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Despite advances in
diagnosis and vaccination, this virus remains one of the
most lethal, contagious infections affecting domestic
cats.
Theresa Bachu hovered
nervously in the exam room. As an avid cat rescuer in
New York City, she’s seen many of the cats and kittens
she’s rescued test positive for feline leukemia virus.
This particular scraggly bunch – a mother and her four
kittens just taken off the street – all had upper
respiratory infections, and all were at increased risk
for having the virus. My staff and I drew blood from
all five felines, and after ten tense minutes, I had the
test results Theresa wanted to hear: all five cats
tested negative, and nowcould be placed for adoption.
Feline leukemia virus (FeLV)
was first recognized in 1964. It quickly became known as
one of the most deadly infections to affect domestic
felines. The virus causes profound suppression of the
cat’s immune system. In addition, the virus can
transform healthy lymphoid cells (cells involved in the
immune response) into malignant cancerous cells.
Although widespread testing and vaccination has markedly
decreased the rate of infection over the last 20 years,
the ability of infected cats to readily shed and
transmit FeLV through saliva ensures that this virus
will continue to plague cat owners, breeders, and
veterinarians for many years to come.
A variety of factors
increase the risk of infection (sidebar 1). Male cats
are at increased risk, as are mixed breed cats, and cats
that go outdoors. Young cats and those that reside in
multicat households are also at increased risk.
Repeated exposure to a cat that is known to be infected
with FeLV greatly increases the risk of contracting the
virus.
FeLV is transmitted in
two general ways: between unrelated adult cats
(“horizontal” transmission), and
from a mother cat to her kittens (“vertical”
transmission). “The virus is present in high amounts in
the saliva and milk of infected queens, and transmission
from an infected queen to her kittens is believed to be
the most significant source of infection”, says Dr.
Michael Stone, board certified internist at the Cumming
School for Veterinary Medicine at Tufts University.
Horizontal transmission occurs most commonly via the
oral/nasal route, by the sharing of food and water
dishes, through mutual grooming, and through sharing a
communal litterbox. “FeLV can also be transmitted
through bite wounds, explaining the increased prevalence
of FeLV in unneutered males that roam outdoors and
engage in territorial battles”, adds Dr. Stone.
Once the virus enters
the body, it replicates in lymph tissue near the site of
entry. Typically, this occurs in the oral, nasal, and
pharyngeal (throat) tissues. During this stage of viral
replication, cats may show signs of illness – fever,
lethargy, diarrhea, and lymph node enlargement. From
here, one of four general scenarios can occur:
- Progressive infection – the
immune system fails to contain the virus. Viral
replication occurs in the oral/nasal tissues, and
infected cells transport the virus to other tissues,
such as the spleen, thymus, and other lymph nodes.
The bone marrow becomes infected, as do the salivary
glands, and many virus particles are shed in the
saliva. These cats frequently succumb to a FeLV-associated
illness in a few months or years.
- Regressive infection – the cat
mounts an effective immune response. Viral
replication occurs near the site of entry of the
virus, but the infection is contained and the virus
does not invade the bone marrow. Viral particles or
viral proteins cannot be detected using the common
screening tests. However, viral DNA can be detected
via PCR testing (described below). These cats are at
little risk of developing FeLV-associated diseases.
- Abortive exposure – after
exposure, the cat mounts an effective immune
response, and the virus is presumably eliminated
from the body. Virus particles or proteins cannot be
detected by any test currently employed.
- Focal infection – a rare
scenario, in which the FeLV infection is restricted
to certain tissues, such as the spleen, a lymph
node, or a mammary gland.
There is some
controversy regarding the four outcomes of FeLV
infection described above. In the past, many cats were
believed to be able to completely clear the virus from
their body – a scenario that fits most closely with
“abortive exposure”. Newer research, however, suggests
that most cats actually remain infected for life
following exposure to the virus. These cats test
negative on the common screening tests, and virus
particles cannot be cultured from the tissues. However,
a specialized DNA test – known as PCR testing – confirms
the presence of viral DNA integrated into the cat’s
genome. These cats are unlikely to ever clear the
virus, since the viral DNA has now integrated itself
into the cat’s own DNA. These cats are unlikely to shed
virus particles in their saliva, and are unlikely to
develop FeLV-associated diseases. The clinical
significance of these cats that test negative on the
screening tests, but test positive on PCR tests, is not
yet clear.
Diagnosis of FeLV is
based on detection of a protein present on the virus
particle. This protein, called p27, is an antigen – a
substance that stimulates the immune system to produce
antibodies. Most cats that are infected with FeLV have
abundant p27 antigen in their body fluids. The most
common in-clinic tests detect p27 antigen in the
bloodstream. Most cats, if exposed to FeLV, will test
positive within 30 days using these antigen tests.
Because a positive test
has potentially dire consequences, any positive test
should be confirmed, in order to rule out the
possibility of a false-positive result. A second blood
test can be performed, ideally using a test from a
different manufacturer. Alternatively, a test can be
performed that looks for the p27 antigen within infected
blood cells, rather than circulating within the
bloodstream. This test, performed on blood smears or on
a bone marrow sample, is known as an IFA (immunofluorescence)
test. A positive IFA test not only confirms that the cat
is infected with FeLV, but that the cat’s bone marrow
has been affected – a progressive infection. Sadly, as
we noted above, these cats are likely to succumb to a
FeLV-associated illness sometime in the future.
A relatively new type
of test, called a PCR test, is perhaps the most
sensitive test for the diagnosis of FeLV. This test can
actually detect the presence of viral DNA that has been
incorporated into the cat’s own genome. A recent study
using PCR showed that 5 - 10% of cats who tested
negative for FeLV (i.e., they had no circulating p27
antigen in their bloodstream) had, in fact, a regressive
infection – they had FeLV DNA in their genome.
The FeLV status of all
cats should be known because of the serious health
consequences of infection. “Sick cats should be tested,
even if they have tested negative in the past”, says Dr.
Tina Waltke, a feline practitioner in New York City.
Cats and kittens should be tested when they are
first acquired. Cats with a known exposure to a FeLV-infected
cat, or to a cat with unknown FeLV status, should be
tested, as should cats living in households with other
cats infected with FeLV. Cats with high-risk lifestyles
(i.e. those that go outdoors or those that like to
fight) should be tested on a regular basis. If
considering vaccinating a cat against FeLV, the cat
should be tested first. “Vaccinating a cat that is
already positive for FeLV, while not harmful, offers no
benefit to the cat”, notes Dr. Waltke.
The clinical signs of
FeLV are varied and non-specific. Many cats can be
infected, yet show no clinical signs. “In cats that are
symptomatic, the most common sign I see is weight loss”,
says Dr. Waltke. The next most common signs are fever,
dehydration, upper respiratory signs, diarrhea,
conjunctivitis, oral infections, swollen lymph nodes,
and abscesses (Sidebar 2). The virus is
immunosuppressive, causing chronic inflammatory
conditions and increasing the susceptibility of
acquiring secondary and opportunistic infections.
FeLV can invade the
bone marrow, resulting in hematologic abnormalities.
Despite the name feline leukemia virus, actual
leukemia (cancer of the blood) is just one of many blood
cell abnormalities that can be caused by the virus.
Severe anemia is often seen in cats with FeLV
infection. Leukopenia (a decreased white blood cell
count) and thrombocytopenia (decreased platelet numbers)
is also commonly reported. The feline leukemia virus is
notorious for causing lymphoma – cancer of the lymph
nodes and lymphoid tissues. “Cats that test positive for
FeLV are sixty times more likely to develop
lymphoma compared to cats that test negative for the
virus”, warns Dr. Stone.
Identification and
segregation of infected cats is undoubtedly the most
effective means of preventing new infections with FeLV.
This, coupled with vaccination, is clearly responsible
for the steady decline in the prevalence of infection
seen in the past 20 years. Because close, intimate
contact – mutual grooming, sharing food bowls, water
bowls, litter boxes – is optimal for transmission of the
virus, any cat that tests positive for FeLV should be
isolated in a separate room to prevent the infected cat
from interacting with its housemates. If isolation is
impossible, uninfected cats should be vaccinated against
FeLV, bearing in mind that no vaccine is 100% effective
and that vaccination is not as effective as isolation.
FeLV-infected cats can
live for many months with proper care. A study of over
800 cats with FeLV infection revealed the median
survival to be 2.4 years. Many cats, however, may live
for several years with good quality life. No cat should
be euthanized based solely on a diagnosis of FeLV
infection. Cats infected with FeLV should be kept
strictly indoors to prevent spreading the virus to other
cats, as well as to reduce their exposure to infectious
organisms carried by animals they may encounter
outdoors. Good nutrition, proper litter box hygiene, and
a stimulating, loving environment are important in
keeping infected cats healthy. FeLV-infected cats
should have more frequent veterinary check-ups compared
to uninfected cats – at least semi-annually. Body weight
should be accurately measured and recorded at each
visit, as weight loss is often the first sign of
deterioration in a cat’s condition. Many treatment
studies have been conducted in cats that have been
infected, either naturally or experimentally, with FeLV.
Unfortunately, many of the results are difficult to
interpret, or they are not well-controlled studies in
which a drug is compared against a standard treatment or
placebo. At present, there is no treatment that has
proven to be effective in clearing a FeLV infection.
Sidebar 1: Risk factors for FeLV
infection
Male sex
Mixed breed
Not neutered
Young age (less than 7)
Outdoor access
Tendency to fight with other cats
Living in a multi-cat household
Living with cats known to be
infected with FeLV
Sidebar 2: The most common clinical
signs seen in cats with FeLV
Weight loss (64%)
Fever (42%)
Dehydration (25%)
Rhinitis (18%)
Diarrhea (17%)
Conjunctivitis (17%)
Oral problems (15%)
Enlarged lymph nodes (13%)
Abscesses (12%)
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