Among all causes of lethal feline disease, none should be taken more seriously by cat owners than those caused by two specific submicroscopic organisms: feline leukemia virus (FeLV) and feline immunodeficiency virus (FIV). Although these viruses are somewhat different in appearance and in the manner in which they exact their toll on feline health, infection with either one of them, if undetected, is capable of causing a cat's premature death. To make matters worse, FeLV and FIV infections are by no means rare. Recent estimates indicate that two percent to four percent of the 83 million or so cats in the U.S. harbor one or both of these two viruses.
At present, several vaccines to prevent FeLV infection are available, says Fred Scott, DVM, PhD, professor emeritus of virology at Cornell University's College of Veterinary Medicine and the founding director of the Cornell Feline Health Center. With rare exceptions, all cats should be vaccinated against FeLV as kittens, then revaccinated one year later. However, notes Dr. Scott, "Some veterinarians feel that, after these initial vaccinations, the benefits of annual vaccinations are outweighed by the risk of the cat's developing cancer-sarcoma-at the vaccination site." As for FIV, a vaccine has been developed, but its efficacy against all strains of the virus is yet to be determined. The key to combating the spread of these infections, therefore, is to identify infected cats and to prevent their coming into contact with uninfected animals.
Diagnosis of FeLV infection, says Dr. Scott, is based on a cat's medical history and clinical signs and the results of tests to determine whether the virus is present in the blood. A positive test will identify the presence of FeLV or its antigens. Two types of tests commonly used for this purpose are an ELIZA (enzyme-linked immunosorbent assay), which can be performed in a veterinary clinic and is routinely carried out as the initial FeLV screening test; and an IFA (immunoflourescent assay), a confirmatory test that, because of the technology required, must be done at a commercial laboratory.
For diagnosis of FIV infection in unvaccinated cats, the ELISA is used to detect specific antiviral antibodies circulating in the blood. These anti-FIV antibodies are produced during the normal immune response to the viral infection. The IFA is not used to confirm FIV infection, says Dr. Scott. Instead, the suspected presence of FIV-associated antibodies can be substantiated by means of a so-called Western blot, in which specific antibodies to FIV can be identified. This procedure, like the IFA, must be carried out at a laboratory that is specially equipped for the test; results will typically be available within a week.
Once cats have been vaccinated against FIV, the antibody assay for the virus is no longer useful, since the test cannot distinguish between antibodies produced as a result of infection and those produced following vaccination. Unfortunately, there is no test available at present to diagnose FIV in FIV-vaccinated cats. [Note: After publication of this article in Cat Watch, a new test called a "discriminant ELISA" was reported by Dr. Levy and co-workers from the University of Florida. This test, not yet available commercially, can differentiate between antibodies produced after FIV vaccination and antibodies produced after FIV infection.]
For a number of years, an innovative kind of assay called the IDEXX Combo test has been routinely used by veterinarians. The assay, which typically costs about $60, can be performed in a local clinic and completed in a matter of minutes. Basically, Dr. Scott explains, the assay kit consists of a small piece of filter paper impregnated with various dots of test material: FeLV- and FIV-positive controls, a negative control, an FeLV sample test spot and an FIV sample test spot. When a few drops of a cat's blood, serum, or plasma are put on the paper, either a change in the color of the various dots-or no change at all-will indicate whether or not the animal is harboring one or both of the viruses. Various other commercial kit tests are available to diagnosis FeLV and FIV.
A Fateful Cycle
FeLV is usually spread when an uninfected cat comes in contact with the saliva or urine of an infected cat-while they groom each other, for example, or when they share food bowls or litter boxes. FIV, on the other hand, is most often spread when an infected cat bites an uninfected cat. A human cannot become infected through contact with an infected cat.
Virologists classify these organisms as retroviruses-a category of viruses that transport their genetic blueprint (single-stranded ribonucleic acid or RNA) from one animal or cell to another in the form of an infectious viral particle. During replication of the virus within an infected cell, the RNA is converted to double-stranded DNA within the nucleus of the newly infected cell by a process called "reverse transcription" - hence the name "retrovirus."
Viral replication in a newly infected cat usually occurs initially in the mouth or pharynx. Subsequently, newly formed virus particles travel to the cat's lymph nodes or bone marrow, where they infect and replicate within various cells, especially T-lymphocytes and other white blood cells. Particularly vulnerable are the T-lymphocytes known as CD4 cells, which play a key role in the animal's immune system.
If the immune response within the infected host is inadequate, this cycle continues as the virus circulates throughout the body, invades additional lymph nodes, and destroys more disease-fighting CD4 cells. Eventually, evidence of the animal's compromised immune system will start emerging as bacteria, viruses, and fungi that would normally be relatively harmless begin to cause illness.
Among the most frequently diagnosed FeLV-caused conditions are lymphoma (cancer or solid tumors of the lymphatic tissues); leukemia (cancer of various bone marrow and circulating white blood cells); and anemia (a deficiency of red blood cells). Immune deficiency resulting from FeLV infection can also diminish a cat's ability to ward off infection with countless bacteria, protozoa, fungi and other viruses. "An FeLV-carrying cat may present with one or more of these diseases," says Dr. Fred 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."
The most commonly observed clinical signs of FIV infection include inflammation of the mucous tissue of the mouth, inflammation of the middle layer of the eye (uveitis) and various cancers. Other frequent consequences include chronic skin infections, gastrointestinal upset and persistent diarrhea, urinary system problems, respiratory tract infections, neurologic disorders, and, in females, reproductive failures such as spontaneous abortion. Some infected cats experience recurrent illness interspersed with periods of relative good health. In most cats, however, susceptibility to opportunistic diseases increases progressively as the animal's immune system is repeatedly challenged by successive episodes of illness.
If your cat has tested positive for one of these viruses, says Dr. Scott, "You don't have to consider it a death sentence. First of all, you'd do well to have the animal retested after about three months, since the original test may have yielded a false positive. Secondly, some FeLV-infected cats develop an effective immune response, which controls the viral infection and results in a transient viremia instead of a persistent viremia. In these cats, subsequent FeLV tests will show that the cat no longer has virus in its blood. Finally, while there is no complete cure for FeLV or FIV infection, newer treatments and supportive care can often result in several years of relatively good health."