Cat FeLV and FIV: testing, what a positive means, and living with it

Quick answer

Feline leukaemia virus and feline immunodeficiency virus are almost always named in the same breath, tested for on the same little plastic cassette, and then treated as if they were one disease. They are not. They spread differently, they carry very different prognoses, and they need completely different household decisions.

The short version: FIV mostly needs a deep bite to pass between cats, and many infected cats live a long and ordinary life. FeLV passes through close friendly contact as well as fighting, and a cat that stays persistently infected has a much harder road. That single difference decides whether a positive cat can share a home with other cats.

Shared bowls, shared trays and mutual grooming matter for feline leukaemia virus. For FIV they matter far less, because it takes a bite to transmit efficiently.

• Never make a permanent decision on one in-house test. Confirm a positive with a second test of a different type.
• The highest-risk cat is an entire male who goes out and fights. Neutering is the single biggest reduction in risk.
• Neither virus infects humans, dogs, or any species other than cats.
• Kittens can test FIV-positive on borrowed antibody from their mother and be perfectly uninfected. Retest when they are older.
• A positive result changes how a cat is managed, not whether it is worth keeping.

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Two viruses, two different problems

Feline immunodeficiency virus.

FIV is a lentivirus, the same broad family as the virus that causes AIDS in people, although it is entirely species-specific and cannot infect a human. It is present in high concentration in saliva, and the efficient route into another cat is a bite that penetrates deeply enough to inject that saliva under the skin.

That makes FIV, in practical terms, a disease of cats that fight. Entire toms who roam and defend territory are the classic patient. Cats that live together peacefully, share bowls, and groom each other pass it on inefficiently, which is why many households keep positive and negative cats together successfully.

After infection the virus establishes itself for life and slowly erodes immune function. The important clinical point is the timescale: that erosion can take years, and a great many FIV-positive cats die of something entirely unrelated at a normal age.

Feline leukaemia virus.

FeLV is a gammaretrovirus, and it behaves differently in two ways that matter.

First, it is shed in saliva during ordinary friendly contact, so mutual grooming, shared food and water bowls, shared litter trays and nose-to-nose greeting all transmit it. Fighting transmits it too, but it does not need fighting. A queen can also pass it to her kittens before or after birth.

Second, exposure does not have one outcome. Some cats mount an effective response and clear the virus entirely. Some contain it, so that viral genetic material stays integrated in the bone marrow while the cat tests negative on the usual antigen test and appears healthy, with the possibility of reactivation later if the cat becomes ill or immunosuppressed. Some stay persistently infected and continuously shed virus, and it is this group that develops the anaemia, the lymphoma and the secondary infections that make FeLV feared.

The practical consequence is that "my cat was exposed" and "my cat is persistently infected" are very different statements, and only repeat testing over time separates them.

Who actually catches them, and when

Risk is not evenly spread across the cat population. It concentrates in a specific profile.

Entire males with outdoor access. Fighting over territory and mates is the main transmission event for FIV and a significant one for FeLV. Neutering reduces roaming and fighting and is the most effective single intervention an owner can make.

Cats reaching social maturity. Kittens do not fight over territory. The behaviour that spreads these viruses starts when a cat begins to range, patrol and defend, which is the same period in which owners notice their cat becoming bolder, staying out longer, and coming home with scratches. If your cat has just started doing that, this is the point at which its risk profile changes.

Cats in dense, unmanaged populations. Colonies, feral groups, and rescue situations with high turnover concentrate both viruses. A cat from that background should be tested regardless of how well it looks.

Kittens born to an infected queen. For FeLV in particular, kittens are more susceptible than adults and more likely to end up persistently infected after exposure.

Indoor-only cats with no contact. Genuinely low risk, which is why keeping a positive cat indoors protects the neighbourhood as well as the cat.

Prevalence differs substantially between countries and between urban and rural populations, so your local practice is a better guide to how common these are near you than any general figure.

What testing does and does not tell you

The mouth is where retroviral disease most often shows itself first. Persistent red, sore gums at the tooth margins is a common route to an FIV diagnosis.

The standard in-clinic test is a combined cassette that looks for two different things at once, and understanding which is which resolves most of the confusion.

The FeLV part detects viral antigen, meaning a piece of the virus itself. A positive means virus is present in the blood at that moment. Because a cat can be transiently positive in the weeks after exposure and then clear it, a positive in a healthy cat needs repeating after an interval before it is called persistent.

The FIV part detects antibody, meaning the cat's immune response, not the virus. Three consequences follow.

A kitten that received antibody in its mother's milk can test positive without ever having been infected. Those borrowed antibodies fade over the first months of life, so the answer is to retest an older kitten rather than to act on the first result.

A cat vaccinated against FIV will test antibody-positive on a standard test for a long time afterwards. FIV vaccination has been available in some countries and withdrawn or never licensed in others, and imported or rehomed cats may carry an unknown vaccination history. If this is a possibility, say so, because it changes how the result is interpreted and may require a different laboratory method to separate vaccine antibody from infection.

Immediately after a bite, antibody has not yet developed. A cat tested the day after a fight can be negative and still become positive later, so a test after a known exposure needs repeating after an interval your vet will specify.

Confirmatory testing exists and should be used before any irreversible decision. Laboratory methods that look for the virus itself rather than the antibody, or that use a different detection principle, are the standard next step for an unexpected positive in a well cat.

What the illness looks like, early and late

Neither virus produces a signature sign. Both produce a cat that keeps having other problems, and the pattern of repetition is the clue.

Early, in both. Often nothing at all. A period of vague unwellness, mild fever, or enlarged lymph nodes soon after infection may pass unnoticed, and then the cat looks entirely normal for a long time.

Established FIV. Chronic gingivitis and stomatitis is the classic picture: gums that are red and painful along the tooth margins and at the back of the mouth, a cat that drops food, salivates, or approaches the bowl and then backs away. Recurrent upper respiratory infections, slow-healing wounds, chronic skin or ear problems, and gradual weight loss follow. Later, immune failure allows opportunistic infections and raises the risk of certain tumours.

Established FeLV. Anaemia is common and produces a cat that is pale in the gums, tires quickly and may breathe faster at rest. Lymphoma is the tumour most associated with it. Persistent or recurring infections that respond to treatment and then return, unexplained fever, poor coat, and progressive weight loss are typical. Kittens from an infected queen may fail to thrive.

What else looks the same. This is the part that stops a diagnosis becoming a conclusion.

A retrovirus test is part of that workup, not a substitute for it. An FIV-positive cat can also have a rotten tooth, and treating the tooth is what makes it comfortable.

Living with a positive cat

A positive cat needs more veterinary contact, not less. A carrier the cat already accepts turns twice-yearly checks from an ordeal into an errand.

The management plan is unglamorous and it works.

Keep the cat indoors. This does two things at once: it stops the cat infecting neighbourhood cats, and it protects an immunocompromised animal from the infections, injuries and parasites that outdoor life delivers. If the cat is used to going out, a secure garden enclosure or a harness routine is a fair substitute.

Neuter, if not already done. It removes the drive to roam and fight, which is both the transmission route and a major source of the bite wounds that turn into abscesses in a cat that heals badly.

Increase the frequency of veterinary checks. Twice yearly is the usual recommendation for a positive cat rather than annually, with weight recorded every time. Weight trend is the earliest measurable sign of decline in these cats.

Treat small problems quickly. In an immunocompromised cat, a minor infection is not minor. The threshold for a phone call is lower than for an ordinary cat.

Take the mouth seriously. Regular dental assessment, and willingness to treat aggressively when gingivitis appears, makes more difference to quality of life than almost anything else in FIV-positive cats.

Keep parasite control current. Fleas, ticks and worms all take a larger toll on a compromised immune system.

Feed a complete commercial diet and avoid raw meat. The argument for cooked food here is specific rather than ideological: an immunocompromised cat handles food-borne bacterial contamination poorly.

Discuss vaccination with your vet. A positive cat still needs protection from the ordinary diseases, but the choice of vaccine type and schedule may change. Modified live vaccines are considered more carefully in immunocompromised animals, and practice differs by country and by product.

In multi-cat homes, split the question by virus. For FIV, a stable household where nobody fights is usually acceptable, and many vets will support keeping the group together with separate feeding stations to reduce tension. For FeLV, the risk to housemates is real even without fighting, and the honest options are separate living space or separate homes. FeLV vaccination of the negative cats reduces but does not eliminate that risk.

What never to do

Do not euthanise a healthy cat on the basis of one in-house positive test. This is the single most consequential error made with these viruses, and it is entirely preventable by confirming the result.

Do not assume a positive kitten is infected. The FIV test measures antibody, and maternal antibody is common and temporary.

Do not treat FIV and FeLV as the same management problem. Rehoming an FIV-positive cat away from a peaceful household is usually unnecessary. Ignoring an FeLV-positive cat's contact with housemates is not safe.

Do not stop looking for other diseases once you have a positive result. Retroviral status explains susceptibility, not every symptom.

Do not let a positive cat outdoors because it seems cruel to confine it. That decision infects other people's cats, and it exposes yours to everything its immune system can no longer handle.

Do not worry about your own health, or your dog's. Neither virus crosses to other species.

When to get help

Book a veterinary appointment promptly for a cat of known or suspected positive status if it shows any of these:

• Pale gums, faster breathing at rest, or unusual tiredness
• Sore mouth, drooling, or dropping food
• Weight loss, however gradual
• A wound or infection that is not healing at the expected rate
• Any lump or enlarged lymph node under the jaw, in front of the shoulder, or behind the knee
• Recurrent respiratory or skin infections

An indoor life for a positive cat is not a punishment. It is the arrangement that protects both this cat and every cat on the street.

Bring to the appointment: the original test result and which test was used, the cat's age when tested, any known fight or bite history, your home weight record, and whether the cat has ever been vaccinated against FIV. That last question is asked far too rarely and explains a surprising number of confusing results.