Why are some infected fish not diseased?

As long as KHV enters the fish’s habitat, there is a good chance for the fish to get infected. However, a small proportion of the infected fish doesn’t fall ill and show symptoms. There are two likely explanations. First, the infected fish may have a good immune system against the virus and stay healthy. However, they can only survive from low-level infection at the permissive temperature. Second, the environment is not suitable for KHV propagation. The most critical condition for KHV breeding is water temperature. As there is slight difference among various reports, the permissive temperature for KHV is from 18°C to 28°C. This is probably the reason why most of the KHV outbreaks have happened in spring and autumn. Out of the permissive temperature, then the virus is suggested to stay dormant (latent) in its host cell for a period of time. Such a period of time could be at least 200 days at 12°C. Dishonet el al. also suggest that koi herpesviurs could persist in fish body, based on his finding that KHV-infected cells became “healthy” after the temperature was elevated to 30°C, but low levels of virus infection was shown to be maintained this way for 30 days. Notably, virus infection was reactivated after the temperature was switched to permissive temperature.



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