A first glance at the 2022 Heartworm Incidence Map from the American Heartworm Society (AHS) might suggest that heartworm rates have stayed steady over the past three years. Dig a little deeper, however, and it’s clear that rates have continued to trend upward in both heartworm “hot spots” and in locales where heartworm cases were once rare.

The AHS conducts a heartworm incidence survey every three years, working with veterinary practices and shelters that voluntarily submit data from heartworm antigen tests run over the course of the previous year. The latest survey was conducted in early 2023 and reflects data from heartworm testing conducted throughout 2022. 

Heartworm maintains grip on Southeast; finds new footholds in cooler, drier regions

The leading states for heartworm incidence continue to be those in and adjacent to the lower Mississippi Delta, which historically experiences a convergence of conditions fostering widespread heartworm infection—a climate conducive to mosquito proliferation along with the presence of untreated dogs and wildlife that serve as reservoirs for infection.

“The states with the highest density of diagnosed heartworm cases in the latest survey were Mississippi, Louisiana, Texas, Alabama, and Arkansas,” says AHS president Jennifer Rizzo, DVM. “Meanwhile, Arkansas, Mississippi, Texas, Georgia, Florida, Tennessee and the Carolinas all saw expansions of high-density areas on our incidence map.”

States with historically low heartworm rates also saw unexpected increases and the development of new “hot spot” areas within their borders. These included Washington, Oregon, Kansas, North Dakota, Massachusetts, and Connecticut. In addition, urban areas such as Seattle and Boise in the Northwest; Bismarck in the Upper Midwest and Tucson in the Southwest all saw significant increases in heartworm rates. While no single reason can be attributed to these changes, factors involved are believed to include a greater influx of heartworm-positive animals from out-of-state, higher testing rates, higher infection rates—or all three.

Heartworm incidence rate changes attributed primarily to “human” factors

Along with submitting their numbers, practitioners completed a brief multiple-choice AHS survey on trends noted since the previous survey, which was conducted in early 2020. In this survey, participants were asked whether heartworm rates in their area have stayed the same, increased or decreased. The findings were as follows:

• Half of respondents (53%) said heartworm rates have remained stable. Significantly more respondents said heartworm rates have increased (29%) than decreased (17%).

• Among those who saw rates rise, almost one-third (32%) blamed the influx of heartworm-positive pets to their area. Poor compliance with heartworm prevention (27%) followed closely, along with weather conditions that caused an increase in mosquitoes (22%).

• Among respondents who saw heartworm rates drop, changes in pet-owner behavior, including increases in pet owners administering heartworm preventives (33%) and improvements on preventive compliance (31%) were the most cited reasons.

“While the past three years have been tumultuous for both the veterinary profession and our clients, the good news is that heartworm disease continues to be almost 100 percent preventable with faithful year-round heartworm prevention,” says Dr. Rizzo. “Whether it’s educating new pet owners about heartworm prevention or reminding long-time owners of its importance, it’s clear that the AHS and the veterinary profession must work together to continue to work together to raise awareness of this devastating disease.”

The new AHS map can now be downloaded and comparedwith maps generated every three years since 2001.