Among all causes of severe feline disease, none is more prevalent than feline leukemia virus (FeLV), a submicroscopic organism associated with the illness and death of more cats worldwide than any other infectious agent. This virus – which is spread through the urine, saliva, nasal secretions or milk of infected animals – is currently recognized as the chief causative agent of fatal infectious disease among U.S. domestic cats. According to Fred Scott, DVM, PhD, professor emeritus of virology Bev Caldwell 288 at Cornell Universitys College of Veterinary Medicine and the founding director of the Cornell Feline Health Center, it is estimated that FeLV is present in two percent to four percent of the overall cat population in America. However, he says, the infection rate can vary significantly among specific feline subpopulations, noting that FeLV is especially common among cats that spend a good part of their time outdoors and are thus more likely to come in contact with infected animals that are shedding the virus. “Depending on whether youre looking at indoor or outdoor cats,” he points out, “the number of infected animals can range anywhere from a fraction of a percent to 15 or 20 percent.” The rate tends to be most noticeably high in cats that are already ill and in those that are very young. Slow-Acting Organism Virologists classify FeLV as a lentivirus, a slow-acting organism that is in the same viral family as the potentially deadly feline immunodeficiency virus (FIV). It is also classified as a retrovirus – a type of virus that converts its genetic blueprint (single-stranded RNA) into double-stranded DNA within the nucleus of a host cell by the use of the enzyme reverse transcriptase. The DNA then produces all the necessary new viral RNA to make new viral particles. “Following exposure, the virus replicates in the local lymphatic areas – in the superficial cells of the tonsils, for example,” explains Dr. Scott. “This is followed by a transient viremia [virus in the blood], with the virus spreading systemically over a few days, reaching other tissues in the body. If the cats immune response is sufficient, thats as far as the virus goes. But if the immune response does not control it, the virus will eventually end up in the bone marrow and will infect other cells there. “Those infected cells will move throughout the cats circulatory system, eventually infecting cells, such as those in the salivary glands, that secrete fluid. Since FeLV does not destroy an infected cell, the cell continues to shed virus into the secretions, and the cat continuously sheds virus into the environment.” Grim Consequences Without an effective immune response to control the virus, the cat will continue to be infected – and to shed the virus into the environment – for the rest of its life. Although some cats that are persistently viremic may remain healthy carriers of the virus for many years, the nonstop viral reproduction taking place within their bone marrow will eventually suppress their immune systems and make them subject to several life-threatening diseases. Among the most frequently diagnosed FeLV-caused conditions are lymphoma (cancer of the lymphatic tissues); leukemia (cancer of the bone marrow); and anemia (a deficiency of red blood cells). Immune deficiency resulting from FeLV infection may also diminish a cats ability to ward off infection with any of countless bacteria, protozoa, fungi and other viruses. “So an FeLV-carrying cat may present with one or more of these other diseases,” says Dr. Scott. “But the underlying cause will be FeLV. It may take many months or years for these other diseases to become evident, although in younger cats it could happen within just a few months.” Some persistently viremic cats may remain healthy carriers of the virus for many years. In other cats, a number of physical and behavioral signs of infection may become evident much sooner, caused either by the primary viral infection or by one of the secondary diseases. These signs include jaundice (a yellowish discoloration of the skin, mucous membranes and whites of the eyes), lethargy and apparent depression, diminished appetite and weight loss, abnormal bowel habits (either diarrhea or constipation) and chronic skin sores. Increased Susceptibility “Age is a risk factor for FeLV,” says Dr. Scott. “But it is a reverse risk factor. That is, younger cats are significantly more likely than older cats to develop a serious form of the disease. As cats mature, they are much more resistant to initial infection and persistent viremia.” Among all risk factors, the most important, he says, is the increased susceptibility to infection among cats that live outdoors, in catteries or in multiple-cat households – “any environment,” says Dr. Scott, “where you have cats coming and going all the time.” In that FeLV can be transmitted in saliva, being licked, groomed or bitten by an infected cat can result in the spread of the virus. Also, he adds, nursing kittens can be infected by ingesting virus-laden milk from FeLV-positive mothers. Spotting FeLV Diagnosis of infection is based on a cats medical history, clinical signs and the results of a test to determine whether the virus is present in an animals blood. A positive blood test will reveal the presence of FeLV antigens – proteins that are unique to the virus. The two most common tests are (1) an enzyme-linked immunosorbent assay (ELISA), which can be conducted in a veterinary clinic and is routinely carried out as the initial FeLV screening, and (2) an immunofluorescent assay (IFA), which, because of the technology that it requires, must be done in a commercial laboratory. According to the American Association of Feline Practitioners, you should have your cat tested for FIV if the animal has never been tested before; is sick, even if it has been tested free of infection in the past; is newly adopted from a shelter or elsewhere; has recently been exposed to or bitten by a cat – whether in your home or outdoors – that is FeLV-positive or whose FeLV status is unknown; shares a household with other cats of unknown FeLV status; or has recently experienced undiagnosed symptoms such as weight loss, diarrhea or decreased appetite.
Vaccination Options
Several vaccines to prevent FeLV infection are available, says Dr. Scott. “Veterinarians generally recommend that most cats be vaccinated when they are kittens and receive a booster shot at one year of age,” he notes. “Some veterinarians feel that the benefits of annual vaccinations beyond the first annual booster are outweighed by the risk of vaccination-site sarcoma and the reduced risk of developing persistent viremia in cats that are older than two years of age. Owners should discuss with their veterinarians the possible benefits and risks of continuous FeLV vaccines beyond one year of age.”
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