Dr Smith relayed your letter of December 20, 1995, with the question for me. Unfortunately, I have had clinical and other responsibilities that kept me from responding until now.

Your question was, "Please explain why it would be unwise to deliberately spread a calicivirus amongst rabbits (which are probably not the viruses usual host as it killed 90% of them according to studies already made) taking into account the close proximity of the target rabbits to livestock,animals,birds,insect vectors and humans. I realise you are not a veterinarian but taking into account your explanation of Caliciviruses and the similarity in appearance genetically of strains affecting people and animals would you please comment on the use of viruses in general to control/destroy animals in the long term and the potential dangers to humans and other species of releasing into the environment viruses that are of such a flexible nature that they could effect (sic) man."

My comments are as follows:

1) The fact that the rabbit calicivirus is killing 90% of rabbits is a clue that this agent is new to rabbits. The rabbit population could not survive if 90% mortality was the routine outcome of exposure. We don’t know from which species the virus arose. I will say that I don’t think the rabbit hemorrhagic disease calicivirus release will work in Australia, or anywhere. Genetically resistant and immune rabbit populations will arise and repopulate the same areas unless a “second hit” mechanism is used to eliminate them.

2) The close proximity of rabbits to other mammals, and other families, increases the possibility of transmission of the agent to species other than rabbits. One scenario would be the consumption of a sick rabbit by a dog. In this instance, one could expect an inoculum to the dog far exceeding that used in the CSIRO experiments. Because inoculum is an important determinant of outcome of exposure, barriers to infection that might have been apparent in experimental settings may be overcome in wild exposures.

3) All caliciviruses share some common genomic features. We anticipate that those common features will result in common utilization of replicative mechanisms in the host cell. We know already that some calicivirus strains can replicate in a wide range of hosts. This suggests the possibility that a virus like the rabbit hemorrhagic disease calicivirus at a minimum will be able to replicate in livers or other tissues from mammals other than rabbits.

4) We know that caliciviruses affecting humans are genomically diverse. In fact, on the basis of genomic comparisons it is difficult to draw the line between different clusters of human calicivirus strains. Rabbit caliciviruses have not been as well characterized as human and other calicivirus strains. We do not know the limits of antigenic types in the rabbit hemorrhagic disease calicivirus and we don’t know the diversity of a population of wild rabbit hemorrhagic disease calicivirus strains. Knowledge of this diversity would be one prerequisite to releasing any calicivirus strain deliberately.