Causes
The virus gains entry to the host's cells through the interaction of the envelope glycoproteins (from the glycoprotein env) of the virus and the target cells' surface receptors. First the SU glycoprotein binds to CD134, a receptor on the host cell. This initial binding changes the shape of the SU protein to one that facilitates interaction between SU and the chemokine receptor CXCR4. This interaction causes the viral and cellular membranes to fuse, allowing the transfer of the viral RNA into the cytoplasm, where it is reverse transcribed and integrated into the cellular genome through nonhomologous recombination. Once integrated into the host cell's genome, the virus can lay dormant in the asymptomatic stage for extended periods of time without being detected by the immune system or can cause lysis of the cell.
CD134 is predominantly found on activated T cells and binds to OX40 ligand, causing T-cell stimulation, proliferation, activation, and apoptosis. This leads to a significant drop in cells which have critical roles in the immune system. Low levels of CD4+ and other affected immune system cells cause the cat to be susceptible to opportunistic diseases once the disease progresses to feline acquired immune deficiency syndrome (FAIDS).
Transmission
The primary mode of FIV transmission is via deep bite wounds, where the infected cat's saliva enters the other cat's tissues. FIV may also be transmitted from pregnant females to their offspring in utero, however this vertical transmission is considered to be relatively rare based on the small number of FIV-infected kittens and adolescents. This differs from FeLV, which may be spread by more casual, non-aggressive contact such as mutual grooming and sharing of food bowls.
Disease Stages
FIV progresses through similar stages to HIV in humans. The initial stage, or acute phase, is accompanied by mild symptoms such as lethargy, anorexia, fever, and lymphadenopathy. This initial stage is fairly short and is followed by the asymptomatic stage. Here the cat demonstrates no noticeable symptoms for a variable length of time. Some cats stay in this latent stage for only a few months, but for some it can last for years. Factors that influence the length of the asymptomatic stage include the pathogenicity of the infecting virus and FIV subtype (A-E), the age of the cat, and exposure to other pathogens. Finally, the cat progresses into the final stage (known as the feline acquired immune deficiency syndrome (FAIDS) stage), wherein the cat is extremely susceptible to secondary diseases that inevitably are the cause of death.
Diagnosis
Veterinarians will check a cat's history, look for clinical signs, and possibly administer a blood test for FIV antibodies. FIV affects 2-3% of cats in the US and testing is readily available. This testing identifies those cats that carry the FIV antibody but does not detect the actual virus. False positives occur when the cat carries the antibody (which is harmless) but does not carry the actual virus. The most frequent occurrence of this is when kittens are tested after ingesting the antibodies from mother's milk, and when testing cats that have been previously vaccinated for FIV. For this reason, neither kittens under eight weeks nor cats that have been previously vaccinated are tested. Kittens and young cats that test positive for the FIV antibody may test negative at a later time due to seroreversion, provided they have never been infected with FIV and have never been immunized with the FIV vaccine.
Cats that have been vaccinated will test positive for the FIV antibody for the rest of their lives owing to seroconversion, even though they are not infected. Therefore, testing of strays or adopted cats is inconclusive, since it is impossible to know whether or not they have been vaccinated in the past. For these reasons, a positive FIV antibody test by itself should never be used as a criterion for euthanasia.
Tests can be performed in a vet's office with results in minutes, allowing for quick consultation. Early detection helps maintain the cat's health and prevents spreading infection to other cats. With proper care, infected cats can live long and healthy lives.
Treatment
In 2006, the United States Department of Agriculture issued a conditional license for a new treatment aid termed Lymphocyte T-Cell Immunomodulator (LTCI). Lymphocyte T-Cell Immunomodulator is manufactured and distributed exclusively by T-Cyte Therapeutics, Inc. It is intended as an aid in the treatment of cats infected with feline leukemia virus (FeLV) and/or feline immunodeficiency virus (FIV), and the associated symptoms of lymphocytopenia, opportunistic infection, anemia, granulocytopenia, or thrombocytopenia. The absence of any observed adverse events in several animal species suggests that the product has a very low toxicity profile. Lymphocyte T-Cell Immunomodulator is a potent regulator of CD-4 lymphocyte production and function. It has been shown to increase lymphocyte numbers and Interleukin 2 production in animals. It is a single chain polypeptide and a strongly cationic glycoprotein, and is purified with cation exchange resin. Purification of protein from bovine-derived stromal cell supernatants produces a substantially homogeneous factor, free of extraneous materials. The bovine protein is homologous with other mammalian species and is a homogeneous 50 kDa glycoprotein with an isoelectric point of 6.5. The protein is prepared in a lyophilized 1 microgram dose. Reconstitution in sterile diluent produces a solution for subcutaneous injection.
Vaccine
As with HIV, the development of an effective vaccine against FIV is difficult because of the high number and variations of the virus strains. "Single strain" vaccines, i.e., vaccines that only protect against a single virus variant, have already demonstrated a good efficacy against homologous FIV strains. A dual-subtype vaccine for FIV released in 2002 called Fel-O-Vax (ATCvet code: QI06AA10 (WHO)) made it possible to immunize cats against more FIV strains. It was developed using inactivated isolates of two of the five FIV subtypes (or clades): A Petaluma and D Shizuoka. The vaccine was shown to be moderately protective (82% of cats were protected) against subtype A FIV, but a later study showed it to offer no protection against sub type A. It has shown 100% effectiveness against two different subtype B FIV strains. Vaccination will cause cats to have positive results on FIV tests, making diagnosis more difficult. For these reasons the vaccine is considered "non-core", and the decision to vaccinate should be made after discussion with a veterinarian and consideration of the risks vs. the effectiveness.
