Feline Leukemia Virus Vaccination in Cats

Feline leukemia virus (FeLV) is a common agent in cats that is
usually transmitted through virus-infected saliva via cat fight
wounds, mutual grooming, or sharing food and water dishes. The
most hazardous situation for FeLV transmission is a multi-cat
household in which a large number of cats are crowded into a
limited space.

The virus causes a fatal infection through development of
malignancy or depression of immunity (inability to fight
infection) with the development of secondary diseases. The virus
lives only a few days in the environment and is easily killed by
common detergent and disinfectants. Thus, most of the risk of
transmission is due to direct contact between cats.

Most cats with FeLV infection die within 2 or 3 years of the
time of infection. The most common problem is depression of the
immune system, which makes the cat susceptible to a variety of
secondary infections. Cats may have persistent and recurring
abscesses, chronic mouth infections, chronic respiratory
diseases, diarrhea, and poor appetite. The virus can also
suppress the cells of the bone marrow that produce red and white
blood cells. Red blood cell suppression produces severe anemia.
Suppression of the white blood cells needed for prevention of
bacterial invasion allows the development of uncontrollable
infections.

Development of malignancies is also a major concern in
FeLV-infected cats. FeLV was originally identified in 1964 in
Scotland in catteries where there was an epidemic of
malignancies. It was later found that months to years after
infection, many cats develop malignancies of the cells of the
lymph nodes and the bone marrow. When these malignant cells are
found in the blood, the malignancy is called a leukemia. These
malignancies can also be found in many organs including the bone
marrow, chest, kidneys, liver, and intestinal tract (called
lymphoma).

There is no treatment to terminate FeLV; only supportive care
and treatment of the secondary bacterial infections with
antibiotics are possible. Some of the malignancies induced by
FeLV can be controlled with chemotherapeutic drugs for a few
months to a year of longer.

Fortunately, there are vaccines that are quite effective in
preventing FeLV infection. Young cats are more susceptible to
Infection than adult cats and should be vaccinated before they
come in contact with possible FeLV carrier cats. The current
recommendation is an initial vaccine after 9 weeks of age and a
second booster dose 3 weeks to a month later. Yearly booster
vaccines are recommended for cats at risk.

Only killed virus vaccines are available for vaccinating cats
against FeLV because of concerns that a modified live virus in a
vaccine could undergo a mutation into a potentially dangerous
virus. Killed virus vaccines of any type can cause the
development of a tumor called a fibrosarcoma. This is an
aggressive malignancy of fibrous tissue that develops in
response to inflammation at the site of vaccination.

The rate of development of malignancies at vaccine injection
sites is estimated to be 1 in 5000 doses of vaccine given. In
cats that are exposed to FeLV, outside cats, and cats that come
in contact with cats that go outside, the infection rate of FeLV
is 2 per 100. In these cats, the benefit of vaccination far
outweigh the possible problems of vaccine induced tumors. In
cats very unlikely to be exposed, the risk/benefit ratio of
vaccination is less evident. Cats that are kept strictly indoors
are not at risk for infection if other cats in the same
household are not infected with FeLV and all cats in the
household are kept indoors.

There are a number of in-office blood tests that your
veterinarian can perform to identify FeLV-infected cats. Cats in
multiple-cat households should be checked to be sure they are
not carriers of FeLV. All new cats introduced into the household
must be quarantined for 3 months and checked twice for FeLV
infection before being admitted into the household. Checking
cats in the household and preventing the entry of infected cats
are effective in developing a FeLV-free environment where
vaccination is not necessary.

FeLV infection is a lethal infection of cats that can be readily
prevented. All cats at risk should be vaccinated regularly.

The above is general veterinary information. Do not begin
any course of treatment without consulting your regular
veterinarian. All animals should be examined at least once every
12 months.