Fish Can Get Diseases Too

Bacterial kidney disease

that affects the ability of 

juvenile Pacific salmon to 

migrate downstream and to 

survive in the ocean. 

 Infectious hematopoietic 

necrosis virus that can 

cause high losses among 

salmon fry. 

 Whirling disease that 

affects wild-spawning 

rainbow trout in the Rocky 

Mountain West. 

 Viral hemorrhagic 

septicemia in marine fishes 

of northern Puget Sound. 

 Ichthyophthirius and 

columnaris disease in 

returning adult salmon in 

the Klamath River. 

Ceratomyxa and 

Parvacapsula infections 

of juvenile salmon in the 

Klamath River. 

 Ichthyophoniasis in 

adult chinook salmon in 

the Yukon River. 

In addition, new studies, using 

state-of-the-art molecular 

biology techniques such as the 

polymerase chain reaction and 

cDNA microarrays, are aimed at 

trying to predict the influence of 

disease in the survival of juvenile 

salmonids exposed to various 

stressors during their outmigration.

How are Bacterial Diseases 

Diagnosed in Fish?

Bacterial and viral diseases can 

have a great negative impact on 

salmonid populations, especially in

a hatchery setting. Some of these 

diseases kill many fish very 

quickly, others are slower to take 

hold but are also very deadly to 

fish. For these reasons, it is 

important to quickly identify any 

potential infection so that 

appropriate action can be taken to 

isolate, and often treat, the 

pathogen before an epizootic (an 

epidemic in animals) can occur. 

U.S. Department of the Interior WFRC Fact Sheet 

U.S. Geological Survey July 6, 2005 

The following diseases or pathogens of free-ranging 

populations of finfish have been studied at the WFRC:

Juvenile Chinook salmon with bacterial kidney 

disease. 

A researcher at the WFRC removes the anterior kidney from a rainbow 

trout at a sterile workstation. Cells from the anterior kidney are used to 

study various aspects of fish health including host/pathogen 

relationships, vaccine efficacy and the effects of hatchery rearing

practices. 

Infectious diseases are increasingly recognized 

as an important component of the ecology of fish 

in the wild. Many of the viral, bacterial, 

protozoan and fungal pathogens of fish that were 

initially discovered in captive fish have their 

origin among wild populations; however, the 

impact of disease among these free-ranging 

stocks has been difficult to study. At the WFRC,

combinations of field and laboratory 

investigations, aided by the tools of molecular 

biology, have begun to provide information on 

the ecology of infectious diseases among natural 

populations of fish in both freshwater and marine

ecosystems.
Gels 

Bacterial Culture Plates 

ELISA Assay 

GramPositive 

Bacteria

GramNegative 

Bacteria

Fluorescing 

Bacterial 

Cell

Bacterial 

Proteins 

Amplified 

Bacterial 

DNA

Bacterial 

Antigen 

Detected

Bacterial 

Colonies 

Microscopic Slides of Bacteria 

We use several diagnostic techniques to identify the presence of specific 

bacteria in salmonids. Some of these methods were developed in our 

laboratory and are now used for diagnosis of bacterial fish diseases in many

parts of the world. (See sidebar) 

Salmonid Immune Function 

Anadromous salmonids are 

susceptible to a variety of disease 

causing (pathogenic) microorganisms, including at least 30 

bacteria and viruses. Whereas the 

impact of these microorganisms on 

salmonids in wild and natural 

rearing areas is difficult to measure, 

losses from disease among hatchery

salmonids are both common and 

well documented. The disease state 

depends on 3 factors, the host (fish), 

the disease organism, and the 

environment. 

Disease is not the result of a single 

event, rather the consequence of an 

imbalance in the host-pathogen 

relationship. The presence of a 

pathogen in a salmon population 

will not always result in losses due 

to disease and fish can remain 

asymptomatic carriers under 

favorable conditions. However, 

stressors exceeding the limits of a 

fish's physiological tolerance will 

diminish its ability to resist disease, 

and consequently, become a threat 

to its health. 

For salmon, the hatchery environment is quite different than the 

environments they inhabit in the 

wild. Successful fish husbandry 

Young salmon fry may have been infected

with the kidney disease bacterium before 

they hatched from the egg. 

requires the minimization of 

environmental stressors such as 

handling, high rearing densities 

and poor water quality, while 

maintaining the carrying capacity

of the facility. Unfortunately, there 

is no simple measure of the health 

status of a fish population. Since 

the immune system of any higher 

organism is essential for its well 

being, measures of the fishes 

defenses provide an attractive 

means to quantify fish health. 

However, we are only beginning to 

understand the immune system of 

salmon, and how it modulates with 

seasonal and life stage changes, or 

differing rearing conditions. 

 

Renibacterium salmoninarum 

Renibacterium salmoninarum is 

the causative agent of bacterial 

kidney disease (BKD), a serious 

disease problem of wild and 

The kidney disease 

bacterium can be

detected in fish tissues by

the fluourescent antibody

test. 
cultured salmonid fishes worldwide. Control of the 

bacteria by use of antibiotics is difficult due to its 

slow growth, and conventional vaccine strategies 

are ineffective or may actually worsen the disease 

state. Furthermore, the bacteria can be passed not 

only between fish in the same water system

(horizontal transmission), but also from one 

generation to the next via infected eggs (vertical 

transmission). During infection, R. salmoninarum

can evade immune surveillance by surviving and 

replicating within host macrophages (a type of 

white blood cell). Also, R. salmoninarum produces 

large amounts of a unique protein (p57) which 

suppresses a number of immune functions. Control 

of R. salmoninarum will require a better 

understanding of the bacteria itself, the salmonid 

immune system and how the bacteria and host 

defenses interact. 

A toolbox of immunological assays is used to monitor the defenses of fish

infected with the kidney disease bacterium.

Whirling Disease 

Myxobolus cerebralis is a 

metazoan parasite that penetrates 

the head and spinal cartilage of 

fingerling trout where it multiplies 

very rapidly, putting pressure on 

the organ of equilibrium. This 

causes the fish to swim erratically

(whirl), and have difficulty feeding 

and avoiding predators. In severe 

infections, the disease can cause 

high rates of mortality in young-ofthe-year fish. Those that survive 

until the cartilage hardens to bone 

can live a normal life span, but are 

marred by skeletal deformities. 

Lightly infected fish can, however 

reproduce without passing on the 

parasite to their offspring. 

Whirling disease has become a 

highly significant problem for 

fisheries managers in federal and 

state agencies throughout the 

nation. Thought to have been 

introduced from Europe, whirling 

disease was first detected in the U. 

S. in 1958 and has now been 

reported from fish in more than 20 

states. The potential for whirling 

disease to cause losses in natural 

stocks of trout became a national 

concern in 1993-1994 when losses 

of up to 90% of the wild rainbow 

trout (Oncorhynchus mykiss) in 

several trout streams in Colorado 

and Montana were attributed to 

whirling disease. Subsequently, a 

large, ongoing survey revealed that 

naturally spawning rainbow and 

brown trout in many western rivers 

were infected with the parasite 

with some of the fish showing 

clinical disease. Of additional 

concern was the presence of

whirling disease in native stocks of 

cutthroat (Oncorhynchus clarki) 

and bull trout (Salvelinus 

confluentus) which are of special 

status due to their declining 

numbers. Improved 

diagnostic methods 

now provide epizootiological information 

about strains of the 

parasite or the alternate 

host.

Bull trout (Salvelinus confluentus)

IHN Virus 

Infectious haematopoietic necrosis 

(IHN) is an infectious viral disease 

of salmon and trout. It was first 

recognized in the 1950s in sockeye 

and chinook salmon. The disease 

has the most economic significance 

for freshwater farms, however 

Pacific and Atlantic salmon (Salmo 

salar L.) in both fresh and sea 

water have been severely affected. 

Deformed skeletons of 8-moonth old rainbow 

trout (Oncorhynchus mykiss) with whirling

disease. 
Fry and small fingerlings become

infected very readily, becoming more 

resistant as they mature. The 

infection is often lethal and the 

mortality rate can be 100% in fry.

Some fish that survive an outbreak of

IHN may become carriers of the 

virus, providing a reservoir of 

infection. In addition, infected 

juveniles shed IHN virus particles in 

the faeces, urine and external mucus.

Researchers at the WFRC are 

creating a set of molecular tools by

cloning each of the viral genes and 

sequencing the entire genome. This 

information is stored in an IHN 

database. When an IHN virus 

epidemic occurs, our researchers use 

molecular genetic methods to 

For additional information, 

please contact: 

Lyman Thorsteinson, Center Director

U.S. Geological Survey

Western Fisheries Research Center 

6505 NE 65th Street 

Seattle, WA 98115

Phone: (206-526-6282

FAX (206) 526-6654 

http://wfrc.usgs.gov

Here at WFRC we work with development of a novel DNA vaccine to protect fish 

against IHN virus. Left: The vaccine, pIHNV-wG, is a circular piece of DNA 

that carries the gene for the IHNV surface protein G. Right: The IHNV DNA 

vaccine will be tested against a placebo (control) vaccine to confirm protection 

against the disease. 

IHNV 

DNA Vaccine

Control mock 

DNA Vaccine

IHNV G Luciferase

pIHNV-wG pLuc

Signs of IHN 

Development of a novel vaccine for IHN 

IHN virus is a severe pathogen of salmon and trout that causes epidemics with 20-

90% mortality. It is in the same virus family as the rabies virus. There is currently no 

commercially available vaccine for IHNV. 

Additional Reading

Miller, K.M., J.R. Winton, A.D. 

Schulze, M.K. Purcell, and T.J. 

Ming. 2004. Major histocompatibility complex loci are associated 

with susceptibility of Atlantic 

salmon to infectious hematopoietic 

necrosis virus. Environ. Biol. 

Fishes 69: 307-316. 

Kocan, R.M., P.K. Hershberger, 

and J. Winton. 2004. 

Ichthyophoniasis: An Emerging 

Disease of Chinook Salmon 

(Oncorhynchus tshawytscha) in the 

Yukon River. J. Aquat. Anim. 

Health 16:58-72. 

Anderson, E.D., H.M. Engelking, 

E.J. Emmenegger, and G. Kurath. 

2000. Molecular epidemiology

reveals emergence of a virulent 

infectious hematopoietic necrosis 

(IHN) virus strain in wild salmon 

and its transmission to hatchery

fish. J. Aquat. Anim. Health 

12:85-99. 

Allen, B.M., E.P. Bergersen. 2002.

Factors influencing the distribution

of Myxobolus cerebralis, the 

causative agent of whirling disease, 

in the Cache la Poudre River, 

Colorado. Dis. Aquat. Org. 49:51-

60. 

A complete list of WFRC 

publications can be found on 

our website at: 

http://wfrc.usgs.gov/pubs/pubs.htm

characterize the specific virus type, 

and compare it with other known 

IHNV types in our database. 

Similar methods are used for other 

fish viruses and for epidemics of 

flu, SARS, and anthrax. Over 600 

IHN virus samples from

throughout the Pacific Northwest 

have been characterized to date. 

Studies have revealed that 

infectious diseases can cause major 

losses in populations of freeranging fish and are significant 

factors in the ability of certain 

stocks to recover. Work has also 

served to highlight the critical role 

played by environmental 

conditions and other sources of 

stress in the ecology of fish 

diseases.