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 FELINE
LEUKAEMIA VIRUS (FeLV):
What is feline leukaemia virus?
We might think the human race has had a hard time with all the
infectious diseases which can attack us but thank goodness we're
not cats. Not only do they have their equivalent of the Human
Immunodeficiency Virus (HIV) in Feline Immunodeficiency Virus
(FIV) but they can also be infected by an even nastier immunosuppresive
virus - Feline Leukaemia Virus (FeLV).
FeLV was first isolated over thirty years ago from a cluster
of cats which were suffering from cancers of the white blood
cells, hence its name. As time progressed, it became apparent
that a more common consequence of infection is a gradual suppression
of the immune system. Most cats have died within two or three
years of contracting the infection.
The virus belongs to a fascinating group of microorganisms
known as retroviruses. These have the ability to integrate into
the host's genetic material, DNA. The viruses have a central
core containing a substance known as RNA. The virus carries
a unique enzyme which creates a copy of DNA from the RNA. When
the virus infects a cell, its DNA copy inserts into the cat's
genetic material. The virus then makes use of the cell's functions
to multiply. The virus core is surrounded by an envelope which
is very fragile. This means the virus is unable to survive for
long outside the cat's body so spread of FeLV relies on close
contact between individuals.
What disease does the virus cause?
The virus, as its name suggests, can cause leukaemia ( a cancer
of the white blood cells) and other cancers such as lymphoma
but a more common consequence of infection is a suppression
of the immune system. Immunosuppression is thought to account
for around half the deaths from FeLV. The affected cat is unable
to fight disease properly and is susceptible to a wide range
of infections that would not normally cause a problem. Cats
may be affected by gum inflammation, skin infections, bladder
infections and diarrhoea. Suspicions that all is not well with
the immune system are raised if a cat has recurring infections
or doesn’t respond as well as expected to treatment. Another
common occurrence in FeLV-infected cats is the development of
a life-threatening anaemia. Sadly most cats will succumb to
the infection within three and a half years of the diagnosis.
If a very young cat is affected, the virus can cause its demise
in a matter of months. Very young kittens may fade and die within
weeks of birth.
How is the virus transmitted?
The virus is fragile and can't survive longer than a few hours
outside the body so transmission between cats usually relies
on direct physical contact. FeLV infected cats shed large quantities
of the virus in body secretions- mainly the saliva but also
urine and faeces. A one-off encounter is highly unlikely to
cause a problem. It generally takes a prolonged period of close
contact between cats, involving activities such as mutual grooming
and sharing of litter trays and food bowls, for sufficient exposure
to the virus to allow transmission to a susceptible cat. Very
young kittens or cats are most susceptible to the infection.
Another potential source of infection is when a queen who is
infected with FeLV is mated and gives birth to a litter of kittens.
In this situation all the kittens will be born infected with
the virus.
What happens when a cat is exposed to FeLV?
Not every exposure to the virus results in disease. A battle
develops between the virus and the cat’s immune system. In most
cases, the cat wins and is successful in eliminating the virus
from the body. In other cases, the virus wins and the cat becomes
persistently infected. It is these cats that develop FeLV-related
disease. The permanently infected cats are responsible for the
transmission of FeLV to other cats. Age is a significant factor
in determining whether or not a cat will become persistently
infected. Cats are quite susceptible to infection up to 4 to
6 months of age. Beyond that, there is a degree of natural immunity
and it can be quite difficult to contract the infection. The
amount of virus a cat is exposed to can also influence the outcome.
In the overall population it's estimated that around 1 to 2
per cent of cats will be persistently infected. If the virus
gets into a multicat household, the proportion is increased
to thirty per cent.
How can FeLV infection be diagnosed?
In the early stages of infection, there is no physical evidence
of ill health. The only way of confirming the presence of FeLV
is by blood testing, a procedure which can be carried out very
quickly and simply at your veterinary practice. In general this
blood test is very reliable, particularly if the cat is unwell.
In some situations we may consider it necessary to send a sample
to a specialist laboratory, where more sophisticated tests are
available. Some cats which have just been exposed to the virus
may test positive but if they are given a little more time will
overcome the infection. We may need to perform a second test
around 12 weeks after the first to confirm persistent infection.
Is there any treatment for FeLV infection?
Many FeLV infected cats will respond to treatment for their
secondary infections but there is currently no specific treatment
for the FeLV itself. What makes FeLV such an unpleasant disease
is that once a cat has become persistently infected, there is
absolutely nothing we can do about it. The problem is the integration
into the host DNA. We can't eliminate the virus because we don't
have any drugs which can specifically zap the viral DNA without
damaging the cat's DNA. There have been some reports of partial
success with certain conventional drugs or with complementary
therapies but to date we have nothing that provides solid, reliable
treatment.
There is obviously massive research into ant-HIV therapy and
because of the similarity in the viruses it is likely that if
an effective HIV drug becomes available it will also work against
FeLV. There have been some reports of the benefits of interferons
in the treatment of FeLV infected cats.
Any attempt to treat a cat should always be carefully considered.
If the cat goes outdoors it is a potential threat to other cats
and is always at risk from secondary infections. If a decision
is made to keep the cat indoors, the change in lifestyle may
be excessively stressful for some individuals.
How can infection be prevented?
Vaccines are now available to protect cats against FeLV infection
and their use is highly recommended in any cat that goes outside
and has contact with potentially infected cats. As with other
vaccines, an initial course of two injections is required, and
annual boosters are necessary to maintain immunity. Although
vaccination is very helpful in controlling FeLV and preventing
infection, it is not 100% reliable (like most vaccines) and
therefore it is important not to deliberately expose a vaccinated
cat to FeLV. A cat which doesn’t meet others has virtually no
chance of picking up the virus. It’s still sensible to consider
the possibility of inadvertent exposure if you or a friend have
recently handled a potentially infected cat and then handle
the isolated cat. If your cat is boarded at a cattery, ensure
they have no direct contact with any other boarders and food
bowls and litter trays are disinfected between cats.
Should I have my cat blood tested before it has the vaccination?
If a cat already has the virus, the vaccine will be ineffective.
It won’t cause any harm but it won’t protect. If a kitten has
been acquired from a breeder, its parents are likely to have
been tested before they were mated so you can be confident it
hasn’t encountered the virus. Rescue cats and kittens tend to
have unknown backgrounds. On examination it’s impossible to
know if they have already encountered the virus. Statistically,
if a cat is healthy on examination, there is a low chance of
it already carrying the virus so vaccination can be performed
quite confidently. If the cat has a health problem, it increase
the chances of being positive. If there is any doubt about the
status of a cat before vaccination it's advisable to carry out
blood tests.
Is the vaccine safe?
In the United States a small number of cats have developed skin
growths at the typical site of injection. It’s been suggested
that leukaemia vaccination is the cause but this hasn't yet
been confirmed. We are monitoring the possible emergence of
similar cases in this country. Although this is naturally a
concern for the cat owner, the condition is rare.
The benefits of vaccination far outweigh any adverse effects.
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