Heartworms Once considered a parasite of southern
climates, the heartworm (Dirofilaria immitis) is now recognized as a major,
global pest affecting
dogs, wolves, coyotes, and
foxes. From its discovery in dogs more than a century ago and the documentation
in cats in the 1920s,
researchers have devised diagnostic tests, preventives and treatments,
but the disease still spreads.
To jog the concern of clients,
veterinary clinics display a preserved heart infected with heartworm in
a jar and hang posters about
heartworm life cycles in examining rooms,
but seeing is not necessarily believing; although clients can view the
infested heart loaded with
long, spaghetti-like
worms every time they visit, many gamble that their dogs will never be
bitten by an infected mosquito.
Heartworm prevention is simple. It involves
a blood draw to determine whether the parasite is present and regular dosing
with preventive
medication. Heartworm infestation is dangerous;
untreated dogs die and treated dogs go through weeks of discomfort while
the worms
are killed and expelled from their bodies.
Parasites go through
several life stages before emergence as adults and often need at least
two hosts to complete the cycle. In
heartworms, a mosquito serves as the intermediate
host for the larval stage of the worm, also known as the microfilariae.
The mosquito
ingests the larva when
it bites an infected dog and deposits its cargo in an uninfected dog when
seeking another blood meal. The
microfilariae burrow into the dog and undergo
several changes to reach adult form. They then travel to the right side
of the heart through
a vein and await the opportunity
to reproduce. Adult heartworms can reach 12 inches in length and can remain
in the dog? heart for
several years.
The time lag between the initial
infestation of microfilariae and reproduction by adult worms living in
the heart is six-seven months in
dogs.
Female heartworms bear live young ? thousands
of them in a day. These young ? the microfilariae ? circulate in the bloodstream
for as
long as three years, waiting to hitch a
ride in a bloodsucking mosquito. They undergo changes in the mosquito that
prepare them to infect
a dog, and they transfer back to the original
host species the next time the mosquito bites. The process of change in
the mosquito takes
about 10 days in warm climates, but can take six weeks in colder temperatures.
The worms grow and multiply, infesting
the chambers on the right side of the heart and the arteries in the lungs.
They can also lodge in
the veins of the liver and the veins entering
the heart. The first sign of heartworm infestation may not manifest for
a year after infection,
and even then the soft cough that increases
with exercise may be dismissed as unimportant by the owner. But the cough
worsens and the
dog may actually faint from exertion; he
tires easily, is weak and listless, loses weight and condition, and may
cough up blood. Breathing
becomes more difficult as the disease progresses.
The progression is traumatic: the dog's quality of life diminishes drastically
and he can
no longer retrieve a
Frisbee or take a long walk in the park without respiratory distress. Congestive
heart failure ensues, and the
once-active, outgoing pet is in grave danger.
Heartworm disease is present on every continent except Antarctica.(1) It
occurs where these four factors are found:
a susceptible host population
a stable reservoir of the disease
a stable population of vector species
a climate that supports the parasite's life cycle
Dogs are considered the definitive host
(2) for the parasite; even though the disease is not transmitted directly
from one dog to another,
untreated dogs provide a stable reservoir for
the disease. Mosquitoes of several different species are the vectors (intermediate
host for the
developing microfilariae). Development
of the microfilariae in the mosquito requires a temperature at or above
80 degrees Fahrenheit for
about two weeks. No larval development takes place in the mosquito below
57 degrees F.
Heartworms can be detected by blood test.
The filtration test finds microfilariae in the blood; the occult test locates
adult worms in the
heart. Many veterinarians prefer to do
both tests as the absence of microfilariae in the blood does not necessarily
mean that there are no
adult worms in the heart. Both tests are
done with a single blood draw, preferably in the early spring before temperatures
warm above 57
degrees F.
Radiographs (X-rays) can also detect the presence of adult heartworms in
the heart and lungs.
If a blood test or the onset of symptoms
alert owner and veterinarian to the presence of this devastating parasite,
treatment is possible
and successful if the disease has not
progressed too far. The first step is to evaluate the dog and treat any
secondary problems of heart
failure or liver or kidney insufficiency
so that he can withstand the treatment. The next step is to kill the adult
worms with an arsenic
compound. Veterinarians now have access
to a Immiticide(3), a new compound that has fewer side effects than the
previous drug and is
safer for dogs with more severe infestations.
The treatment is administered in
two doses each day for two days, followed by several weeks of inactivity
to give the dog's system a
chance to absorb the dead worms. Exertion can cause the dead worms to dislodge,
travel to the lungs, and cause death.
At least three-to-four weeks
after the administration of the drug to kill the adult worms, further treatment
to kill the microfilariae is
needed. The dog is dosed daily for a week,
then the blood test is repeated. If microfilariae are still present, the
dose can be increased.
Follow-up studies should be done in a year.
Surgical removal of the adult heartworms is possible and may be indicated
in advanced cases with heart involvement.
Preventive doses come
in oral and topical versions and are only available from a veterinarian.
Diethylcarbamazine is given daily.
Ivermectin (Heartguard(3), Milbmycin (Interceptor(4))
and Moxidectin (ProHeart(5)) are given monthly. Selamectin (Revolution(6))
is a
new preventive applied topically. Some
of these drugs also kill other parasitic worms, and Revolution also acts
against fleas, ticks, and
mites.
Many veterinarians recommend and
many owners use a year-round heartworm prevention program to guard against
the occasional
mosquito flying about in southern Ohio's
relatively mild winters. If Fido has already had his yearly check-up, call
your veterinarian to
schedule a heartworm check. If he's due for yearly vaccination, be sure
to include a heartworm check in the visit.
What we use:
We use Ivomec(Ivermectin) 1% Orally @ .1cc per every 10lbs, every month,
year round.
