PETS AND PEOPLE: Beware of Heartworm Disease


More prevalence of heartworm predicted in New England this spring.

 I don’t have any “favorite” diseases, but my least “favorite” is Canine Heartworm Disease.  It is a very risky disease to treat and requires a very long period of inactivity and isolation. It can be prevented by using anti-microfilarial medicine such as Heartguard (Macrocyclic lactones) and in addition, additional mitigation such as topical insecticides which prevent the carrier of heartworms, the mosquito.

A Brief History of Canine Heartworms: 

In 1586, Chez Jean Wolfe sketched the grotesque creature found inside a horse’s heart. That “monster,” as he called it, turned out to be Dirofilaria immitis1 —heartworms. Almost 300 years later, canine heartworms were discovered in dogs on the coast of the southeastern United States. The earliest cases of heartworms in dogs occurred in South America in 1847, but the official report wasn’t published until 1875. Nearly 100 years later, in 1974, the American Heartworm Society was established.

Macrocyclic lactones first came to the veterinary market in the 1980s as an option for regularly clearing heartworm infections before dogs developed full-blown heartworm disease.

Those have been the main highlights in the history of canine heartworms—until now.

Research conducted relatively recently looked at the efficacy of approaching canine heartworm with mitigation and mosquito-bite prevention strategies like those used around the world for preventing the spread of mosquito-borne diseases in people. The results look promising.

By adding a layer of mosquito repellent insecticide to macrocyclic lactone use, the transmission of D. immitis microfilariae drops significantly—even with resistant strains of canine heartworm, such as JYD34 (short for “junk yard dog 34”), which is one of six known resistant strains.

Despite more than 30 years of increased awareness and preventive efforts incorporating the use of macrocyclic lactones, more dogs continue to become infected by mosquitoes carrying D. immitis microfilariae.

This increased prevalence across the United States may be due to:

  • Gaps in pet owner compliance with giving macrocyclic lactones on schedule every time
  • Weather conditions ripe for mosquito breeding
  • Emergence of macrocyclic-lactone-resistant heartworm strains

The Companion Animal Parasite Council’s (CAPC) annual heartworm prevalence forecast looks even worse due in part to above average precipitation and above average temperatures in a wide swath of the country. The early heat we are seeing this Spring is an example. CAPC predicts several endemic areas will see even more cases than usual, including the Lower Mississippi River Valley, New England, and the Ohio River Valley.

The reality is that veterinary testing finds canine heartworm infections in all 50 states, and the American Heartworm Society survey of veterinary practices reported seeing more heartworm in recent years.

Understanding Heartworm Transmission Risk Factors and Assessment:

A number of factors affect the heartworm transmission risk in our geographic area. Local bodies of water and other breeding grounds for mosquitoes are the primary problem on Cape Ann. Other factors include:

  • Weather conditions (both seasonal and year to year)
  • The number of dogs in our community receiving regular veterinary care, including heartworm testing and preventives
  • Geography plays a significant role.Some areas hold the dubious distinction of being heartworm infection hot zones. It’s obvious that places near river and stream valleys and near wetlands see higher mosquito populations. Greater mobility in the human population, including people traveling with their dogs, can introduce heartworm into a community with a previously low infectious burden. Additionally, as more pets are adopted from southern States, the risks increase.  People want to be outside with their dogs. They want to enjoy nice weather, and preventing mosquito bites makes everyone happier, more comfortable, and in the case of heartworm prevention in dogs, healthier too.

Environmental management to eliminate mosquito breeding grounds in typical backyards include these strategies:

  • Empty, wash, and refill outdoor water bowls daily to remove any mosquito eggs before they hatch (typically in 24–48 hours after being laid).
  • Empty and turn over baby pools, bird baths, and yard items, such as buckets, when not in use.
  • Resolve any landscaping areas where water from sprinklers or rain can pool.
  • Keep screens on any open windows and doors.

Dr. Lamb is the Veterinarian at the Manchester Animal Hospital.

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