Infections by feline leukaemia virus (FeLV) nd feline immunodeficiency virus (FIV) re definitely the most common infectious diseases of domestic cats. The seroprevalence varies greatly depending on the age, sex, lifestyle, physical condition and geographical location of the population studied.
The behaviour of FeLV is fairly controversial. In the past it was thought that more than a third of infected cats could eliminate the infection from the body, but recent research suggests that in fact most cats, although no longer viraemic, remain infected for their whole life. Indeed, even though many cats tend to become serologically negative after an initial phase of viraemia, the presence of a FeLV provirus can be demonstrated in most, if a polymerase chain reaction (PCR) technique is used. In contrast, FIV infections, like other lentiviral infections, are considered persistent with only a very low possibility of the virus being eliminated from the body.
FeLV is mainly transmitted by infected females to their litters, but other forms of horizontal transmission are possible. It usually takes about 4-6 weeks after the infection for a persistent viraemia to be established or for the virus to be eliminated from the body. The route of infection of FIV is almost always through bites; the production of antibodies useful for the diagnosis occurs within the 2 months following the infection.
The basis of the clinical diagnosis of FeLV and FIV is the presence of circulating antigen (FeLV), antibodies (FIV) or nucleic acids (PCR).
A correct diagnosis of the presence of infection is important not only for infected cats, but also for uninfected ones. A missed or mistaken diagnosis in infected cats can lead on the one hand to the free circulation of carriers that can transmit the virus and on the other hand to the euthanasia of healthy subjects. The American Association of Feline Practitioners (AAFP), like the ABCD, has, therefore, drawn up guidelines to help clinicians perform and interpret the available tests.
INFECTION BY FELINE LEUKAEMIA VIRUS
Infections caused by FeLV can be controlled by the identification and isolation of persistently viraemic cats, which are the main source of the spread of the disease. It is important that the correct aetiological diagnosis is made, in order to provide adequate prophylaxis against this retrovirus.
FeLV is characterised by a substantial antigenic load; consequently, tests that evaluate the presence of circulating FeLV p27 antigen are influenced very little by external factors such as the presence of antibodies of maternal origin.
The standard tests are an enzyme-linked immunosorbent assay (ELISA) and immunochromatography: these detect the presence of p27 antigen in the circulation and are more accurate if carried out on serum rather than on whole blood.
There are various circumstances in which a veterinarian should perform tests to determine whether an animal is infected or not by FeLV:
- in all sick cats, independently of their age, of previous tests with a negative result, and whether or not the animals have been vaccinated. It should, in fact, be remembered that the infections caused by FeLV are associated with a vast range of clinical manifestations;
- whenever a cat whose epidemiological status is unknown is introduced into a domestic setting, in which no other cats are present: the tests should be carried out even if the cat is apparently healthy, as it could subsequently manifest the disease. Furthermore, even if kept indoors, the potentially infected animal could escape and become a source of exposure to infection for other cats.
Periodic controls are advised for those cats considered to be at continuous risk of exposure to infection, such as cats with free access to the external environment and stray animals. In the case of suspected contact with the virus and negative results of the tests commonly used for the diagnosis of FeLV infection, the veterinarian should re-examine the animal about 1 month after the last potential exposure to infection, because the tests can give negative results during the pre-viraemic stage of the disease.
The key to a correct use of the tests available by the practising veterinarian is, therefore, to understand how to evaluate the results and to know if, and when, confirmatory tests are needed.
A positive result of an ELISA or immunochromatographic test indicates one of the following situations:
- a persistently viraemic cat. Cats that cannot eliminate the virus within 12 weeks of being infected remain infected. Such cats are susceptible to the development of FeLV-associated pathologies within a few years; furthermore, they are a source of risk for other cats because they shed the virus into the environment.
- a cat with transient viraemia. Following a prompt immune response, cats can eliminate FeLV and after 4-6 weeks, cats with a transient viraemia can become FeLV-negative. Consequently, tests should be repeated after 6 to 8 weeks in all FeLV-positive cats. In other words, no cat should be euthanized on the basis of a single positive test.
- a false positive or discordant result. Every positive result in an apparently healthy cat should raise doubts; these doubts should be particularly strong when the positive result is for a cat that lives in very low risk context, such as bred cats and cats that live isolated in flats. In these groups of cats the risk of a false positive test result can reach 50%. When a false positive result is suspected, the test should be repeated in a commercial laboratory provided with the information on which test gave the first result. If the result is confirmed, the animal is considered to be positive. If the result is discordant, other investigations can be carried out.
- Up to 30% of cats with a positive ELISA may not be viraemic, but may have a local infection of the mammary or salivary glands or of the related regional lymph nodes. In these cases it is recommended that the animal is retested after 6 to 8 weeks with an ELISA or immunofluorescence assay.
A negative result of an ELISA or immunochromatographic test may mean that:
- the cat has not been exposed to FeLV;
- the cat has been infected in the past, but has eliminated the virus;
- the cat has been infected very recently and the virus is, therefore, not yet detectable;
- the cat has a latent infection;
- the cat has a localised infection;
- the result is a false negative: as noted in the third point, a test may be falsely negative if the cat has been infected but has not yet developed a viraemia that can be detected by the test. At least 2 to 4 weeks must pass from the time of the infection for a test to become positive. Subjects that live outdoors must be isolated for at least 28 days before carrying out the test and then checked again after 90 days, given that some animals take much longer to develop viraemia.
INFECTION BY FELINE IMMUNODEFICIENCY VIRUS
The tests used to diagnose FIV infection are based on finding the antibodies produced against the virus, since FIV does not produce sufficient quantities of viral particles to be detected by routine immunological tests of the blood or other body fluids. The standard diagnostic tests exploit an ELISA or immunochromatography to demonstrate antibodies circulating in the serum of the patients and many different assays are now present on the market.
Some kits show antibodies against the p24 core protein, while others detect the antibodies against the gp40 envelope proteins. The most appropriate samples are serum and plasma, while samples containing anticoagulant should generally be avoided because they can cause colour changes that can increase the risk of false positive results.
Seroconversion occurs 2-4 weeks after infection and antibodies can be detected by the tests after this period. Most of the commercially available diagnostic kits are extremely sensitive: false-negative results are rare, while results are falsely positive in about one-third of cats if the seroprevalence in the population is very low. For this reason a subject with a positive ELISA, especially if healthy, should be retested by western blotting.
A positive test can indicate one of the following three situations:
- a persistently infected cat – anti-FIV antibodies are associated with life-long infection by this virus.
- a kitten born from an infected queen: it is worth remembering that although a kitten delivered by a positive female is unlikely to be infected, it has undoubtedly absorbed maternally derived antibodies against FIV in the colostrum. Given that the tests normally used to diagnose FIV (ELISA, immunochromatography) demonstrate the presence of antibodies, and the maternal antibodies can remain for many months, at least 4, before declining, kittens will be falsely positive. If the kitten has actually been infected, another 2 months are needed for seroconversion to occur. A diagnosis of FIV cannot, therefore, be made on the basis of antibody detection in a kitten before it has reached 6 months of age.
- false positive result. No test is 100% accurate, so it is always necessary to evaluate the clinical and social characteristics of the animal tested. A positive result in a cat that has had a very low possibility of being infected (a bred cat that has never left the house) has up to a 50% likelihood of being a false positive.
A negative test can indicate one of three possibilities:
- the cat is not infected by FIV;
- the cat is infected but has antibodies that cannot be detected by the test;
- the cat is infected by FIV but is not producing antibodies or has not yet produced them. This can occur in very sick animals, in the final stage of the infection or in cats in the acute stage of the infection (within less than 2 months after the infection). In this last case, if the time of the potentially infective traumatic event (bite) is known, the animal should be tested again 6 to 8 weeks later.
Suggested readings
- American Association of Feline Practitioners (AAFP) e Accademy of Feline Medicine (AFM), 200117