Feline Leukemia Virus Disease
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. 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. The term
"leukemia" is used 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 theway 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:
A. The amount of
the virus (Did someone sneeze directly in your face?).
B. The strain of the virus (Some strains are more potent
than others).
C. The status of our immune system (are immune
suppressing drugs being taken?).
D. Age (the very young and very old are more likely to
become infected).
E. 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 ½ 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. 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. 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 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. 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 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. In most cats, the risk of contracting a feline
leukemia virus disease and dying of it is considered far
greater than for development of
vaccine-related tumors.
Resource References:
Much, much appreciation to Dr. Raymond Van Lienden, DVM
for his extensive research in locating the material found on
this page. Visit his clinic, the
Animal Clinic of Clifton, VA,
|
|
|
|
