Heartworm

What is it?
Canine heartworm disease is a serious and potentially fatal disease that is spread
by mosquitoes. The disease is also called Dirofilariasis. When the mosquito bites
the dog it ingests a microscopic, immature form of heartworm, called microfilaria.
The microfilaria develops into young, infective larvae that are transmitted to another
dog when that mosquito bites another dog. Therefore, heartworm disease is more
prevalent in areas where mosquitoes are numerous – such as near lakes, golf
course, swimming pools. In Arizona, with the addition of such water areas, there
has been an increase in cases of heartworm. In the Phoenix vicinity, in the year
2000, there were 86 cases. And, in Tucson (and vicinity), in the year 2000, there
were 15 cases. Most cases were in local dogs that had never left Arizona.

Signs
The signs of heartworm disease in dogs are related to the life cycle of the larvae
within the dog’s body. The infective larvae enter the dog’s body and goes into the
bloodstream where it develops into an adult worm. This process can take six
months. The adult worms live in the right side of the heart and in the nearby blood
vessels. Their accumulation impairs the blood’s circulation, which causes damage
to the dog’s heart, lungs, liver and kidneys. It can take several months for this
damage to take place before clinical signs occur. Signs noted in dogs include
coughing, difficulty breathing, exercise intolerance, reduced endurance,
listlessness and weight loss. Without treatment, the disease can lead to
congestive heart failure and death.

Diagnosis
There is a simple blood test we can perform to detect the heartworm larvae after
they have been in the dog’s body for six months. Some cases can be more difficult
to detect. For example, if the dog is infected for less than six months, and has as
yet a low worm burden, then the blood test may be falsely negative. These cases
may require additional tests such as x-rays or other blood tests to diagnose
heartworm disease.

Treatment
Infected dogs can be successfully treated for heartworm disease, especially if
detected early. The treatment is to kill the adult worms with a series of injections,
while the pet is hospitalized. If the dog has other complications, such as liver
disease, then the dog may need intravenous fluids and other forms of supportive
care. Reactions to the drug can occur, and more severely infected dogs are at a
greater risk for complications resulting as a consequence to the injections. After all
adult worms are eliminated, as noted by repeat blood tests, a second drug is used
to eliminate the microfilaria and is used as a preventive.

Prevention
It is certainly easier and safer to prevent heartworm disease than it is to treat it.
Most preventive medications are given orally on a monthly basis. Untreated dogs
serve as a constant source of infection. Therefore, it is essential that regular,
annual heartworm tests be performed, even if the pet is on preventives. In Arizona,
where we have mild winters, we have mosquito season all year long. We
recommend using the preventive all year round. And, it is safe to keep your dog on
the preventive for the pet’s entire life.