By Leslie Manis, Health/Genetics Chairman, ASTC
[Every effort has been made to ensure accuracy of information. However, this is not a substitute for prompt veterinary care. Any similarity to other publications is unintentional. Published online at Sealyhealthguard.org, 11/23/10]
Download PDF version
Dr. Betsy Sigmon, founder of Creature Comfort Veterinary Hospital,
discussed heartworms in a recent CHF podcast. Heartworm is
caused by a filarial nematode, Dirofilaria immitis. It
is a roundworm, a bloodborne parasite.
It requires two hosts, an intermediate (mosquito), where the larvae
live for a short transition period in order to become infective (capable
of causing disease). They reach maturity and reproduce inside the
definitive host (a mammal, such as a dog, raccoon, coyote). Heartworms
can affect over 30 mammalian species, rarely people. Dogs are
the preferred host.
Signs: Heartworm disease can be silent for
2-3 years. It begins with signs of heart failure--a cough, lethargy,
weight loss. When it becomes advanced, symptoms include difficulty
breathing, exercise intolerance, distended belly. An adult heartworm
can grow up to 12" long.
Diagnosis: Blood test (antigen test). There
is a kit available for the veterinarian to use in the office. The
kit shows negative or positive results, but only as of 6 months prior
to the test. For example, if you test a rescue dog with an unknown
history in June, a negative test means it is negative up to the previous
January. It could have immature worms in it, so must be tested again
in 6 months to be clear. Then the dog may be tested once a year if
it is on monthly prevention.
Treatment: Most veterinarians recommend puppies
begin heartworm prevention, such as Heartguard, at six months of age.
Then the vet will test for heartworm beginning at age one at the dog's
annual exam. Most heartworm preventives are over 95 percent effective.
There are topical, oral and even injectible products, all with similar
effectiveness. They must be prescribed by a veterinarian for proper
dosage. The most common reasons for loss of effectiveness are improper
dosage and infrequent dosage. It is important to give the monthly
preventive every 30 days rather than once a month (a dose April 1
then May 25 is much different than April 1 and May 1).
The environment has to be 57 degrees or higher and takes 8 days or
longer for a mosquito to bite a positive animal then transmit larvae
at the proper age to infect another animal. As the temperature cools,
it takes the mosquito longer to be able to transmit them (longer time
for heartworm larvae development). When there are a lot of warm days
during the winter, it's much riskier to withhold heartworm preventive
during the winter months. Dr. Sigmon strongly recommends giving heartworm
prevention all year round.
Heartworm is such an easily prevented disease, and our preventives
are so effective, it's easy to become complacent. John W. McCall,
MS, PhD, Professor Emeritus in the Dept. of Infectious Diseases at
the University of Georgia's College of Veterinary Medicine says that
heartworm disease continues to spread throughout the United States
for several reasons:
Relocation of heartworm-positive dogs, especially after
Hurricane Katrina in 2005;
Natural and man-made environmental changes, leading
to more breeding sites for mosquitoes and resulting in an increase
in mosquito populations;
Introduction of non-native mosquito species, and native
mosquito species expanding their territories;
Enlarging pool of unprotected canid populations, for
example, pet dogs that are not on heartworm prevention and wild canids,
such as coyotes which are very susceptible to heartworms. The more
animals carrying heartworms, the more likely heartworm disease will
Medication Shortage: Until about seven months
ago (mid-2011), veterinarians were treating heartworm with Immiticide
by Merial (melarsomine dihydrochoride). It was an arsenical product
and was very effective. Many of you may not realize that on August
4, 2011, Merial announced that there was an Immiticide shortage. It
was the only heartworm adulticide approved by the US FDA for use in
dogs. Immiticide hit the market in 1996. Notice of the shortage
caused a run; by August 9, Merial was officially "out" of the drug.
The US supplier could no longer produce the active ingredient. The
FDA was reluctant to allow Merial's plant in Brussels to fill American
orders. The situation "related to technical issues providing finished
product to us. The finished product is made by a manufacturing company
in the US," explains Natasha Mahanes, a Merial spokesperson. On
September 30, the FDA announced that it would allow Merial to import
limited quantities of Immiticide from a European supplier to address
concerns over a shortage of the drug. The FDA says this is temporary
while Merial works out technical issues in the US plant.
The European supplier is an approved source of the product for international
markets, but only has a limited supply for importation to the US.
It will only satisfy a fraction of the US demand. It will only be
available through a restricted distribution program directly from
Merial. Veterinarians treating only severe cases of heartworm
disease could access the drug on a case-by-case basis with approval
from the company.
Some rescue organizations have had trouble placing dogs with a positive
heartworm test since Immiticide has become unavailable.
Alternative Treatment: Without Immiticide, veterinarians
must now treat adult heartworm by giving an antibiotic, usually Doxycycline,
plus monthly heartworm preventive. This treatment lasts 60-90 days.
It prevents future infections and the worms will die in 2-3 years.
They do not have a way to kill the adult heartworms. The risks with
this treatment are a chance of reaction and producing drug resistance.
Previously, when a dog had problems during heartworm treatment, it
was thought to be a reaction to the worms. Recently, it has been discovered
it is a reaction to a bacteria that lives inside the heartworms. This
bacteria is called Wolbachia. It is in the family Richettsiales.
It does not live outside the host, and seems to be necessary for many
filarial nematodes, including heartworm, to develop, reproduce and
survive long-term in the definitive host. When a worm dies inside
of the host, Wolbachia are released in massive numbers
from the nematode's cells, exposing the host to the bacteria.
A protein found of the surface of the bacteria, called the Wolbachia surface
protein or WSP, may cause the dog's body to mount a specific immune
response. This response may worsen the heartworm disease. Researchers
think this may also worsen the lung and kidney inflammation seen in
dogs with heartworm disease.
As you can see, it's so important to take good care of our dogs & prevent
this terrible disease.
For more information:
Heartworm life cycle:
Parasite prevalence maps: