leukaemia virus (FeLV) is a common and important cause of
illness and death in pet cats. Cats that become persistently
(permanently) infected with this virus are at risk of developing
many severe illnesses such as anaemia and cancer. Between
80 and 90% of infected cats die within three and a half years
of being diagnosed as having FeLV.
most common effect of infection is immunosuppression. The
virus infects cells of the immune system (the white blood
cells) killing or damaging them. This leaves the cat vulnerable
to a wide variety of other diseases and infections (secondary
infections). FeLV is a member of the same virus family as
feline immunodeficiency virus (FIV).
is at risk?
is a fragile virus which is not able to survive for long in
the environment so spread of infection between cats relies
on prolonged and close contact. For this reason infection
is most common in situations where there is a high population
density of cats. It is estimated that 1-2% of cats in the
UK are infected with FeLV. This figure tends to be higher
in more densely populated city cats and lower in rural cat
populations. In multi-cat households and catteries where FeLV
infection is endemic (constantly present in the household),
up to 30% of the cats may be infected.
cats, particularly those less than six months old, are especially
vulnerable to becoming persistently infected.
is it spread?
major source of virus is in saliva from a persistently infected
cat. Virus is spread by activities where saliva is exchanged
between cats, such as mutual grooming or sharing of food bowls.
Alternatively, FeLV infection of other cats may be caused
by biting or contact with urine and faeces containing the
virus. It is also possible for virus to be passed from a queen
to her kittens either in the womb or after the kitten is born,
via infected milk. However, it is uncommon for FeLV-infected
cats to give birth as FeLV usually causes pre-natal death
of the kittens which results in resorption or abortion.
all cats which are exposed to FeLV become persistently infected.
Either they have not been exposed to enough virus or their
body's immune system is successful in eliminating the infection.
The majority of cats become infected with the virus entering
the body via the mouth or nose. The virus multiplies at these
sites before spreading, in the bloodstream, to the rest of
the body and in particular to the bone marrow. If the cat
is able to eliminate the virus, this will occur during the
initial stages (4 - 12 weeks) of infection. Once significant
infection of the bone marrow is present, the cat remains infected
for the rest of its life.
rarely FeLV infection may be limited to certain parts of the
body such as the mammary (breast) tissue. This is known as
a 'localised infection'.
variety of chronic and/or recurrent disease develops in cats
persistently infected with FeLV. There is a progressive deterioration
in their condition over time. Clinical signs are extremely
diverse but include fever, lethargy, poor appetite and weight
loss. Respiratory, skin and intestinal signs are also common.
Cats may suffer from several illnesses at the same time. Anaemia
occurs in about a quarter of infected cats. FeLV can infect
the red blood cells in the bone marrow causing a reduction
in numbers of these cells or production of abnormal red blood
cells which do not work properly. In other cases, FeLV may
cause destruction of red blood cells by the cat's own immune
system. Anaemic cats show clinical signs such as weakness
develops in around 15% of cats infected with FeLV. The most
common is lymphoma, a cancer of lymphocytes (a type of white
blood cell) resulting in solid tumours or leukaemia (tumour
cells in the blood stream). Solid tumours can be seen at various
sites including the intestine, kidneys, eyes or nasal chambers.
In multicentric lymphoma, the tumour involves multiple lymph
nodes and other sites.
of FeLV infection
is currently no treatment that is able to eliminate an FeLV
infection. Treatment must therefore be aimed at maintaining
quality of life and managing the effects of infection such
as immunosuppression, anaemia and cancer.
and effective supportive care and management of secondary
infections is essential in the ill FeLV-positive cat. Because
of the failing immune system, much longer courses of antibiotics
are needed. The response to therapy is usually much slower
and less likely to be successful. Relief
of symptoms may be provided by non-specific therapies such
as corticosteroids, anabolic steroids and multivitamins (which
encourage appetite). Antiviral agents, such as AZT (azidothymidine),
which have been used in people with HIV, do not appear to
be beneficial in the FeLV-infected cat. A further treatment
called recombinant feline interferon has recently been licensed,
and there have been anecdotal reports of its use in cases
of feline leukaemia. At this point, the value of the drug
remains somewhat controversial until further scientific evidence
becomes available. Some cats with lymphoma may show temporary
improvements when treated with cancer chemotherapy drugs.
This usually involves a number of drugs given by mouth and
with FeLV infection should not eat raw meat because of the
increased risk of Toxoplasma gondii infection. This
parasite is usually only a problem in immunosuppressed cats
where it can cause uveitis (inflammation of the internal structures
of the eyes), neurological signs such as seizures (fits) and
ataxia (drunken gait).
particularly for cat flu and infectious enteritis, is recommended
for any cats staying in a high risk situation such as a veterinary
hospital or cattery. Flea treatment is recommended to minimise
the risk of Mycoplasma haemofelis (a blood parasite
which can cause anaemia) transmission. Routine worm treatment
is also recommended.
vaccines are available for FeLV. The aim of these is to prevent
cats exposed to the virus from becoming persistently infected.
All of the vaccines aim to do this by stimulating a successful
immune response to FeLV. Unfortunately, no vaccine is likely
to be 100% effective at protecting against infection. Vaccination
is recommended in situations where cats have a high risk of
exposure to the virus. This includes free-roaming cats and
those in contact with potentially infected individuals.
is unwise to assume that a vaccinated cat is necessarily free
of infection, and where it is important to know the FeLV status
of a cat (for example, introducing a new cat to a breeding
colony) it is vital that a vaccination certificate is not
accepted in place of a negative FeLV test. Vaccination of
cats does not interfere with the FeLV blood tests.
lack of a totally effective vaccine means that it is also
inadvisable knowingly to mix an FeLV-infected cat with a vaccinated
the virus is highly infectious and readily transmitted by
prolonged close contact, other cats in the household are at
risk of becoming infected via mutual grooming and sharing
of food bowls. Uninfected cats should be kept away from the
persistently infected cat where possible. It is also recommended
that FeLV-positive cats are kept indoors to minimise spread
to other cats in the area. This may be difficult for some
cats who will not tolerate being kept inside permanently.
It is important to carefully weigh up the risks (to your own
cat and others) with the welfare implications. It is feasible
to fence in the garden or to construct a run where cats can
go out, thereby preventing any risk to your own or other cats.
the breeding cattery, the 'test and removal' system has been
extremely successful at eliminating FeLV infection. This system
relies on FeLV testing of all the cats with separation of
any that test positive. After a period of 12 - 16 weeks, all
of the cats are tested again as some cats initially testing
negative may have been incubating the disease and some of
those testing positive may have been transiently infected
with subsequent elimination of the virus.
cats repeatedly testing positive should be removed from the
unit, while only those with two consecutive negative tests
are kept. All new additions to the colony, whether vaccinated
or not, should be tested for FeLV before joining the cattery.
All cats in a breeding colony should be tested every 6 - 12
months in order to maintain the negative status of the colony.
an aside, single cats that are confined indoors should not
be at any risk of exposure to FeLV. It is however possible
for an adult cat to succumb to disease from FeLV infection
despite having spent their whole life since kittenhood isolated
from other cats. This can be explained by the often protracted
course of FeLV infection; if a kitten was infected by his/her
dam prior to rehoming, the signs of FeLV infection may develop
months to years later after the time of initial infection
with FeLV. The risk of spread of FeLV at cat shows is
for FeLV infection
use test kits to detect one of the viral proteins (p27) present
in the bloodstream of FeLV-infected cats. Often the kits simultaneously
test for FIV, as many of the clinical signs of FIV infection
are similar to FeLV infection. Occasional false positive and
negative results occur, so it is vital that a confirmatory
test is performed if an unexpected result is obtained. This
is especially important if a positive result is obtained in
a healthy cat and conversely if a negative result is obtained
in a sick cat with signs compatible with FeLV infection.
isolation - detects the virus within the blood plasma
- this tests for viral antigens (proteins) in white blood
(polymerase chain reaction) - this test detects the genetic
material of the virus, however it can only be performed
by selected laboratories
the results of a confirmatory test are pending the cat should
be isolated to avoid any possible risk of transmission to
following test protocol is generally followed by vets:
- If the cat tests antigen negative
on an in-practice test, there is a high probability that
it is genuinely negative
- If a healthy cat test is antigen
positive on an in-practice test, the cat is retested using
a confirmatory test
- If the cat tests antigen positive
but is negative on a confirmatory test, the cat is retested
after 12 weeks to confirm the status
cats in the rescue situation
all cats entering a rescue centre should be tested for FeLV
infection, however this may not be feasible due to the cost
implications involved. It is advisable to enquire whether
a cat has been tested prior to agreeing to rehome it. A cat
that has tested positive to FeLV should only be rehomed when
all of the risks have been explained to the new owner.
for infected cats
prognosis for a sick FeLV positive cat is poor; the associated
disease problems are usually serious. For cats that have been
identified to be FeLV positive but are healthy at the time
of diagnosis, the prognosis is guarded. Most of these cats
subsequently develop a fatal FeLV-associated problem, however
the time course for this to occur can be variable (months to years).
It is vital that these cats are isolated to prevent further
transmission of FeLV infection to other cats and secondly
to reduce the risk of the FeLV positive cat contracting infections;
if that cannot be guaranteed, euthanasia should be considered.