Cats infected with FeLV may show one ore more of these signs or none of them at all.
Stages of Infection
Following exposure to FeLV, infections may progress in several different ways.
In some instances, a cat can mount an effective immune response against the virus and completely eliminate it, but these abortive infections are rare.
In approximately 10% of infections, a cat mounts an immune response that is sufficient to eliminate the virus from the bloodstream, but not robust enough to completely eliminate FeLV DNA from the body. Cats with these regressive infections carry FeLV DNA in their bone marrow, and although they are not infective for other cats during this stage of infection, the virus may be reactivated at some time in the future, at which point they may begin to show clinical signs and become a source of infection for other cats.
Cats with progressive FeLV infections have FeLV virus particles in their blood streams, may serve as sources of infection for other cats, and often begin to show clinical signs of disease.
Feline leukemia virus (FeLV) is one of the most common infectious diseases in cats, affecting between 2 and 3% of all cats in the United States. Infection rates are significantly higher (up to 30%) in cats that are ill or otherwise at high risk (see below). Fortunately, the prevalence of FeLV in cats has decreased significantly in the past 25 years since the development of an effective vaccine and accurate testing procedures.
Cats persistently infected with FeLV serve as sources of infection for other cats. The virus is shed in saliva, nasal secretions, urine, feces, and milk of infected cats. Cat-to-cat transfer of the virus may occur from a bite wound, during mutual grooming, and (rarely) through the shared use of litter boxes and feeding dishes. Transmission can also take place from an infected mother cat to her kittens, either before they are born or while they are nursing. FeLV does not survive long outside a cat's body – probably less than a few hours under normal household conditions.
Cats at greatest risk of FeLV infection are those that may be exposed to infected cats, either via prolonged close contact or through bite wounds. Such cats include cats living with infected cats or with cats of unknown infection status, cats allowed outdoors unsupervised where they may be bitten by an infected cat, and kittens born to infected mothers.
Kittens are much more susceptible to FeLV infection than are adult cats, and therefore are at the greatest risk of infection if exposed. However, even healthy adult cats can become infected if sufficiently exposed.
FeLV adversely affects a cat's body in many ways. It is the most common cause of cancer in cats, may cause various blood disorders, and may lead to a state of immune deficiency that hinders a cat's ability to protect itself against other infections. Because of this, common bacteria, viruses, protozoa, and fungi that usually do not affect healthy cats can cause severe illness in FeLV-infected cats. These secondary infections are responsible for many of the diseases associated with FeLV.
During the early stages of infection, it is common for cats to exhibit no signs of disease at all. Over time, however, (weeks, months, or even years) an infected cat's health may progressively deteriorate or he/she may experience repeating cycles of illness and relative health. Signs can include:
Two types of blood tests are commonly used to diagnose FeLV, both of which detect a protein component of the virus called FeLV P27. One of these tests, called an enzyme-linked immunosorbent assay (ELISA), is usually performed first as a screening tool, and can be run in a veterinarian's office. ELISA-type tests detect the presence of free FeLV particles that are commonly found in the bloodstream during both the early and late stages of infection.
The indirect immunofluorescent antibody assay (IFA) test is usually sent out to a diagnostic laboratory after a positive ELISA test to confirm FeLV infection and determine whether the cat has reached the later stages of infection. IFA tests detect the presence of virus particles within white blood cells, usually an indication of a more advanced infection. The majority of cats that test positive by IFA remain infected for life. In some cases, isolating the whole virus or detecting DNA of the virus using a test called a polymerase chain reaction (PCR) may be recommended to determine whether FeLV has infected the bone marrow. Always consult with your veterinarian to determine which tests are appropriate for your cat.
Treatment and Prevention
Although there are some therapies that have been shown to decrease the amount of FeLV in the bloodstream of affected cats, these therapies may have significant side effects and may not be effective in all cases. Unfortunately, there is currently no definitive cure for FeLV. Veterinarians treating and managing FeLV-positive cats showing signs of disease usually treat specific problems (like prescribing antibiotics for bacterial infections, or performing blood transfusions for severe anemia).
The only sure way to protect cats from FeLV is to prevent their exposure to FeLV-infected cats. Keeping cats indoors, away from potentially infected cats is recommended. If outdoor access is allowed, provide supervision or place cats in a secure enclosure to prevent wandering and fighting. All cats should be tested for FeLV prior to introducing them into a home, and infection-free cats should be housed separately from infected cats. Food and water bowls and litter boxes should not be shared between FeLV-infected cats and non-infected cats. Unfortunately, many FeLV-infected cats are not diagnosed until after they have lived with other cats. In such cases, all other cats in the household should be tested for FeLV. Ideally, infected and non-infected cats should then be separated to eliminate the potential for FeLV transmission.
A relatively effective vaccine against FeLV is available, although it will not protect 100% of cats vaccinated, and it is not considered a core vaccine. Owners contemplating FeLV vaccination for their uninfected cats should consider the cats' risk of exposure to FeLV-infected cats and discuss the advantages and disadvantages of vaccination with a veterinarian. Since not all vaccinated cats will be protected by vaccination, preventing exposure remains important even for vaccinated pets. FeLV vaccines will not cause false positive FeLV results on ELISA, IFA, or any other available FeLV tests.
Although a diagnosis of FeLV can be emotionally devastating, it is important to realize that cats with FeLV can live normal lives for prolonged periods of time. The median survival time for cats after FeLV is diagnosed is 2.5 years. Once a cat has been diagnosed with FeLV, careful monitoring of weight, appetite, activity level, elimination habits, appearance of the mouth and eyes, and behavior is an important part of managing this disease. Any signs of abnormality in any of these areas should prompt immediate consultation with a veterinarian.