Dirofilaria immitis: The monster then and now

dvm360dvm360 April 2024
Volume 55
Issue 4
Pages: 23

Heartworm disease is more prevalent than ever, but prevention methods can help defeat the parasitic beast

grethental / stock.adobe.com

grethental / stock.adobe.com

In 1586, Chez Jean Wolf created a sketch of a “monster” that had been found inside the heart of a horse. It was later identified as Dirofilaria immitis, also known as heartworm.1

Canine heartworm was discovered in dogs in the southeastern United States in 1856 and in cats in the 1920s.1 D immitis is now diagnosed in all 50 states and around the globe.2

Heartworm disease is a serious and potentially fatal disease for dogs, cats, and other species. The worms live in the heart, lungs, and associated blood vessels, causing severe lung disease, cardiac failure, and damage to other organs in the body. Heartworm’s definitive hosts are canids both wild and domestic: dogs, coyotes, foxes, wolves, and wolverines. It can also be found in sea lions, harbor seals, bears, ferrets, horses, muskrats, beavers, raccoons, red pandas, leopards, domestic cats, and, in rare incidences, humans.1

Four hundred years after the parasitic roundworm was discovered, heartworm cases are on the rise. Despite veterinary awareness, 2022 data from the American Heartworm Society show an increase in heartworm prevalence in some areas of the US, with the leading number of cases in the states in or adjacent to the lower Mississippi Delta region.3

These increased cases can be attributed to gaps in adherence, weather changes in climate, humidity, microclimates in urban areas around our homes, increased vector species, increased favorable breeding grounds for the mosquito vector, expansion of microfilaremia wild canid territories, urban sprawl, relocation or travel of the microfilaric dog, Wolbachia contamination, worm adaptations, and macrocyclic lactone resistance.1-3

A water dish with as little as 1 in of water, can become a mosquito breeding ground. Add in increased temperatures radiating from our homes and you have mosquito paradise: a created microclimate. Consider that 25 of the 180 known mosquito species carry heartworm, and each female can lay several hundred eggs that can hatch in 24 to 48 hours.2

According to the American Heartworm Society, the mosquito season decreases during the winter, but the presence of these microenvironments in urban areas suggests the risk of infection will never reach zero. The good news is that the development of the larvae within the mosquito cannot occur in temperatures less than 57 °F.2

Disease is also influenced by microfilaremic dogs and wild canids moving into areas where previously there were few to no heartworm cases. These canids act as a nidus for infection to noninfected dogs. Results of a recent study suggest that heartworm-infected dogs attract mosquitos because of volatile organic compounds that these dogs exhale. Another study found that mosquitos are 10 times more likely to find and feed on infected dogs and wild wolves or coyotes than noninfected ones.4

Cats, although susceptible hosts, seem more resistant to infection with D immitis than dogs are.2 Most heartworm infections in cats are considered “light infections,” with a maximum of 6 adult worms, typically of a single sex. Circulating microfilaria is seldom found in infected cats.2

So how do we defeat a monster that has been going strong for centuries? We get craftier than the beast.

We stop the mosquitos from picking up heartworm larvae from infected dogs and stop infected mosquitos from transmitting to noninfective dogs. We do this using a multimodal approach with repellent, insecticides, and macrocyclic lactones. We use repellent and insecticide strategies like those employed in human medicine: spraying, baits, and sterilization. We give our pets repellents and use macrocyclic lactones year-round in both cats and dogs. We keep our pets inside during high mosquito feeding times of dawn and dusk; a dog can get more than 80 mosquito bites in this 2-hour window.

There are 6 strains of D immitis resistant to macrocyclic lactones. One of the most recognized is JYD34. In a study, a multimodal prevention strategy proved effective against JYD34.1

What else can we do to defeat this monster? We become informed and aware of our microenvironments and destroy these habitats. We increase our vigilance. We test for antigens and microfilaria in the dog and antibodies and antigens in the cat. We limit exposure to resistant strains such as JYD34, and we combat Wolbachia with sterilization and antibiotics. We are the guardians of the gate, and monsters die when we take away their power.

Gale Savino-Eason, DVM, is a partner doctor at Caroline Veterinary Clinic, a Heart + Paw family practice in Ridgely, Maryland.


1. Ceva and the American Animal Hospital Association. Heartworm update: fighting mosquitoes and heartworms for a double punch of protection. 2018. Accessed March 18, 2024. https://www.aaha.org/globalassets/05-pet-health-resources/ceva_heartworm- booklet.pdf

2. American Heartworm Society. Current canine guidelines for prevention, diagnosis, and management of heartworm infection in dogs. July 2014. Accessed March 18, 2024. https://www.heartwormsociety.org/images/pdf/2014-AHS-Canine-Guide- lines.pdf

3. New heartworm incidence map shows increase in parasitic cases. dvm360. April 20, 2023. Accessed March 14, 2024. https://www.dvm360.com/view/ new-heartworm-incidence-map-shows-increase-in-parasitic-cases

4. Lederhouse C. Map shows heartworm rates continue to increase in hot spots, new locations. American Veterinary Medical Association. Updated May 8, 2023. Accessed March 18, 2024. https://www.avma.org/news/ map-shows-heartworm-rates-continue-increase-hot-spots-new-locations

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