Downeast Dog News

Heartworm update

By Judith K. Herman DVM, CVH | Aug 01, 2019

Q.  Can you tell me about heartworm disease?  My little dog is mostly indoors, so how would my dog get it?

A.  Heartworm disease use to be rare in Maine especially north of Bangor. With the higher temperatures, wetter environments, and the large influx of southern dogs into Maine, more dogs are at risk for heartworm (Dirofilaria immitis) infection than in the past. To understand heartworm disease we need to know the life cycle of this parasite.

So what is heartworm?  It is a spaghetti-like worm that lives in the heart of the dog. Dogs, coyotes, and other canids are the natural host, which means the worm will thrive and reproduce in them. Any canid that has adult male and female heartworms can infect another dog through mosquito bites.

A mosquito bites an infected dog and ingests the heartworm baby (microfilaria). The parasite develops into the infective stage in the stomach of the mosquito. Your dog is bitten by the mosquito, which transmits the larvae into your dog.  The developing larvae are in the tissues for the next one to two months. After that, the developing adult is in the bloodstream and heart for four to five months.  Heartworms need to have male and female adults to produce the microfilaria. In the heart, the mature adult produces microfilariae for the next 5 to 7 years.

The cycle then repeats itself.

The recommendations from the American Heartworm Society (AHS) have been updated in 2018.  These guidelines are based on the 2016 Triennial Symposium of the American Heartworm Society.

Here is a summary of the updates and changes:

Diagnostics: Most clinics have the antigen test made by Idexx or Abaxis for heartworm. These tests can test for heartworm alone or combined with testing for tick borne diseases. The AHS is also recommending doing microfilaria testing.  Testing for microfilaria is what veterinarians used to do before the advent of the antigen test.

Prevention: The AHS recommends year-round administration of an approved heartworm preventive drug because of the documentation of resistant subpopulations of heartworm. They also recommend using a bug repellent to reduce the risk of your dog being bitten by mosquitoes. It is also recommended to avoid taking your dog outside at peak mosquito activity, which is dawn and dusk. Another way to reduce the risk is by eliminating mosquito friendly environments. This is done by removing standing water whereever possible.  Breeding grounds for mosquitos are found in buckets, birdbaths, stagnant ponds just to name a few.

Heartworm has been diagnosed in all 50 states and around the world. Relocation of microfilaremic dogs and expansion of the disease in our wild canids are contributing factors.

Another extremely important reason for the spread of heartworm disease is the rise in temperature and humidity needed to support heartworm. With these changes, the ingested microfilaria matures into the infective stage (third stage larvae) in the intermediate host, the mosquito.

Current heartworm preventative medicines are effective, but there is concern for the growing pockets of resistant heartworm populations.


Judith K. Herman DVM, CVH

Animal Wellness Center

Augusta, Maine