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Canine Heartworm Disease

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Heartworm Disease (HWD) refers to the condition caused by the
parasite Dirofilaria immitis, carried by mosquitoes and
affecting dogs, cats, and ferrets. In the United States, it is a
major problem in the Southeast, East, and the Mississippi River
valley. PARASITE AND LIFE CYCLE

The adult worm is large, up to 12 inches long, and lives
predominantly in the pulmonary arteries (Pas, the large vessels
that carry blood to the lungs from the heart). When an infection
consists of both male and female mature worms, reproduction
occurs with resultant microscopic circulating baby forms called
microfilariae (L1). These are an important part of the life
cycle because they allow infection of other animals to occur
and, when found upon microscopic examination of the blood, allow
the diagnosis of HWD. For transmission of heartworms (HWs) to
occur, a mosquito sucks blood containing L1 from infected dogs.
The L1 develops in the mosquito, becoming infective in about 2
weeks. The mosquito then transmits the infective larvae to
another dog. Further development occurs with migration to the
heart and PAs 3 to 4 months later. Adult HWs are thought to live
for 5 to 7 years. CLINICAL SIGNS

>From 1 to over 200 HWs may reside in the heart and PAs. The PAs
become thickened and inflamed, increasing the work of the heart
as it pushes blood past the worms into the lungs. In addition,
the lungs themselves become inflamed. Mild infestations may
produce no signs. The earliest clinical signs are typically
exercise intolerance, cough, and weight loss. More severe signs
may include severe cough, labored breathing, and heart failure
(usually manifested as abdominal swelling). Once HWD has reached
this stage, the dog may die. DIAGNOSIS

The diagnosis can be made by finding L1 in blood. An
enzyme-linked immunosorbent assay (ELISA, a test that identifies
proteins [antigens] produced by adult female HWs) readily
detects infections with two or more adult females. A diagnosis
may also be suspected on the basis of radiographs (x-rays).
TREATMENT

Adulticidal Therapy

After tests to ensure that a dog is healthy enough, arsenical
drugs are used to kill adult HWs. A newer, more expensive agent
(melarsomine) is safer than arsenamide (Caparsolate), allowing
gradual destruction of HWs so that the lungs can gradually
“clean up” the infection. Both drugs can cause irritation at the
site of injection and could damage the liver and kidneys. By far
the greatest concern is dead HWs, producing a severe reaction in
the lungs 1 to 3 weeks after administration. This can be
prevented or minimized with melarsomine given in three doses
(one initially and two separated by 24 hours in 1 month) and by
severely restricting exercise for at least 1 month after
adulticidal therapy. Exercise restriction is imperative after
adulticidal therapy! Therapy with steroids may be needed to
reduce lung inflammation and resultant cough but is typically
discontinued before adulticidal therapy. Aspirin may be used to
reduce the vascular damage caused by HWs but is controversial.
Microfilarial (L1) Therapy

After killing the adult worms, the L1 forms should be killed,
thereby lessening the risk to other pets. A dosage of
ivermectin, milbemycin, or moxidectin can be given approximately
6 weeks after the adults have been killed. Although often
effective, this treatment may produce severe reactions. An
alternative is to use ivermectin at lower doses, thereby
gradually eliminating L1 over about 6 months. This may be done
before or after the adulticide and adverse reactions are rare.
Ideally the pet is closely observed the day of the first dose
with either method. PREVENTION

Heartworm infection is clearly better prevented than treated.
Prevention is instituted at 6 to 8 weeks of age or as soon
thereafter as climatic conditions dictate. Prevention of HWD can
be accomplished by daily administration of diethylcarbamazine
(DEC) or monthly administration of ivermectin, milbemycin, or
moxidectin. Although each of these drugs is effective when given
as directed, even brief lapses in DEC therapy may result in
infection. The monthly drugs, however, provide protection
despite lapses of up to a month. Both DEC and monthly drugs are
extraordinarily safe if administered before infection but may
produce severe, even fatal, reactions if administered to dogs
with L1. Such reactions are more severe with DEC. Heartworm
testing should be performed in all dogs older than 6 months of
age (if there has been seasonal potential for exposure) before
institution of preventative. Seasonal and Geographic
Considerations

There are areas in the United States in which no HW preventative
is necessary. In the deep South and California, preventative is
typically administered all year. In the North, the season is
shorter; DEC is used from the first mosquito sighting until 2
months after the first hard frost, and the monthly drugs should
be administered from the onset of mosquito season until 1 month
after the first hard frost. Your veterinarian knows the
appropriate preventative schedule for you region. Yearly Testing

Yearly testing is required for dogs receiving DEC because of the
potential for adverse reactions in dogs that become infected and
are restarted with preventative. The need for yearly testing
with monthly treatment is less certain. Many veterinarians still
advocate yearly testing because pets may not receive or may not
swallow the necessary preventative. If it is certain that the
medication is administered and swallowed for the entire HW
season, then testing every 2 years is an option.

The above is general veterinary information. Do not begin
any course of treatment without consulting your regular
veterinarian. All animals should be examined at least once every
12 months.