Feline Leukemia

What is feline leukemia ?

Feline leukemia is a cancerous disease caused by the feline leukemia virus (FeLV). FeLV will also cause diseases other than leukemia including immunodeficiency and additional cancers. Feline leukemia virus (FeLV)is a retrovirus. It is related to the feline immunodeficiency virus (FIV) and human immunodeficiency virus (HIV). All of these viruses cause disease by inserting copies of their own genetic material into that of the cells they have infected.

How common is 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 13% of cats that are ill, very young, or otherwise at high risk of infection carry the disease.

How is it transmitted?

FeLV 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 the virus may occur from a bite wound, during mutual grooming, and 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.

FeLV once contracted goes through 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.

What are the signs of FeLV disease?

FeLV 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. These secondary infections are responsible for many of the diseases associated with FeLV.

Clinical signs of these conditions 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
• 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

What diagnostic tests are available?

Two types of FeLV blood tests are available: the ELISA, or kit test, which can be performed in the veterinarian's office and the IFA or slide test, which must be sent out to a diagnostic laboratory. Both tests detect the FeLV as it circulates in the bloodstream, either free in the blood (ELISA test) or within infected white blood cells (IFA test).

The ELISA test can detect the primary (or transient) viremia stage, before the bone marrow has become infected, when the cat's immune system has an opportunity to ward off the virus. Transiently viremic cats characteristically test ELISA-positive and then revert to negative status within about eight weeks. It is important that a positive FeLV test be repeated in eight to twelve weeks to determine whether the viremia is transient or persistent. The ELISA test can also detect the virus in the second or persistent viremia stage.

The IFA test detects the circulating virus primarily during the second stage. Cats testing positive by the IFA test remain positive for life. These cats are shedding FeLV in the saliva and are infectious to other cats. Occasional discrepancies between the two FeLV tests have been noted. When such a discrepancy arises, it is important to remember that the two tests detect FeLV in two different "compartments" of the blood (blood fluid versus white blood cells).

If a cat is positive by ELISA and negative by IFA at the same time, it may mean that the virus is at the first or primary viremia stage. These cats are still carrying FeLV but are apparently not shedding it in saliva (and thus will not transmit it to companion cats), and most appear to be resistant to the disease-producing effects of FeLV.

What is the treatment?

There is no effective treatment for the immunosuppressive (bone marrow) form of leukemia. Treatment is mainly supportive, and may require blood transfusions, prednisone, and anabolic steroids. FeLV cancer (lymphoma) has a better response to therapy than the myeloproliferative diseases do. Treatment may include chemotherapy, glucocorticoids, interferon, Protein A, and supportive treatment. Eighty-five percent of cats with FeLV infection die within 3 years of the diagnosis.

How is it prevented?

There are several preventive measures that can be taken to decrease the risk of contracting FeLV. Cats should be FeLV tested, and then vaccinated if they are negative. FeLV vaccination of infected cats does not affect the carrier state, the capacity to infect other cats, or the development of disease in the infected cats. Kittens may be tested at any age. However, infection in newborn kittens may not be detected until weeks to months after birth. After negative testing the FeLV vaccination can be started at 9 weeks. All kittens or adult cats that test negative by the first ELISA screening test - but with a known or suspected exposure to FeLV - should be retested. This is done to rule out possible negative results obtained during incubation of the FeLV virus. Although the majority of cats will test positive within several weeks, final retest of negative cats should be no sooner than 90 days post-exposure.