FELINE LEUKEMIA VIRUS DISEASE COMPLEX: a Detailed Account
Until the development of a vaccine to protect against Feline Leukemia Virus (FeLV) infection in the mid-1980’
s, the complex of diseases associated with FeLV was the most frequent cause of death in cats.
What is Feline Leukemia Virus (FeLV)?
Feline leukemia virus (FeLV) is one of the most important infectious viruses of cats. It was discovered in cats
with a form of leukemia, hence its name. FeLV is the cause of a variety of diseases, not just leukemia. Like all
viruses, FeLV is a minute micro-organism consisting of nucleic acid and a few proteins and glycoproteins in a
simple structure. Viruses can only replicate themselves inside living cells. FeLV is specific to members of the
cat family and does not pose a risk to other animals or people.
How common is FeLV?
FeLV infection is found worldwide. In general, around 1-2% of the cat population is persistently infected with
this virus, and many more are exposed. The proportion of cats infected differs according to the geographical
location, environment and the life-style of the cat. Infection is more common in colonies of cats where there is
close contact between individuals.
What disease does the virus cause?
FeLV invades and replicates in various cells of the cat’s immune system and blood-forming cells, and less
commonly in other cells. In its replication, the nucleic acid (genetic code) of FeLV inserts itself into the nucleic
acid of the cat cells it has invaded. The result can be death of the cell or change in its genetic code. Such a
change can make the cell potentially cancerous and lead to neoplastic disease (cancer). The cancerous change
may not occur for months or years after infection.
Depending on the type and location of cells infected with FeLV, cancers can occur in a variety of tissues,
organs and body sites. Such cancers can involve any type of the circulating white blood cells (leukemia) or
other cells of the reticulo-endothelial system. The most common tumor associated with FeLV is that of
lymphoid cells known as lymphoma or lymphosarcoma. These tumors may occur at single or multiple sites in
Although the development of cancer is one outcome of FeLV infection, other diseases more commonly
develop. In many cats, FeLV infection results in a moderate to severe suppression of the immune system
(immunosuppression). This means that the infected cat is less able to defend itself against a wide range of
infections; infections that would not normally cause a problem in healthy cats. A variety of clinical signs of
chronic and/or recurrent disease develops in these cats, and there is a progressive deterioration in their health
Another common occurrence in FeLV-infected cats is the development of a profound and life-threatening
anemia. Other diseases including abortion, severe enteritis (causing diarrhea), neurological (nervous) signs,
and ocular (eye) disease are commonly associated with FeLV infection.
FeLV is usually fatal. Studies have shown that 80-90% of FeLV-infected cats will die within 3 to 4 years of
How is FeLV transmitted?
Direct contact between cats is the most frequent method of FeLV infection. The virus is fragile and cannot
survive longer than a few hours in the environment outside of the cat. A cat with FeLV sheds a large quantity
of the virus in its saliva as well as in other body secretions and excretions such as urine and feces. However,
FeLV is not a highly contagious virus, and transmission generally requires a prolonged period of close contact
between infected and susceptible cats. Close contact activities include mating, mutual grooming, and sharing
of litter trays and food bowls. Cat bites by an infected cat can readily transmit infection.
Another potential source of infection is when a pregnant cat infected with FeLV gives birth. In this situation,
all the kittens will likely be born with FeLV virus. Fortunately, it is more likely that a queen infected with
FeLV becomes infertile or there is pre-natal death of the kittens with abortion or resorption of the fetuses.
What happens when a cat is exposed to FeLV?
Not all cats exposed to FeLV will develop persistent infections. Many of the cats that are infected are able to
mount an immune response to the virus. This immunity is successful in eliminating the virus in
approximately 30% of the cats exposed to FeLV. However, there will have been a period of some weeks when
the virus has been replicating in those cats before they cleared the infection. During the period when FeLV
was replicating inside those cats’ cells, there may have been cell changes that could lead to disease later in life.
Approximately 70% of cats are unable to mount an immune response to eliminate the virus. Following
infection, these cats become persistently and permanently infected with the virus and are at the highest risk of
developing FeLV-related disease. Furthermore, it is the permanently infected cats that are primarily
responsible for the transmission of FeLV to other cats. It can be many months or even years between the initial
virus infection and the onset of related clinical disease problems.
How can FeLV infection be diagnosed?
Diagnosis of FeLV infection is relatively simple. A rapid blood test can be performed which is able to detect
viral proteins of FeLV in the blood of an infected cat. This test is very accurate and reliable; although false
results occur on occasion. In some situations it may be necessary to confirm infection through further blood
testing at a specialist laboratory where more sophisticated tests are available.
Additionally, some cats with only a transient FeLV infection will be positive on the blood test and a second
test performed 8-12 weeks after the first test may be required to confirm persistent infection.
Diagnosis of FeLV disease is more difficult because of the variety of signs and symptoms and the frequent
complication of other disease problems co-existing with, and promoted by, the FeLV infection.
Is there any treatment for FeLV infection or disease?
There is currently no specific treatment for FeLV-infected cats (no treatment to eliminate the virus from the
body). Most FeLV-infected cats will eventually die of diseases related to their infection or require euthanasia.
However, many cats showing FeLV-related disease will improve on symptomatic treatment, at least for a
period of time. For example, if FeLV is causing disease in an individual through immunosuppression and the
development of secondary infections, these other concurrent diseases may be treatable, leading to
improvement of the clinical signs.
How can infection be prevented?
Vaccines are now available to protect cats against FeLV infection. Their use is highly recommended in any cat
that goes outside at any time and therefore could have contact with FeLV- infected cats. As with other
vaccines, an initial course of two injections is required, and annual boosters are necessary to maintain best
immunity. Your veterinarian will discuss the most appropriate options for your cat with you. We recommend
testing all cats for FeLV prior to vaccination.
Although vaccination is very helpful in preventing infection with FeLV and thus controlling FeLV-related
disease, no vaccine is 100% protective. Where possible do not allow your cat, particularly if a kitten, to come
into close contact with known FeLV-infected cats or cats without a known history of proper vaccinations.
In larger colonies of cats, such as breeding households, it is possible to control FeLV infection by regular
blood testing. A combination of routine FeLV testing, quarantine and vaccination strategies can help prevent a
FeLV outbreak in these situations. Fortunately, vaccinating a cat does not interfere with subsequent blood
testing for FeLV (except possibly in the week immediately following vaccination).
This client information sheet is based on material written by Ernest E. Ward Jr., DVM.
© Copyright 2002 Lifelearn Inc. Used with permission under license. January 25, 2008.
Animal Clinic, PA