April 11, 2018

Heartworm: Where Are We Today?

Experts share their insight on why this disease is on the rise, whether resistance is a concern, and fresh approaches to prevention.
By Amanda Carrozza
Image courtesy of American Heartworm Society
Warnings and reports about pets infected with Dirofilaria immitis in the United States are not unusual. In fact, canine heartworms were first discovered on the southern US coast in 1856.1 What should be making headlines is the fact that despite the preventive medications that have been widely available for decades, the incidence of heartworm disease continues to rise across the nation.

According to the triennial American Heartworm Society (AHS) incidence survey, the average number of dogs diagnosed with heartworm per clinic in 2016 was 21.7% higher than the number diagnosed in 2013.2 The 5 states with the highest heartworm incidence: Mississippi, Louisiana, Arkansas, Texas, and Tennessee. But no state in the country is heartworm-free, and infection in dogs is considered at least regionally endemic in every state except Alaska.

Veterinary parasitologists continue to study heartworm infections extensively and periodically issue updated guidelines for the best methods of prevention, testing, diagnosis, and treatment. But what will it take to eradicate this potentially deadly disease?

Although longstanding preventive and treatment methods are highly effective, veterinarians should be aware of heartworm disease advances and updates, regardless of the prevalence of heartworm in their area.

Multimodal Approach to Prevention

In human medicine, the common approach to averting a parasite-derived disease starts with thwarting contact with the vector. Consider Zika virus, for which the first line of defense is preventing the bite of mosquitoes. Although the vector for Zika and heartworm is the same, veterinary medicine historically has taken an entirely different approach to prevention. To date, the mosquito—which the World Health Organization called one of the deadliest animals in the world—has been an aspect of the heartworm life cycle that generally has been ignored.3

“In the past, [veterinarians] were under the impression that heartworm preventives were 100% effective, and so whether a mosquito bit or not wasn’t really considered important,” explained Susan Little, DVM, PhD, DACVM (parasitology), a regents professor and Krull-Ewing chair in veterinary parasitology at Oklahoma State University. “We were allowing the natural infection cycle to take place and then relying on those ‘perfect’ preventives to keep the adult heartworms from developing.”

But with the understanding that preventives are not always 100% effective and pet owners are not always compliant in giving preventives to their pets, many veterinarians and parasitologists have begun calling for a stronger focus on the mosquito. “We should probably have been including mosquito prevention all along,” Dr. Little said. “For all other parasites we talk about the life cycle, but for heartworm, we hadn’t.”

This line of thinking has sparked a movement in veterinary medicine known as the Double Defense Heartworm Protocol, an initiative from Ceva Animal Health. The corresponding website is a resource veterinarians and pet owners can use to educate themselves on the advantages of a multimodal approach to prevention. As the practice gains in popularity, continued research has been published in support of double defense.

In a 2017 study, a group of investigators led by John McCall, PhD, professor emeritus in the Department of Infectious Diseases at the University of Georgia College of Veterinary Medicine and past president of AHS, assessed the effectiveness of a topical ectoparasiticide combined with a macrocyclic lactone on transmission of heartworm from mosquitoes to dogs and subsequent development of worms in treated dogs exposed to infected mosquitoes.4 In the study, 32 beagles were allocated evenly to 4 groups:
  • Group 1: untreated controls
  • Group 2: treated topically with an ectoparasiticide on day 0
  • Group 3: treated orally with a macrocyclic lactone on day 51
  • Group 4: treated with an ectoparasiticide on day 0 and a macrocyclic lactone on day 51

Dogs were sedated and exposed to D immitis–infected Aedes aegypti mosquitoes for 1 hour on days 21 and 28. Upon study completion, the investigators concluded that the Double Defense protocol of a topical ectoparasiticide combined with a macrocyclic lactone had better efficacy for protecting dogs against heartworm transmission and infection than a macrocyclic lactone alone.

Still, preventing mosquitoes from biting isn’t a one-size-fits-all approach. As many as 80 different species of mosquitoes can transmit heartworm. Instead, Tanja McKay, PhD, director of environmental sciences at Arkansas State University, said it opens up a variety of channels for studying how these tiny insects play a pivotal role in intensifying the spread of such a pervasive disease. “Each mosquito species is very different in its biology, ecology, and habitat,” she explained. “It is a very complex issue, but one that must be part of the heartworm conversation.”

In the multimodal approach, preventing mosquitoes from biting doesn’t negate the need for continued use of preventives. Repellents should be used an adjunct, not as a replacement.

Rising Temperatures, Shifting Distribution

Environmental temperature is another contributing factor to the increasing rates of heartworm infection, and investigators have paid particular attention to climate change in the wake of continuing debates about global warming and the trend of once rural areas turning into densely populated cities.

Temperature Increases
As Dr. McKay explained, even the slightest temperature changes have the potential to escalate the mosquito life cycle and, in turn, the spread of heartworm. “The higher the temperature, the faster a mosquito will develop from egg to adult,” she said. “The same is true for the temperature requirement for the nematode within the mosquito. A higher temperature will decrease the amount of time it takes for a nematode to develop into the necessary larval stage for infection.”

Elevated temperatures also present the potential for an increase in the number of mosquitoes that have the capability of carrying the parasitic roundworm. “If it’s warmer outside, maybe you have 1 or more days on the front end and on the back end of the time period of when heartworm might be more of an issue,” she said. “Does that have an effect on the number of canine heartworm cases?”

Temperature fluctuations not only affect how quickly mosquitoes and nematodes develop but may also have an impact on the migration patterns of mosquito species. “We know that in the past 30 to 40 years, we’ve had a dramatic expansion of mosquitoes and new mosquito species introduced,” Dr. Little added.

Urban Heat Island Effect
Temperatures in urban areas are typically a few degrees higher than in neighboring rural regions, an occurrence known as the urban heat island effect. In fact, the annual mean air temperature in a city with 1 million people or more can be 1.8°F to 5.4°F warmer than the surrounding areas. In the evening, the difference can be as high as 22°F.5

Not only does the urban heat island effect impact energy demand, but it has been linked to the increased prevalence of mosquitoes where the average temperature might not have historically supported breeding. This is particularly true when manmade bodies of water, such as ponds or fountains, are introduced. “Mosquitoes take advantage of that and survive well in urban, developed areas,” Dr. Little pointed out.

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