Regarding the question of fish and infection with H5N1 [4] influenza virus: This is not the first time this question has come across my desk. While it is not wise to say “absolutely never” in biology, I find actual infection of fish with influenza viruses to be extremely unlikely. I’ve never encountered any study to show that fish (or insects, before someone asks) can be infected with or transmit influenza viruses. I am a public health M.D./epidemiologist, not a virologist, so I am prepared to be wrong. But I’d be surprised. The issue is not so much that certain animals are “cold blooded” — although that perhaps plays a role, since viruses have preferred temperatures for carrying out the molecular steps for infection — but rather involves the specific molecular biology of the influenza virus. In particular, there are specific proteins (hemagglutinin or “HA”) on the outside of each virus that determine what sorts of animal cells the virus can attach to and thereby infect. I don’t think any HA molecule subtypes will attach to the cell receptors found in fish or insects. Further, the various other components of the virus are also necessary for an infection cycle to be successful, and these components would probably not be expected to function properly in non-mammal, non-avian cells.
Having said all that, the issue of environmental “contamination” with this virus has indeed been raised with regard to fish farming. Influenza viruses in general are able to remain viable in the environment for extended periods, especially if cold and wet. There have been concerns raised (and refuted, but with little direct evidence one way or another) about chicken manure used as fish food in integrated operations in Asia. The real concern is that transport and spread of untreated manure from infected poultry could contribute to local spread of the virus, not so much that the farmed fish are directly infected. It’s worth reminding everyone that cooking easily kills influenza virus and other pathogens. If preparing raw meats, always practice safe kitchen procedures and avoid cross-contamination of cutting boards etc. Then WASH YOUR HANDS. I eat sushi, for what it’s worth. – A Public Health Physician