RESULTS PART IV: ADDITIONAL INVESTIGATIONS 

 Whirling disease status 



Whirling disease, caused by the exotic myxosporean parasite Myxobolus 

 cerebralis, was first detected in the Blackfoot River in 1995 near Ovando, MT. Since 

 then, the disease has increased in both distribution and intensity. It now infects 

 salmonids in the entire main stem Blackfoot River and continues to expand in the lower 

 reaches of most tributaries (Figure 47; Table 8). The highest infections are found in 

 tributaries of the middle Blackfoot River as well as certain spring creeks. Many of the 

 lower river tributaries, those with higher gradients and streams with colder summer 

 temperatures currently support lower infection levels. The low-elevation distribution of 

 the disease currently overlaps with the distribution of many salmonids. 



Myxobolus cerebralis , . .« 



has a complex, two-host life ' 



cycle involving a salmonid ^^^ / > ■ .- 



and the aquatic oligochaete 

 worm, Tubifex tubifex. There 

 are also two spore forms of 

 the parasite; a fi-agile 

 triactinomyxon (TAM) that is 

 released by the worm and 

 infects young trout and a 

 hardy myxospore later 

 released by infected fish and 

 ingested by the worm host, 

 where the myxospore is then 

 converted back to the TAM 

 stage. The development and 

 severity of whirling disease 



■>ft/^ 



"i-i. 



V 



Figure 47. Generalized distribution of whirling disease in the 

 Blackfoot Watershed. 



in exposed salmonids is dependent on many factors involving: 1) the fish host (species, 

 strain, age, size) (Thompson et al. 1999; Vincent 2002; Ryce 2003); 2) the worm host 

 (Granath et al. 2002); 3) the environment (water quality parameters, water temperature, 

 flow rates) (MacConnell and Vincent 2002; Smith et al. 2002); and 4) the overlap of 

 contact with both spore types (overlap of TAM with susceptible fiy species and 



myxospore being encountered by the worm) 



(Kerans and Zale 2002). 



Sentinel cages provide an indirect 

 measure of TAM abundance and disease 

 severity. They were first deployed in the 

 Blackfoot Watershed in 1998 {see Procedures). 

 Sentinel cage monitoring has continued through 

 2005 at established Blackfoot River sites and 

 throughout tributaries in order to assess disease 

 expansion. A mean grade infection is 



Mean grade 

 category 



0.0-2.0 



2.01-2.74 



2.75-3.7 



3.71-5.0 



Infection Level 



Description 



Low 



Medium 



High 

 Very High 



Table 6. Mean grade category 

 descriptions (Baldwin et al. 2000). 



determined ft-om histology results ft^om sentinel fish exposed in each cage to determine 



57 



