Lacking a sure vaccination or cure for feline leukemia, the best way to halt the number-one viral killer of cats is to prevent contact with pets that have it, veterinarians at Cornell University's Feline Health Center have concluded.

"I hate to give pet owners this news, but the fact is: Vaccines against feline leukemia virus are not 100 percent effective," said James R. Richards, D.V.M., the Cornell College of Veterinary Medicine's telephone "cat answer man." Some of the most heartbreaking calls to Richards at 1-800- KITTY-DR involve feline leukemia, he noted. "People say, 'I've fallen in love with a cat infected with feline leukemia virus; can I keep her with my other uninfected cats?' I'm sorry, but the answer has to be 'no.'"

FeLV, the retrovirus that causes feline leukemia, is spread by infected cats in saliva, urine and milk when cats groom each other, fight, eat from the same dish, nurse or share the same litter box. Once in the bloodstream and bone marrow, the virus produces, among other symptoms, anemia, jaundice, decreased appetite and weight loss, diarrhea or constipation, enlarged lymph nodes, depression, decreased stamina, respiratory distress and, as the name implies, leukemia. The disease is almost always fatal, and 80 percent of infected cats are dead within three years.

Between 3 and 4 percent of the general cat population in the United States is infected with feline leukemia virus, but high-risk groups, such as multiple-cat households, cat- breeding facilities and outdoor cat populations, suffer about a 7 percent infection rate. Some 15 to 20 percent of sick cats seen by veterinarians are infected with the virus, said Richards, director of the Camuti Memorial Feline Consultation Service at Cornell Veterinary College. Vaccination, although useful, protects only 30 to 90 percent of cats, he reported.

"The best way to keep this virus from spreading is to prevent exposure. Keep your cats inside, keep them from fighting outside and keep them away from cats that you know are infected," Richards said. "That's one reason why tests are so important."

Taking that advice, students in the Cornell College of Veterinary Medicine volunteer at the local animal shelter, giving physical exams and FeLV tests to cats that are up for adoption. The FeLV testing program earned the Cornell Student Chapter of the American Association of Feline Practitioners national recognition, the 1994-95 Hill's Outreach Program Award.

"Until a sure-fire vaccine comes along, the only way we're going to control this disease is by testing and preventing exposure," Richards said.

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NOTE: More information on feline leukemia is in a brochure by the Cornell Feline Health Center, available in veterinarians' offices or from the center, College of Veterinary Medicine, Ithaca, N.Y. 14853-6401.

Questions to 1-800-KITTY-DR are answered for a nominal fee.