What Is Feline Immunodeficiency Virus?
Feline Immunodeficiency Virus, or FIV, is a lentivirus that affects the immune system of cats. It belongs to the same broad family as the human immunodeficiency virus (HIV) in people, though FIV cannot infect humans and HIV cannot infect cats. For many years, an FIV diagnosis carried a bleak connotation, but the reality for most FIV-positive cats is far more hopeful than many owners initially fear.
With appropriate management, many FIV-positive cats go on to live long and comfortable lives, sometimes reaching old age with a good quality of life. An FIV diagnosis alone is not a reason to consider euthanasia, and it is important that owners understand this from the outset.
How Does FIV Spread?
FIV is spread primarily through deep bite wounds. The virus is present in high concentrations in the saliva of infected cats, and biting that penetrates the skin deeply enough to introduce saliva into the wound is by far the most efficient route of transmission. This is why intact male cats — toms that roam, mark territory, and fight with other cats — are at the highest risk of both acquiring and transmitting FIV.
Casual contact such as mutual grooming, sharing food bowls, or living peaceably alongside another cat in the same household carries a very low risk of transmission. Transmission from a mother cat to her kittens can occur but is not common. The key risk factor is fighting, and the key preventive measure is therefore neutering combined with keeping cats indoors.
The Three Stages of FIV Infection
Acute Phase
In the weeks following initial infection, some cats develop a brief acute phase characterised by mild fever, swollen lymph nodes, and a temporary reduction in white blood cell count. This phase often goes unnoticed or is mistaken for a minor illness, and the cat typically appears to recover fully.
Asymptomatic Carrier Phase
After the acute phase, cats enter a prolonged asymptomatic carrier phase that can last for many years — sometimes the rest of the cat's natural life. During this phase, the cat shows no signs of illness and the immune system, though gradually being compromised, is still functional enough to protect against most infections. Many cats live well throughout this entire period with no intervention beyond routine veterinary care.
AIDS-Like Phase
In some cats, over time, the cumulative damage to the immune system becomes significant enough that the cat begins to suffer from recurrent or severe secondary infections. This late stage is sometimes described as feline AIDS, though not all FIV-positive cats reach this stage. Weight loss, chronic infections, dental disease, and poor coat condition may become apparent. Even at this stage, supportive care and prompt treatment of infections can maintain quality of life for a meaningful period.
An Important Note on the FIV Vaccine in Europe
An FIV vaccine was previously available in some markets but has been withdrawn from the European Union market. This is an important point for owners in Europe to be aware of: there is currently no licensed FIV vaccine available in the EU. Prevention therefore relies entirely on management strategies — primarily neutering and keeping cats indoors. Owners in some other parts of the world should check with their vet about vaccine availability locally, and should also be aware that the vaccine historically caused false positives on standard antibody tests, which complicated screening.
Diagnosis
FIV is typically diagnosed using an in-clinic snap test that detects antibodies to the virus in the cat's blood. A positive result indicates that the cat has been exposed to FIV and has mounted an immune response. It is worth noting that kittens born to FIV-positive mothers may carry maternal antibodies for up to six months, giving a false positive result. Any kitten testing positive should be retested at around six months of age before a definitive diagnosis is confirmed.
As mentioned above, the previously available FIV vaccine could also cause false positive antibody results. The ABCD guidelines recommend confirmatory testing in uncertain cases.
Why FIV-Positive Cats Should Not Be Euthanised on Diagnosis
This point bears emphasis: receiving an FIV-positive diagnosis is not grounds for euthanasia. Many FIV-positive cats spend years — often the majority of their lives — in the asymptomatic carrier phase with an excellent quality of life. Euthanasia should only ever be considered on welfare grounds based on the individual cat's current quality of life and clinical condition, not on the basis of a positive test result alone. Rescue organisations, rehoming centres, and owners are encouraged to manage FIV-positive cats appropriately rather than assuming their situation is hopeless.
Managing an FIV-Positive Cat
Management of FIV focuses on reducing exposure to secondary infections and monitoring the cat's health over time.
- Indoor-only living is strongly recommended. This protects the FIV-positive cat from picking up infections from other cats, and prevents transmission of FIV to cats in the neighbourhood through fighting.
- Regular veterinary check-ups — at least every six months — allow early detection of any emerging health issues. Blood tests to monitor immune cell counts and general organ function are valuable.
- Any signs of illness should be investigated and treated promptly, as infections that a healthy cat might shake off easily can be more serious in an immunocompromised individual.
- Dental disease is particularly common in FIV-positive cats and can be a significant source of chronic infection and discomfort. Regular dental checks and professional cleaning under anaesthetic when needed are an important part of ongoing care.
- Raw food diets should be avoided in FIV-positive cats due to the risk of foodborne pathogens such as Salmonella or Toxoplasma, which an immunosuppressed cat is less able to deal with safely.
Co-Infection with FeLV
Cats that are positive for both FIV and Feline Leukaemia Virus (FeLV) face a substantially worse prognosis than cats positive for either virus alone. The combined effect on the immune system is significantly greater than either infection individually. For this reason, cats in high-risk groups should be tested for both viruses, and management decisions should account for co-infection status.
Prevention
The most effective prevention strategies for FIV are neutering and keeping cats indoors. Neutering dramatically reduces the likelihood of roaming and fighting, which are the primary drivers of FIV transmission. An intact tom that roams freely is the single highest-risk profile for both acquiring and spreading FIV. Neutering also benefits the cat's overall health and behaviour, making it a straightforward recommendation for the vast majority of pet cats.
Living with an FIV-Positive Cat
Many owners find that managing an FIV-positive cat is far less burdensome than they initially imagined. With an indoor lifestyle, regular vet care, and attentive monitoring, these cats can be wonderful companions for many years. The key is early diagnosis, informed management, and a vet team that understands the realistic and often positive long-term outlook for these cats.