Feline Leukemia Virus (FeLV)
Feline leukemia virus (FeLV) is a retrovirus, a family of
viruses which has many members that infect cats and cause disease and
death. This family of viruses includes
another well-known feline retrovirus, the feline immunodeficiency virus (FIV)
as well as human immunodeficiency virus (HIV).
The retroviruses are species specific, which means that a feline
retrovirus will only infect cats and a human retrovirus will only infect
humans. Retroviruses are fairly fragile
meaning they are inactivated by ultraviolet light, heat, detergents, and
drying. FeLV and FIV can be found
together in the same cat and can produce some similar symptoms, but they are
two separate viruses.
Transmission
Viremia is the condition when feline leukemia virus is
present in the blood. There are two
stages of viremia during the course of disease, and these are important to know
because this is how an owner can get two different, conflicting test
results.
The primary viremia is the stage before the virus infects
the bone marrow. It is at this point
that the cat’s immune system is still trying to fight off the infection. The secondary viremia is when the virus has
invaded the bone marrow and established a permanent infection in the cat.
Transmission of FeLV requires intimate moist contact with an
infected cat. The virus is shed in
saliva, tears, urine, and feces. The
most common route is contact with infected saliva through grooming, licking,
biting, and shared dishes and litter pans.
Kittens can be infected by their mother in utero or during nursing, and
FeLV can be obtained through a blood transfusion.
If a cat has been exposed to the virus, there are several
possible outcomes, which depend on how well the cat’s immune system responds to
the virus challenge.
1. IMMUNITY The cat’s immune system mounts a response and
eliminates the infection.
During this period, the cat may actually develop a mild form
of illness with fever, poor appetite, lethargy, and swollen neck lymph nodes
lasting for three to ten days. This
scenario is more likely to occur in an adult cat than a kitten. Natural immunity is developed that will last
for an unknown period of time.
2. INFECTION The cat’s immune system is overwhelmed by the
virus.
The cat becomes permanently infected with the virus and
excretes it in its saliva. After an initial
sickness, the cat usually recovers for a period of weeks to years. These cats test positive on a screening test
for FeLV. Vaccination of these cats does
not cause problems but will not help the cat in any way.
3. LATENCY The cat harbors the virus, but it is not
easily detected by testing.
The FeLV inserts a copy of its own genetic material into the
cat’s cells. These cats do not shed
virus in their saliva or other body secretions, but queens may still pass the
virus in utero and through the milk.
These cells may later be transformed into cancer cells or cells which do
not function normally. This genetic
change remains undetected for some period of time during which the cat appears
normal.
Another 30% of cats do not produce immunity but also do not
become persistently infected immediately.
In these cats, the virus hides in the bone marrow for up to 30
months. Eventually, these cats either
overcome the virus or become persistently infected.
4. CARRIER The cat becomes an immune carrier.
The
virus becomes hidden in the cat’s epithelial cells. The FeLV is multiplying but is not able to
get out of the epithelial cells because the cat is producing antibodies against
the virus. The cat appears normal. “Carrier" cats that are vaccinated but
develop a FeLV-related disease will appear to be a "vaccine failure.”
Finally, some cats can develop a latent or sequestered
infection. This probably happens to less than 5-10% of cats. These cats, whose
virus is hiding in sites such as the bone marrow, will rarely be contagious and
are unlikely to develop illness. They will not test positive on routine
testing.
Age is a very important factor in determining what will
happen after a cat is exposed to FeLV.
Almost all FeLV-exposed kittens less than 8 weeks of age will have
persistent infection and show signs of disease during the acute phase. As kittens get older, the probability of
becoming persistently infected after exposure lessens as they reach adulthood.
There are three disease categories associated with FeLV:
- Leukemia
– white blood cell cancers.
- Lymphosarcoma
– a lymph tissue cancer involving various different organs such as lymph
nodes, kidneys, liver, gastrointestinal tract, bone marrow, and blood to
name a few. The location of the
tumor will dictate the cat’s clinical signs.
- Noncancerous
disease – a wide variety of unrelated diseases usually related to the
immunosuppressive action of this virus.
Because FeLV interferes with the cat’s ability to fight off
infectious diseases, the cat may experience chronic infections, chronic
gingivitis and tooth problems, and slow wound healing.
Here is a short list of some of the complications of FeLV
infection and the reasons they affect FeLV- infected cats.
Anemia:
FeLV often affects cells in the bone marrow and as a result, most
FeLV-infected cats have a nonregenerative anemia.
Immune-mediated Diseases: Large amounts of the FeLV antigens combine
with the cat’s own antibodies to form complexes, which can be deposited in the
kidneys, blood vessels, or joints.
Gastrointestinal Disease: Cancers of the stomach or intestines, as well
as FeLV-associated changes in the intestinal wall, can cause anorexia,
vomiting, diarrhea, and weight loss.
Parasites and bacteria can multiply more easily and cause diarrhea in
FeLV-infected cats.
Neurologic Disease: Seizures, blindness, paralysis, and changes
in behavior can be seen in FeLV-infected cats.
These symptoms may be caused directly by FeLV or by parasites (Toxoplasma
gondii) and fungi (Cryptococcosis), which occur more often in
immunosuppressed animals.
Platelet Disorders: A decrease in the number of platelets
(thrombocytopenia) or a dysfunction of platelets sometimes occurs in cats
infected with FeLV.
Neoplasia:
Usually the cancer is in the form of tumors of white or red blood cells
and includes lymphosarcoma, lymphoid leukemia, and myeloid leukemia.
Diagnosis
Testing is the basis for diagnosing and managing FeLV
infections. There are several types of
tests used to diagnose the infection.
The enzyme-linked immunosorbent assay (ELISA) test is the most common
screening test and can be done in the veterinarian’s office. This test detects a protein component of the
virus as it circulates freely in blood serum.
That is why this test is best done using serum rather than tears, blood,
or saliva because the results are more reliable. The ELISA test detects the primary and secondary
viremia stages. Tests using tears or
saliva are recommended only as screening tests for catteries and should not be
used to screen individual cases.
Any positive ELISA test results should be confirmed using
the immunofluorescent antibody test
(IFA) test, usually done at a commercial laboratory. The IFA test detects circulating viruses in
the secondary viremia stage only. This
test detects a protein component of the virus as it circulates in the
bloodstream within infected white blood cells. It is for this reason that this test uses
blood smeared on a slide onto which a fluorescently tagged antibody is
reacted. This special antibody is made
in the laboratory and can be seen through a special microscope.
Because both of these tests look for actual viral antigens,
vaccinated cats or cats that are immune due to previous infection will not show
a positive test. Also, kittens can be
reliably tested because maternal immunity does not affect test accuracy.
The American Association of Feline Practitioners (AAFP) has
published recommendations for FeLV testing.
Since it is possible to have results on ELISA and IFA tests that do not
agree, the AAFP testing protocol should be followed in order to determine the
status of all cats. The FeLV status of
all cats should be known because this is the mainstay of FeLV control and is
not replaced by vaccination. When cats
should be tested:
- Cats
that are sick, regardless of previous negative results, should be
tested. While many signs such as
fever, stomatitis, vomiting, and diarrhea are obvious indicators of
illness, other signs are subtle and may include changes in behavior,
grooming, and eating habits. This
includes sick cats that have been vaccinated for FeLV.
- All
new adoptions, cats and kittens, should be tested whether they will be
entering a household with other cats or not.
- All cats
in multi-cat households because infected cats can remain asymptomatic for
years, during which time they may transmit the virus to uninfected cats.
- Testing
should be done when cats have had known or possible exposure to other cats
of unknown infection status. This
may include cats that go outdoors unsupervised, or cats that have been in
fights and have been bitten. These
cats should be tested no sooner than 60 days post-exposure.
- If the
ELISA and IFA test results are not the same, the cat should be tested at
4-8 week intervals for at least 90 days.
- All
cats with a negative result should be retested 90 days later to be sure
they were not recently infected prior to the first test, when viremia was
not yet present.
- If an
FeLV infected cat is found in a multi-cat household or cattery, that cat
should be removed from the others. All other cats should be tested at
90-day intervals until all cats have had two consecutive negative tests.
Treatment
Cats that test positive for FeLV may live for months to
years depending on the stage of the viremia and illness status of the cat. Owners should never assume that euthanasia is
the only option for a cat with FeLV.
That should be addressed on an individual basis in consultation with the
veterinarian. In many cases, it is
possible and feasible to keep a FeLV-positive cat and ensure good quality of
life through the combined efforts of the owner and the veterinarian. Because positive cats are capable of
transmitting their infection to other cats, they should not live with other
cats nor should they be allowed to roam outside. This not only protects other cats from FeLV,
but also protects the positive cat against the many diseases and illnesses they
may contract due to their increased susceptibility.
The mainstays of treating FeLV positive cats are protecting
them from exposure to other diseases, ensuring good nutrition, reducing stress,
controlling parasites, and early and aggressive treatment of any symptoms that
appear. There is no specific treatment
for FeLV and no known cure. A large
number of therapies have been investigated for FeLV positive cats, but most
have not shown encouraging results. A
drug that stimulates the immune system, known as interferon, can be given
orally to cats without side effects and may be helpful in many cases.
Specific cancers associated with FeLV have their own
chemotherapy treatment protocols. Some
forms of leukemia (blood cancer) are responsive to some chemotherapy and some
are not. Because the virus is not
affected by treatment, the cat will always remain infected with FeLV and
relapse is possible and should be expected.
Prevention
None of the available vaccines provide 100% protection. Almost all of them are derived from an
inactivated (killed) virus or portions of the virus. Most of these vaccines have one or two
additives called adjuvants that enhance the antigenicity (ability of the virus
to cause an immune response) of the killed virus. In general, it is recommended that cats who
go outdoors, to shows, are boarded, or otherwise have contact with cats other
than those in their household should be vaccinated against FeLV. Remember, all cats should be FeLV tested
before they are vaccinated. Cats that
are entirely indoors may not need to be vaccinated against FeLV. Vaccination is something you should discuss
with your veterinarian.
Cats already infected with the feline leukemia virus will
not be helped by the vaccine, but it does not hurt them either. Vaccination does not affect feline leukemia
test results, so a newly adopted cat with unknown vaccination status can be
tested with confidence in the results.
It is essential that the risk of exposure of a FeLV-negative
cat to a potentially FeLV-positive cat be minimized. In single cat households, this is best
accomplished by keeping the cat indoors.
When a new cat is being introduced to a house after the loss of a feline
leukemia positive cat, extensive environmental disinfection is not
necessary. The feline leukemia virus
only lives at most a few hours outside the cat.
Normal cleaning of dishes, litter boxes, and bedding should be
sufficient.
In multi-cat households, all FeLV-positive cats should be
removed or totally separated from the other cats. Movement of cats in and out of the household
or cattery should be minimized. All cats
should be retested at 90-day intervals until all cats have had two consecutive
negative tests. In cattery situations,
only FeLV-negative cats should be used for breeding animals.
We hope this article helps you better understand how feline
leukemia virus infection affects cats.
The information provided here should not substitute for a visit to your
veterinarian.
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Hartmann K, et al. “Treatment
of feline leukemia virus (FeLV) infection.” Vet Microbiol 1999 Sep
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Hofmann-Lehmann R, et al.
“Feline leukaemia provirus load during the course of experimental infection and
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