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Feline immunodeficiency virus (FIV)

Virus classification

Group: Group VI (ssRNA-RT)

Family: Retroviridae

Subfamily: Orthoretrovirinae

Genus: Lentivirus

Species: Feline immunodeficiency virus

Feline immunodeficiency virus (FIV) is a lentivirus that affects cats worldwide. From 2.5% up to 4.4% of cats worldwide are infected with FIV. FIV differs taxonomically from two other feline retroviruses, feline leukemia virus (FeLV) and feline foamy virus (FFV), and is more closely related to human immunodeficiency virus HIV. Within FIV, five subtypes have been identified based on nucleotide sequence differences coding for the viral envelope (env) or polymerase (pol). FIV is the only non-primate lentivirus to cause an AIDS-like syndrome, but FIV is not typically fatal for cats, as they can live relatively healthily as carriers and transmitters of the disease for many years. A vaccine is available although its efficacy remains uncertain, and cats will test positive for FIV antibodies after vaccination.


FIV was first isolated in 1986 by Pedersen et al. of the UC Davis School of Veterinary Medicine in a colony of cats that had a high prevalence of opportunistic infections and degenerative conditions and was originally called Feline T-lymphotropic Virus (FTLV). It has since been identified as an endemic disease in domestic cat populations worldwide.​



The primary modes of FIV transmission are deep bite wounds and scratches, where the infected cat's saliva enters the other cat's bloodstream. FIV may also be transmitted from pregnant females to their offspring in utero. This differs from FeLV, which may be spread by more casual, non-aggressive contact since the virus is also present at mucosal surfaces such as those in the mouth, rectum, and vagina, so casual contact cannot be ruled out as a potential transmission.



Veterinarians will check a cat's history, look for clinical signs, and possibly administer a blood test for FIV antibodiesFIV affects 2-3% of cats in the US and testing is readily available. *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.



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. **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.



FIV can attack the immune system of cats, much like the human immunodeficiency virus (HIV) can attack the immune system of human beings. FIV infects many cell types in its host, including CD4+ and CD8+ T lymphocytes, B lymphocytes, and macrophages. FIV can be tolerated well by cats, but can eventually lead to debilitation of the immune system in its feline hosts by the infection and exhaustion of T-helper (CD4+) cells.


FIV and HIV are both lentiviruses; however, neither can infect the other's usual host: humans cannot be infected by FIV nor can cats be infected by HIV. FIV is transmitted primarily through saliva (bites) such as those incurred during territorial battles between males. 


***Cats housed exclusively indoors are much less likely to be infected, provided they do not come in contact with infected cats.


There is a quantifiable risk that cats living outside of a home can spread the disease to others and can also spread the disease in a group setting in a shelter. 


FIV infects other feline species, and in fact is endemic in some large wild cats, such as African lions. Unlike domestic cats, these species do not necessarily exhibit symptoms, perhaps because they have developed evolutionary mutations that confer resistance.


Disease Stages

FIV progresses through similar stages to HIV in humans. The initial stage, or acute phase, is accompanied by mild symptoms such as fever, lethargy, anorexia, pyrexia, 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 where the cat is extremely susceptible to secondary diseases which inevitably are the cause of death.



The primary modes of FIV transmission are deep bite wounds and scratches, where the infected cat’s blood-tainted saliva enters the other cat’s bloodstream. 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 numbers of FIV-infected kittens and adolescents.

Risk factors for infection are being of the male sex, adulthood, and outdoor access. One case study conducted in São Paulo found that 75% of the FIV-infected cats were males. Higher rates of infection in males than females makes sense because the primary means of transmission is from biting and most instances of biting in cats occur in fights over territory an activity more common for males than females. 

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