FrançaisTwo different viruses, two different tests
| FeLV (feline leukemia virus) | FIV (feline immunodeficiency virus) | |
|---|---|---|
| What the quick test detects | The p27 antigen, a piece of the virus itself, in blood [1]. | Antibodies the cat makes against the virus [1]. |
| When it turns positive | Most cats within 30 days of exposure, some later [1]. | Most cats within 60 days of exposure [1]. |
| Affected by vaccination? | No: FeLV vaccination does not cause a positive antigen test [1]. | Yes, with some tests: vaccinated cats make antibodies that look like infection [1]. |
| Affected by the mother’s antibodies? | No [1]. | Yes: kittens of an infected queen can test positive up to 6 months of age [1]. |
The AAFP guidelines advise testing every cat at risk: as soon as possible after it is acquired, after exposure to an infected cat or a cat of unknown status, before FeLV or FIV vaccination, and whenever the cat is ill [1]. Saliva and tears should not be used for FeLV testing [1].
When a positive result needs confirming
False positives are more likely in cats at low risk, such as healthy indoor-only cats, and can also come from a test run incorrectly [1]. For FeLV, the guidelines recommend confirming a positive result, especially in low-risk cats and cats without symptoms, with a laboratory test (ELISA or PCR for the provirus) or an in-clinic test of a different brand [1].
For FIV, a PCR test can help, but it misses some infected cats: one PCR had 92% sensitivity and 99% specificity [1]. Western blot is the traditional confirmatory test [1].
FeLV: the three possible outcomes
- Abortive infection: the cat clears the virus; the antigen test stays negative [1].
- Regressive infection: the immune system contains the virus, which hides in the cat’s DNA. Antigen and PCR results can disagree, and the virus can reactivate years later [1].
- Progressive infection: the cat stays antigen-positive and sheds the virus. Only antigen-positive (viremic) cats shed it [1].
Cats that are positive on both the antigen test and PCR can shift to a regressive pattern, usually within 16 weeks, which is one reason to retest [1].
Kittens and vaccinated cats
- Kittens and FIV: antibodies from an infected mother can make a kitten test positive up to 6 months of age. A kitten still positive after 6 months is likely to be truly infected; a reliable PCR can be used sooner [1].
- FIV-vaccinated cats: some in-clinic tests still identified true status when the primary vaccination was at least 6 months earlier, while the SNAP Combo detected vaccine antibodies [1]. Tell your vet if your cat was ever vaccinated against FIV.
What the blood count may show
FeLV-infected cats are at increased risk of anemia, bone marrow suppression, lymphoma, secondary infections and mouth inflammation [1]. Anemia in FeLV cats is often non-regenerative, because the virus acts on the bone marrow, but it can also come from infections such as Mycoplasma [1]. Here is an example line from an FeLV-positive cat.
- Abbreviation PCV or HCT: the share of the blood made up of red cells.
- Result 18% in this illustrative line: a moderate to marked anemia.
- Unit Percent (some laboratories report L/L: 0.18).
- Laboratory interval An illustrative cat interval; each laboratory sets its own.
- Flag L: below the interval. The reticulocyte count tells whether the marrow is responding. See anemia in cats.
Outlook
In one German group of cats, those with progressive FeLV lived a mean of 312 days after diagnosis, against 732 days for uninfected cats; FIV-positive cats lived a mean of 785 days, with no significant difference from uninfected cats (620 days) [1]. Many FIV-positive cats live for years. The guidelines state that a positive result alone should not be the reason for euthanasia [1].
Related articles
En français : FIV et FeLV chez le chat.
Related diseases: FIP in cats and lymphoma in cats.
Questions to ask your veterinarian
- Should this positive result be confirmed with another test?
- Could my cat have been exposed too recently for the test to be reliable?
- Is it FeLV progressive or regressive, and when should we retest?
- Are the red cells, white cells and platelets normal?
- How often should my cat have check-ups and blood tests from now on?
Get your cat’s blood count explained alongside the test.
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Frequently asked questions
What is the difference between FeLV and FIV?
Both are retroviruses. The FeLV test detects the virus itself and is not affected by vaccination; the FIV test detects the cat’s antibodies, which can come from the mother or a vaccine [1].
Can dogs get FIV from cats?
No. FIV and FeLV infect cats; they are not known to infect dogs or people.
Can a kitten test positive for FIV and not be infected?
Yes. Antibodies from an infected mother can make a kitten test positive up to 6 months of age. A kitten still positive after that is likely to be infected [1].
Is a negative test always reliable?
Negative results from good in-clinic tests are generally reliable, except early after exposure: before about 30 days for FeLV and 60 days for FIV. Retesting is then advised [1].
Should a cat be put down if it tests positive?
No. The AAFP guidelines state that a positive result alone should not be the reason for euthanasia; many infected cats live for years [1].
Sources
- Little S et al. 2020 AAFP Feline Retrovirus Testing and Management Guidelines. J Feline Med Surg. 2020;22(1):5-30. doi.org/10.1177/1098612X19895940
Laboratory methods and reference intervals vary. This article explains; it does not diagnose. For a decision about your pet, use the original report and talk to your veterinarian. Our editorial policy.