Memorandum

To: Glen Knowles, Bureau of Reclamation, HFE Technical Team Lead

From: Scott VanderKooi, Grand Canyon Monitoring and Research Center, Acting Deputy 

Chief

CC: Shane Capron, GCDAMP Technical Work Group, Chair

Date: October 22, 2012

Subject: Whirling disease in Glen Canyon, Arizona and implications for resource management 

in the Colorado River

The confirmation of the presence of whirling disease in Glen Canyon has repercussions for the 

management of fisheries and other resources in the Colorado River below Glen Canyon Dam.

The risk of spreading the disease must be taken into consideration as the recently completed 

Environmental Assessments (EA) for Nonnative Fish Control and High-Flow Experiments are 

implemented. Of the actions proposed, live removal and relocation of rainbow trout 

(Oncorhynchus mykiss) represents the greatest risk for spreading whirling disease, as these fish 

are highly susceptible to infection and can carry large numbers of the parasite. Risks associated 

with experimental floods further spreading the disease are low as the downstream movement of 

infected fish is already occurring. Higher flows may actually decrease the prevalence of whirling 

disease through disruption of the parasite’s life cycle by displacing its alternate host and reducing 

its preferred habitat.

Whirling disease was initially detected in Glen Canyon in 2007 (Makinster and others, 2008) and 

re-detected in 2011 (B. Stewart, AZGFD, pers. comm.). The 2011 results showed 22 percent of 

samples (90 fish pooled into batches of five fish each) tested positive for the disease with 

positive groups collected from both upstream and downstream reaches. The presence of whirling

disease has implications for a number of proposed management actions related to Glen Canyon 

Dam and the Colorado River in Glen, Marble, and Grand Canyons. Two potential actions that 

have raised concerns due to their perceived potential to spread whirling disease are: 1) live 

removal and relocation of rainbow trout associated with the Nonnative Fish Control EA (Bureau 

of Reclamation, 2011a); and 2) experimental floods conducted as part of the High-Flow 

Experiment EA (Bureau of Reclamation, 2011b). Below, we summarize available literature 

concerning the risk of spreading whirling disease through these potential management actions.

United States Department of the Interior

U.S. GEOLOGICAL SURVEY

SOUTHWEST BIOLOGICAL SCIENCE CENTER

2255 NORTH GEMINI DRIVE, MS-9394

FLAGSTAFF, ARIZONA 86001-1600

928 556-7094 Telephone

928 556-7092 Fax
Background: Whirling disease biology and life cycle

Whirling disease only infects salmon and trout species, and is caused by Myxobolus cerebralis, a 

myxozoan parasite introduced to North America from Europe in the 1950s (Bartholomew and 

Reno. 2002). Elwell and others (2009) provide a thorough description of the parasite and 

summarize the disease and its effects on fish in the United States in a white paper prepared as 

part of the Whirling Disease Initiative. Myxozoan parasites exhibit complex life histories 

requiring both an invertebrate and vertebrate host to complete their life cycle (Figure 1). In the 

case of M. cerebralis, the invertebrate host is the oligochaete worm Tubifex tubifex and the 

vertebrate host is a salmonid fish (e.g., salmon, trout and whitefish). 

Figure 1. The life cycle of Myxobolus cerebralis, the parasite that causes whirling disease. Myxospores (A) are 

ingested by the oligochaete worm Tubifex tubfex (B) which are infected and subsequently produce (C) and release 

(D) triactinomyxons (E). Triactinomyxons infect a salmonid fish through their skin (F) which then produce 

myxospores (G) that are released following the death of the fish (H) completing the life cycle (Figure from Elwelland

others,. 2009).

No other worm or fish species can be infected, but susceptibility to whirling disease does vary by 

strain of T. tubifex as well as species and stock of salmonid. For example, rainbow trout are quite 

susceptible to the disease although some stocks are resistant, while brown trout (Salmo trutta)

can carry the parasite and show few clinical signs of infection. Susceptibility in fish also varies 
by life stage and size with young and small fish most vulnerable, because the parasite targets 

cartilage as the infection develops. Resistance to whirling disease increases in developing fish as 

cartilage is replaced with bone. Infection is also influenced directly and indirectly by a number of 

environmental factors including water temperature, substrate, and flow. Water temperatures 

between 10 and 15 °C are most suitable for triactinomyxon (TAM) production (the life-stage of 

whirling disease produced in T. tubifex that subsequently infects fish) and are associated with the 

highest levels of infection and disease severity. Substrates composed of finer materials like silt 

and clay are more favorable for T. tubifex reproduction and TAM production in those worms. 

Higher levels of organic material in streams lead to higher worm abundance which, in turn, may 

be related to an elevated risk of infection in fish. In the short term, flows high enough to scour 

substrates can disadvantage T. tubifex by displacing fine sediment and the worms residing in it. 

Scouring high flows can also disadvantage T. tubifex in the long term by limiting amounts of 

suitable habitat. Anthropogenic changes in watersheds can also affect whirling disease 

prevalence. The tailwaters downstream from dams often have conditions favorable to T. tubifex,

which could increase M. cerebralis infection risk in fish.

Risk of spread through relocation of trout

Live removal and relocation of rainbow trout or brown trout from the Colorado River to other 

waters poses a substantial risk of unintentionally spreading whirling disease as both species can 

carry the parasite. An infected fish can release millions of myxospores (Hallet and Bartholomew,

2008), which can then infect T. tubifex, thus completing the life cycle of the parasite. There is no 

effective treatment of whirling disease in fish once a population has become infected (Gilbert and 

Granath, 2003). Because of this, possessing or transporting fish from whirling disease infected 

waters is explicitly forbidden by state law (see Arizona’s Aquatic Invasive Species Interdiction 

Act; A.R.S. 17-255.02). Absent human intervention, the natural movement of infected fish 

appears to be a likely mechanism by which M. cerebralis is spread within rivers and watersheds 

(Zielinkski, 2008).

Risk of whirling disease spread through High-Flow Experiments

Controlled floods, administratively called High-Flow Experiments (HFEs), appear to pose a 

minor risk of spreading whirling disease, particularly since there are several mechanisms that 

allow the disease to spread to downstream reaches in the absence of controlled floods. 

Triactinomyxons are neutrally buoyant (Gilbert and Granath, 2003) and are easily dispersed 

downstream in rivers and streams. In addition, TAMs have been shown to be viable for as long as 

15 days at temperatures ranging from 7 to 15 °C (El-Matbouli and others, 1999). These findings 

are particularly relevant to the current situation in the Colorado River downstream from Glen 

Canyon Dam. Triactinomyxons released from infected worms in Glen Canyon clearly have the 

potential to infect fish throughout the length of Marble and Grand Canyons during normal dam 

operations given that water transport times are short (e.g., 4.5 d to travel 235 miles at 15,000 

ft3/s; Graf, 1997) and water temperatures throughout Grand Canyon are usually between 7 and 15 

°C (Voichick and Wright, 2007). This is why many agencies consider the length of the Colorado 

River between Glen Canyon Dam and Lake Mead to be infected by whirling disease. 
Higher flows are thought to limit whirling disease (Hallet and Bartholomew 2008). In fact,

flushing flows have been identified as a potential management tool for decreasing whirling 

disease infections (Elwell and others, 2009). Higher flows scour fine sediments and associated 

organic material from larger substrates and displace T. tubifex living there, thus reducing sources 

of TAMs that can infect fish, as well as limit suitable habitat for worm recolonization. A flushing 

flow with a peak approximately 6 times higher than base flows on the San Juan River in New 

Mexico temporarily reduced organic matter and T. tubifex densities in downstream deep water 

habitats, which may have decreased the prevalence and severity of whirling disease there (DuBey 

and Caldwell, 2004). The last controlled flood on the Colorado River in March2008 resulted in a 

temporary decrease in the density of tubificid worms in Glen Canyon (Cross and others, 2011). 

Thus, future HFEs are likely to result in a decrease of whirling disease prevalence and severity in 

Glen Canyon. 

Controlled floods on the Colorado River could facilitate the spread of whirling disease to 

downstream reaches by temporarily increasing M. cerebralis myxospore and TAM abundance in 

the water column, but an increase in infection risk due to these floods seems unlikely. Floods of 

the magnitude proposed result in short travel times through Grand Canyon (e.g., 2.4 d to travel 

235 miles at 45,000 ft3/s; Graf, 1997) and Hallett and Bartholomew (2008) found higher flow 

rates resulted in lower M. cerebralis infection prevalence in both T. tubifex and rainbow trout,

and reduced disease severity in rainbow trout. Another factor that makes it unlikely that future 

HFEs will facilitate the downstream spread of whirling disease, is that most rainbow trout in

downstream reaches are relatively large (Yard and others, 2011). Fish are most susceptible to 

infection when they are young and small, thus the large rainbow trout that are common in Grand 

Canyon naturally have low susceptibility to whirling disease.

Conclusions

The implications of the presence of whirling disease in Glen Canyon for proposed management 

actions in the Colorado River vary by action. Live removal and relocation of trout from the 

Colorado River represents, by far, the greatest risk of spreading the disease. Relocation of trout 

from an infected population without any risk of fish escapement or spread of myxospores is 

virtually impossible. There is a low risk of spreading whirling disease as a consequence of 

conducting experimental floods. The disease is already present downstream from Glen Canyon 

Dam, and infected fish are already moving into Marble and Grand Canyons. It is likely that 

HFEs will result in a decrease in the prevalence and severity of the disease through reductions in 

the abundance of the intermediate host T. tubifex and its preferred habitat of fine sediment and 

organic matter.
References

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Bartholomew J.L., and Wilson, J.C., eds., Whirling disease--Reviews and current topics:

American Fisheries Society, Symposium 29, Bethesda, Maryland, p. 3-24.

Bureau of Reclamation, 2011a, Environmental assessment--non-native fish control downstream 

from Glen Canyon Dam: Salt Lake City, Utah, Bureau of Reclamation, Upper Colorado 

Region, 102 p. + appendices, accessed on March 21, 2012, at 

http://www.usbr.gov/uc/envdocs/ea/gc/nnfc/NNFC-EA.pdf. Bureau of Reclamation, 2011b, 

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