Q Recently, I took in an older, unneutered stray cat who was very social. As is the normal procedure for me, I took him to my vet, and he was tested for feline leukemia virus.

The test was negative. So he was neutered and vaccinated for FeLV, rabies, etc, and he lived among our household of five other cats.

He continued to fight with any stray cat that had the misfortune to walk past, however, and we had several subsequent visits to the vet for cat bites.

I thought maybe he needed an inside home where he was the only cat but was unable to find someone to adopt him. This went on for about a year. Then suddenly, he became ill, and a trip to the Emergency Clinic revealed that he had FeLV. I was wondering about your recommendation for FeLV testing in stray cats.

A. FeLV is a retrovirus that affects pet cats. Infection is most commonly spread from the saliva, but blood from a bite can spread it as well.

The fact that your cat was social is significant, as this virus is often transmitted by social contact, such as grooming and sharing of food/water bowls.

Once a cat is infected, two things can happen: Either a full-immune response is mounted and the infection is eliminated, or an ineffective-immune response is mounted, resulting in a lifelong infection.

The lifelong infection can be present in two forms. One is a latent infection where the virus incorporates into the DNA of cells in the bone marrow. These cats do not have circulating virus and are not infectious to other cats.

However, the infection can be reactivated following stress or drugs that block the immune system. The second type of infection is a persistent viremia where the virus is replicating in the bone marrow, spleen, lymph node and salivary glands. These cats are constantly infectious.

As you can imagine, all these forms of infection can make testing and the interpretation of these tests difficult. There are two readily available tests, the enzyme linked immunosorbent assay (ELISA) and the fluorescent antibody (FA).

ELISA was developed as a quick method that could be performed at any veterinary clinic. It screens for a protein (antigen) produced by the virus. The ELISA test may be positive in a cat that eventually will eliminate infection, or it may be negative in a cat with a latent infection.

The FA test detects the virus once it has infected the blood cells, so it becomes positive only when the bone marrow is infected. If positive, it is likely the cat is permanently infected.

A cat that is viremic may be missed. This is a test that can be done only at a diagnostic laboratory, so you must wait several days to get the results.

As you can see, testing and interpretation are not easy and will vary with each catís situation. The two reasons to test are to prevent infection of the cats already in your home when introducing a new cat and to determine if an illness is associated with FeLV infection.

Here are some general guidelines to help.

1. All new cats should have an ELISA test.

2. The new cat with a negative test should be retested in eight weeks.

3. An ELISA-positive cat should have an FA to confirm persistent infection.

4. If ELISA positive but FA negative, retest in eight weeks.

5. Recommend annual ELISA if ever positive and before FeLV vaccination.

6. Recommend an ELISA if there is an unexplained illness, as it may have had a long-term latent infection.

Leukemia is not the only disease FeLV-infected cats can have. It may affect their immune system, preventing their bodies from fighting off other infections like viral upper respiratory infections, blood parasites and FIV infections.

Anemia, low platelet counts and low white blood cell counts are common since these are all produced in the bone marrow. As the name implies, it is known to cause leukemia and many other tumors in cats.

At one point, up to 80 percent of cats with lymphoma also were FeLV positive. However, improved testing and vaccination have dramatically reduced this association. Cats can live years with the FeLV infections, and even in positive cats, not every problem is FeLV-related.

Since there is no effective treatment once infected, prevention is the best medicine. Spreading is best prevented by appropriate testing to insure uninfected cats are never exposed.

A vaccine is available for those cats that may be exposed.

However, the overall effectiveness of the vaccines is controversial, and some have been associated with cancer formation at the vaccine site.

You must talk to your vet and come up with the plan that best suits your cats. Fortunately, we see fewer infected cats than we did 20 years ago. Proper testing to prevent exposure is the best method to protect your cats.

Dr. Perry H. Jameson and Dr. Henri Bianucci are with Veterinary Specialty Care LLC. Send questions to petdocs@postandcourier.com.