Feline immunodeficiency virus
Profs. Margaret Hosie and Regina Hofmann-Lehmann
Published September 2015
Feline immunodeficiency virus (FIV) was discovered in California in 1986 and is now recognised in domestic cats worldwide. The virus is very similar to human immunodeficiency virus (HIV); indeed the cat from which the virus was first isolated displayed signs similar to those observed in HIV-infected patients with acquired immunodeficiency syndrome (AIDS). Infection tends to be more common in older, male and feral cats, as FIV is transmitted by biting, although kittens born to infected queens may also become infected. FIV infection is common in hoarding situations, because cats living in close confinement, under stressful conditions, are likely to exhibit aggressive behaviour.
It is generally accepted that FIV infection can induce immunodeficiency. This leads to infections with other viruses, bacteria or fungi and sometimes the development of tumours; a common finding is inflammation in the mouth, with painful gums. However, in some cats the clinical signs are mild and such cats might have a normal life expectancy. These different outcomes of FIV infection are likely associated with the variability amongst field isolates which we are only just beginning to understand, as well as other differences in the ways that cats respond to infection.
FIV infection is usually diagnosed using in-house serological assays to detect antibodies against major viral proteins. It is important that such results are properly interpreted and confirmatory tests are recommended; for example, a positive result in a young indoor cat may be a false positive and should be confirmed using a different test, for example Western blot or virus isolation. Young kittens may test seropositive owing to the persistence of maternally derived antibody and should be retested at 6 months of age to confirm whether they are infected. Some cats, especially those homed with FIV-infected cats, may not test positive until several weeks (or even months) following infection. In immunodeficient cats, in the late stage of infection with high viral burdens, antibody may be sequestered in virus-antibody complexes, leading to false negative serological test results.
At present, it is not possible to predict whether or not an infected cat will develop AIDS and therefore surrogate markers are required to provide an objective assessment. Recent research at Glasgow has shown that viruses dominating in early infection display distinct properties and that the emergence of viruses with altered properties coincides with the onset of immunodeficiency. Accordingly, tests may be developed to assist in the clinical staging of individual cats diagnosed with FIV infection.
Several laboratories offer PCR tests for the diagnosis of FIV but these tests are not necessarily more reliable than serological tests. PCR tests are highly prone to contamination and must be conducted in reliable laboratories under rigorous conditions with appropriate controls to minimize false positive results. False negative results may arise in situations where the primers do not match the infecting field strain and infection is not detected, or in cases with very low viral loads.
FIV-infected cats pose an infection risk to other cats and therefore should not have free access outdoors. If FIV-infected cats are to be housed together with uninfected cats, with unrestricted access and sharing food and water bowls, litter trays and bedding, the risk of FIV transmission can be decreased by spay/neutering the cats in order to reduce the risk of territorial aggression, which can result in penetrating bite wounds and consequently FIV transmission. Younger kittens are less at risk of FIV infection before territorial aggression develops. Similarly, neutered cats are less likely to display territorial aggression than intact cats.
Recent research at Glasgow has shown that FIV infected cats kept in single cat households with regular veterinary care are more likely to remain healthy than FIV-infected cats in multi-cat households lacking regular health checks. Prompt supportive treatment dramatically increases the survival time for FIV infected cats.
Few antiviral drugs are effective against FIV and do not induce adverse side effects. Treatment with nucleoside analogues that inhibit the reverse transcriptase, such as zidovudine or PMEA, may improve clinical signs, but these drugs, as well as derivatives such as PMPDAP, induce anaemia. Treatment with the bicyclam AMD3100 inhibits viral replication and leads to a significant decrease in proviral load in cats naturally infected with FIV. While this may be a useful therapy, this treatment should not be combined with PMEA.
A whole inactivated virus vaccine has been available commercially to veterinarians in the USA since 2002 and is now marketed also in Australia, New Zealand and Japan. However, its use is not recommended in Europe by the European Advisory Board on Cat Diseases since the vaccine interferes with the serological diagnosis of infection and vaccination has not yet proven effective against primary field strains of FIV circulating within Europe.
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