Heartworm disease: The most important parasite of dogs

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Heartworm disease, caused by Dirofilaria immitis, is the most important parasite of dogs in North America, explained Christopher B Adolph, DVM, MS, DACVM (Parasitology), senior veterinary specialist-parasitology at Zoetis, during a lecture at the 2025 American Veterinary Medical Association (AVMA) Convention in Washington DC.¹ Although it is not the most common parasite of canines, the significant damage the parasitic worm does to its host’s organs and arteries, as well as its challenging nature to treat, render it the most important parasite of dogs.¹ By the time the potentially fatal disease is detected in a dog, the disease has already started, and the damage it has caused can be permanent.¹

Although not the most prevalent parasite of canines, the parasite poses a growing threat. During the session, Adolph showed the audience a map of melarsomine dihydrochloride (Diroban; Zoetis) sales distributions in the US, which broadly mirror human population density across North America, revealing the disease’s widespread presence.

Contributing factors in its growth across the US, as well as its increasing resistance include natural disasters, changes in natural climate conditions, online adoption sources with pet transportation options, non-compliance with medication, and an increase in pet owner travel with dogs. Currently, more than 40 species of mosquitoes are known vectors of the disease, making full vector control impossible, further contributing to the parasite’s increasing presence and resistance.¹

Dirofilaria immitis’ life cycle and why missed treatment doses are critical

The parasite’s life cycle involves adult heartworms living in the pulmonary arteries of dogs, other canids, or cats, and producing microfilaria—ie, baby worms—that circulate in the dog’s bloodstream.^1,2 When an infected dog is bitten by a mosquito, the mosquito ingests the microfilaria, which then undergo 3 molts inside the mosquito. Once the microfilaria reaches the third molt, L3, it becomes an infective larva that can then be transmitted to a new host when the mosquito bites another animal. At that point, the L3 larvae will develop into L4 within 3 to 12 days. Afterwards, the L4 will develop into immature adults, formerly called L5. The L4 parasites molt into immature adults at days 50 to 70.

As Adolph explained during the session, all heartworm preventives target the parasite’s L3 and L4 stages, killing them before they can mature into adults and cause disease. “They're L4 guys for about 50 to 70 days, so if you're giving your monthly product every 30 days on time, and you're encountering susceptible worms...[the medication is] killing all of those L4s and hopefully the 30-day stage before they can continue their term,” said Adolph.

However, because all preventives are labeled for the L3 and L4 stages, missing just one dose can allow L4 larvae to develop into adults as early as day 50, bypassing the medication’s target window. “One missed dose is a hole in the net, and that can lead to infection,” stressed Adolph. “We can't be missing doses. [We need to dose] every month, year-round.”

The importance of testing

“These immature adults are going to start arriving in the vasculature around days 67 to 70, and that is when you start the clock on damage. That's when damage starts,” Adolph continued. Around day 120, the immature adults will start arriving in the pulmonary arteries, with veterinarians still being 2 months from being able to detect them.

The more worms a patient has, the pathology there will be, emphasized Adolph. “Every beat of the heart is slamming worms into the vascular endothelium, creating progressive, irreversible damage,” he said.

“This is [the]... reason why I...test all the time,” he continued. “Test at least once a year if they're compliant, and every time they have a gap, test now [and] test again in 6 months. Test, test, test because you want to find these things early in the [disease] process and make these worms dead as soon as possible with [melarsomine dihydrochloride] and all the other ancillary treatments.”

References

1. Adolph CB. Mapping the Path Forward: Key Findings from Heartworm Surveillance Research. Presented at: American Veterinary Medical Association Convention; Washington, DC: July 18-22, 2025.
2. What is heartworm disease? American Heartworm Society. Accessed July 18, 2025. https://heartwormsociety.org/pet-owner-resources/heartworm-basics