April 2022

In this VETgirl online veterinary continuing education blog sponsored by the American Heartworm Society, Dr. Chris Duke, DVM, reviews the state of heartworm disease in the United States, and why heartworm season should be seen at 12 months long! Please note the opinions in this blog are the expressed opinion of the author and not directly endorsed by VETgirl.

By Dr. Chris Duke, DVM, President, American Heartworm Society, Bienville Animal Medical Center, Ocean Springs, Mississippi

The State—and States—of Heartworm Disease in the U.S.: Why Heartworm “Season” Should be Seen as 12 Months Long

Over the past couple of years, we’ve all become much more familiar with terms like epidemic, pandemic and endemic as we’ve coped with COVID-19 and its far-reaching effects on public health and life. And while “endemic” has been positioned as the least of these three evils in the context of COVID, an endemic disease is, in fact, consistently present in a particular population or region, regardless of severity. One of the most widespread and well-recognized endemic diseases of pets is heartworm disease.

The prevalence of heartworms varies across the U.S. due to factors that affect the presence of mosquito vectors and infected hosts. Heartworm infection, which can be life threatening to pets, is considered at least regionally endemic in all 48 contiguous states in the U.S., as well as Hawaii, Puerto Rico, the Virgin Islands and Guam. This fact has been repeatedly demonstrated over the decades during which the American Heartworm Society (AHS) has surveyed U.S. veterinary practices on heartworm incidence.* The latest survey, which aggregated heartworm testing data from more than 6,000 U.S. veterinary practices, confirmed this fact.

Given the endemicity of heartworms, the AHS Guidelines on Heartworm Disease recommends heartworm prevention for all dogs and cats in this country. Meanwhile, rather than suggesting a seasonal approach that limits prevention to warm-weather months, the AHS recommends year-round prevention—effectively approaching heartworm disease as having a 12-month “season.” Here’s why:

• It’s impossible to predict a beginning or end to a locale’s heartworm “season.” The period when heartworm can be transmitted is when temperatures are warm enough to incubate infective, third stage (L3) heartworm larvae in the mosquito. The warmest months of the year in the Northern Hemisphere—July and August—are peak months for heartworm transmission, while maturation of larvae ceases at temperatures below 57oF (14 oC).

heartworm in dog heart

However, predicting the first and last days of the year when mosquitoes can survive is nearly impossible, given the wide variability of weather in many parts of the country. Furthermore, mosquitoes tend to gravitate to warmer environments such as crawl spaces, entryways and even indoors as the weather turns cooler. A veterinarian I know who lives in Indiana tells me he changes his furnace filter the first of every month and has never had a month when there aren’t at least a few mosquitoes trapped on the filter. He attributes this to the sheltering presence of shrubbery near the fresh air intake on the outside of his house.

Meanwhile, the “heat island” effect extends the number of days heartworms can be transmitted. This phenomenon is produced by the presence of buildings, parking lots and other urban structures that retain heat during the day, creating microenvironments with the potential to support the development of heartworm larvae in mosquito vectors during colder months. This is a significant factor when you consider that 2020 census data revealed almost 83% of the U.S. population lives in cities and towns.

• Mosquito vectors are expanding their territory and adapting to their environments. The lifespan of mosquitoes varies by species and environment. According to the American Mosquito Control Association, while most female mosquitoes live for 2-3 weeks, some species that over-winter in garages, culverts and attics can live up to 6 months. Several mosquito species, including Aedes albopictus, Aedes sticticus, Aedes vexans, Ochlerotatus canadensis, and Anopheles quadrimaculatus, that are known to be heartworm vectors are also known to be long-lived, surviving from 2 or 3 months up to 4 or 5 months.

American Heartworm Society 4 Seasons Infographic

In addition, the distribution of mosquito species that transmit heartworms is dynamic. A perfect example is Aedes albopictus, the Asian Tiger Mosquito. This species was introduced into the U.S. in the 1980s and continues to expand its range. Initially documented in Texas, it can now be found as far north as New England.

• Many heartworm preventives prevent more than heartworm. Numerous common heartworm preventives are broad-spectrum parasiticides. Some protect pets against intestinal parasites, such as roundworms, hookworms, tapeworms and whipworms, while others also provide protection again external parasites like fleas, ticks and mites. The life cycle of these different parasites varies significantly, with some surviving indoors and others transmitted year-round and/or in cooler seasons than heartworms. Administering broad-spectrum preventives year-round protects the pet’s health while also protecting the owner from possible infection with zoonotic parasites.

• Year-round administration aids adherence. I once heard a speaker say, “An action, if repeated, becomes a pattern; a pattern, if repeated, becomes a habit.” As practitioners, we want heartworm prevention to be habit forming, and year-round preventive administration can help owners be successful.

Social psychologist Phillippa Lally, PhD, conducted a study published in 2009 that revealed that it can take anywhere from 18 to 254 days for a habit to develop, depending on the individual and the concept. While we can’t know how long a particular client will take to form a heartworm prevention habit, we can be confident knowing that year-round behavior repetition is more conducive to habit formation that on-again, off-again administration. Creating context for the behavior also help clients get in a heartworm prevention groove. Whether that translates to giving a heartworm preventive on the first Monday of every month or an annual injection at the veterinary office each April, such cues can help establish the behavior as a habit and improve adherence. Fortunately, we have oral, topical and injectable products with different intervals for administration. Taking the time to understand the owner’s lifestyle and preferences can increase the compliance odds.

Given the ubiquity of mosquitoes in much of the U.S. and the widespread presence of wildlife and domestic dogs that carry infective heartworm larvae, it is unlikely that heartworm disease will lose its “endemic” status anytime soon. Fortunately, with year-round administration of convenient and affordable medications, heartworms are almost 100% preventable in pets.

This VETgirl online veterinary continuing education blog is sponsored by the American Heartworm Society, whose mission it is to lead the veterinary profession and the public in the understanding of heartworm disease. Every three years, the AHS sponsors its Triennial Heartworm Symposium—a premier C.E. event that draws speakers and attendees from around the world to the New Orleans Ritz Carlton Hotel. An early-bird discount in registration fees is available to attendees who register by July 15th. The 2022 symposium will be held September 8th through the 11th.

Please note the opinions in this blog are the expressed opinion of the author and not directly endorsed by VETgirl.

American Heartworm Society logo

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