FIV is not feline leukemia. They are two different viruses that both infect cats, and they’re often confused because they share some similarities. Both are retroviruses, both suppress the immune system, and both are tested for on the same screening panel at the vet. But Feline Immunodeficiency Virus (FIV) and Feline Leukemia Virus (FeLV) differ in how they spread, how they damage the body, and how long cats can live with them.
How the Two Viruses Differ
FIV works much like HIV does in humans. It targets immune cells directly, gradually weakening the cat’s ability to fight off infections. Over time, this can lead to opportunistic infections, neurological problems, and certain tumors. Cats with FIV often go years without any symptoms at all.
FeLV is more aggressive. It attacks the bone marrow, which is the factory that produces blood cells and immune cells. This means FeLV can cause cancers (especially lymphoma), severe anemia, and a deeply compromised immune system. FeLV is considered the more pathogenic of the two viruses, and it typically causes serious illness faster than FIV does.
Transmission Routes
The viruses spread in fundamentally different ways, which matters if you have multiple cats. FIV is classically a disease of male aggression. It’s transmitted through deep bite wounds, the kind that happen during serious territorial fights. Sharing food bowls, grooming each other, and even minor spats do not appear to transmit FIV. A study tracking FIV-positive and FIV-negative cats living together found no transmission through casual contact, shared food and water, or even from mothers to kittens.
FeLV spreads through close, prolonged contact. Mutual grooming, shared litter boxes, shared food and water dishes, and nose-to-nose greeting can all pass the virus along. This makes FeLV much more dangerous in multi-cat households, shelters, and colonies. A cat doesn’t need to be bitten to catch it.
What Each Virus Looks Like in Cats
FIV-positive cats can appear completely healthy for years. When symptoms eventually show up, they tend to reflect a weakened immune system: recurring upper respiratory infections, chronic mouth and gum inflammation, slow wound healing, and weight loss. Some cats develop neurological symptoms like changes in behavior or coordination.
FeLV symptoms are broader and often more severe. Because the virus targets bone marrow, cats may become severely anemic, producing too few red blood cells. Lymphoma is the most common cancer associated with FeLV. Cats can also develop secondary infections from immune suppression, similar to FIV, but the bone marrow damage adds a layer of risk that FIV alone doesn’t carry.
Lifespan and Prognosis
This is where the two viruses diverge most sharply. A referral clinic study in Italy tracked survival times and found that FIV-positive cats had no statistically significant difference in longevity compared to cats testing negative for both viruses. The median survival for uninfected cats was about 10.8 years in that study, and FIV-positive cats came in at about 5.6 years, but the difference was not statistically significant because of variation among individual cats. Many FIV-positive cats live full, normal lifespans indoors.
FeLV tells a different story. The median survival for FeLV-positive cats in the same study was roughly two years. Cats infected with both viruses simultaneously fared worst, with a median survival of just 77 days. FeLV-positive status significantly shortened life expectancy in a way that FIV alone did not.
How Each Virus Is Diagnosed
Vets typically screen for both viruses at the same time using a point-of-care blood test, which is one reason the two get confused. But the tests detect different things.
The FeLV screening test looks for a viral protein (an antigen) that circulates in the blood. Most infected cats test positive within about 30 days of exposure. The FIV screening test, by contrast, looks for antibodies the cat’s immune system produces against the virus. Antibodies can take 60 days or longer to appear, so a recently exposed cat may test negative even though it’s infected.
Both tests can produce unclear results. For FeLV, a positive screening test doesn’t always mean the cat will stay sick. Some cats fight the virus into a “regressive” state where it hides in their DNA but isn’t actively replicating. Others develop a “progressive” infection with ongoing viral activity and shedding. A DNA-based lab test can distinguish between the two by measuring how many viral copies are present: fewer than one million copies per milliliter suggests a regressive infection, while more than one million suggests progressive disease. For FIV, false positives can occur, particularly in kittens who carry their mother’s antibodies, so retesting after six months of age is standard.
FeLV Has Four Possible Outcomes
Not every cat exposed to feline leukemia develops the same type of infection. The current classification recognizes four courses:
- Abortive infection: The cat’s immune system defeats the virus before it ever integrates into the cat’s DNA. The cat clears the infection entirely.
- Regressive infection: The virus integrates into the cat’s cells but is kept in check by the immune system. The cat is not actively shedding virus, but stress or illness could potentially reactivate it later.
- Progressive infection: The virus replicates freely. The cat sheds virus continuously and is at high risk for developing FeLV-related diseases.
- Focal infection: The virus replicates in a limited area of the body, which is uncommon.
FIV doesn’t follow this pattern. Once a cat is infected with FIV, the virus persists for life, but it typically progresses slowly enough that many cats never develop serious illness.
Vaccines and Prevention
An effective vaccine exists for FeLV and is now considered a core vaccine for all kittens. The current veterinary guidelines recommend vaccinating kittens every three to four weeks until they’re 16 to 20 weeks old. After the first year, FeLV vaccination becomes optional for adult cats, with boosters recommended annually for cats regularly exposed to FeLV-positive animals and every two to three years for lower-risk cats.
The situation for FIV is less encouraging. A vaccine called Fel-O-Vax FIV was once available in the United States and Canada, but it was pulled from those markets in 2017. It remains available in Japan, Australia, and New Zealand. Field studies showed it offered only about 56% protection in real-world conditions, and vaccinated cats did not develop the broadly protective antibodies needed to fend off currently circulating strains. The vaccine also created a practical problem: vaccinated cats tested positive on standard FIV antibody screening tests, making it impossible to distinguish vaccinated cats from truly infected ones.
For FIV, the most reliable prevention is keeping cats indoors or preventing fights with unknown cats. Since deep bite wounds are the primary route of transmission, neutering male cats significantly reduces risk by curbing territorial aggression.
Living With an Infected Cat
If your cat tests positive for FIV, the prognosis is genuinely good. Keep the cat indoors, feed a high-quality diet, stay current on veterinary checkups, and watch for signs of secondary infections. An FIV-positive cat can safely live with FIV-negative cats as long as they get along and aren’t fighting. The key is careful introductions and monitoring for aggressive behavior. If there’s any suspicion of real fighting (not just play), separating the cats is the safest approach.
FeLV-positive cats need more caution. Because the virus spreads through casual contact, an FeLV-positive cat should ideally be the only cat in the household, or live only with other FeLV-positive cats. All cats in a multi-cat home should be tested, and any new cats coming in should be tested and vaccinated before introduction. FeLV-positive cats benefit from more frequent vet visits to catch complications early, since the disease tends to progress faster and the range of potential health problems is wider.