What is feline leukemia virus?
Feline leukemia virus (FeLV), a retrovirus, so named because of the way it behaves within infected cells. All retroviruses, including feline immunodeficiency virus (FIV) and human immunodeficiency virus (HIV), produce an enzyme, reverse transcriptase, which permits them to insert copies of their own genetic material into that of the cells they have infected. Although related, FeLV and FIV differ in many ways, including their shape: FeLV is more circular while FIV is elongated. The two viruses are also quite different genetically, and their protein constituents are dissimilar in size and composition. Although many of the diseases caused by FeLV and FIV are similar, the specific ways in which they are caused differs.

How common is the infection?
FeLV-infected cats are found worldwide, but the prevalence of infection varies greatly depending on their age, health, environment, and lifestyle. In the United States, approximately 2 to 3% of all cats are infected with FeLV. Rates rise significantly—13% or more—in cats that are ill, very young, or otherwise at high risk of infection.

How is FeLV spread?
Cats persistently infected with FeLV serve as sources of infection. Virus is shed in very high quantities in saliva and nasal secretions, but also in urine, feces, and milk from infected cats. Cat-to-cat transfer of virus may occur from a bite wound, during mutual grooming, and (though 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 doesn't survive long outside a cat's body—probably less than a few hours under normal household conditions.

What cats are at greatest risk of infection?
Cats at greatest risk of 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
• Kittens born to infected mothers

Kittens are much more susceptible to infection than are adult cats, and therefore are at the greatest risk of infection if exposed. But accompanying their progression to maturity is an increasing resistance to FeLV infection. For example, the degree of virus exposure sufficient to infect 100% of young kittens will infect only 30% or fewer adults. Nonetheless, even healthy adult cats can become infected if sufficiently exposed.

What does FeLV do to a cat?
Feline leukemia virus adversely affects the cat's body in many ways. It is the most common cause of cancer in cats, it may cause various blood disorders, and it may lead to a state of immune deficiency that hinders the cat's ability to protect itself against other infections. The same bacteria, viruses, protozoa, and fungi that may be found in the everyday environment—where they usually do not affect healthy animals—can cause severe illness in those with weakened immune systems. These secondary infections are responsible for many of the diseases associated with FeLV.

What are the signs of disease caused by FeLV?
During the early stages of infection, it is common for cats to exhibit no signs of disease at all. However, over time—weeks, months, or even years—the cat's health may progressively deteriorate or be characterized by recurrent illness interspersed with periods of relative health. Signs can include:

• Loss of appetite
• Slow but progressive weight loss, followed by severe wasting late in the disease process
• Poor coat condition
• Enlarged lymph nodes
• Persistent fever
• Pale gums and other mucus membranes
• Inflammation of the gums (gingivitis) and mouth (stomatitis)
• Infections of the skin, urinary bladder, and upper respiratory tract
• Persistent diarrhea
• Seizures, behavior changes, and other neurological disorders
• A variety of eye conditions
• In unspayed female cats, abortion of kittens or other reproductive failures

I understand there are two stages of FeLV infection. What are they?
FeLV is present in the blood (a condition called viremia) during two different stages of infection:

• Primary viremia, an early stage of virus infection. During this stage some cats are able to mount an effective immune response, eliminate the virus from the bloodstream, and halt progression to the secondary viremia stage.
• Secondary viremia, a later stage characterized by persistent infection of the bone marrow and other tissue. If FeLV infection progresses to this stage it has passed a point of no return: the overwhelming majority of cats with secondary viremia will be infected for the remainder of their lives.

How is infection diagnosed?
Two types of FeLV blood tests are in common use. Both detect a protein component of the virus as it circulates in the bloodstream.

• ELISA (enzyme-linked immunosorbent assay) and similar tests can be performed in your veterinarian's office. ELISA-type tests detect both primary and secondary stages of viremia.
• IFA (indirect immunofluorescent antibody assay) tests must be sent out to a diagnostic laboratory. IFA tests detect secondary viremia only, so the majority of positive-testing cats remain infected for life.

Each testing method has strengths and weaknesses. Your veterinarian will likely suggest an ELISA-type test first, but in some cases, both tests must be performed—and perhaps repeated—to clarify a cat's true infection status.

How can I keep my cat from becoming infected?
The only sure way to protect cats is to prevent their exposure to FeLV-infected cats.

• Keep cats indoors, away from potentially infected cats that might bite them. If you do allow your cats outdoor access, provide supervision or place them in a secure enclosure to prevent wandering and fighting.
• Adopt only infection-free cats into households with uninfected cats.
• House infection-free cats separately from infected cats, and don't allow infected cats to share food and water bowls or litter boxes with uninfected cats.
• Consider FeLV vaccination of uninfected cats. (FeLV vaccination of infected cats is not beneficial.) Discuss the advantages and disadvantages of vaccination with your veterinarian. FeLV vaccines are widely available, but since not all vaccinated cats will be protected, preventing exposure remains important even for vaccinated pets. FeLV vaccines will not cause cats to receive false positive results on ELISA, IFA, or any other available FeLV tests.

I just discovered that one of my cats has FeLV, yet I have other cats as well. What should I do?
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.