Feline Leukemia Virus Diseases
Feline leukemia virus infection was, until recently,
the most common fatal disease of cats. Because we can now protect cats with a
leukemia virus vaccine, we are seeing fewer cases of the disease. However, it
still remains a major cause of death in cats.
"Leukemia" means cancer of the white blood
cells. This was the first disease associated with the feline leukemia virus (FeLV)
and, thus, the source of its name. We often use the term "leukemia"
rather loosely to include all of the diseases associated with the virus, even
though most are not cancers of the blood. This virus causes many other fatal
diseases, in addition to leukemia.
What diseases are caused by the FeLV?
There are three major disease categories associated
with the FeLV:
- The Leukemias are cancers of the white blood cells.
- Lymphosarcoma is a cancer which begins in lymphoid
tissue, such as a lymph node. Almost any tissue may be affected; organs
commonly involved include lymph nodes, intestinal tract, kidneys, liver,
spinal cord, brain, bone marrow and blood.
- The Non-Cancerous Diseases include a variety of
somewhat unrelated diseases. Anemia, abortion, arthritis, and immune
suppression are examples. When the immune system is suppressed, the cat
becomes susceptible to many diseases it would ordinarily resist and mild
diseases, such as respiratory infections, may become fatal.
How is the virus transmitted?
The main means of transmitting the virus is through cat
fights. Because large quantities of the FeLV are shed in cat saliva, puncture
wounds associated with fighting result in injection of the virus into other
cats, Other less frequent routes of viral spread include sharing food and water
bowls, cats grooming each other, and transmission from mother to kittens before
birth.
What is a "leukemia test"?
The "leukemia test" is used to determine if a
cat harbors the virus. Any of three different tests may be used to detect one
particular virus protein in the cat. Some tests detect earlier stages of
infection, whereas others are used to detect later (i.e., irreversible) stages
of infection.
- The blood ELISA test is performed on a blood sample
and detects the FeLV at any stage of infection. This test turns positive
within a few days of infection and, in some cases, may later turn negative
if the cat's immune system eliminates the infection.
- The IFA test is performed on a blood smear and turns
positive only after the FeLV infection has progressed to a late stage of
infection. Once positive, the IFA test usually means that the cat has a
permanent infection. A cat who tests IFA positive is only rarely able to
successfully eliminate the virus.
- The tears/saliva ELISA test is performed on a sample
of tears or saliva. It turns positive only in a late stage of infection;
therefore, it may yield a false negative result in cats who are in the early
stage of FeLV infection. It also has been associated with some false
positive results due to inherent errors in the way the test is performed.
Because of these problems, the tears and saliva tests are not used
routinely.
What can happen if a cat is infected with the FeLV?
When we are exposed to a virus, such as a flu virus,
there are two possible outcomes. Either our immune system responds to the
challenge and protects us, or it is unable to respond successfully and we
develop the flu. A number of factors determine which outcome occurs and whether
or not we will get sick.
- The amount of the virus (Did someone sneeze directly
in your face?)
- The strain of the virus (Some strains are more
virulent than others)
- The status of our immune system (Are immune
suppressing drugs being taken?)
- Age (the very young and very old are more likely to
become infected)
- The presence of other infections which might cause
debilitation
The behavior of the feline leukemia virus in the cat's
body is not so black or white. Instead of the two possible outcomes described
above (i.e., we get sick or we get well), there are four possible outcomes for
cats with FeLV. Understanding these allows one to more fully comprehend some of
the unusual situations which may arise in cats.
OUTCOME 1: IMMUNITY - The cat mounts an immune
response, eliminating the infection.
This is the most desired outcome because it means that
the cat will not become persistently infected with the virus. During this period
of virus challenge, the cat may actually develop a mild form of illness. Fever,
poor appetite, lethargy, and swollen glands (lymph nodes) in the neck may
develop and last for 3 to 10 days. Outcome 1 occurs about 40% of the time after
a cat is challenged by the FeLV. Immunity to the virus is more likely to develop
in the adult cat than in the kitten.
OUTCOME 2: INFECTION - The cat's immune system
is overwhelmed by the virus.
This is the least desired outcome because the cat
becomes permanently infected with the virus. Although the cat may be sick for a
few days initially (as described above), it usually recovers and appears normal
for weeks, months, or years. Ultimately, most of these cats die of FeLV-related
disease, but as many as 50% will still be healthy after 2-3 years and 15% after
4 years. Vaccination of these cats will not cause any problems, but doesn't help
the cat, either, Outcome 2 occurs an estimated 30% of the time after a cat is
challenged by the FeLV.
OUTCOME 3: LATENCY - The cat harbors the virus
but we cannot easily detect it.
Unlike other viruses, the FeLV does not directly kill
the cat's cells or make them become cancerous. Instead, it inserts a copy of its
own genetic material (called DNA) into the cat's cells; these cells may later be
transformed into cancer cells or cells which will no longer function normally.
In Outcome 3, the genetic change in the cat's cells will remain undetected for
an average of 2 1/2 years, during which time the cat will appear completely
normal.
In the early stages of infection, the blood ELISA and
IFA tests will remain consistently negative. The PCR test, a recently available
diagnostic tool, will detect the latent infection. However, this test is
somewhat expensive and not widely available so it is not used for routine
testing.
OUTCOME 4: IMMUNE CARRIER - The cat becomes an
immune carrier.
The FeLV becomes hidden in some of the cat's epithelial
cells, Although the FeLV is multiplying, it is not able to get out of these
cells because the cat is producing antibodies against the virus. The cat will
appear normal in every way. This situation is uncommon and probably occurs only
1-2% of the time.
How are cats with leukemia treated?
Some forms of leukemia (blood cancer) are unresponsive
to all available forms of cancer treatment. Other types of leukemias may respond
to chemotherapy, though most of these have an average survival time of less than
one year. Because the virus is not affected by treatment, the cat will always
remain infected with FeLV. Also, relapse of leukemia is possible (and expected).
These factors cause us to recommend treatment of leukemia in very few
situations.
What should I do to disinfect my house?
The FeLV lives, at most, only a few hours outside the
cat if the environment is dry. Therefore, extensive environmental disinfection
is not necessary. If you wait even two days to get a new cat, you can be assured
that none of the virus from a previous cat will remain in your house.
I have a healthy cat that is infected with the
virus. What does that mean?
Healthy infected cats may remain apparently unaffected
by the virus for several years. However, such cats should be considered
infectious and potentially dangerous to other cats. Such cats should be isolated
from non-infected cats to prevent spread of infection. Many people find this
undesirable or impossible and elect euthanasia to protect non-infected cats.
Is there any danger to my family?
Extensive tests have been conducted for over 15 years
to determine if the FeLV can be transmitted to humans. Thus far, no conclusive
evidence has shown any FeLV-related disease in humans or other animal species,
including the dog. However, persons with compromised immune systems are of
concern to many researchers. Newborn babies, persons on chemotherapy, AIDS
patients or transplant recipients on anti-rejection drugs should probably not be
unnecessarily exposed to this or any other virus.
Can I protect my other cats?
A vaccine is available to protect cats from the FeLV.
Although not 100% of cats are totally protected, the vaccine is strongly
recommended for cats who are exposed to open populations of cats (i.e., outdoor
cats). We have seen a definite decline in the incidence of feline leukemia virus
infection and related diseases since vaccine use became widespread. We strongly
recommend it. If your cat stays indoors at all times and is not in contact with
another cat that goes outdoors, the need for the vaccine is minimal.
Cats who are already infected with the FeLV will not be
helped by the vaccine. (They will not be hurt by it either). We recommend
pre-vaccination testing for the FeLV for:
- Cats with a history of cat fights or fight wounds
(i.e., abscesses)
- Cats exposed to FeLV-infected cats
- Cats from unknown backgrounds (particularly animal
shelters, humane societies, or pet shops)
- Routine health care, especially in multicat
households
Will vaccinating my cat with the FeLV vaccine cause
the leukemia test to be positive?
No. The vaccine will not cause a cat to test positive
for the virus, While the history of vaccination is important for us to know, it
does not alter our ability to interpret the feline leukemia virus test.
Are there any possible adverse effects associated
with the leukemia virus vaccine?
Possibly. In the last 10 years, several million doses
of leukemia vaccine have been given without any adverse side-effects. However, a
form of cancer that will arise at the injection site has been found in a small
subset of cats (estimated at between 1 in 10,000 and 1 in 100,000) that have
received the leukemia vaccine. This tumor is called a fibrosarcoma, or tumor of the
connective tissue. In many cats, it is not possible to remove the tumor with
surgery. However, in most cats, the risk of contracting a feline leukemia virus
disease and dying of it is considered far greater than development of
vaccine-related tumors.
