FIV and FeLV are the two retroviruses a new cat is screened for, and most clinics run them as one in-house snap test on a few drops of blood. They are not the same disease, they do not spread the same way, and a positive result carries a different weight for each. Two rules cover most of the confusion: a negative test in a cat with a known exposure is not final until 4 to 6 weeks have passed, and a positive test in a healthy cat is not final until a second, different test agrees.
What each virus actually does
Feline immunodeficiency virus (FIV) infects white blood cells, mainly the T lymphocytes that coordinate the immune response. Damage accumulates slowly, so an FIV-positive cat often shows nothing for years and may die of ordinary old-age causes. Progression is measured in years, and many positives live a normal lifespan indoors.
Feline leukemia virus (FeLV) is faster and broader. It infects bone marrow precursors, suppressing immunity and triggering anemia, lymphoma, and reproductive failure. Roughly 30 to 40 percent of exposed cats clear the virus and become immune; the rest develop a persistent or latent infection that can reactivate under stress or steroids. Outcome is largely set in the first 4 to 16 weeks, which is why kittens under 4 months are far more likely to become persistently infected than adults.
How each one spreads
FIV travels in saliva and enters almost only through a deep bite from an infected cat. It is a disease of intact outdoor males that fight, which is why prevalence is much higher in that group. Casual contact does not transmit it: shared bowls, mutual grooming, and shared litter boxes have not been shown to pass FIV between stable, friendly cats.
FeLV spreads far more easily through sustained close contact: grooming, shared food and water bowls, a crowded household. It also passes from mother to kittens before birth or through milk. That difference is why one FIV cat can live with others while an FeLV cat usually should not.
The test window: timing decides the result
Snap tests look for viral antigen in FeLV and antibody in FIV, so the blind spots differ.
- FeLV antigen appears only after the virus reaches the bone marrow, so a cat infected yesterday tests negative for roughly 3 to 4 weeks, sometimes 6.
- FIV antibody takes 2 to 4 weeks, occasionally 8 to 12, to reach detectable levels. A cat bitten last week can be infected and read negative.
- In kittens under 6 months, maternal antibodies from a vaccinated or infected mother produce positives in kittens that are not infected. Any positive that young should be repeated after 6 months.
For a cat with a known exposure, the protocol is a test at intake, isolation for 4 to 6 weeks, then a repeat test before any introduction.
Why false positives cluster in healthy cats
No test reaches 100 percent specificity, and the arithmetic is unforgiving. Where true prevalence is 1 percent, a test at 98 percent specificity produces more false positives than true ones, because 99 uninfected cats generate nearly as many flags as the single infected cat. That is the situation of a healthy indoor cat in a single-cat home. In a colony of free-roaming males the same test is far more trustworthy. The result is not wrong; the prior probability changed.
Practical consequence: never euthanize or rehome on one positive in a low-risk cat. Confirm first.
How a positive gets confirmed
- FeLV: a different antigen test from another maker, or immunofluorescent antibody (IFA) on a blood smear, which reads antigen inside white cells and is more specific than a snap test. PCR suits discordant results and suspected latent infection, though a positive PCR alone does not prove the cat is infectious.
- FIV: IFA, Western blot, or PCR. PCR is the weakest option, because viral load can sit below detection in a genuinely infected cat, so a negative PCR does not clear an antibody-positive cat.
- Timing: repeating the same snap in 4 to 6 weeks is reasonable when the first result surprises you, but a positive confirmed by a second method is much stronger evidence.
Multi-cat households: the 4 to 6 week protocol
An FIV-positive cat joining friendly cats that do not fight poses a low risk, and most vets allow it with three changes: no outdoor access for anyone, meals in bowls at least a meter apart, and immediate separation if hissing turns into a fight. For FeLV the usual advice is a home of other FeLV-positive cats, or a single-cat home.
A cat of unknown status should be kept in one room, tested, and retested 4 to 6 weeks later before mixing. The introduction itself follows introducing a new cat.
Vaccines, and what they do not do
FeLV has a vaccine, core for kittens and any cat with outdoor access, inside the feline vaccine schedule. It is not sterilizing immunity: it cuts the chance of persistent infection substantially without removing it.
FIV vaccine is licensed in only a few countries and absent from most of North America and much of Europe, partly because it wrecks antibody testing. A vaccinated cat reads positive on the standard snap for life, and that one side effect is why many vets dropped it.
What a positive means for daily life
| FIV positive | FeLV positive | |
|---|---|---|
| Typical survival | Years; often a normal lifespan | 2 to 3 years from diagnosis on average |
| Household | Can live with friendly negatives | Best with other positives, or alone |
| Monitoring | Exam and bloodwork every 6 to 12 months | Every 6 months, watching for anemia |
| Dental care | High priority | High priority |
Keep both groups indoors, feed a complete diet, and skip raw meat and unpasteurized dairy. Fever over 48 hours, weight loss across 2 to 3 weeks, or a wound unhealed at 5 days all warrant a visit rather than another week of watching.
Five steps for a new cat
- Test at intake and record the date.
- Keep the cat in one room with its own bowls and litter box.
- Repeat the test 4 to 6 weeks after the last possible exposure.
- If positive and healthy, confirm with IFA or PCR before deciding anything.
- If negative with no exposure risk, mix the household and file the record with the vaccine paperwork, as listed in what to check when adopting.
Frequently asked
My new cat tested positive for FIV. Should I return it?
Not on the basis of one test. In a healthy indoor cat with no bite history, a single positive is more likely to be a false positive than a true infection, so the standard next step is confirmation by IFA, Western blot, or PCR. An FIV-positive cat also often lives a normal lifespan indoors and can share a home with friendly cats that do not fight.
How long after exposure will the test turn positive?
For FeLV, antigen usually becomes detectable 3 to 4 weeks after exposure, and occasionally up to 6 weeks. For FIV, antibodies need 2 to 4 weeks and sometimes 8 to 12 weeks to reach a detectable level. That is why a cat with a known exposure is retested after 4 to 6 weeks of isolation rather than trusted on the intake result.
Can a vaccinated cat test positive?
Yes, and this is the main reason the FIV vaccine fell out of use. A vaccinated cat carries antibodies that the standard in-house test cannot distinguish from infection, so it reads positive for life. The FeLV vaccine does not cause this problem, because the routine test looks for viral antigen rather than antibody.