Feline immunodeficiency virus (FIV) is an important viral infection of cats that occurs worldwide.
The virus was first discovered during the investigation of a disease outbreak in a previously healthy colony of rescue cats in the USA, that had been showing similar signs to people with the acquired immunodeficiency syndrome (AIDS) caused by human immunodeficiency virus (HIV) infection. Although HIV and FIV are very similar, the viruses are species specific, which means that FIV only infects cats and HIV only infects humans. Thus there is no risk of infection for people in contact with FIV-positive cats. Cats are most commonly infected with FIV through bite wounds. Once infected, a cat will remain infected with the virus for life, and after a period that may last several years, the virus may damage the cat's immune response and lead to signs of disease.
What is FIV and how is it spread?
Feline immunodeficiency virus belongs to the retrovirus family of viruses in a group called lentiviruses. Lentiviruses typically only cause disease slowly and thus infected cats may remain healthy for many years.
Once a cat has been infected with FIV, the infection is virtually always permanent (cats cannot eliminate the virus), and the virus will be present in the saliva of an infected cat. The most common way for the virus to be transmitted from one cat to another is via a cat bite, where saliva cottoning the virus is inoculated under the skin of another cat. The virus does not survive long in the environment and is readily killed by common disinfectants. Rarely, the virus may also be spread by non-aggressive contact between cats (eg, mutual grooming), from a pregnant queen to her kittens; and it can also be spread through blood transfusions.
It is not known if blood sucking parasites such as fleas can spread infection so it is wise to maintain a regular flea control programme.
How does FIV cause disease?
FIV infects cells of the immune system (white blood cells, mainly lymphocytes). The virus may kill or damage the cells it infects, or compromise their normal function. This may eventually cause a gradual decline in the cat's immune function.
In the first few weeks after infection the virus replicates and may cause mild signs of disease such as a mild fever and swollen lymph nodes. Usually these signs are so mild they go unnoticed. An immune response will develop which does not eliminate the virus, but keeps viral replication at a relatively low level.
After a period of time, in some infected cats viral replication increases again, and it is typically these cats that go on to develop signs of disease. In most cases this will probably be around 2-5 years after the cat was first infected. Increased replication of the virus leads to progressive damage to the immune system.
How common is FIV infection?
The prevalence (frequency) of FIV infection varies in different cat populations. It tends to be more common where cats live in more crowded conditions (and thus where cat fights are more common) and tends to be much less common where cat populations are low and where cats are kept mainly indoors. In general, among healthy cats around 1-5% will be infected with FIV but in high risk cats (for example in cats with signs of recurrent disease suggesting immunosuppression) the prevalence may be as high as 15-20%. Infection is much more common in outdoor cats, and is about twice as common in male cats compared with female cats. Although cats of all ages can be infected, it is most commonly middle-aged cats (5-10 years of age) where infection is diagnosed.
What are the clinical signs of an FIV infection?
FIV usually causes disease through immunosuppression - the normal immune responses of the cat are compromised, leading to an increased susceptibility to other infections and diseases. There are no specific signs associated with FIV, but typically infected cats will develop recurrent bouts of infections or diseases that gradually get worse over time, and infections may not respond to treatment as well as would normally be expected.
Some of the most common signs seen in FIV infected cats are:
- Weight loss
- Recurrent fever
- Enlarged lymph nodes
- Gingivitis and stomatitis (inflammation of the gums and mouth)
- Chronic or recurrent respiratory, ocular and intestinal disease
- Chronic skin disease
- Neurological disease (in some cats the virus can affect the brain)
Other disease may also develop such as neoplasia (eg, lymphoma) and other infectious agents may be more problematic in FIV infected cats (such as toxoplasmosis, haemoplasma infections, feline infectious peritonitis, etc).
How is FIV diagnosed?
There are several tests available for diagnosing FIV infection, some of which can easily be performed in your own vet's clinic. Most tests involve collecting a blood sample and detecting the presence of antibodies in the against the virus (usually there is not enough virus in the blood itself to be able to readily detect it).
Antibodies against FIV are produced by the cat's immune system during infection, and the test works on the principle that cats cannot eliminate the virus so if antibodies are present in the blood then the virus will also be present. These tests are generally highly reliable, but no test is 100% accurate. If there is any doubt about the validity of the test result, your vet may want to do a follow-up confirmatory test using a different method (such as a different test kit, or sending blood to a laboratory to check for antibodies using a more sophisticated assay such as 'western blotting' or to look for virus using a molecular test such as PCR).
It is important to remember that kittens born to FIV-infected queens will receive antibodies from the queen via the milk, and so will test positive early in life though they may not be infected. Kittens with a positive test result should always be retested when they are 5-6 months of age. Additionally, in countries where the FIV vaccine is available (see below), cats that have been vaccinated will also test positive on the routine antibody tests, so alternatives (such as a PCR test) are needed.
Many FIV infected cats are able to live happily with the virus for a long period of time, and indeed the virus will not necessarily ever cause clinical disease. Whether disease develops depends on many factors including the strain of FIV a cat is infected with, the cat's immune response and the presence or absence of other infectious agents. In one study, FIV-infected cats were found to survive just under 5 years on average (from the time their disease was diagnosed) compared with about 6 years for a similar group of non-infected cats.
The main aims of managing an FIV-infection are to prevent further spread of infection to other cats and to maintain a good quality of life for the infected cat. Some antiviral medications used in human patients with HIV infection have also been shown to help some cats with FIV infection.
General and supportive treatment should include:
- Neutering all FIV-infected cats to reduce the risk of fighting and spreading infection
- Confining FIV-positive cats indoors where possible, and keeping them away from non-infected cats. This helps prevent spread of infection to other cats and reduces exposure of the FIV-infected cat to other infectious agents. Alternatively, create a cat-proof enclosure to allow your cat some access to the outdoors without coming into contact with other cats – see fencing in your garden
- Maintaining good quality nutrition - using a good commercial food and avoiding raw meat, eggs and unpasteurised dairy products helps reduce the risk of exposure to parasites and bacteria that might cause disease
- Maintaining good routine preventive healthcare (regular flea and worm control, routine vaccinations etc.)
- Ideally veterinary health checks twice yearly - your vet may suggest certain blood tests occasionally to monitor your cat's health
- Prompt diagnosis and appropriate treatment of any secondary or concurrent diseases. Longer courses of antibiotics may be needed to treat bacterial infections if they are significantly immunosuppressed.
Two forms of antiviral therapy are sometimes used in FIV-infected cats:
- Interferons are a group of naturally-produced compounds that have anti-viral effects and modify immune responses. A recombinant feline interferon (feline interferon omega) is available in some countries and it is possible that using this may have some helpful anti-viral and immune modulating effects. It is unlikely to have a profound effect in FIV-infected cats, but your vet may suggest trying this as a treatment.
- Antiviral drugs such as AZT - some of the human antivirus drugs used to treat HIV are also effective against FIV and can be safe to use (although careful monitoring of cats is needed). These drugs cannot 'cure' a cat with FIV, but especially if the signs of disease are quite severe, this may be a form of therapy that your vet will offer. Treatment is expensive though, and many cats appear to do just as well with good supportive therapy.
Prevention and control
A vaccine against FIV has been licensed and is available in a number of countries. The available data suggests that the vaccine gives a useful degree of protection and that it might therefore be useful in cats at appreciable risk of being exposed to FIV. The vaccine cannot be expected to give complete protection though, especially as there are multiple different strains of FIV. Also, a vaccinated cat will subsequently test positive on the routinely used antibody tests for FIV infection (see above).
FIV-infected cats should ideally be separated from other cats, but this can sometimes be difficult in a multicat household. As the risk of transmission by social contact such as sharing food bowls and mutual grooming is low, some owners elect to keep the household as it is. However, it may be helpful to at least feed cats using separate food bowls, as large amounts of virus are present in saliva. Litter trays and food bowls should be disinfected after use to kill the virus.
Advice for breeding colonies
To minimise the risk of introducing FIV into the colony, breeders should prevent their cats having free access outdoors, or having contact with cats allowed outdoors. Annual testing of breeding cats is ideal, but testing any new cats before being introduced to an FIV-free colony is vital. If any cats test positive for FIV they should be removed, the colony isolated, and the remaining cats retested after 3-6 months.
Advice for cat rescue centres and organisations
Ideally routine screening should be performed in all cats before homing but financial constraints mean this is not always possible. Priority should be given to testing any cats at high risk (cats showing clinical signs suggesting FIV or aggressive cats). Ideally, cats in a rehoming facility should be housed separately and, if not, then kept in the smallest groups possible. Neutering all cats before rehoming will help in reducing transmission of FIV.
Advice for boarding catteries
Boarding catteries should be properly constructed and managed so that cats from different households can never come into contact with each other, or use common exercise areas. In this situation there is no risk to other cats, and no reason not to board an FIV-positive cat.
Prognosis for infected cats
The prognosis for FIV-infected cats is guarded, but depends on the stage of disease. If FIV is diagnosed early, there may be a long period during which the cat is free of clinical signs related to FIV, and not all infected cats go on to develop an immunodeficiency syndrome. Infection is almost invariably permanent, but many infected cats can be maintained with a good quality of life for extended periods.