Despite the widespread use of vaccines, feline leukemia virus remains one of the most important causes of illness and death in cats. It causes a variety of cancers. Persistent infection can also lead to severe suppression of the immune system and severe anemia. The virus is present worldwide. Young kittens are much more susceptible than adults.
The rate of feline leukemia virus infection is directly related to the population density of cats. Infection rates are highest in catteries and multiple cat households, especially when cats have access to the outdoors. In the United States, 1 to 2% of healthy stray urban cats persistently have the virus; this figure is higher for sick, “at risk” cats.
Persistently infected healthy cats are carriers of the virus. These carriers excrete large quantities of virus in their saliva. Lesser amounts of virus are excreted in tears, urine, and feces. Mouth and nose contact with infectious saliva or urine is the most likely method of transmission. Nose-to-nose contact, mutual grooming, and shared litter boxes and food dishes make it easy for the disease to spread to uninfected cats. Bite wounds from infected cats are also an efficient method of transmission. This virus may also be transmitted from mother to kitten either in the womb or in the milk.
The damage to the immune system caused by feline leukemia virus is similar to that caused by feline immunodeficiency virus (see Immune Disorders of Cats: Immunodeficiencies Caused by Viruses). With a damaged immune system, a cat has an increased susceptibility to bacterial, fungal, protozoal, and other viral infections.
Disorders Caused by Feline Leukemia Virus
Feline leukemia virus-related disorders are numerous. They include suppression of the immune system, cancer, anemia, immune-mediated diseases, reproductive problems, and inflammation of the intestines.
Tumors, including lymphoma, lymphoid leukemia, and erythremic myelosis, develop in up to 30% of cats infected with feline leukemia virus. Cats that test negative for the feline leukemia virus (called nonviremic cats) also develop these tumors. However, they may still be caused by feline leukemia virus that is no longer detectable in the bloodstream.
Lymphoma is the most frequently diagnosed cancer of cats. Most American cats with lymphoma of the spine, chest, or multiple locations are feline leukemia virus-positive. However, in some parts of the world, these forms of lymphoma are becoming much less common. The proportion occurring in feline leukemia virus-positive cats is decreasing. This may be related to effective control of feline leukemia virus.
Feline lymphoma can be treated with anticancer drugs, often called chemotherapy. The use of chemotherapy has improved in recent years. Most cats do not experience significant adverse effects and enjoy a good quality of life.
About 50% of cats with lymphoma that are treated will have a complete remission (no clinical evidence of disease). Feline leukemia virus-negative cats that attain a complete remission live an average of 9 months. Feline leukemia virus-positive cats have an average survival period of 6 months. Cats not treated or those not responding to treatment survive only about 6 weeks.
Leukemia is a cancerous proliferation of blood cells originating in the bone marrow. In cats, leukemia is strongly associated with feline leukemia virus infection. Leukemia is sometimes (but not always) associated with cancerous cells circulating in the blood.
Reproductive problems, including infertility and abortion, are common in cats with the feline leukemia virus. Fetal death and reabsorption of the placenta may occur between 21 and 42 days into the pregnancy. The normal gestation period in cats is 63 to 67 days. It is likely that, in these cases, the virus was transported across the placenta and fatally infected the developing kittens. Occasionally, infected queens give birth to live kittens that were infected with the virus while in the womb. Queens that are infected but have not tested positive for the virus may pass the virus on to their kittens in their milk.
Inflammation of the Intestines
Inflammation of the intestines may develop in feline leukemia virus infections. This inflammation often resembles feline panleukopenia (see Disorders Affecting Multiple Body Systems of Cats: Feline Panleukopenia). Signs include loss of appetite, depression, vomiting, and diarrhea (which may be bloody). Because immune system damage is associated with feline leukemia virus infection, blood poisoning may develop. Evidence suggests that the feline leukemia virus and the feline panleukopenia virus may act together to produce this syndrome.
Other disorders may also develop as a result of infection with feline leukemia virus. It occasionally causes problems in the nervous system, leading to inequality in the size of the pupils, loss of bladder control, or hind limb paralysis. Certain lymphomas induced by the feline leukemia virus can produce identical signs. Thus, your veterinarian will be careful to confirm whether the cat has a form of cancer or problems with the nervous system. The treatment required for these conditions is different and, for the health of your cat, it is important that the correct treatment be provided.
Ideally, a feline leukemia virus-infected cat would be identified early and treated to totally eliminate the infection before virus-related diseases had time to develop. Unfortunately, most infected cats have a longterm infection by the time the disease is diagnosed.
Many treatments have been administered in an attempt to reverse the presence of the virus in the blood or control the signs associated with feline leukemia virus infection. However, most of these treatments have not been found to be effective.
Cats with feline leukemia virus infection can live without major disease for several years if they have good supportive care. Stress and sources of secondary infection should be avoided. To avoid infection from bacteria and other agents, water should be changed at least daily, uneaten food should be removed at least daily, and all dishes should be thoroughly cleaned daily. The cat should remain indoors 100% of the time. This will reduce the risk of exposure to infectious agents and prevent transmission of the virus to other cats. Routine preventive care for feline leukemia virus-infected cats is more important than for uninfected cats. Infected cats should receive health checkups every 6 months or whenever you notice any health changes. Your veterinarian will provide routine vaccinations based on the risk to the cat. Rabies vaccinations will also be given to comply with local laws. Feline leukemia virus vaccinations are not administered to infected cats as there is no evidence that these vaccines provide a benefit. If the infected cat is not already neutered, this should be done.
If you have a cat with feline leukemia virus, you should ask your veterinarian to provide you with a list of signs of developing disease. This will allow you to watch for signs of virus-related disease, particularly secondary infections. Treatment for any infections or other illnesses will be started earlier and may last longer due to the cat's weakened immune system.
Prevention and Control
Testing for feline leukemia virus infection is recommended for all kittens at their first veterinary visit. If the kitten(s) test positive, your veterinarian will talk with you about the steps you should consider. Testing is also recommended for all cats prior to entering a household with existing uninfected cats, for cats in an existing household prior to admission of a new, uninfected cat, and for all cats prior to their first feline leukemia virus vaccination.
Feline leukemia virus vaccines are intended to protect cats against infection and/or to prevent persistent presence of virus in the blood. Vaccines are recommended only for uninfected cats; there is no benefit in vaccinating a feline leukemia virus-positive cat.
Your veterinarian will assess each cat's risk of exposure to feline leukemia virus and prescribe vaccines only for those cats at risk. Uninfected cats in a household with infected cats should be vaccinated. In addition, other means of protecting uninfected cats (for example, by physical separation) should also be used. Constant exposure to feline leukemia virus-infected cats is likely to result in viral transmission to a previously uninfected cat whether it has been vaccinated or not.
Some strains of feline leukemia virus can be grown in human tissue cultures. This has led to concerns about possible transmission to people. Several studies have addressed this concern; none have shown any evidence that people can be infected with this virus by exposure to infected cats.
Last full review/revision July 2011 by Otto M. Radostits, CM, DVM, MSc, DACVIM (Deceased); Max J. Appel, DMV, PhD; David A. Ashford, DVM, MPH, DS; Stephen C. Barr, BVSc, MVS, PhD, DACVIM; J. P. Dubey, MVSc, PhD; Paul Ettestad, DVM, MS; Craig E. Greene, DVM, MS; Delores E. Hill, PhD; Johnny D. Hoskins, DVM, PhD; Eugene D. Janzen, DVM, MVS; Jodie Low Choy, BVMS; Dennis W. Macy, MS, DACVIM; Dudley L. McCaw, DVM, DACVIM (Small Animal, Oncology); Barton W. Rohrbach, VMD, MPH, DACVPM; J. Glenn Songer, PhD; Richard A. Squires, BVSc (Hons), PhD, DVR, DACVIM, DECVIM-CA, GCertEd, MRCVS; Bert E. Stromberg, PhD; Joseph Taboada, DVM, DACVIM; Charles O. Thoen, DVM, PhD; John F. Timoney, MVB, PhD, Dsc, MRCVS; Ian Tizard, BVMS, PhD, DACVM