Heartworm
Heartworm disease:
Is your
dog really at risk?
 For
many responsible dog guardians, the onset
of Spring means a visit to the veterinarian
for a check-up and an annual prescription
of heartworm medicine. Most people never
question the necessity of these preventatives.
One look at a photo of a dog’s heart
infested with adult heartworms is enough
to scare anyone into using these drugs.
But are are all dogs at the same risk
for this disease?
The simple answer
is no, says Dr. Konrad Kruesi of the Cold
River Veterinary Center in New England.
“We have clients all over the map
– in four provinces, 25 states and
six different countries – and I
can’t make a blanket statement about
heartworm. It’s important to do
a risk assessment on the dog to determine
his lifestyle.”
Dr. Kruesi’s
comments echo a growing number of veterinarians
who are counselling their clients to help
them make informed decisions about their
dog’s health, including the risk
of heartworm disease.
Since the infection
is passed on by mosquitoes, one of the
biggest risks is exposure. “If you
have 12 beagles whose job it is to chase
rabbits and they’re outdoors all
the time, they’re more at risk than
the family companion who’s basically
a couch potato,” says Kruesi.
Dr. Ian Buffet, a
Toronto, Ontario-based veterinarian agrees.
“Here in Toronto, especially if
it’s a dog that’s just walking
around the block, the risk is really,
really small,” says Buffet. “In
the six years I’ve been practising
in Toronto, I haven’t seen any dogs
with clinical heartworm symptoms. But
when I worked in the Simcoe, Ontario area
(west of Toronto), we might see 30 cases
a year, mostly outdoor farm dogs. Once
you get into a pocket, it feeds itself,
so it’s important to ask your vet
how much heartworm there is in your area.”
Heartworm is a cyclical
disease. Adult female heartworms that
live inside an infected dog release microfilariae,
or pre-larvae heartworms into the bloodstream.
When a mosquito partakes of a blood meal
from an infected dog, it ingests the microfilariae.
After two weeks maturing inside the mosquito,
the parasite is sufficiently developed
to grow to its next stage when its mosquito
host bites another dog and infects it
with the matured microfilaria. In about
four or five months, the larvae show up
in the heart or pulmonary arteries and,
two months later, the adult heartworm
is ready to produce microfilariae of its
own (unless the heartworms are all of
the same sex, in which case no microfilariae
are produced). Thus, the cycle begins
again.
According to studies
published by the Ontario Veterinary College
at the University of Guelph, the majority
of dogs who become infected live in rural,
outdoor settings. They also tend to be
middle-aged, male and large breed dogs.
Kruesi adds that most of the cases he
sees are typically dogs from shelters,
who are also infested with fleas, and
those from canine rescues, who have a
long history of being neglected.
Most cases of heartworm
infection are actually asymptomatic, meaning
the dog has no clinical signs of the disease.
These cases typically turn up after annual
bloodwork to test for the presence of
the heartworm antigen. Treatment at this
point can be relatively straightforward.
“If you catch it early, you can
deal with it before it becomes a significant
problem,” says Buffet. For Buffet,
“dealing with it” means putting
the dog on a heartworm preventative with
ivermectin to kill the microfilariae and
shorten the lifespan of the adult worms.
Letting the worms die off gradually allows
the dog’s internal systems to better
cope with getting rid of the parasite.
Killing off the microfilariae at this
point will keep any of the parasites from
growing into adult worms and will stop
the dog from becoming a source for spreading
heartworm in the area. Buffet prefers
this method to using the more serious
adulticide medication, which may bring
complications.
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Published
in the Summer 2002 issue of Animal Wellness
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