CONDITION
Feline Immunodeficiency Virus (FIV)
Feline immunodeficiency virus (FIV) is a virus that infects cats and gradually weakens the immune system over months to years. The virus belongs to the same family as HIV — both are lentiviruses — but FIV is entirely species-specific and cannot infect humans, dogs, or other animals. Many cats live for years after infection without obvious illness, though they tend to become more vulnerable to infections and certain other conditions as time passes. Owners often arrive on this page after a positive test result at a routine appointment, or because a cat has been unwell repeatedly with infections that seem slow to clear. Some cats show very little at first; others may have recurring mouth inflammation, skin infections, or weight loss. The virus spreads mainly through deep bite wounds, so outdoor cats and those in multi-cat households where fighting occurs face higher exposure. This page explores what FIV infection can look like over time, how the virus affects the immune system, how infection is confirmed, and the approaches used to support affected cats through different stages of the disease. It also addresses common questions about transmission, testing, and what infection may mean for an individual cat's life.
Why this matters now
Feline immunodeficiency virus is a lentivirus that affects cats worldwide. Infection is most common in outdoor, unneutered male cats due to transmission through bite wounds during fighting. Once infected, cats carry the virus for life. However, many FIV-positive cats live for years with good quality of life, and the diagnosis often carries more anxiety than the actual disease warrants.
After initial infection, cats may experience a brief period of mild illness including fever and lymph node enlargement. This is followed by a prolonged asymptomatic phase that can last many years—often the entire lifespan of the cat. Some infected cats eventually develop acquired immunodeficiency, making them more susceptible to secondary infections, but this progression is variable and often slow.
Signals & patterns
Early signals
Often no visible signs
Most cats show no signs during the long asymptomatic carrier phase.
Mild, self-limiting illness after initial infection
Some cats experience transient fever and lymph node swelling shortly after becoming infected.
History of bite wounds
Cats that have been bitten during fights may have been exposed to the virus.
Discovery on routine screening
FIV is often diagnosed incidentally when testing new cats or during health checks.
Later signals
Recurrent infections
Cats with progressed FIV may develop recurring respiratory, skin, or oral infections.
Chronic oral disease
Gingivitis and stomatitis can be more severe in immunocompromised cats.
Weight loss
Gradual weight loss may occur as the disease progresses.
Persistent lymph node enlargement
Chronically enlarged lymph nodes can indicate ongoing immune stimulation.
Click to read about the biological mechanisms
How this is usually investigated
Diagnosis typically begins with history—outdoor access, fighting, known exposure to infected cats—and physical examination findings that may prompt testing. Because many FIV-positive cats appear outwardly healthy, the infection is often discovered during routine screening or when investigating another illness. The investigation then focuses on confirming infection, assessing immune function, and identifying any secondary complications that may have developed.
FIV antibody test (ELISA)
Western blot or immunofluorescence assay
Complete blood count
Chemistry panel
Physical examination
Options & trade-offs
Management focuses on supporting general health, minimising exposure to infections, and addressing secondary illnesses as they arise. No treatment eliminates the virus, so the approach is typically a combination of environmental measures, monitoring, and medical intervention when complications develop. What works well for one household and cat may differ from another, depending on the cat's environment, health status, and the owner's circumstances.
Indoor housing and environmental control
Keeping infected cats indoors reduces exposure to infectious diseases, parasites, and fighting, which can introduce new infections or stress the immune system. It also prevents transmission to other cats. Some owners create enriched indoor environments with vertical space, puzzle feeders, and companionship from other FIV-positive cats.
Trade-offs: Transition from outdoor to indoor life can be difficult for cats accustomed to roaming, and some cats show signs of stress or frustration. Indoor housing requires commitment to environmental enrichment and may not be practical for all households.
Prompt treatment of secondary infections
When bacterial, fungal, or parasitic infections develop, they are treated with appropriate antimicrobials or antiparasitics. Treatment courses may need to be longer than in immunocompetent cats, and monitoring for recurrence is often necessary. Some cats benefit from regular dental care to manage chronic gingivostomatitis.
Trade-offs: Frequent or prolonged courses of antimicrobials carry cost and the risk of developing resistant infections. Some secondary conditions, particularly severe oral inflammation, can be difficult to control even with aggressive treatment.
Regular monitoring and preventive care
Many vets suggest routine examinations and blood work every six to twelve months to detect problems early. Weight tracking, parasite control, and maintaining vaccination against respiratory pathogens may also be part of the plan. Some owners find this schedule reassuring and useful for catching issues before they become advanced.
Trade-offs: Frequent veterinary visits incur ongoing cost and can be stressful for cats that find travel and handling difficult. In asymptomatic cats, monitoring may identify changes that do not require intervention, leading to uncertainty about when to act.
Supportive and palliative care
This includes nutritional support for cats losing weight, anti-inflammatory medication for chronic pain or inflammation, and fluid therapy for dehydration. The aim is to maintain comfort and quality of life rather than to cure the underlying infection. Adjustments are made as the cat's condition evolves.
Trade-offs: Intensive supportive care can be time-consuming and emotionally demanding. Some interventions, such as appetite stimulants or subcutaneous fluids, require daily handling that not all cats tolerate well.
Antiviral therapy
Some antiviral drugs used in human medicine, such as certain reverse transcriptase inhibitors, have been tried in cats with FIV. Evidence for clinical benefit is limited, and these medications are not widely used. Interferons have also been explored, with variable results reported in the literature.
Trade-offs: Antiviral drugs can be expensive, may cause side effects, and do not reliably slow disease progression or improve survival in all cats. Access may be limited, and long-term efficacy data in cats are sparse.
Common misconceptions
"FIV is a death sentence and infected cats cannot live long, healthy lives."
Many FIV-positive cats live for years—often a decade or more—with good quality of life, particularly when kept indoors and monitored for secondary illnesses. The progression from infection to immune compromise is highly variable, and some cats never develop severe disease. The diagnosis often provokes more concern than the infection itself warrants in many cases.
"FIV-positive cats must be kept completely isolated from all other cats."
Transmission requires deep bite wounds, so casual contact such as grooming, sharing food bowls, or living peacefully in the same household poses very low risk. FIV-positive cats can often live safely with non-infected cats if the household is calm and fighting does not occur. Isolation is typically unnecessary in harmonious multi-cat homes.
"FIV can spread to humans or dogs."
FIV is closely related to HIV—both are lentiviruses in the same viral family—but each is highly species-specific, so FIV cannot infect humans, dogs, or other non-feline species, and HIV cannot infect cats. There is no risk to people or other pets in the household from an FIV-positive cat.
Related conditions
Feline Leukaemia Virus (FeLV)
Feline leukaemia virus is another retrovirus that can compromise immune function in cats, and the two infections share similar routes of transmission through close contact. Cats infected with FIV may be at higher risk of co-infection with FeLV, and the presence of both viruses can complicate the clinical picture and affect long-term outlook.
Feline Stomatitis
Cats with FIV often develop severe, persistent inflammation of the gums and mouth tissues, and feline stomatitis is one of the more common clinical problems seen in cats whose immune defences have been weakened by the virus. The inflammation in stomatitis can be more difficult to manage when the immune system is compromised.
Chronic Upper Respiratory Disease in Cats
Chronic upper respiratory signs—sneezing, nasal discharge, recurrent infections—can persist or recur more frequently in cats with FIV, as the weakened immune system may struggle to fully clear or contain viral or bacterial pathogens in the airways. This pattern is one reason respiratory symptoms may prompt testing for FIV in cats with a history of lingering or repeated illness.
Lymphoma in Cats
Cats infected with FIV have a higher risk of developing lymphoma compared to uninfected cats, likely because the virus disrupts normal immune surveillance and contributes to chronic immune activation. Lymphoma may appear years after FIV infection is established, and the association is one of the reasons FIV-positive cats are monitored over time.
Toxoplasmosis
Toxoplasmosis is an infection that many cats carry without signs, but cats with compromised immune function—such as those with FIV—may be less able to keep the parasite dormant, and clinical illness can sometimes emerge. The overlap is one reason toxoplasmosis is considered in FIV-positive cats presenting with neurological or systemic signs.
If a cat has tested positive, understanding what the immune system does—and what happens when it is gradually compromised—can help make sense of patterns that develop over time. The broader context of immune and inflammatory health may also be relevant if secondary infections or inflammatory conditions arise. For cats living with FIV, the focus often shifts from the virus itself to the practical question of how to support wellbeing over the years ahead.
Last reviewed: 24 April 2026 · Dr Alastair Greenway MRCVS