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Heartworm Disease In Arizona


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 aftermath of Hurricane Katrina in 2005, many heartworm positive homeless dogs found their way to Arizona via rescue groups and relocation of families, further spreading the disease. In Maricopa County, in the year 2009, there were 472 reported cases. Most cases were in local dogs that had never left Arizona.  For information on the geographical distribution of Heartworm Disease in Arizona and the United States, visit the website of the Companion Animal Parasite Council at

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.

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.

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.  URGENT! In August 2011 it was announced that the only available prescription medication to kill adult heartworms, Immiticide, would be unavailable for an undetermined duration. Because there are no other approved products available for killing the adult heartworm in dogs, the American Heartworm Society Board and Scientific Committee has developed and approved a management plan for heartworm positive dogs during this period of unavailability. Talk to your Veterinarian or visit for more information.

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.

Click on the picture above for a larger map of Heartworm Cases confirmed in 2009.