Feline Leukemia Vaccine--an AIDS Clue?

Feline leukemia, a viral disease that kills an estimated 1 million cats in this country each year, can now be prevented by a vaccine that became commercially available this week, according to an Ohio State University veterinarian who developed the preventive.

In addition to being a lifesaver for the nation’s 50 million cats, researchers say the knowledge gained may be useful in shedding more light on how the virus that causes acquired immune deficiency syndrome (AIDS) damages the human immune system. Feline leukemia and AIDS are caused by the same family of viruses, called “retroviruses.”

The vaccine was developed by Dr. Richard Olsen, and in the last five years it has undergone tests that show it to be 80% effective in protecting cats that receive three shots over a 10-week period plus a single booster shot annually. An announcement that the vaccine is now available nationwide was made Tuesday by the manufacturer, Norden Laboratories of Lincoln, Neb.

Unlike most vaccines--which are made by either chemically destroying a virus or altering it to a point at which it no longer causes disease--the feline leukemia vaccine is composed of certain proteins, called “antigens,” from the virus. When injected into a cat, the proteins stimulate the animal’s immune system to the degree that it is able to fend off any feline leukemia viruses that it encounters thereafter.

But, according to Dr. William Beckenhauer, research and development director for Norden, the vaccine is not effective in cats that have been infected by the virus before vaccination.

In a telephone interview, he said that between 3% and 10% of cats in the United States are already infected, although not all of them may be showing symptoms. The virus is spread from one cat to another through saliva--for example when the two cats share the same feeding bowl. Typically, according to Beckenhauer, an infected cat may go for months or even years without showing symptoms before it eventually develops leukemia and dies.

Feline leukemia researchers have learned that an infected cat dies because the virus destroys the same elements of its immune system, called “T-cells,” that are destroyed in humans by the AIDS virus.

Although cat leukemia and AIDS are caused by viruses from the same family and both attack T-cells, researchers warned that this does not mean that the cat vaccine would be useful as an AIDS preventive.

“We don’t want people to think they can coerce veterinarians to give them the cat vaccine thinking that it might work,” said one AIDS researcher, who asked to remain anonymous. The advantage for humans is that the cat disease may serve as a model to learn more about what the virus does when it invades T-cells, researchers say.

Besides the AIDS virus, which is called “human T-cell leukemia virus” or “HTLV-III,” another retrovirus called “HTLV-I” is responsible for an unusual kind of leukemia in humans. The virus is most common in parts of southern Japan and in the Caribbean.