Deluge of questions prompts AAFP to develop FIV vaccine brief

Published on October 15, 2002
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The prevalence of feline immunodeficiency virus infection in the feral cat population is similar to that in the general cat population.

Since March, when the Department of Agriculture licensed the first commercial vaccine for feline immunodeficiency virus, the American Association of Feline Practitioners and the Cornell Feline Health Center have been fielding a flood of questions about the vaccine.

So much so that the AAFP decided it would be worthwhile to develop a resource that addresses the fundamental questions that have arisen.

Various AAFP panels on vaccines and retroviruses were involved in the extensive development process. Outside input was sought from certain experts and the vaccine manufacturer.

Dr. James Richards is director of the Cornell Feline Health Center, and a board member of the AAFP and co-chair of several of its feline guidelines panels. "The big question we've been receiving has been on testing"—an area he said neither the vaccine label nor the initial advertising material addresses. "Veterinarians just did not realize that cats that were vaccinated would test positive for antibody. Many of those who did understand that cats would test positive on the in-house kits failed to realize that they would also test positive on western blot."

As veterinarians become more educated consumers, he added, they are seeking independent information when new vaccines are released.

IDEXX Laboratories, the manufacturer of the FIV diagnostic test, said it is evaluating methods that would allow differentiation of vaccinated cats from infected cats when necessary. Meanwhile, the manufacturer sponsored several teleconferences in which Dr. Richards discussed FIV infection, diagnosis, and vaccination. Staff from a thousand veterinary hospitals reportedly listened in to the ones in August.

At the Sept. 19 teleconference, which stretched to two hours to accommodate audience questions, some important points about testing and vaccination were clarified.

There is undertesting for FIV in cats that are taken to a clinic with vague signs of illness, Dr. Richards said. He also encourages veterinarians to test cats that a client intends to adopt—whether found homeless or at a shelter or pet shop. This is true regardless of whether the household has other cats.

Periodic testing is indicated for cats that initially test negative but live with an FIV-infected cat or with cats of unknown infection status, and those that are allowed unsupervised outdoor activity.

Postexposure testing of a cat that has been bitten is also justified, Dr. Richards said. Most cats that become infected will test positive within 60 days postexposure, usually in two to six weeks.

One reason the AAFP decided to revisit its retrovirus testing guidelines was to emphasize the importance of testing kittens.

"A misimpression was given in the first set of guidelines that you cannot test kittens because the tests are meaningless in kittens less than six months of age," Dr. Richards noted. "That was an erroneous conclusion, one that required some clarification."

Most kittens will test negative, indicating they're uninfected. But the confusion is because kittens can test positive on the antibody test if an infected queen passes on antibody to them.

Even if they test positive, infection is unlikely, Dr. Richards said. Test every 60 days up to six months, he said. If they become seronegative, it's likely the kittens are not infected.

A caller asked whether it's possible and useful to test for FIV subtype. A laboratory could sequence the isolates, Dr. Richards replied, but it would be of no practical value. There is no compelling evidence of a correlation between subtype and severity of illness.

So far, five subtypes or clades of FIV have been identified—A, B, C, D, and E. Worldwide, 80 percent of the subtypes are A and B. In this country, A and B predominate. Subtype D is common in Asia.

Most of the cats that have tested positive in the United States have subtype B virus, and most have been in the East. In the West, subtype A is seen most often. The question then arises, can FIV tests detect all the field strains?

Dr. Richards said antibody-based tests should detect infection, irrespective of the FIV subtype, but he is concerned about a high number of false-negative test results when using polymerase chain reaction-based assays. A recent study showed a PCR-based assay was false-negative on one of 10 tests for subtype A and one of 12 for subtype B. In general, he considers the PCR assay a wonderful tool, but he is concerned about its current usability as a commercial diagnostic test for FIV.

Asked about the potential for developing an antigen test, Dr. Richards said that, unlike feline leukemia virus-infected cats, FIV-positive cats are not highly antigenemic, so he doesn't foresee one.

The discussion then shifted from testing to vaccination. The inactivated, killed FIV vaccine contains isolates of two subtypes or clades—A and D.

It's important to note that cats can be both vaccinated and infected, Dr. Richards noted. They could have been infected prior to vaccination or despite vaccination. For that reason, many people have asked whether antibody tests are worthless.

"I'd be quick to point out that, despite the difficulties that we have with positive test results in vaccinated cats and interpreting those, negative test results still remain very meaningful to us," Dr. Richards said, "and at this point, most cats are going to test negative."

A caller who works with rescue groups asked what advice to give them. Dr. Richards suggested advising them it may not be cost effective to routinely test the feral cat population, since the prevalence of FIV infection there is similar to that of the general cat population.

The efficacy of the FIV vaccine has raised many questions. For the laboratory study of efficacy required by the Department of Agriculture, the FIV manufacturer vaccinated 25 specific pathogen-free kittens. Approximately a year following vaccination, the vaccinated kittens and a control group of 19 (initially 20) kittens were challenged intramuscularly with subtype A virus. "The efficacy figure, when we look at something called the preventable fraction, was around 82 percent, so there was quite a difference between the vaccinated cats and nonvaccinated cats," Dr. Richards said.

Given that the cats in the efficacy study were challenged with subtype A, however, no information is available about protection from subtype B. A frequent concern is whether the vaccine is then less effective on the East Coast, where subtype B appears more prevalent. Although no data were available, Dr. Richards noted this is a big concern about HIV and FIV vaccines, because the diversity of the viruses demands broad protection against field isolates.

Addressing veterinarians, Dr. Richards said, "The question then comes up: do I use the vaccine or not use the vaccine? In my view, the major concern is testing confusion. When clients come to you with questions about using the vaccine, it's a situation where you need to spend some time talking about the pros and cons of vaccination.

"If the client decides, under your counsel, that vaccination is something they want to do, I would certainly make sure to test that cat beforehand."