Feline Immunodeficiency Virus (FIV)
Fred Scott, Former Director, Cornell Feline Health Center
James Richards, Director, Feline Health Center
Feline immunodeficiency virus (FIV), previously called feline T-lymphotropic lentivirus (FTLV), is a feline virus belonging to the family Retroviridae. Although it is related to feline leukemia virus (FeLV), FIV does not cause cancer and is not classified with FeLV in the oncornavirus subfamily of retroviruses. instead, FIV has been placed in the lentivirus subfamily, along with the viruses causing progressive pneumonia in sheep, infectious anemia in horses, arthritisencephalitis in goats, and acquired immunodeficiency syndrome (AIDS) in human beings.
THE CAUSE. The genetic material of FIV, like that of other retroviruses, consists of single-stranded RNA. The production of a double-stranded DNA copy of this RNA is an essential step in the replication (reproduction) of FIV within the host. This step requires a special viral enzyme, reverse transcriptase, which the virus carries with it when it infects a cell. The doublestranded copy of the viral genetic material then is inserted into the DNA of the host cell, where it may remain in an inactive state for some time before production of new virus particles is initiated.
Antibodies to FIV do not bind to FeLV, nor do antibodies to FeLV bind to FIV, so that the two viruses are antigenically unrelated. in addition, FIV is not antigenically related to human immunodeficiency virus (HIV), the lentivirus responsible for AIDS.
OCCURRENCE AND TRANSMISSION. The immunodeficiency syndrome associated with FIV, with its array of secondary infections, anemia, and low white blood cell counts, is indistinguishable from the noncancerous syndromes associated with FeLV infection. Prior to the first identification of FIV in California, cats that presented with what seemed to be an FeLV-associated disease, but that repeatedly tested negative for FeLV, were nevertheless assumed to be infected with FeLV. Now that a test for FIV is available, it is apparent that at least 15 percent of such FeLV-negative sick cats are in reality infected with FIV. The prevalence of FIV infection in the general, healthy, United States cat population, however, is estimated to be about I to 3 percent.
In addition to infecting domestic cats, FIV infects several species of nondomestic cats, including lions, tigers, snow leopards, pumas, bobcats, Pallas' cats, and cheetahs. Each species apparently is infected with its own unique type of FIV. The potential for disease production associated with FIV infection in the nondomestic cats is not clear at this time.
The primary mode of FIV transmission is through bite wounds. Casual, nonaggressive contact among cats rarely results in the spread of FIV. Transmission from an infected queen to her kittens occurs infrequently, as an in utero event, during the birth process, or through ingestion of infected colostrum or milk. This type of maternal transmission occurs primarily when the queen is exposed to FIV and becomes infected during gestation or lactation. Spread of FIV through sexual contact is theoretically possible but does not occur frequently during either experimental or natural infections.
PATHOGENESIS. Following initial infection, FIV is carried to regional lymph nodes, where it replicates in a subpopulation of white blood cells known as T lymphocytes or T cells. These cells are primary target cells of FIV (culture techniques often require the use of feline T lymphocytes to grow the virus in the laboratory). The virus then spreads to lymph nodes throughout the body, resulting in a generalized lymphadenopathy (enlargement of the lymph nodes). At this point, much of FIV replication occurs in another white blood cell, the macropbage. This stage of the disease usually passes unnoticed by an owner unless the nodes are greatly enlarged. Some time later-perhaps days but possibly weeks to months-the cat may develop a fever and a drop in the white blood cell count (leukopenia). A low red blood cell count (i.e., anemia) also may develop. The cause of these sometimes precipitous declines in blood cell types is not clear, but it may result from suppression or loss of precursor cells in the bone marrow. There then follows a largely enigmatic period (sometimes measured in years), during which the cat appears normal, despite being persistently infected with FIV. However, during this clinically quiescent period, there is a progressive decline in the number of a type of T lymphocyte known as a CD4 + T helper cell. The CD4 + T cell is extremely important in maintaining proper immune system function. Eventually, signs of immunodeficiency begin to develop, compromising the cat's ability to protect itself against infection. Bacteria, viruses, protozoa, and fungi, which can be found in the cat's everyday environment and generally are innocuous to a healthy animal, can cause severe illness in an immunocompromised individual. These secondary infections are responsible for most of the clinical signs associated with FIV infection.
CLINICAL SIGNS. The clinical progression of FIV infection can be divided into five stages, using the system described for HIV infection in humans: (1) acute infection (the first four to sixteen weeks of infection); (2) asymptomatic carrier (this stage may last months to years); (3) persistent generalized lymphadenopathy (PGL-this stage is usually very short and difficult to observe in FIV-infected cats); (4) AIDS-related complex (ARC-signs of gastrointestinal and respiratory disease are usually apparent); and (5) acquired immunodeficiency syndrome (AIDS cats develop opportunistic infections accompanied by fever and wasting). Cats in the ARC or AIDS stage of infection generally live for less than one year.
The clinical signs of the immunodeficiency syndrome are diverse in nature because they involve an array of secondary infections. General unthriftiness (failure to thrive) may be the first outward sign that a problem exists. Fever of 103'F or greater is often present in the ARC and AIDS stages of disease. A very common presenting complaint is a loss of appetite or evidence of pain while eating, caused by infection of the gums (gingivitis) and mouth (stomatitis). These conditions can lead to inflammation of the tissue around the teeth (periodontitis), with eventual loss of the teeth.
Chronic, nonresponsive, or recurrent infections of the skin, urinary bladder, and upper respiratory tract are often seen. Persistent diarrhea and ocular disease are also frequent problems. Abortion of kittens or other reproductive failures have been seen in infected queens. Some infected cats have experienced seizures, dementia (mental deterioration), sleep disorders, and other neurologic manifestations. Slow but progressive weight loss also is common, with severe wasting occurring late in the disease process.
DIAGNOSIS. Diagnosis is based on the history, the clinical signs, and the result of an FIV antibody test. Detection of FIV antibody is the diagnostic test of choice, because the levels of virus in the blood of an infected cat are frequently so low as to be undetectable by conventional means. An ELISA test is available in kit form for use in private veterinary clinics. All positive results should be confirmed by a second test, preferably using a different procedure. Confirmatory tests, which are performed at certain commercial diagnostic laboratories, include IFA assays and a test called the Western blot or immunoblot procedure. A positive FIV antibody test indicates that a cat is infected with FIV (probably for its lifetime-established infections are rarely cleared) and is capable of transmitting the virus to other susceptible cats. it should be noted that eight to twelve weeks (and occasionally more) may elapse after infection before detectable antibody levels appear.
Because FIV tests depend on the detection of antibodies to the virus rather than the virus itself, kittens of FIV infected (antibody-positive) queens will almost always test positive for FIV antibodies during the first three to four months of life. in this case, a positive FIV test does not mean that the kitten is actually infected with FIV. Such kittens should be tested for FIV at six to eight months of age, after the loss of maternally derived antibodies, to determine their true infection status.
TREATMENT. Therapy of the secondary infections associated with FIV is based on the clinical signs and the nature of the infectious agent. Use of antiniicrobial (antibacterial or antifungal) drugs to control bacterial and fungal infections has been 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 (those that promote growth and tissue repair) may help to combat weight loss and wasting. it must be kept in mind that these measures are not directed at combating FIV itself. The drug AZT, useful in therapy for AIDS patients, may also be of use against FIV, but the drug is expensive and difficult to obtain, and its side effects are apparently greater in cats than in humans. Drugs designed to enhance or modify the immune system, such as interferons and acemarman, appear to be useful in treating some FIV-associated diseases; however, such drugs do not eliminate the infection.
PREVENTION. No vaccine is available to prevent FIV infections. 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. Catteries and multiple-cat households should test all their cats and isolate or remove any positives. Once FIV-negative status has been achieved, all prospective additions should be quarantined for eight to twelve weeks, then tested prior to introduction to the household.
PUBLIC HEALTH SIGNIFICANCE. Although FIV is similar structurally to HIV and causes a disease in cats similar to AIDS in humans, it is a highly species-specific agent. Only cells of feline origin have been found to support replication of most primary FIV isolates. There is no significant cross-reactivity between FIV and any of the other lentiviruses. Initial studies indicate that veterinarians, owners, and researchers who have had close contact with FIV infected cats show absolutely no evidence of FIV infection. It appears at this time that FIV infections are restricted solely to cats.
Even though the risk of transmission of FIV from cats to humans is minimal, immunocompromised persons (such as those undergoing chemotherapy, HIV-infected persons, and some pregnant women and newborn infants) should not be exposed to cats with FIV or FeLV infections. Cats with retrovirus-induced immunosuppression are especially susceptible to infection with several parasitic organisms that can cause severe disease in immunosuppressed individuals.
Excerpted from The Cornell Book of Cats
Available from The Cornell Feline Health Center