EPIDEMIOLOGY
Feline Leukemia
Virus infects domestic
cats. It is a recognized virus worldwide,
but the
incidence in cat populations varies with population density. The
overall rate of disease has decreased
within the last 20 years (2), due to the ability to better test for
FeLV,
thereby
allowing removal of infected cats from the population.
Vaccination has also helped to lower the
incidence of disease.
FeLV incidence is
most prevalent in
high-density populations of cats where the cats have direct contact
with each
other (3). Such conditions exist in
catteries and multi-cat households. As
well, cats allowed outdoors become infected more often than cats that
remain
confined indoors. This is due to the
direct contact that is required to spread the virus from cat to cat. The infection rate in free-roaming healthy
cats has been calculated to be 1 – 8% (3).
These healthy infected cats are viremic and shedding the
virus,
but then clear it from their system
quickly.
The infection rate in free-roaming sick cats
can reach 21% (4). Sick cats are more “at
risk” of becoming infected because their immune system is slightly
depressed.
Infection between male
and female
outdoor cats is fairly even. Males may
have a slightly higher incidence only because they tend to roam more
and
therefore may come into contact with more cats (2).
But transmission of FeLV is not primarily based on cat scratches
(from
territorial males getting into cat fights).
Although FeLV can infect cats through scratches, this is much
more rare
than “social” activities, such as sniffing or licking one another (3).
There are no breed
predispositions for
infection. The virus attacks all cats
alike. There is a notable trend in which
purebreds show
FeLV less often, not because they are more immune to infection, but
rather
purebreds are usually kept indoors (3). Likewise, due
to the general public becoming more aware of the disease, breeders are
testing
their cats more frequently for FeLV to keep their cats free from
infection.
The age of the cat when infected has a
huge influence
on successful infection. Kittens are
more susceptible than adult cats.
Young
kittens less than 16 weeks are more susceptible than older
kittens. There is an age-related resistance to
the virus, but it is NOT absolute (4). For
example, adults cats have an increased chance of infection with
continual
exposure to a FeLV cat shedding the
virus than if they only
contact the
FeLV cat once. The age-resistance is based
on cellular receptor number. Young
kittens have an increased number of cellular receptors on their cells
that the
virus attaches to for entry into the cell compared to older cats
(3). Furthermore, older cats have more mature macrophages,
which are more functional.
Studies have shown that the
prevalence of
FeLV
antibodies in cats increases with time, indicating that cats receive
continual
exposure to the virus over their lifetime, but are able to mount a
proper
immune response to clear it from their
systems (4). Likewise,
susceptibility to persistent viremia
decreases
with age,
because an older cat should have more FeLV antibodies
and is better
able to
resist the development of persistent viremia.
Persistent viremia is most common in cats less than 2 years of
age (3).
TRANSMISSION
There are several ways for FeLV to
spread among the cat population:
Horizontal
Transmission: Viral spread
between cats is called horizontal transmission.
A susceptible cat needs to come into
direct contact with an
infected cat
that is actively shedding the
virus.
Saliva and nasal secretions contain millions of virus particles,
which
are able to infect susceptible cats.
“Social cats” have an increased susceptibility compared to
isolated
cats. Social cats partake in activities
such as nose-nose contact and mutual grooming.
They share litter boxes and food bowls.
Cat bites are of less importance for transmission (3).
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Fecal, urinary, fomite,
and aerosol transmission are less likely because the virus cannot
survive in
the environment for a prolonged period of time. It
quickly becomes
inactivated (4).
The direct contact
requirement for transmission is why animal shelters do not see
infection
spread. As long as the cats are housed
individually and cages are routinely disinfected, horizontal
transmission is
avoided.
IATROGENIC
Transmission - A type of horizontal
transmission between cats that is induced by the veterinarian.
This can occur through contaminated needles,
contaminated instruments, and blood transfusions (2).
INDIRECT
Transmission - It requires that
another species become infected with FeLV and then pass it on to a
susceptible
cat (6). This cannot happen with FeLV (3). (Eg. a human cannot become infected with the
virus and spread it to susceptible cats.)
Vertical
Transmission: This occurs when a
queen passes the infection to her kittens.
There are four possible methods (3):
a)
Transplacental – the virus infects the kittens in utero by
crossing
the placenta. Reproductive failure
results from early embryonic death and resorption of the
embryo, or from
abortion. 20%
of kittens will
survive to birth, but they become persistently infected kittens. These kittens are born infected and will
never be free from infection. They will
eventually die. Some kittens may not
show a positive FeLV test immediately after birth because it may take
the virus
some time to reproduce itself. These
kittens will show a positive FeLV test at a later date.
b) Lactation – the virus
enters the queens milk which the kittens drink.
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c) Grooming – the queen can
infect her kittens by licking them.
d)
Latently infected queen – a queen that is FeLV negative according
to the
FeLV test does not have the virus in her, but can have the provirus in
her
DNA. Stress during the pregnancy causes
the virus
to reactivate, causing the queen to have circulating virus in her body. Now she can infect her kittens
transplacentally.
