Is leukemia still #1 disease threat to cats?

From ANIMAL PEOPLE, December 1993:

Shelter          Cats Screened             Cats FeLV+              %
Americ an SPCA (NY) 20,000+ 400/500 >2.5%
Anti-Cruel ty Society (Chicago, IL) 3,600 “few”
Bennington County Humane Society (VT) (“we want adoptors to take cats to a vet)
Bloomington Animal Control (MN) (“costs too much”)
Knoxville County Humane Society (TN) 30 0 0
Mari n Humane Society (CA) 1,500 30/40 >2.5%
North Shore Animal League (NY) 20,000 400/500 >2.5%
Progressive Animal Welfare Society (WA) 1,125/1,500 25/30 >2.0%

The 30-year-old feline leukemia pandemic may be
ending, at least in the U.S., suggests an ANIMAL PEOPLE
fax poll of animal shelters. Though the sample base was lim-
ited and the results perhaps ambiguous, the convergence of
numbers from shelters across the country suggests the actual
current risk of this reputed riskiest of all cat diseases may be
quite low. Collectively, the six responding shelters that do
feline leukemia screening reported finding fewer than 1,000
cases in 45,000 cats screened.
First identified by Scottish researcher W.F.J. Jarrett
in 1964, feline leukemia has appeared in cats around the
world, and is believed to cause more deaths than any other
cat illness. In 1991, IDEXX Corporation, of Portland,
Oregon, found that among 27,000 sick cats at 1,500 veteri-
nary clinics, 13.3% had feline leukemia––twice as many as
had the next most common terminal ailment, feline immune
deficiency syndrome. Clinical studies indicate that of the cats
who get feline leukemia, which is contagious, 40-50% die
within a year; 30-40% die of specific related cancers within
three years, and meanwhile carry the disease; and 30-40%
develop an immunity, recovering to lead a normal life.
Genetic susceptibility appears to determine the outcome of
exposure, while exposure, at least in concentrated popula-
tions, may be inevitable, since the feline leukemia virus can
be transmitted with just a sneeze.
Cat experts have accordingly theorized that feline
leukemia will ravage the cats of any given area for as long as
it takes for the genetically susceptible cats to die out, while
the resistant cats become dominant. But conventional wis-
dom has held that this would be a long, slow process, owing
to the prolific and rapid reproduction of cats, coupled with
their high degree of mobility. The ANIMAL PEOPLE fax
poll indicates that the growing popularity of neutering and
immunization may be speeding the process up.
What we actually set out to learn was whether feline
leukemia testing might be an economical means of easing the
stress of triage for euthanasia at animal shelters. Many shel-
ters euthanize more than 80% of the cats they receive due to
lack of adoptive homes. If 70-80% of all cats are genetically
susceptible to feline leukemia, and if shelter cats are likely to
have already been exposed, we reasoned, it would make
sense to select those cats for euthanasia first, thereby thin-
ning the reservoir for the disease as well as euthanizing
“doomed” cats before those likely to remain healthy.
To test the theory, in early November we surveyed
a loosely representative cohort of a dozen shelters to find out
what their feline leukemia screening experience is. Of the
eight that responded by deadline, five screen all cats for
feline leukemia before putting them up for adoption––but
none are finding many cases. The four who provided hard
figures agreed they are finding fewer than 2.5 victims per
The survey results are not necessarily incompatible
with the IDEXX data, since IDEXX investigated only sick
cats. It is also possible that a highly contagious and usually
fatal disease can persist for decades while afflicting only
2.5% of a host population. Indeed, epidemiological evidence
suggests that many fatal diseases mutate to avoid causing
severe reduction of host animal numbers, which could
deprive them of their habitat. In addition, four of the five
shelters that do routine feline leukemia screening reserve it
for cats who are either up for adoption or already have been
adopted. The exception, the North Shore Animal League,
tests all cats received but receives many cats from other shel-
ters, which may already have euthanized any with feline
leukemia symptoms. It is thus possible that the actual inci-
dence of feline leukemia may be twice what the shelters are
finding––or more.
Immunization vs. risk
But immunizing 70% of a population at risk is usu-
ally sufficient to stop the spread of a contagious disease. The
combination of neutering and vaccination may already have
cut the number of potential hosts and carriers among owned
cats by much more than 70%. Surveys of pet owners by the
Massachusetts SPCA and the Tufts Center for Animals and
Public Policy have separately confirmed that at least in
Massachusetts, eight out of 10 owned cats are neutered. A
neutered cat may either contract or spread feline leukemia,
but cannot pass along either genetic risk or immunity.
The rate of immunization against feline leukemia is
completely undocumented. However, the first effective
feline leukemia vaccine, Leukocell, came on the market in
1985, and the more generally effective Leukocell II was
introduced in 1990. Both have now been in use long enough
to have protected a significant percentage of several genera-
tions of pet cats, leaving the homeless cat population as the
primary feline leukemia reservoir. And among homeless
cats, feline leukemia itself may have been reducing the rate
of genetic susceptibility for decades by taking out the vulner-
able cats before they reproduce.
Undoubtedly many cat populations––in
colonies, in shelters, and in homes––are still susceptible and
at risk. Certainly there is no reason for anyone to relax vigi-
lance. At prices ranging from $1.25 per screen performed by
NSAL to $2.00 per screen performed by the Anti-Cruelty
Society of Chicago, the cost of doing routine feline leukemia
testing is substantially lower than the cost of keeping a cat in
a reputable shelter overnight, and so long as any cats who
aren’t immediately suffering are to be euthanized, euthaniz-
ing first those who have contracted a debilitating terminal
disease would seem only reasonable.
––Merritt Clifton
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