Feline leukaemia virus (FeLV) can cause a variety of neoplastic diseases, the most important of which is lymphoma, a solid tumour of lymphocytes (a type of white blood cell) and the most common tumour of cats. Lymphoma may take several forms depending on the tissues involved. Less commonly, the neoplasia arises in the bone marrow, causing true leukaemia or related myeloproliferative disorders. However, not all cases of lymphoma or leukaemia in cats are associated with FeLV infection – this virus is just one potential underlying cause.
Clinical disease with FeLV infection can also result from immunodeficiency and the associated clinical picture can be very similar to that caused by feline immunodeficiency virus (FIV). FeLV also causes disease in other ways and is linked to a variety of clinical syndromes including anaemia, neurological conditions, haemorrhagic enteritis and infertility.
FeLV does not survive for long periods outside the cat, and transmission of the infection requires close contact between cats. Following exposure, FeLV may establish a persistent infection. In
this state, virus is continuously present in the blood (viraemia) and is excreted in all bodily secretions, the most important of which is saliva. Transmission is often associated with prolonged exposure through close, extended contact with an infected cat and is thought to result frequently from mutual grooming, leading to ingestion of virus.
FeLV-infected queens are often infertile but if an infected queen is able to breed, her kittens are very likely to be persistently viraemic at birth. Kittens may also acquire the infection from
ingesting infected milk during suckling.
Animals become more resistant to infection with age, so that adult cats are less likely to become persistently infected than young kittens. Cats that are exposed to the virus and then recover are generally solidly immune and resistant to further infection.
The prevalence of FeLV varies in different groups of cats. The lifestyle of cats living in groups or multicat households, especially breeding catteries, is particularly conducive to the spread of FeLV. However, the virus has been eliminated from most pedigree breeding catteries using a test and isolation system so that infection is now unusual in such colonies.
A low prevalence, usually less than one per cent, is found on testing healthy pet cats. Surveys of sick cats, in which FeLV is likely to be more common, have recorded a prevalence of infection of up to 18 per cent. While these results probably overstate the importance of FeLV as a cause of disease, they indicate that FeLV is important as a cause of serious disease. There is some evidence that there are geographical variations in the UK in the prevalence of FeLV.
Since older cats are known to be less susceptible to infection than young kittens, there is a common misconception that adults do not become infected. It is clear that older cats can become infected.
The results of surveys of the prevalence of FeLV in stray cats have been contradictory. In some surveys, a high prevalence has been found, whereas in others only a small proportion of infected cats have been identified. Geographical differences may explain some of this variation.
FeLV-infected cats generally have a short life expectancy and about 90 per cent die (or euthanasia is carried out) within four years of being infected. Often, cats that are tested and found to be positive have had the infection for some time and may be expected to have an even shorter life expectancy. A small proportion of viraemic cats may live for longer periods. However, there is no prognostic test that will predict how long an infected cat will survive.
Persistent FeLV infection results in the continuous presence in the blood of infectious virus, soluble viral antigens and antigencontaining white blood cells. These can be detected by a variety of test systems including test kits designed for use in veterinary practices as well as tests that are used in specialist laboratories. In-practice tests for FeLV detect viral p27 antigen, the major viral core protein, and are based on either enzyme linked immunosorbent assay (ELISA) or immunochromatography (IC) technologies. Combination tests that enable simultaneous testing for both FeLV and FIV are popular since both viruses may cause similar clinical signs. There is reasonable agreement between results of antigen detection obtained using these different systems.
Unlike tests for FIV, FeLV tests are not affected by maternally-derived antibodies, so they can be used for cats of any age.
Many commercial laboratories also offer tests for FeLV. Some carry out ELISAs or IC tests, while other more specialised laboratories offer confirmatory tests, including virus isolation, immunofluorescence and polymerase chain reaction (PCR). Virus isolation detects infectious virus in the plasma while immunofluorescence demonstrates viral antigen (proteins) in the white blood cells. Either of these methods is generally regarded as the definitive (gold standard) test for FeLV and there is very good agreement between the results of the two. PCR is not widely available. This test detects FeLV-specific DNA in the white blood cells. A problem is that although it identifies viraemic cats, PCR also detects residual DNA in some cats that have recovered from FeLV infection and will continue to be completely healthy. Before PCR can be used routinely for FeLV diagnosis, further research is required to clarify how the results should be interpreted.
There are two main issues of concern relating to in-practice test results for FeLV. Firstly, while these tests are rapid and simple to perform, they can lead to erroneous results. It is very important to avoid technical errors by closely following the manufacturers' instructions. Most in-practice test kits require blood samples and are best performed with serum or plasma, rather than whole blood. Tests designed for using saliva samples, collected using mouth swabs, are also available, but are less sensitive than blood tests and are generally not recommended.
The second problem is that even with a very accurate test, when an infection is of low prevalence (as with FeLV in healthy cats), an appreciable number of false positive results will be obtained. It is widely agreed, therefore, that a single positive result in a healthy cat should not be considered to be indicative of persistent infection and should be confirmed by a more specialised test such as virus isolation or immunofluorescence. While the status of such cats is being confirmed, they should be maintained in isolation to avoid any possible risk of transmission to other cats.
A small proportion of healthy cats that test positive for antigen but negative by virus isolation or immunofluorescence may continue in this state for a long period of time. These ‘discordant' cats most likely have a focus of infection somewhere in the body which releases antigen, but not virus, into the blood. While it is unlikely that these cats transmit virus, if they are in contact with other cats, they should be retested at intervals to ensure they remain free of virus.
Negative in-practice test results for FeLV are generally regarded as more reliable provided the test has been performed carefully. However, if FeLV infection is suspected in a sick cat that tests negative on an in-practice test, this result also should be confirmed by virus isolation or immunofluorescence.
Cats that have recovered from infection and are immune will often have virus neutralising antibodies, which can be detected at specialist laboratories. This test can be useful when deciding how to segregate cats in a multi-cat household where FeLV has been diagnosed.
FeLV-positive cats should be considered individually depending on their circumstances; what to do with these cats requires careful discussion between the veterinary surgeon and cat owner.
They may be treated symptomatically, and by chemotherapy if they have a lymphoma. However, euthanasia must be seriously considered because these cats have a short life expectancy and in their remaining time may not have a good quality of life.
These cats may continue to live in good health for a considerable period of time, but the vast majority will have a very much shortened lifespan (months or a few years). They should be given the same level of care as an uninfected cat but additionally should be shielded from cats with other infectious diseases and they should not be stressed. Since they excrete the virus, they should not be allowed contact with other cats to whom they might spread the virus.
Several different types of vaccine are available for FeLV. The vaccines that are available in the UK provide good protection against natural infection but, as with other vaccines, cannot be relied upon to be 100 per cent effective. Therefore, the possibility of FeLV infection should not be discounted in a sick, vaccinated cat with possible exposure to FeLV. Prior vaccination will not interfere with testing for the presence of infection. Since vaccination cannot be relied upon to provide complete protection, close contact with other cats that are known to be infected with FeLV, or are of dubious viral status, should be avoided. Although vaccines are very unlikely to induce virus neutralising antibodies, they may still be protective.
1. The pet cat
Single indoor/outdoor cats
Single cats should be tested if they are sick and showing signs compatible with FeLV infection. A pet cat infected with FeLV poses a threat to other cats. Close contact is generally an important risk factor for transmission of FeLV, but the possibility of spread through biting must also be considered. In order to prevent it transmitting the virus to other cats, it would need to be kept indoors. On welfare grounds this may not be applicable to some cats which will not tolerate being kept indoors permanently. Owners must weigh up the risks (to their own and other cats) with the welfare implications. It may be possible to fence in the garden or provide a run where cats can go out and present no risk to themselves or others.
Single cats confined indoors
Single cats confined indoors should not be at any risk of exposure to FeLV. However, there can be a long delay between initial infection and development of disease, so there may be a possibility of earlier infection, for example as a kitten from their dam. Consequently, adult cats could succumb to disease from FeLV infection, despite having spent their whole life since kittenhood isolated from other cats.
2. Multicat households
The same considerations about possible risks to neighbouring cats from infected cats from single cat households also applies to multicat households. The issue of possible spread of infection within the household of cats must also be addressed. This is a particular concern with FeLV, which is readily transmitted within a household of cats that socialise together. If an FeLV-infected cat is identified in a household, all the other cats should be tested and positive and negative cats segregated to avoid further spread of infection. Cats should then be retested 12 weeks later to establish if any cats have eliminated the infection, and to identify any cats that had just become infected and subsequently become positive.
Testing of cats before introducing new individuals to a multicat household should be done to avoid inadvertent spread to incoming cats or to cats already established in the household.
3. The rescue situation
What should be the test protocol for cats coming in to the rescue facility?
Ideally all cats should be tested so that their FeLV status is known. However, as this is not feasible economically, most rescue organisations have adopted the approach of testing cats considered to be at high risk of being infected. These include sick cats, stray cats and feral cats. If any are found to be positive, all those cats with which they have been in contact should also be tested. Pregnant queens should be tested since all kittens of FeLV-infected queens will almost certainly be viraemic.
What are the responsibilities/legal considerations for the rescue facility?
Rescue facilities have a duty of care and should undertake what is reasonable in the circumstances. As outlined above, it may not be financially viable to test every cat or kitten, but whether they have or have not been tested should be explained to the new owner, along with any results if tests have been undertaken. No cat that has tested positive for FeLV should be rehomed without the new owner being fully informed of the risks.
What is the suggested testing protocol for kittens and cats?
- If a healthy cat tests antigen-negative on an in-practice test there is a high probability that it is genuinely negative.
- If a sick cat tests antigen-positive, it is very likely to be infected.
- If a healthy cat tests antigen-positive, then retest using a confirmatory test such as immunofluorescence or virus isolation. If positive on this test, the cat is almost certainly persistently infected.
- If a healthy cat tests antigen-positive but is negative on a confirmatory test, retest after 12 weeks to confirm that it is still virus-negative. (Unfortunately, it may not be possible for rescuers to maintain cats for this length of time, in which case prospective owners should be advised to have the cat retested).
What should be done with FeLV-positive cats?
Two facts should be considered: cats that test positive are likely to have a much shorter life expectancy than uninfected cats; and viraemic cats may spread the virus to other cats in contact. For these reasons, it is difficult for rescuers to rehome an FeLVpositive cat. Sick cats that test positive by virus isolation or immunofluorescence should be euthanased. Healthy cats may be rehomed under certain conditions: they should not be introduced into a house in which they would be in contact with FeLV-free cats; and they should not be allowed to roam freely. If these conditions cannot be met, and it is impossible to house positive cats within the rescue facility long-term, they should be euthanased.
4. Feral cats
Feral kittens young enough to be rehomed can be considered as rescue kittens above. All feral cats should be tested and positive cats euthanased rather than returned to the colony.
5. Breeding/show cats
The risk of spread of FeLV at cat shows is minimal. FeLV-positive cats should never be used for breeding. Potential owners and new breeders should ensure that cats they intend to acquire are FeLV free. Most breeders test for FeLV on a regular basis and most breeding cats are kept indoors or in runs where FeLV cannot be contracted from outside cats. To maintain an FeLV-negative household, all cats should be tested before entry.
Reviews of FeLV biology and associated diseases
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Hardy WD Jr, McClelland AJ, Zuckerman EE, Hess PW, Essex M, Cotter SM, MacEwen EG, Hayes AA (1976) Prevention of the contagious spread of feline leukaemia virus and the development of leukaemia in pet cats. Nature 263, 326-328
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Updated May 2006