Feline Leukemia
Feline
leukemia is a high-mortality disease caused by the feline leukemia
virus (FeLV). FeLV produces an initial immunosuppressive infection
followed by various other disease (e.g., respiratory disease, diarrhea,
anemia) affecting the immunosuppressed cat. Cats that survive these
initial diseases may develop some form of cancer, hence the name feline
leukemia. The disease is transmitted by direct contact with infected
cats or with contaminated food dishes or litter boxes. Feline leukemia
vaccination is now commonplace.
Additional Information About Feline Leukemia
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 one of the most frequent causes 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 first 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 tissues, as well as other cells. To replicate, the
nucleic acid (genetic code) of FeLV inserts itself into the nucleic
acid of the cells it has invaded. The result can be death of the cell
or a 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.
Cancers
can occur in a variety of tissues, organs and body sites, depending on
the type and location of cells that have been infected with FeLV. 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 the body.
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. This means that
the infected cat is less able to defend itself against a wide range of
infections that would not normally cause a problem in healthy cats. A
variety of clinical signs develop in these cats, and there is a
progressive deterioration in their health over time.
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 three to four years of initial diagnosis.
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 bodily fluids 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. 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 may rarely occur. In addition, some cats with only a
transient FeLV infection will be positive on the blood test. A second
test performed eight to twelve weeks after the first test may be
required to confirm persistent infection. In some situations it may be
necessary to confirm infection through further blood testing at a
specialized laboratory where more sophisticated tests are available.
Diagnosis
of FeLV disease is more difficult because of the variety of signs and
symptoms. It is common to have a complicated situation in which other
diseases or conditions co-exist with, and are promoted by, the FeLV
infection.
Is there any treatment for FeLV infection or disease?
There
is currently no specific treatment for FeLV-infected cats. There is no
treatment to eliminate the virus from the body. Most FeLV-infected cats
will eventually die or be euthenized because of diseases related to
their infection. However, many cats showing FeLV-related disease will
improve with symptomatic treatment, at least for a period of time. For
example, if FeLV is causing immunosuppression and the patient develops
secondary infections, the secondary infections may be treatable,
leading to clinical improvement.
How can infection be prevented?
Vaccines
are available to protect cats against FeLV infection. Their use is
highly recommended for 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 regular
boosters are necessary to maintain immunity. Your veterinarian will
discuss the most appropriate options for your cat with you. All cats
should be tested 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, it is possible to control FeLV infection
through a combination of routine FeLV testing, quarantine and
vaccination programs. Fortunately, vaccinating a cat does not interfere
with subsequent blood testing for FeLV.
