American Heartworm Society Urges Better Understanding of Heartworm Disease

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April is Heartworm Awareness Month, but the threat of heartworm disease—which affects more than a million pets is the U.S.—is year-round. The American Heartworm Society urges pet owners to (1) learn more about this common but preventable disease; (2) ensure their pets are routinely tested for heartworm infection; and (3) keep their pets on heartworm preventives 12 months a year.

2013 Heartworm Incidence Map, American Heartworm Society

““Providing year-round heartworm prevention is one of the best, easiest and most cost-effective steps pet owners can take to protect their pets’ long-term health.” -- Dr. Stephen Jones, President, American Heartworm Society

Noting that deadly heartworm disease remains far too common in the U.S., American Heartworm Society (AHS) President Dr. Stephen Jones urges pet owners across the U.S. to get their pets tested for heartworm infection and keep them on year-round heartworm prevention. Following are five reasons why:

1.    More than a million pets in the U.S. have heartworm disease. This is a conservative estimate, based on testing data from veterinarians across the country. A look at the most recent AHS heartworm incidence map* reveals that in most clinics in the U.S., at least one to five cases per veterinary clinic were diagnosed in 2013, while numerous regions reported 100 cases per clinic or more. These reports do not reflect the status of the millions of dogs and cats that are not regularly seen by a veterinarian or tested for heartworm.

2.    Heartworm disease has been diagnosed in all 50 states. Heartworms are spread by mosquitoes, so heartworm disease tends to be associated with regions that have warm, humid weather and high counts of pesky bloodsuckers. Nevertheless, heartworm infection is widespread in most states of the country, including states like California and Arizona where the disease was once considered rare. Why?

  • Dog mobility. Infected dogs are moved from areas where heartworm disease is common to areas where it isn’t—and bring the disease with them.
  • Proliferation of heartworm-carrying mosquitoes. More than 20 species of mosquitoes in the U.S. can carry heartworm infection, and that number may be on the rise. In 2014, Los Angeles vector control specialists found a mosquito new to the U.S.; it was determined to be an Australian mosquito species that commonly carries heartworm as well as several human viruses.

3.    Both dogs and cats get heartworm disease. In dogs, the disease is caused by adult heartworms that develop from heartworm larvae deposited by mosquitoes. Cats can also harbor adult heartworms, but it is more common in cats for the worms to die before reaching maturity. Meanwhile, even immature worms can cause respiratory disease in cats.

4.    Heartworm disease can be fatal. Heartworm disease affects the heart, lungs and pulmonary blood vessels of pets and can cause death in both dogs and cats. Annual testing and monitoring is important, because infected dogs can be effectively treated--and the earlier the better. There are no approved treatments for cats, but supportive care can help manage complications.

5.    Prevention is safe, effective and cost-effective. The American Heartworm Society recommends year-round prevention for dogs and cats in the U.S., even in regions that experience cold winters. Heartworm preventives work retroactively, so an animal that acquires an infection one month must be given its heartworm prevention in the months that follow to be protected against disease. And with unpredictable weather patterns and the ability of hardy mosquitoes to survive in protected areas—as well as indoors—it’s difficult to predict when heartworms aren’t in season. Fortunately, heartworm prevention is highly effective when given faithfully, and the year-round cost of preventing the disease in dogs is a small fraction of the cost of heartworm treatment.

“The reality is that every season is heartworm season,” concludes Dr. Jones. “Providing year-round heartworm prevention is one of the best, easiest and most cost-effective steps pet owners can take to protect their pets’ long-term health.”

*The 2013 American Heartworm Society Heartworm Data is based on a survey of heartworm testing results from more than 4,500 veterinary clinics and shelters across the U.S. The American Heartworm Society is a non-profit organization whose mission is to lead the public in the understanding of heartworm disease. Through their “Think 12” program, they remind owners to give their pets heartworm preventives 12 months a year and test pets for heartworm every 12 months. Owners can learn more about heartworm protection for their pets by checking out the resources at

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Sue O'Brien
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