I am often asked why I recommend a heartworm check on dogs yearly if their owners have them on the preventative faithfully. Is that necessary? Yes. It’s a really good idea. Here are a couple of reasons: Compliance and resistance.
Let’s face it, few people remember to give (or take their own) medication 100% of the time. A review by veterinary practices of drug sales to regular clients revealed that heartworm prophylactic products are given less than once monthly. One such drug, Heartgard, is sold at a greater than 6 month interval, even though it is sold as a 6 month supply. Most people give their dogs monthly chewables like Heartgard, Interceptor and Sentinel. These are outstanding products, but people do forget occasionally, or they skip a month. Or a dog may vomit up the preventative, obviously rendering it ineffective. Revolution is a spot-on topical monthly prophylactic which, when applied correctly, works to prevent heartworm disease. But if not applied appropriately, there will be decreased absorption which may leave a dog vulnerable to infection. And many owners stop giving the preventative through the winter. Obviously, there are not clouds of mosquitos in the cold months of the year, but we do have the occasional seventy-degree day in January when the pest can become active.
Heartworm disease (HWD) is spread by mosquitos. If a mosquito carrying HWD bites a dog, then microscopic larvae will travel from the bite site, through the muscles to the heart and great vessels of the lungs. There they grow to adult worms becoming roughly 6-8 inches in length. Obviously, the worms interfere with the functioning of the heart and damage blood vessels in the lungs. Because HWD causes so much cardiovascular damage, veterinarians emphasize disease prevention. Hence the once monthly tablets.
Diseases do have regional prevalence rates. HWD is quite common in the South, but the incidence has increased in more northern and western states over the last decade. Many rescue organizations save dogs from kill shelters in the South and some of those animals bring disease with them. Animals rescued from Hurricane Katrina certainly were responsible for an influx of heartworm-infected dogs to northern states. Then of course, people move, travel, visit around the country. This interstate movement of dogs allows for shifts in disease patterns. This is true for many diseases, not just HWD. If a heartworm positive dog moves next door, then he serves as a potential reservoir of infection for the neighborhood. The American Heartworm Society estimates that two-thirds (2/3rds!) of pets are not on heartworm prevention.
HWD is on the rise. The number of dogs diagnosed in 2017 is 21% higher than the number diagnosed in 2013. The World Health Organization calls the mosquito one of the deadliest animals on the planet. Cats can get HWD too, but the parasite doesn’t behave the same way in their bodies so cats can have bad reactions to the worm. Cats can be put on heartworm preventative also, either with a chewable pill or a spot-on topical.
There has been a concern in recent years that heartworms are developing a resistance to the common prophylactic drugs like Heartgard and Interceptor. This resistance seems to be confined to the lower Mississippi valley so far. Still, transport of dogs from the South for humanitarian or other reasons could inadvertently cause these resistant strains to spread. This is a powerful reason to focus on mosquito bite prevention and diligent administration of heartworm medications. And perform yearly testing.