Feline leukemia virus (FeLV) is a
that infects cats. As
a retrovirus, the genetic information of FeLV is carried by RNA instead of DNA.
FeLV is usually transmitted between infected cats when the transfer of saliva or
nasal secretions is involved, for example when sharing a feeding dish. If not
defeated by the animal’s immune system, the virus can be lethal. There is currently no known cure for
Cats infected with
FeLV serve as sources of infection. The virus is released from the body in
saliva and nasal secretions. It is also transmitted, though not as often, in
urine, feces, and milk from infected cats. Cat-to-cat transfer of the virus may
occur from a bite wound, during mutual grooming, and sometimes even through the shared use of litter boxes and feeding dishes.
Transmission can also take place from an infected mother cat to her kittens,
either before they are born or while they are nursing. FeLV doesn't survive long
outside a cat's body, normally less than a few hours under normal household
FeLV causes immunosuppression in pet cats, and there is also evidence for
existence of the virus in larger wild cat populations also (e.g. Lynx, Cheetah,
and Lion). Overwhelming epidemiologic evidence suggests FeLV is not
transmissible to either humans or dogs. This statement is based on the fact that
approximately one pet dog in five lives with a cat, and all pet cats live with
humans (some 60 million pet cats in the USA). Not
one verifiable report exists in the medical/veterinary literature for FeLV
infection in either a puppy or adult dog, and there is no report of an FeLV
infection in a human child or of an immunocompromised adult pet owner.
Approximately 0.5% of pet cats are persistently infected with FeLV, but many
more pet cats (>35%) have specific IgG antibodies which indicate prior
exposure and subsequent development of immunity instead of infection.
Transmission of FeLV is mainly via saliva and
friendly behaviours, such as sharing feeding bowls and mutual grooming (as
distinct from fighting and biting).
There is strong evidence kittens under 4 months of age are susceptible to
infection, but by eight months are resistant - hence it is a good idea to keep
young pet kittens indoors where virus exposure is minimal or non-existent until
about 8 months of age.
Four subgroups of FeLV exist: A; B; C, and T, but only subgroup A is
transmissible between cats. The other subgroups arise ‘’’de novo’’’ and as
recombination with an endogenous DNA feline sequence. Hence, there is very
good evidence this virus is quite ancient, and may well have evolved more than
one time over the last 10,000,000 years.
There are many possible outcomes as to how successfully the cat’s
immune system will react to the virus. About forty percent of cats
extinguish the virus. Sixteen percent of these fight it off due to minimal
exposure to it. The other twenty-four percent resist the virus at phase four,
which will be described later. All of this usually occurs between sixteen to
eighteen weeks after the FeLV infection begins. About twenty percent are able to
put the virus into a latent stage, in which the virus will remain until the cat
becomes stressed causing the FeLV to re-emerge. About five to ten percent of
cats go through a sequestered stage in which
viremia is limited, intermittent, or absent altogether. Approximately thirty
percent of cats go through the disease from start to finish, normally resulting
Once the virus has entered the cat, there are six phases to a FeLV infection:
Phase one is when the virus enters the cat, usually through the pharynx
where it infects the epithelial cells and infects the tonsillar
B-lymphocytes and macrophages. These white blood cells then filter down to
the lymph nodes and begin to replicate.
In phase two, the virus enters the blood stream and begins to distribute
throughout the body.
Phase three starts when the lymphoid system (produces antibodies to
attack infected and cancerous cells) becomes infected with further
distribution throughout the body.
Phase four is the main point in the infection, where the virus can take
over the body's immune system cause viremia. During this phase the
hemolymphatic system and intestines become infected.
If the cat's immune system does not fight off the virus, then it goes
onto phase five where the
marrow becomes infected. At this point, the virus will stay with the cat
for the rest of its life. In this phase, the virus replicates and is
released four to seven days later in infected neutrophils (white
blood cells), and sometimes lymphocytes, monocytes (white blood cell
formed in the bone marrow), and eosinophils (another white blood cell).
At phase six the cat's body is overwhelmed by infection and mucosal and
glandular epithelial cells (tissue that forms a thin protective layer on
exposed bodily surfaces and forms the lining of internal cavities, ducts,
and organs) become infected. The virus replicates in epithelial tissues
including salivary glands, oropharynx, stomach, esophagus, intestines,
trachea, nasopharynx, renal tubules, bladder, pancreas, alveolar ducts, and
sebaceous ducts from the muzzle.
Cats diagnosed as persistently infected by
ELISA testing may die within a few months or may remain asymptomatic for up to 4
years. The fatal diseases are leukemias,
non-regenerative Anemias. There is no known cure for the virus infection and
no known palliative pharmaceutical therapy. Vaccines for FeLV are available, but
their efficacy remains unproven.
Feline Leukemia Virus (FeLV) is an RNA
retrovirus or oncornavirus first described by W. Jarrett (et al, Nature 202:566)
at University of Glasgow, School Veterinary Medicine, in 1964. The virus is
comprised of 5' and 3' LTR's and three genes: Gag (structural), Pol (enzymes)
and Env (envelope and transmembrane); the total genome is about 9,600
The replication process of the FeLV virus is fairly straightforward. First
the virus attaches to receptors on surface of the target cell. Then the virus
penetrates the cell membrane, sheds its envelope and capsid, and releases its
single stranded RNA and reverse transcriptase. Reverse transcriptase, with the
help of the cat's cellular enzymes, makes the ssRNA into double stranded DNA.
The dsDNA circulates and integrates into the nucleus and attempts to incorporate
into the cat's chromosomes. If the viral DNA is successful in incorporating into
the cat's DNA it transcribes to mRNA. The new viral RNA, proteins, and reverse
transcriptase are synthesized, assembled, and then bud from the plasma membrane
as a new FeLV virus to infect another cell. When the dsDNA is integrated into
the nucleus it can have four possible effects: productive infection, latent
infection, insertional mutagenesis (in which the viral DNA is incorporated into a critical part of
the DNA and mutates the cell), and finally failure of integration (where viral
DNA accumulates in the cell and is toxic to it).
Comparison with FIV
Feline immunodeficiency virus are in the same family, and are sometimes
mistaken for one another. However, the viruses differ in many ways. Their shapes
are quite different: FeLV is more circular while FIV is elongated. The two
viruses are also quite different genetically, and their protein coats differ in
size and composition. Although many of the diseases caused by FeLV and FIV are
similar, the specific ways in which they are caused differs also differs.