Indications for vaccination against feline leukaemia virus

By Pr. Oswald Jarrett, University of Glasgow, 2007.
(Reprinted with permission).

Introduction: why is FeLV important?

FeLV is the cause of several serious, ultimately fatal, diseases in the cat, particularly lymphomas, anaemia and immunodeficiency. Of cats that develop a persistent infection, almost 90% die within 4 years of becoming infected. Young kittens are very susceptible to infection but older cats are much more resistant: thus, a dose of virus that will infect 100% of 8-week-old kittens will infect only around 20% of 6-month-old cats. Cats that are exposed but recover from infection are solidly immune to natural re-infection by FeLV. The virus grows particularly well in haemopoietic tissues, such as the bone marrow, and in mucosal epithelial cells, and is transmitted primarily from cat to cat in the saliva. Since such transmission requires very close contact between cats, FeLV is almost exclusively an infection among groups of cats. When the virus is endemic in a household of cats, on average around 30% of the cats are found to be infected; however, among free-ranging cats from mainly single-cat households, less than 1% are now infected.

Why has the prevalence of FeLV declined?

This latter figure is much less than was observed 20 years ago, when the prevalence in that population was over 5%. Why has the prevalence of FeLV declined so dramatically in this period? The reason is that the infection has been effectively controlled in multi-cat households by test-and-isolation programmes, and latterly by vaccination. The most dramatic change has been in pedigree cats in which the virus has been eradicated. A further consequence of the resulting decrease in FeLV incidence in these populations has been a reduction in the risk of infection for the general free-ranging cat population from cats that were infected in an endemic multi-cat household.

This has been a great success story.

Can vaccination be used to continue this decline?

Vaccination should be used in combination with testing to maintain the reduction in the risk of FeLV infection. The classic method of FeLV control, particularly in multi-cat households, is test-and-isolation. In an infected household, FeLV-positive and negative cats are identified and then isolated from each other. If further cats are to be introduced into the house they should be tested to ensure they are free of the virus before being mixed with the existing negative cats. It is very important that reliable diagnostic tests are used to identify infected cats and that a positive result obtained with an in-practice test, particularly in a healthy cat, is confirmed by a test such as immunofluorescence, virus isolation or polymerase chain reaction. Positive cats should not be allowed to range freely, with the attendant danger of infecting other cats. Negative cats that are allowed to go out should be vaccinated to prevent them becoming infected and then re-introducing the virus into the household. Vaccination does not interfere with subsequent testing for FeLV. Positive cats need not be vaccinated because the vaccine will have no beneficial effect on them.

Should FeLV-negative cats in single cat households be vaccinated? Clearly, if they are kept entirely inside the house, there is no need to vaccinate because they are not at risk. However, if they range freely outside, they should be vaccinated to protect them from becoming infected.

Some myths about FeLV vaccination.

There is a common misconception that because vaccines are only around 80% effective in experimental conditions that they are not worth using in pet cats. In fact, it is quite likely that the level of protection achieved by vaccination in nature is greater than this figure suggests. Vaccines are tested in laboratory systems where the level of challenge to which they are exposed is extremely high. In natural conditions, cats are probably exposed to lower doses of virus, so the level of protection in the field may be higher than in the laboratory. Another reason for believing that vaccination can help reduce the risk of infection for the whole population is that it has been predicted from mathematical modelling that even if a relatively low proportion of the population is vaccinated with a vaccine that is only 80% effective, then the risk of transmission of the virus is very low.

Another myth is that since adult cats are more resistant to FeLV infection than kittens, they do not need to be vaccinated. Although it is true that older cats are less susceptible to FeLV infection than young kittens, they are still at considerable risk of being infected. Therefore, the best practice for the welfare of cats of all ages is to have them vaccinated.

How should FeLV vaccines be used?

I believe that a FeLV vaccine should be considered as a 'core vaccine' in situations where a cat is at any risk of exposure to the virus. Essentially this means that all cats should be vaccinated if they go outside and are in contact with other cats, whether or not they are from single- or multi-cat households.

What calendar is appropriate for FeLV vaccination?

An essential plan is a primary course of vaccination (for kittens, two inoculations at 8-9 and 12 weeks of age; for adults, two inoculations 3-4 weeks apart), followed by re-vaccination 12 months later. Duration of immunity studies with some vaccines indicate that cats are protected very well for at least 12 months after primary vaccination. Cats that recover from natural FeLV infection are resistant to re-infection for periods in excess of three years. Whether vaccination induces the same quality of immunity is not yet known. Although current advice from the manufacturers is that cats should be revaccinated annually, it is likely that protection can be maintained by vaccinating at intervals of three years.