 

and I? R. Bowser. 1991, A large unintegrated 

retrovirus DNA species present in a dermal tumor of 

walleye Stizostedion vitreum. Diseases of Aquatic 

Organisms 10:153-158. 

Mavor, J. W., and S. M. Feinberg. 1918. Lymphocystis 

&e/. A new sporozoan from the pike-perch, 

Stizostedion vitreum Mitchill. Transactions of the 

Wisconsin Academy of Sciences 19:559-561. 

McAllister, K. W., and P. E. McAllister. 1988. Transmission of infectious pancreatic necrosis virus from 

carrier striped bass to brook trout. Diseases of 

Aquatic Organisms 4:lOl-104. 

McAllister, P. E. 1993a. Freshwater temperate fish 

viruses. Pages 284-300 in M. K. Stoskopf, editor. 

Fish medicine. W. B. Saunders Co., Philadelphia. 

McAllister, I? E. 1993b. Salmonid fish viruses. Pages 

380-408 in M. K. Stoskopf, editor. Fish medicine. W. 

B. Saunders Co., Philadelphia. 

McAllister, P. E., and W. J. Owens. 1995. Assessment of 

the virulence of fish and molluscan isolates of 

infectious pancreatic necrosis virus for salmonid fish 

by challenge of brook trout, Salvelinus fontinalis 

(Mitchill). Journal of Fish Diseases 18:97-103. 

Middlebrooks, B. L., R. D. Ellender, and J. H. Wharton. 

1979. Fish cell culture: a new cell line from 

Cynoscion nebulosus. In Vitro 15:109-111. 

Middlebrooks, B. L., C. M. Po, and R. D. Ellender. 1980. 

Properties of an established cell line from the Atlantic 

croaker (Micropogon undulatus). Proceedings of the 

Society of Experimental Biology and Medicine 

165: 123-1 28. 

Middlebrooks, B. L., D. L. Stout, R. D. Ellender, and S. 

Safford. 1981. Fish cell lines: two new cell lines 

derived from explants of trunk musculature of 

Cynoscion arenarius. In Vitro 17:427-430. 

366 NCRAC Culture Series 101- Walleye Culture Manual 
Diseases and Chemotheraneutics - Chanter 16 

Midlige, F. H., Jr., and R. G. Malsberger. 1968. In vitro 

morphology and maturation of lymphocystis virus. 

Journal of Virology 2:830-835. 

Nigrelli, R. F., and G. D. Ruggieri. 1965. Studies on virus 

diseases of fishes. Spontaneous and experimentally 

induced cellular hypertrophy (Lymphocystis disease) 

in fishes of the New York Aquarium, with a report of 

new cases and an annotated bibliography (1 874- 

1965). Zoologica 50:83-96. 

Pritchard, N. H., and R. G. Malsberger. 1968. A cytochemical study of lymphocystis tumors in vivo. 

Journal of Experimental Zoology 169:371-380. 

Ryder, R. A. 1961. Lymphocystis as a mortality factor in 

a walleye population. Progressive Fish-Culturist 

23: 1 83-1 86. 

Smith, I. R., A. F. Johnson, D. MacLennan, and H. 

Manson. 1992. Chemical contaminants, 

lymphocystis, and dermal sarcoma in walleyes 

spawning in the Thames River, Ontario. Transactions 

of the American Fisheries Society 121 :608-616. 

Stephens, J. S., R. K. Johnson, G. S. Key, and J. E. 

McCosker. 1970. The comparative ecology of three 

sympatric species of California blennies of the genus 

Hypsoblennius gill (Teleostomi, Blenniidae). Ecology 

Monograph 40:213-233. 

Templeman, W. 1965. Lymphocystis disease in American plaice of the eastern Grand Bank. Journal of the 

Fisheries Research Board of Canada 22:1345-1356. 

Walker, D. P., and B. J. Hill. 1980. Studies on the culture, 

assay of infectivity and some in vitro properties of 

lymphocystis virus. Journal of General Virology 

51 :385-395. 

Walker, R. 1958. Lymphocystis warts and skin tumors of 

walleyed pike. Rensselaer Review of Graduate 

Studies 14: 1-5. 

Walker, R. 1961. Fine structure of a virus tumor of fish. 

American Zoologist 1 :395-396. 

Walker, R. 1966. Viral DNA and cytoplasmic RNA in 

lymphocystis cells of fish. Annals of the New York 

Academy of Sciences 126:386-395. 

Walker, R. 1969. Virus associated with epidermal 

hyperplasia in fish. National Cancer Institute Monograph 31 :195-207. 

Wharton, J. H., R. D. Ellender, 6. L. Middlebrooks, P. K. 

Stocks, A. R. Lawler, and H. 0. Howse. 1977. Fish 

cell culture: characteristics of a cell line from the 

silver perch Bairdiella chrysura. In Vitro 13:389-397. 

Wolf, K. 1962. Experimental propagation of 

lymphocystis disease of fishes. Virology 18:249-256. 

Wolf, K. 1988. The viruses and viral diseases of fish. 

Cornell University Press, Ithaca, New York. 

Wolf, K., M. Gravell, and R. G. Malsberger. 1966. 

Lymphocystis virus: isolation and propagation in 

centrarchid fish cell lines. Science 151 :1004-1005. 

Wolf, K. E., M. C. Quimby, C. P. Carlson, and W. J. 

Owens. 1979. Lymphocystis virus: infectivity of lesion 

preparations stored after lyophilization or simple 

desiccation. Journal of Fish Diseases 2:259-260. 

Yamamoto, T., R. K. Kelly, and 0. Nielsen. 1985a. 

Morphological differentiation of virus-associated skin 

tumors of walleye (Stizostedion vitreum vitreum). Fish 

Pathology 20:361-372. 

Yamamoto, T., R. K. Kelly, and 0. Nielsen. 1985b. 

Epidermal hyperplasia of walleye, Stizostedion 

vitreum vitreum (Mitchill), associated with retroviruslike type-C particles: prevalence, histologic and 

electron microscopic observations. Journal of Fish 

Diseases 8: 425-436. 

Yamamoto, T., R. D. Macdonald, D. C. Gillespie, and R. 

K. Kelly. 1976. Viruses associated with lymphocystis 

disease and dermal sarcoma of walleye (Stizostedion 

vitreum vitreum). Journal of the Fisheries Research 

Board of Canada 33:2408-2419. 

NCRAC Culture Series 101 - Walleye Culture Manual 367 
Chapter 16 - Diseases and Chemotherapeutics 

368 NCRAC Culture Series 101- Walleye Culture Manual 
Marcino, J. 1996. Observed diseases of walleye in Minnesota. Pages 369-370 in R. C. Summerfelt, editor. Walleye culture manual. NCRAC Culture Series 101. North Central Regional 

Aquaculture Center Publications Office, Iowa State University, Ames. 

Observed Diseases of Walleye in Minnesota 

Joe Marcinq Minnesota Department of Natural Resources, Fish and Wildlife Pathology Laboratory; Box 25, 

500 Lafayette Rd., St. Paul, MN 55155 

Introduction 

Minnesota has long been known as one of the largest 

producers of pond-reared (extensive culture) walleye in 

North America. In 1993, 5 10 million walleye eggs were 

taken to produce 355 million fry. A total of 2.4 million 

fingerlings were raised by extensive culture. Disease 

occurrence in the culture of fish, whether intensive or 

extensive, seems inevitable, and walleye are no exception. Remarkably, over the last five years the pathology 

laboratory received only 43 diagnostic cases for 

walleye, and fish health inspections were conducted on 

only nine lots of fish from private fish hatcheries. The 

present report describes the hnds of infectious &seases 

observed in these samples. The responsible pathogens 

can be grouped into viruses, bacteria, parasites, and 

mysterious maladies. 

Bacterial diseases 

Only two types of bacteria were observed in the nine 

lots of fish from private hatcheries. 

Cytophaga (flexibacfer) columnaris-‘‘Columnaris’’ 

disease 

Columnaris dnease is caused by the bacterium 

Cytuphaga columnaris, is responsible for the majority 

of infectious disease cases associated with walleye 

culture. This disease can be triggered by high density, 

high temperature, and mechanical damage from 

handling. If not treated promptly, mortality may reach 

60-90%. 

Clinical signs may or may not be evident depending on 

the virulence of the pathogen. Externally, pathological 

changes are seen on the body surface, gills or both. On 

scaled fish, lesions occur initially as grayish white 

cutaneous foci on the fins, head, and body. The lesion 

may grow and the slun in the affected area may become 

eroded, resulting in small ulcerations. On the gills, the 

gill tissue becomes pale and necrotic, but fusion of the 

lamellae docs not occur (Thoeson et. al. 1994). There 

have been instances where saddleback or a discoloration encircling the dorsal fin has also been evident. 

Yersinia ruckeri-“Enteric Redmoufh Disease” 

Bacterial isolates from a routine inspection of walleye 

from a private grower in northwestern Minnesota 

revealed pure colonies of the bacterium Yersinia 

ruckeri, the pathogen responsible for enteric redmouth 

disease, a systemic bacterial disease primarily known 

for its occurrence in rainbow trout. This bacterium is a 

certifiable disease of salmon and trout but its effects arc 

relatively unknown to walleye. The bacterium may 

occur naturally in some areas and the &sease can 

develop when susceptible fish are subjected to stress. 

There are no reports of ths pathogen in natural outbreaks among free-ranging fish (Nelson 1989). 

Parasitic diseases 

Although a great variety of protozoan diseases occur, 

the most common are “Ich” and trichodinas. 

lc h fhyop fhirius mulfifiliis- “lch ’’ 

Ich is a large ciliated protozoan parasite responsible for 

major losses in the cultured catfish industry. It is a 

common parasite of warmwater fishes, but can also be a 

serious parasite of cool and coldwater fish. Ich can be 

identified microscopically by its horseshoe shaped 

nucleus and by the “white spot” appearance macroscopically. This parasite has no host specificity; both 

fish and amphibians can be affected. Ich is an obligate 

fish parasite that requires a host to survive; however, 

amphibians may be reservoirs of infection. 

The parasite lives in, or under the epithelial cells of the 

host. The trophozite is visible on the body surface as 

white specks up to 1 mm in size. The trophozoite feeds 

on cells by hgesting away the cell membrane. The 

adult stage, cannot reinfect a fish, but it secretes a 

NCRAC Culture Series 101 - Walleye Culture Manual 369 
Chapter 16 - Diseases and Chemotherapeutics 

mucoid adhesive over its exterior surface that will 

adhere to anything. The nuclear material then forms a 

dense mass in its center and undergoes multiple fission. 

Up to 1,000 tiny new bodies are formed (tomites), each 

with enough genetic material to produce a clone. The 

tomites seek fish and penetrate the epithelium. Tomites 

must find a fish within about 24 hrs or they die. Tomites 

have a penetration gland at the anterior end that secretes 

a material which dissolves membranes and allows the 

tomite to enter the cell. Once inside the cell a cytosome 

(“cell mouth”) appears as the parasite consumes the cell 

contents, and the parasite develops its distinctive 

horseshoe nucleus. 

The life cycle of Ich is temperature dependent. At 

optimal temperatures of 69-73°F (20.5-22.7”C), the 

life cycle may take as few as 3-4 d. The life cycle 

requires 2 weeks at 59°F (1 5°C) and more than five 

weeks at 50°F (10°C). The disease is difficult to control 

because not all adults emerge at the same time. There 

are no treatments known that will hull trophozoites 

imbedded in the fish. Multiple treatments are required 

to gain control. 

In Minnesota, epizootics of ich seem to be common in 

recirculating systems. These problems have been 

persistent and chronic. Water temperatures in intensive 

systems are usually near optimum for parasite reproduction and outbreaks have been devastating to walleye 

culture and production. Ich has been the most pervasive 

disease problem we have encountered in the intensive 

culture of walleye. Clinical signs of the disease include 

fish rubbing on surfaces, twitching, gathering at the 

water inlet, tight schooling, seelung cover, failure to 

feed, white pustules, frayed fins, petechiae on body 

surface, pale gills, low hematocrit, excess mucus, and 

mortality. The cause of death is usually attributed to 

osmoregulatory failure, complicated by tissue destruction, respiratory damage, and severe anemia. 

Trichodina sp. 

Trichodinids are saucer shaped protozoans with rows of 

cilia around the margin of the body. These protozoans 

live on the skin, fin, and gill tissues of fish and, when 

abundant, cause considerable irritation to the fish. It 

seems that the large adult stage is less pathogenic than 

the smaller juveniles. Reproduction is by simple fission. 

Because they are less likely to be imbedded in the flesh 

of their host, than ich, so a single treatment usually 

relieves the infection. 

Clinical signs of the disease are failure to feed, tail 

chasing in small fingerlings, emaciation, pinheads, 

erosion of fins, excess mucus production, and mortality. 

Infections of Trichodina spp. may lead to secondary 

bacterial infections by opportunistic pathogens. 

Viral diseases 

A variety of skin lesions of viral origin have been 

found in fishes from Minnesota (Economon 1957). 

Some of the more conspicuous are tumorous growths, 

to which walleye seem to have a hgh susceptibility. 

Walleyes can be affected by various slun tumors, 

including lyrnphocystis, dermal fibroma, and epidermal 

hyperplasia. A different virus is associated with each of 

these surface lesions. These lesions are benign and are 

confined to the external surfaces of the body. They are 

cyclical in nature, with the relative frequency appearing 

to fluctuate in successive years. They are also seasonal 

with the incidence most obvious during the coldwater 

months (Wyatt 1981). A thorough description of these 

viral diseases of walleye are given by McAllister 

(1 996). 

References 

Economon, P. P. 1957. Studies on bacterial diseases of 

Minnesota fish. Proceedings of the Minnesota 

Academy of Science 25-26:233-238. 

Economon, P. P. 1975. Myofibrogranuloma, a muscular 

dystrophy- like anomaly of walleye. Minnesota 

Department Of Natural Resources Special Publication 113. 

McAllister, P.E. 1996. Viruses associated with diseases 

of walleye, sauger, and walleye x sauger hybrids. 

Pages 355-367 in R.C. Summerfelt, editor. Walleye 

culture manual. NCRAC Culture Series 101, North 

Central Regional Aquaculture Center Publications 

Office, Iowa State University, Ames. 

Nelson, R. C. 1989. Introduction to Fish Health. 

Thoeson, J. C., Editor. 1994. Suggested procedures for 

the detection and identification of certain finfish and 

shellfish pathogens. 4th ed., Version 1, Fish Health 

Section, American Fisheries Society. 

Wolf K. 1988. Fish Viruses and Fish Viral Diseases. 

Wyatt E. 1981. Parasites and selected anomalies of 

some fishes of the North Central United States and 

Canada. 

370 NCRAC Culture Series 101- Walleye Culture Manual