Why Year-Round Heartworm Prevention Medication Matters
Both the American Heartworm Society (AHS) and the U.S. Food and Drug Administration (FDA) recommend that all dogs and cats in the U.S. are actively on heartworm preventives year-round. Here’s why.
Pet and Mosquito Mobility
Heartworm transmission requires both a heartworm-positive animal—usually a dog or a wild canid, such as a coyote—and mosquitoes that feed on this animal and then transmit heartworms to other pets through a bite.
While heartworms were once considered uncommon in large areas of the country—including the western U.S. and Northern states—that is no longer the case.
The movement of heartworm-positive animals is a factor, thanks to our mobile society and the practice of pets being rescued and transported from areas with lots of heartworms to areas where they previously were uncommon.
For example, rescue dogs are frequently transported from Southern states to Northern states due to overcrowded shelters down South and better adoption opportunities up North.
Mosquitoes also migrate. More than 20 different species of mosquitoes are known to transmit heartworms in the U.S., with some of the most common being relatively new arrivals in many parts of the country.
Climate Change
Heartworm transmission can only occur when mosquitoes are active and feeding on pets. However, the U.S. Environmental Protection Agency (EPA) states that average temperatures have been rising across the U.S., with more rapid increases noted since the late 1970s.
Winter seasons in particular are becoming shorter and warmer, which leads to mosquitoes remaining active longer.
Human Impact to Environment
More than 80% of the U.S. population lives in urban areas, where temperatures can be 10–20 degrees warmer than surrounding rural areas.
City living fosters the development of heat islands, where buildings and concrete retain and trap warmth. Heat from fossil fuels also causes temperatures to rise in urban areas. The result means heartworm transmission is possible to pets living in protected urban and suburban environments when the temperature just a few miles away may have dropped to freezing.
Even if heartworm transmission is significantly lower in late fall and winter than it was during spring and summer months, it may never drop to zero.
Standing Water Sources
While pet parents living in desert climates may assume their dry conditions are protecting pets from mosquito-borne disease, rising rates of heartworm disease in states like Arizona indicate otherwise.
Mosquitoes need standing water to breed, but common urban-dwelling mosquito species known to carry heartworms can breed in something as small as a flowerpot holding an inch of water.
Not only do desert areas experience winter “monsoon” seasons with rainstorms, but irrigation practices and standing water in urban and suburban areas have made it easy for mosquitoes to breed.
Sources of standing water can range from a water hazard on a golf course, to birdbaths, to small puddles left when lawns or crops are watered.
Mosquitoes Can Relocate Indoors
Mosquitoes carrying heartworm larvae can move into garages, crawl spaces, and other indoor environments, regardless of the season.
Mosquitoes can enter the warm environments of homes through doorways and window screens, where they pose a threat to both pets and pet parents.
Pets Need Protection from Other Parasites
Pet parents have many options when choosing a heartworm preventive, and most medications protect pets from other parasites besides heartworms.
Intestinal parasites, such as roundworms and hookworms, present a serious threat to pets and pet parents—since these parasites can be transmitted to people, too.
Intestinal parasite infections thrive—and may even peak—during cold-weather months, thus creating the need for year-round prevention. Many heartworm preventives also protect pets from common external parasites like fleas, ticks, and mites that thrive both indoors and outdoors.
Heartworm Preventives Work Retroactively
Heartworm preventives don’t work by preventing mosquitoes from transmitting heartworms to pets; instead, they work by eliminating new infections from heartworm larvae that were acquired the previous month.
A pet parent may assume it’s reasonable to stop prevention for their dog in November because area temperatures have dropped below freezing, without realizing that 30 days earlier the temperature was 70 degrees F and mosquitoes were alive and feeding on their pet.
Year-Round Prevention Fosters Good Habits
Pet parents who give monthly heartworm preventives to their pets need to do so on a schedule, whether that means administering the medication on the first day of the month or setting up a calendar reminder to do so.
However, starting and stopping this monthly practice can derail the habit, and result in missed doses—which in turn can lead to heartworm disease in a pet.
