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Feline Leukemia & Feline Aids Virus

Feline leukemia and aids are complex diseases of viral origin, both somewhat contagious. The viruses that cause these illnesses are related. One is the feline leukemia virus (FeLV); the other is the feline aids virus (FIV).


Both diseases have similar symptoms. They include: weight loss, persistent fever, skin problems, frequent infections, difficulty breathing, gum disease, and reproductive problems including miscarriage, small litters and weak kittens. Any affected animal may have only one, or several, of these symptoms.

Cats particularly at risk for leukemia/aids are kittens of infected mothers, cats of unknown background, cats that fight with other cats or live in an area with a high stray cat population, and breeding cats.


FeLV can be spread fairly easily from an infected mother to her kittens before and during birth. It is also spread by fighting (bite wounds). In rare cases, it can be spread by close, long-term contact. FIV is spread primarily by bite wounds, but transmission from a positive mother is possible in rare cases.


We recommend a blood test to check for both viruses (FeLV/FIV Combo test). We recommend testing all kittens (unless their mother recently tested negative and they have no history of contact with other cats), and for all cats of unknown background. We also recommend testing unvaccinated cats 1-2 months after cat fights. The The Combo test can distinguish FeLV infected cats from previously vaccinated cats, but it cannot do this for FIV. Antibodies from an FIV vaccine are expected to interfere with the test for approximately one ear.                             Any cat or kitten testing positive for FeLV or FIV, particularly if they are symptom-free, should be re-tested 2 months later to confirm the persistence of the infection.


FeLV can be prevented with an initial series of two vaccines given one month apart, then yearly boosters. The vaccine does not give 100% protection, but it is expected to be 70-80% effective. To be very safer, it is a good idea to keep cats inside at night, when most fighting occurs. Since cats less than 3 years old are more susceptible to FeLV, we recommend vaccinating indoor cats that are under 3 years of age if there is any chance of escape. (The FIV vaccine was discontinued in 2016, because of insufficient demand nationally).


Cats positive for FeLV or FIV should be kept strictly indoors, both for their own safety and to avoid exposing other cats.

FIV positive cats may live symptom-free for many years, but FeLV positive cats usually develop signs of illness within one to two years. Both FeLV and FIV affect the immune system, so most virus-related illnesses are the result of an immune system that no longer functions well. Frequent infections, weight loss, gum disease, chronic fevers and cancer are common FeLV/FIV related illnesses.

Cats living with FeLV and FIV should be monitored closely and even minor illnesses should be treated aggressively. Housemates of positive cats have a very low risk for infection, but should be vaccinated and kept indoors.

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