BY: SAMANTHA BARTLETT, DVM
The American Heartworm Society conducts a survey of heartworm positive cases in dogs every 3 years. The lasts survey, published in 2023, shows the numbers from the end of 2022. Over the last 20 years, the numbers of heartworm positive dogs has continued to climb and the incidence of reported heartworm positive cases is increasing in states with traditionally low numbers. According to AHS data, the number of heartworm positive dogs increased from around 800,000 in 2001 to over 1.2 million in 2022.
The spread of heartworm to traditionally non-endemic areas can be explained by several factors, including lack of owner compliance in giving prevention, transport of positive dogs out of state, climate change contributing to weather conditions conducive to increased mosquito populations and expansion of territories of wild canids. States known for having the highest incidence of heartworm disease include Georgia, Mississippi, Texas, Arkansas and the Carolinas. Although the rates of heartworm are historically low, Kansas, Connecticut, Massachusetts, North Dakota, Washington and Oregon saw unexpected increases in their numbers this last survey.
Dr. Tanja McKay, a professor at Arkansas State University, published study findings showing that mosquitoes are more attracted to infected canids than non-infected. The changes that occur once a dog is infected contribute to the attraction of mosquitoes that feed on the infected dogs and subsequently spread the infection to another dog. Once a dog is infected with heartworm, the pulmonary tissue already starts to become necrotic long before the dog tests positive. The necrosis of tissue causes the dog to exhale VOCs (volatile organic compounds) that attract the mosquitoes. This also occurs within populations of wild canids such as wolves and coyotes.
Disease density and spread are influenced by weather conditions such as extreme drought and hurricanes. Seasonality can also affect populations of mosquitoes, with decreased incidence of heartworm positive cases in cooler months. However, areas that artificially warm such as congested urban areas, can contribute to increases in mosquito populations in cooler months. Further contributing to the spread to other states is the practice of shipping stray pets to other states for adoption. Many of these pets are tested for heartworm at the time of spay or neuter and are negative, but follow-up is lost in the process of transport and rehoming. These pets that area actually heartworm positive, then move into a new community and serve as reservoirs for mosquitoes to spread infection.
The veterinary community can contribute to combating the spread of heartworm disease by explaining the ease by which the disease is spread and the role of preventatives in protecting pets from heartworm infection. Reasons pet owners do not routinely purchase and give prevention involve several factors including, forgetfulness, cost, fear of side effects and lack of knowledge about the disease process and its transmission within the community. Veterinarians can help by ensuring clients leave the clinic with a prescription or dose(s) of prevention in hand. In addition, monthly reminders to give prevention may be helpful for clients that tend to forget to give the medication routinely. Other helpful reminders include yearly reminders for testing as well as monthly, twice yearly or yearly reminders to buy prevention based on the clients’ purchase habits.
More information on treatment and prevention as well as marketing and education resources can be found on the American Heartworm Society website at www.heartwormsociety.org.
