What is Heartworm Disease?
Heartworm Life Cycle
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Heartworm Incidence Map
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Heartworm disease is a serious
and potentially fatal condition caused by parasitic worms living
in the arteries of the lungs and occasionally in the right side
of the heart of dogs, cats and other species of mammals,
including wolves, foxes, ferrets, sea lions and (in rare
instances) humans. Heartworms are classified as nematodes
(roundworms) and are filarids, one of many species of
roundworms. Dogs and cats of any age or breed are susceptible to
infection.
Filarids rely on different insect species to be transported from
animal to animal. The specific filarid causing heartworm in dogs
and cats is known as Dirofilaria immitis.
Dogs or other animals harboring adult worms are the recognized
reservoir of heartworm infection. The disease is spread by
mosquitoes that become infected with microfilariae while taking
a blood meal from an infected dog. Within the mosquito, the
microfilariae mature into the infective larval stage. When the
mosquito then bites another dog, cat, or susceptible animal, the
larvae are deposited on the skin and actively migrate into the
new host. For about 2 months the larvae migrate through the
connective tissue, under the skin, then pass into the animal's
venous blood stream and are quickly transported to the arteries
of the lung. It takes a total of approximately six months for
the infective larvae to mature into adult worms that begin
producing offspring, microfilariae. Adult heartworms can live
for five to seven years in the dog.
In the dog, the larvae progress in their development to an adult
form of the worm, and live in the pulmonary vessels, where they
continue the life cycle and cause extensive injury. The period
of time when heartworms are reproductively capable is referred
to as patency. In cats, it takes seven to eight months before
adult worms potentially reach patency in the pulmonary vessels,
and this is referred to as transient patency, as reproductive
capability in the cat is usually very short (months) compared to
that of dogs (years). In most cases the cat is not an effective
reservoir host, since microfilaria are produced in less than 20%
of the cats.
In the cat, the larvae molt as well, but fewer worms survive to
adulthood. While dogs may suffer from severe heart and lung
damage from heartworm infection, cats typically exhibit minimal
changes in the heart. The cat's primary response to the presence
of heartworms occurs in the lungs.
Preventives
While treatment of canine heartworm disease is usually
successful, prevention of the disease is much safer and more
economical. There are a variety of options for preventing
heartworm infection, including monthly tablets and chewables,
monthly topicals and a six-month injectable product. These
products are extremely effective and when administered properly
on a timely schedule, heartworm infection can be prevented.
The American Heartworm Society is now recommending year-round
prevention, even in seasonal areas. One reason for this is
compliance – to make sure the medicine has been given properly
by the pet owner. In addition, most monthly heartworm
preventives have activity against intestinal parasites. Many of
these same intestinal parasites that infect dogs can also infect
people, with estimated infections occurring in three to six
million people every year. So this added benefit of monthly
deworming makes great sense.
Before starting a preventive program,
all dogs that could possibly be infected with mature heartworms
should be tested.
Ivermectin
Ivermectin (Heartgard® & Heartgard® Plus by Merial, Iverhart®
Plus & Iverhart MAX™ by Virbac and Tri-Heart® Plus by
Schering-Plough) was the first in this family of drugs to be
approved for preventing heartworm infection. An infection with
larvae as long as two months prior to the initiation of
ivermectin treatment will be blocked from development.
Milbemycin
Milbemycin oxime (Interceptor® & Sentinel® by Novartis) has
benefits, which are similar to ivermectin.
Selamectin
Selamectin (Revolution® by Pfizer) is applied topically to
prevent heartworm disease.
Information obtained
from American Heartworm Society
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