Question

A breeder wrote with concerns about the calicivirus :

I bred a litter of six Burmese kittens last summer which developed calicivirus very mildy a couple of weeks after their first vaccination.  As they all tested positive I sold five of them as pets, asking the future owners to sign a declaration to say that they would not show them or breed from them.  I kept one kitten.  I also have four adult cat (three of them are neutered) who are fully vaccinated.  They showed no signs of the disease but I am aware that at least one of them must have been a carrier.  They have not at this point been tested.   

1    You state that the majority of cats eventually stop shedding the virus although they may later pick up a different strain.  However, other advice I have read suggests that it is necessary to separate carriers from non-carriers or the non-carriers can become re-infected.  As this is not really practical in my small house, will the disease just go round and round the five of them indefinitely?

2    Can you tell me anything about the effectiveness of interferon (although I think it is currently not licensed for animal treatment) or of any undesirable side effects that might occur with its use?  (I gather it is very expensive but could be worthwhile if it eliminates the virus completely.)

I would love to resume showing my cats and occasionally breeding a litter but I don't want to risk spreading the virus further afield.  But my cats are also much loved pets and I don't want to rehome them.

I do not know at this point how many of them are carriers as it would be expensive to keep getting them tested and I want to have a plan of action in place to deal with the problem once I know the extent of it. However, I do know that the largest kitten (who has gone to a home where he is the only cat) was tested recently and is still positive, three and a half months after first being infected, having spent two of these in his new home.

I would appreciate it if you could answer my queries to help me decide what steps to take next with my cats.

Answer

Following infection with calicivirus, most cats will continue to shed virus for some time – 50 per cent are still shedding 75 days after infection, and most will stop shedding by a year after infection. Some will go on to become lifelong shedders, even though they appear clinically healthy.

Unfortunately, if they are only shedding at a low level, detection of the virus on oral swabs can be difficult, and a false negative test could be given unless a series of swabs are taken over a number of weeks. Also, positive tests do not necessarily mean that a cat is a chronic carrier. The cat may have been recently infected, and shedding organisms prior to becoming immune. Twenty to thirty per cent of the feline population is estimated to be shedding calicivirus. Vaccination will tend to protect against disease, but does not stop cats shedding the virus.

I would suggest that you do nothing for about six months. Keeping the population stable in this time will reduce stress and therefore maximise the chance of recovery from the disease. Separating them into different groups may just contribute to stress by altering the social hierarchy. I would then have your cats swabbed to identify if any of them are shedding, although I would recommend two to three swabs two to three weeks apart in case they are low-level shedders. If any are identified as positive, then they should not be bred from, and removed from the household. Any
negative cats could be bred from, but should be:

1 vaccinated prior to mating
2 moved into isolation three weeks before kittening, so that if the mother does start to shed, she should be recovered from this by the time the kittens are born

3 maintain the kittens and mum in isolation until four to five weeks old, during which time the kittens should be protected by antibodies from mum
4 separate kittens from mum at four to five weeks, and maintain in isolation until vaccinated

With regard to interferon: a licensed feline interferon is available in this country. Protocols have been devised for treatment of calicivirus, but there are few extensive trials. The cases in which this drug has been used have been actively infected, rather than it being used for elimination of the carrier state. A course of treatment would probably be about £300 per cat - a lot to spend when we have no evidence that it would eliminate the disease from your household.

Unfortunately, calicivirus is a very difficult infection to eliminate.