If there is a previous history of IHN at the facility and the fish are showing characteristic clinical signs, IHNV may be the cause of a disease outbreak. However, the preliminary diagnosis should be confirmed by isolation and identification of IHNV. Recommendations for fish sampling, processing and diagnosis have been outlined by fish health officials in the USA and Canada (Department of Fisheries and Oceans, 1984; Ganzhorn and LaPatra, 1994; Thoesen, 1994). Fish tissues to be tested are dependent on fish size and disease status. The preferred tissues are the kidney and spleen; mucus has also been used in non-lethal sampling (LaPatra et al., 1989c). For testing brood stock, the ovarian fluid is preferred, since the virus is less frequently detected in the milt. Sampling of postspawning females, storage of ovarian fluid or incubation of ovarian fluid cells enhances the sensitivity of viral detection (Mulcahy et al. 1984b; LaPatra and Groberg, 1985; Mulcahy and Pascho, 1986; Mulcahy and Batts, 1987; LaPatra et al., 1990c). If milt is tested for virus, it should be centrifuged and, after the pellet is incubated in water, the water is assayed for virus. This procedure enhanced IHNV detection by at least fourfold (Batts, 1987; Meyers et al.,1990).
Methods used for diagnosing IHNV should be rapid, simple, sensitive, specific, inexpensive and able to test a large number of samples. Ideally, they should also be suitable for field use. The only method initially available for the diagnosis of IHNV was to detect virus in cell culture and then identify the virus by a serum neutralization test (Amend, 1970b). This classical method is widely accepted and is still commonly used. The discovery that rabbit antiserum has higher levels of IHNV-binding antibodies than IHNV-neutralizing antibodies (Hsu and Leong, 1985) allowed the development of several more rapid serological methods. There are also non-serological methods available for diagnosing IHNV. Each of the available methods are discussed below.
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