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Diagnosis of a virus which is always virulent, e.g. FMDV, is definitive. Diagnosis of a virus which can be avirulent, e.g. Newcastle disease, is more difficult because the virus may not have caused the disease but be a subclinical infection.
Diagnosis of a new virus from a new disease is the most difficult because the virus must be shown to cause disease by fulfilling Koch's 3 postulates: isolating the virus in pure culture; using the pure virus to reproduce the new disease in a healthy host, e.g.PRRS into a healthy piglet; recovering the virus from the lesions of the experimental animal.