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The
Heartworm Story Provided by :
The American Heartworm Society
The Heartworm Story
The first published description of heartworm in dogs appeared more
than 100 years ago in an issue of "The Western Journal of Medicine
and Surgery."1 Heartworm in cats was first described in
the early 1920's.2, 3 Since then, naturally acquired
heartworm infection in cats and dogs is identified as a worldwide
clinical problem. Despite improved diagnostic methods, effective
preventives and increasing awareness among veterinary professionals
and pet owners, cases of heartworm infection continue to appear in
pet dogs around the world. The diagnosis of the disease is still
complex and elusive in cats.
1 Osborne, TC. Worms found in the
Heart and Bloodvessels of a Dog; Symptoms of Hydrophobia. The
Western Journal of Medicine and Surgery, 1847.
2 Riley, WA. Dirofilaria immitis in the heart of a cat. J
Parasitol 1922;9:48
3 Travassos, LP. Notas Helminthologicas. Brazil-Med. An.
1921;35 2(6):67
What do we know about it?
Heartworm disease is a serious and potentially fatal condition
caused by parasitic worms living in the arteries of the lungs and 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 but one
of many species of roundworms. The specific roundworm causing
heartworm in dogs and cats is known as
Dirofilaria immitis.
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Adult female heartworms, that may grow to be
14-inches long, living within the infected animal's body release
their young, called
microfilariae (pronounced: micro fil ar ee), into
their host's bloodstream. The mosquitoes become infected with
these microscopic worms after biting into an infected adult
animal for a blood meal. During the next 10 to 14 days, the
microfilariae mature to the infective larval stage within
the mosquito. When the mosquito bites another host (dog, cat or
other susceptible animal), the infective
larvae enters a new host through the bite wound. It then
takes a little over six months for the infective
larvae to mature into adult worms that may live for five to
seven years (in a dog). The
microfilaria cannot mature into adult heartworms without
first passing through a mosquito. |
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Dogs or other animals harboring adult worms are the
recognized reservoir of infection. Adult worms produce the
offspring that circulate in the blood, and are then transmitted
to mosquitoes once they bite the infected animal. These
offspring (microfilariae)
undergo development to an infective larval stage within 14 days
in the mosquito, and can then be transmitted to another host
(such as a cat) or back to another dog, when the infected
mosquito bites again. The infective heartworm
larvae travel through a tubular organ within the mosquito's
head and are injected into the skin of a new host animal through
the mosquito bite wound. In the dog, the larvae progress in
their development to an adult form of the worm, and live in the
heart and pulmonary vessels, where they continue the life cycle
and cause extensive injury. In the cat, the larvae molt as well,
but fewer worms survive to adulthood. While dogs suffer 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.
Within the dog, the time frame between initial infection and
growth to adult worms is approximately six to seven months,
eventually arriving in the heart and pulmonary vessels where
they begin to produce new offspring. This period is referred to
as patency. In cats, it takes seven to eight months before adult
worms arrive in the heart and pulmonary vessels, and this is
referred to as transient patency. In most cases the life cycle
of the heartworm ends here since
microfilaria are produced in less than 20% of the cats.
Go to American Heartworm Society Canine Heartworm section...
Go to American Heartworm Society Feline Heartworm section...
View Dogs vs. Cats comparison at American Heartworm Society
- For dogs, there are a variety of options for preventing
heartworm infection, including an injectable administered by
your veterinarian that provides protection for six months,
daily and monthly tablets and chewables, and monthly topicals.
- In cats, there are three products currently approved to
prevent heartworm infection, two oral medications and one
topical medication.
- Pet owners should discuss the proper product selection for
their pet and dose timing with their veterinarian.
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Links to Heartworm
Preventive Websites:
  
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