Heartworm Incidence on the Rise: What Can Veterinarians Do?

Although the hotbeds of heartworm disease haven’t changed dramatically in the past 3 years, new data from the American Heartworm Society (AHS) show that the average number of positive cases per veterinary practice has been inching upward. What’s more, the Companion Animal Parasite Council (CAPC) predicts that the incidence of heartworm infection will be above average nationwide this year. American Veterinarian™ spoke with AHS President Chris Rehm, Sr, DVM, and CAPC President Craig Prior, BVSc, CVJ, to learn more about this news and how veterinarians can help keep their patients protected against the potentially deadly disease.

“It’s timely that both organizations are coming out with information at the same time,” says Dr. Rehm, “because it provides ammunition to help veterinary practitioners step up their educational efforts with pet owners.” While the AHS incidence study is based on a retrospective look at heartworm incidence in 2016 and the CAPC information is predictive for 2017, both sets of information are based on diagnostic data that identify trends in heartworm incidence.

“Unfortunately,” says Dr. Rehm, “both models raise cause for concern, as these data indicate that heartworm incidence is on the rise and may continue to rise. It’s more important than ever that veterinarians recommend year-round heartworm prevention and stress on-time preventive administration with their clients.”

Current Incidence Numbers

The AHS 2016 Incidence Survey revealed that the average number of dogs diagnosed with heartworm disease in 2016 rose by 21.7% compared with 2013 (the AHS conducts its survey every 3 years). Of practices responding to the question, 23.3% reported seeing more heartworm cases in 2016 than in 2013 while 19.8% reported a decline in their practice areas.

No US state is heartworm-free. The survey showed that the top 5 states in heartworm incidence in 2016 were Mississippi, Louisiana, Arkansas, Texas, and Tennessee—all states that have been in the top tier since the AHS began tracking incidence data in 2001. Rounding out the top 10 states with the highest incidence were South Carolina, Georgia, North Carolina, Alabama, and Florida. Among the top 10, only Alabama, Louisiana, and Texas saw decreases in the per-practice averages of dogs diagnosed, while increases were noted in the other 7 states.

Among veterinarians who reported a decrease in heartworm incidence since the 2013 survey, 64% attributed the change to owner behavior, including increased use of heartworm preventives and improved owner compliance. Veterinarians who reported incidence increases agreed; almost half (47.8%) cited failure to give preventives, skipped doses, or failure to give preventives year-round as contributing factors. Dr. Prior concurs. “A huge part of the problem,” he says, “is that owners are not giving the preventives as prescribed—every

30 days, year round.”

Heartworm disease is complex and involves many factors, from weather patterns conducive to transmission to the presence of vectors to the movement of infected animals fromendemic to nonendemic areas to pet owner behavior and the on-time administration (or lack thereof) of heartworm preventives. According to CAPC, the emergence of resistance to macrocytic lactones may also be also a contributing factor. A simple fact remains, however: Heartworm disease is present in every state in the country, and where there are infected dogs or other canids, such as coyotes, native pets are at risk.

Looking to The Future

According to CAPC’s 2017 parasite forecast, the only area of the country expected to see below-normal heartworm activity this year is western Texas. The forecast also predicts that the hyper-endemic prevalence seen in the lower Mississippi River region will be even higher than normal. Veterinarians in the Rockies and westward, where heartworm is traditionally not seen, may see a problematic rise in infections among their patients.

CAPC and the AHS agree that shifting weather patterns, including milder temperatures and increased precipitation, have created ideal breeding conditions for mosquitoes across the country. Warm and wet weather is often associated with increases in mosquito populations and longer periods of mosquito exposure and heartworm transmission.

“Looking at areas such as the western United States,” says Dr. Rehm, “it’s very possible that an uptick in heartworm incidence can be expected.” The heartworm incidence rate in states such as California was down in 2016, which is not surprising given the extended drought conditions in much of the west. “Now that those same areas have experienced a dramatic reversal in weather conditions and above-average precipitation, it’s highly possible that growth in mosquito populations will lead to increases in heartworm transmission in the future,” notes Dr. Rehm. “Higher incidence rates could very well be expected in 2017—and perhaps even more so in 2018.”

Reducing Heartworm Incidence

To prevent further cases of heartworm infection, veterinarians must convince more pet owners to use preventives and to use them throughout the year— with no lapses. How can veterinarians convince pet owners to get on board?

“The message to pet owners,” says Dr. Rehm, “is this: Not only is treatment of heartworm infection expensive, while treatable in dogs, infection can be fatal or have a life-long impact on the dog’s health (it can also be fatal in cats and ferrets—and there’s no approved treatment for these species).”

What’s more, Dr. Prior notes, many of the preventives available today not only prevent heartworm disease,”but they also treat and/or prevent intestinal parasites (some of which are zoonotic) and other parasites.” He recommends that veterinarians discuss heartworm disease with every client, during every visit. “Open up conversations regarding the risk of heartworm for their pets—dogs and cats,” he suggests. “Use Facebook, Instagram, and Twitter to engage your clients, then use the CAPC maps to educate them.” Practices can also sign up with CAPC to receive automatic monthly parasite updates for Facebook. Likewise, the AHS shares a continuous stream of infographics, facts sheets, videos, and other engaging client education resources— as well as resources for veterinary and staff education—through its Facebook and Twitter pages.

In addition to statewide data, CAPC offers prevalence data that localizes reported parasitic disease activity at the county level for veterinarians to use in their discussions about annual testing and year-round protection. This information is available for free at the CAPC website (capcvet.org). The annual CAPC parasite forecasts provide important information to help veterinarians and pet owners understand that parasites are a true risk to both pets and people. With significant shifts in prevalence, the CAPC maps can serve as a critical educational tool for veterinary practices.