Virologists classify feline immunodeficiency virus (FIV) in the same Retroviridae family as the Feline Leukemia Virus (FeLV), but with one major difference: FIV is not considered to be in the oncornavirus subfamily of retroviruses, as is FeLV. Instead, FIV is classified as a lentivirus (or “slow virus”), along with the viruses that cause progressive pneumonia in sheep, infectious anemia in horses, arthritis–encephalitis in goats, and acquired immunodeficiency syndrome (AIDS) in human beings. FIV and FeLV often cause similar types of disease, but the viruses themselves differ in many ways, including the shape of the viral particle. The FIV particle is elongated, while FeLV is more circular. In addition, the two viruses are quite different genetically, and their structural proteins are dissimilar in size and composition. This means that the two viruses are antigenically unrelated; antibodies to FIV do not bind to FeLV, nor do antibodies to FeLV bind to FIV.

How prevalent is the infection?

FIV–infected cats are found worldwide, but the prevalence of infection depends on geographic locale and the population of cats tested. In the United States, approximately 1.5 to 3% of healthy cats are infected with FIV. Infection rates rise significantly in cats that are sick; up to 15% of cats with clinical signs of other disease also are infected with FIV. Free–roaming male cats –– especially aggressive ones –– are the most frequently infected, while cats housed exclusively indoors are much less likely to be infected.

How is FIV spread?

The primary mode of transmission is through bite wounds. Casual, nonaggressive contact among cats does not appear to be an efficient route of spreading FIV. On rare occasions, the disease is transmitted from an infected mother cat to her kittens during gestation, during passage through the birth canal, or when the newborn kittens ingest infected milk. Sexual contact probably is not a primary means of spreading FIV.

How does the virus affect the cat?

Following initial infection, the virus is carried to regional lymph nodes, where it may replicate in white blood cells known as T–lymphocytes. The virus then spreads to lymph nodes throughout the body, resulting in a generalized enlargement of the nodes. This stage of the disease usually passes unnoticed by an owner unless the nodes are greatly enlarged. Some time later––perhaps days but usually weeks to months––the cat may develop a fever and a drop in the white blood cell count. This decrease in white cells is due primarily to a lack of neutrophils, the white blood cells that help protect cats against bacterial infections, and to a loss of certain types of lymphocytes called T–helper cells, which play an important role in almost every aspect of immune protection. Anemia (or low red blood cell count ) also may develop, especially later in the disease.

Persistently infected cats may appear normal for years. Eventually, signs of immunodeficiency begin to develop, and the cat’s ability to protect itself against infection is compromised. The same bacteria, viruses, protozoa, and fungi that are found in cats’ everyday environment––where they usually do not affect healthy animals––can cause severe illness in cats with weakened immune systems. These secondary infections are responsible for most of the clinical signs associated with FIV infection, and are the major cause of death in FIV–positive cats.

What are the signs of the disease?

Clinical signs of the immunodeficiency syndrome appear throughout the body. Poor coat condition and persistent fever with a loss of appetite are commonly seen. Infection and inflammation of the gums (gingivitis) and mouth (stomatitis) are present in about half of the cats infected with FIV. Chronic or recurrent infections of the skin, urinary bladder, and upper respiratory tract are often present.

Persistent diarrhea can be a problem. Inflammation of the tissues of the eye occurs frequently, but in most cats the damage is not clinically apparent. Slow but progressive weight loss also is common, followed by severe wasting late in the disease process. FIV–infected cats have an increased risk of developing certain types of cancer, such as lymphomas, although the cancer risk is greater with FeLV infection. Abortion of kittens or other reproductive failures have been noted in infected queens. Some infected cats experience seizures, mental deterioration, and other neurologic disorders. Some FIV–infected cats have recurrent illness interspersed with periods of relative health between episodes. In those cases, low white blood cell counts and anemia also appear to cycle, with episodes of low cell counts followed by recovery to nearly normal levels. However, the overall trend seems to be progressive, with cell counts dropping lower with each subsequent episode.

How is a diagnosis of the disease made by our veterinary team?

Diagnosis is based on the history, clinical signs, and results of an FIV antibody test. This test is performed by most commercial and university veterinary diagnostic laboratories, and also is available in kit form for use in private veterinary clinics. The presence of FIV antibody in “positive ” test results indicates that a cat is infected with FIV—probably for its lifetime—and thus is capable of transmitting the virus to other cats. Since false–positive test results can occur, positive results should be confirmed using a test with a different format. Occasionally, an FIV test is reported as equivocal or indeterminate. Retesting in 8 to 12 weeks usually results in either a positive or a negative result. However, a few cats will continue to have equivocal results, often because of some factor in the blood that interferes with the test.

Young kittens may have positive test results for 12 to 16 weeks after birth, without actually being infected with FIV, because of passive transfer of FIV antibodies from the mother. Only a small percentage of these kittens actually are or will become infected. For this reason, kittens with positive antibody tests must be retested when they are 6 to 8 months of age to determine their true infection status.

A negative test result indicates that antibodies directed against FIV have not been detected. In most cases, a negative test result means that the cat is not infected. However, it takes 8 to 12 weeks after infection (and sometimes even longer) before detectable levels of antibody appear. If the test is performed during this interval, inaccurate results might be obtained. Therefore, antibody–negative cats with either an unknown or a known exposure to FIV–infected cats should be retested about 8 to 12 weeks after their most recent exposure in order to allow adequate time for development of antibodies. (On very rare occasions, cats in the later stages of FIV infection may test negative because their immune systems are so compromised that they no longer produce antibodies.)

What can be done if my cat is infected with FIV?

One of the most important things you can do is to protect your cat from exposure to the infectious agents that cause severe disease and death in immunosuppressed animals. Keep your cat strictly indoors and away from other cats, not only to protect your cat, but also to prevent transmission of FIV to other cats as well. The use of antimicrobial drugs to control bacterial and fungal infections may be moderately successful but must be continued for long periods, or reinstituted as new infections occur. Supportive care, including intravenous fluids, blood transfusions, and feeding of high–caloric dietary supplements, is frequently required. The use of corticosteroids or other anti–inflammatory drugs may be indicated in some cases to control gingivitis and stomatitis. Anabolic steroids may help to combat weight loss and wasting. Keep in mind that these measures are not directed at combating the virus itself. Some of the drugs useful in therapy for AIDS patients may also have the potential against FIV, but they are expensive and difficult to obtain, and side–effects appear to be greater in cats than in humans. Drugs designed to enhance or modify the immune system might be of benefit in treating FIV infections. However, most potentially effective treatments are still in the experimental stages of development and testing. Even these drugs only suppress the effects of the virus and do not destroy it.

I just discovered that one of my cats has FIV, yet I have other cats as well. What do I do now?

Cat–to–cat transmission of FIV in multiple–cat households where there is no fighting among cats appears to be quite uncommon. Many FIV–positive cats are not diagnosed until after they have lived in the multicat environment for years. Ideally, any infected cats in such households should be separated from the non–infected ones, but in reality, if fighting or rough play is not taking place, the risk to the non–infected cats appears to be low.

How long can I expect my FIV–positive cat to live?

It is impossible to accurately predict the life expectancy of a cat infected with FIV. Under ideal conditions, such as isolation of the FIV–infected cat from other cats, many cats will remain in apparent good health for many months to a number of years after the initial infection. If your cat has already had one or more severe illnesses as a result of FIV infection, or if persistent fever and weight loss are present, a much shorter survival time can be expected. The immune status of an FIV–infected cat can be determined by testing the ratio of CD4+ to CD8+ lymphocytes. This test, available through several veterinary laboratories including the Veterinary Diagnostic Laboratory at Cornell, is a measurement of the degree of immunosuppression. The CD4+:CD8+ ratio is often a useful prognostic indicator: the lower the ratio, the more dire the prognosis.

My FIV–positive cat died recently after a long illness. Is it safe to bring a new cat into my home?

Feline immunodeficiency virus is fairly unstable outside the cat and will not survive for more than a few hours in most environments . In addition, transmission of FIV occurs primarily through bites, so a waiting period between cats is not required to prevent FIV infection. However, FIV–positive cats are frequently infected with other infectious agents which may pose some threat to a newcomer, so precautions should be taken. Thoroughly clean and disinfect or replace food and water dishes, bedding, litter pans and toys. A dilute solution of household bleach (4 oz. bleach in 1 gal. water) makes an excellent disinfectant. Vacuum carpets and mop floors with an appropriate cleanser. Any new cats or kittens should be properly vaccinated against other infectious agents before entering the household.

How can I prevent my new cat from contracting FIV?

No vaccine against FIV is available. Owners can protect their cats only by preventing them from contacting infected cats. Pets kept indoors and away from free–roaming cats are highly unlikely to contract FIV infection. Ideally, catteries and multiple–cat households should test all their cats and remove any that are infected. Once FIV–negative status of resident cats has been established, all prospective feline newcomers should be tested for FIV antibodies, and only FIV–negative animals should be brought into the household or cattery. A quarantine period of about 8 weeks to 12 weeks, followed by a repeat test, is recommended for a cat with an uncertain history of exposure to the virus, such as a stray cat.

Can I become infected with FIV from an infected cat?

No, almost certainly not. Although FIV is structurally similar to HIV (the human immunodeficiency virus) and causes a disease in cats similar to AIDS in humans, it is a highly species–specific agent and affects only felines. In several studies, veterinarians, owners, and researchers who have had close contact with FIV–infected cats have shown absolutely no evidence of infection. Based on current evidence, it appears that FIV infections are restricted solely to cats.