FISH HEALTH MANAGEMENT 297 



isolation, positive identification requires neutralization of the virus by a 

 specific antiserum. 



Fish that survive an infection become carriers; both sexes shed the virus 

 primarily with sex products. Gonadal fluids are used in bioassays to detect 

 carrier populations. Natural transmission occurs from infected fish to 

 noninfected fish through the water, or from the exposure of susceptible fry 

 to sex products of carrier adult broodfish. The virus also can be transmit- 

 ted with eggs or by the feeding of infected fish products. 



Only rainbow trout and chinook and sockeye salmon have been shown 

 to be susceptible to IHN. Coho salmon apparently are resistant to the 

 virus. Resistance increases with age and deaths are highest among young 

 fry and fingerlings. However, natural outbreaks have occurred in fish rang- 

 ing from yolk-sac fry to 2 years of age. The incubation and course of the 

 disease are influenced strongly by water temperature. At 50°F, mortality 

 may begin 4 to 6 days after exposure. Numbers of dead usually peak 

 within 8 to 14 days, but mortality may continue for several weeks if the 

 water temperature remains near 50°F. Below 50°F, the disease becomes pro- 

 longed and chronic. Above 50°F, the incubation time is shorter and the 

 disease may be acute. Some epizootics have been reported at temperatures 

 above 59°F. 



Outbreaks of IHN disease have occurred along the Pacific Coast from 

 the Sacramento River in California to Kodiak Island, Alaska. Although the 

 virus may not exist in all populations of sockeye salmon, the virus has 

 been detected in all major salmon production areas. Among chinook sal- 

 mon, the disease is a particularly serious problem in the Sacramento River 

 drainage; it has been found also in fish of the Columbia River. Outbreaks 

 of IHN in rainbow trout have been much more restricted. Isolated 

 hatcheries where carriers and outbreaks were identified are known from 

 South Dakota, Minnesota, Montana, Idaho, Oregon, Washington, Colora- 

 do, and West Virginia. All involved fish or eggs from a known carrier 

 stock. However, there has been no recurrence of the disease at most of 

 these hatcheries after the original outbreak. IHN also occurred in Japan in 

 sockeye salmon from eggs transported from Alaska. 



An effective method of control is to maintain the water temperature 

 above 59°F while fish are being reared. This principle has been used suc- 

 cessfully to control IHN in chinook salmon along the Sacramento River. 

 However, it is expensive to heat large volumes of water. Furthermore, rear- 

 ing infected fish at elevated temperatures does not eliminate the carrier 

 state. 



In rainbow trout, IHN virus is believed to be transmitted with eggs as a 

 contaminate. Disinfection of eggs with iodophors usually will destroy the 

 virus. 



