Pet Health

Heartworm Disease
& Prevention

Dogs are the natural host for heartworms. Dogs can become infected after an infected mosquito bites them. Once infected, the larval stages of the heartworm make their way through the tissues to eventually reach the heart and pulmonary arteries. This migration can take 3-4 months. Once in the heart and pulmonary arteries, the larvae mature into adults. This maturation can also take several months. Adult heartworms can reach up to 14 inches in length. Once mature, adult male and female worms reproduce (if both male and females present), and the female worms will produce microfilaria that are released and circulate in the bloodstream. The cycle begins again when a mosquito takes a blood feeding from an infected dog. Once inside the mosquito, the microfilaria incubate, and after a few weeks of warm weather, the mosquito can bite and infect another dog or cat.

The severity of heartworm in dogs is directly correlated with the number of worms or “worm burden”, i.e the greater the number of worms, the worse the disease. The parasites can cause respiratory and heart disease, with coughing and exercise intolerance as the most obvious clinical symptoms. If untreated, heartworm disease leads to congestive heart failure and death.

Heartworm infection can be treated, but treatment is expensive and risky for the pet. The treatment involves giving a chemical compound, melarsomine, which kills the worms. When the worms die, they are released into the bloodstream and pumped from the heart to the lungs. The dead worms become foreign material in the lungs to which the dog’s immune system must react. This process carries risks. If there are large numbers of worms that die all at once, the dog can suffer severe distress leading to respiratory and cardiac arrest. The most recent protocol for treatment of heartworm disease involves giving the melarsomine in 3 injections, in intervals designed to decrease the potential for causing distress to the dog. During this process, the dog is kept confined for 1 month or longer.

At EEVMC, we follow the American Heartworm Society guidelines by recommending annual screenings for Heartworm disease. The test, called “4dx”, requires a few drops of blood, takes 10 minutes to get results, and also screens for tick diseases such as Lyme disease, Ehrlichiosis, and Anaplasmosis.

Heartworm can be prevented with a monthly preventative. Here is a list of some heartworm preventatives currently available:

  • Heartgard Plus oral chewable tablets
  • Interceptor Plus oral chewable tablets
  • Revolution Topical Solution
  • Advantage Multi Topical Solution
  • Trifexis Oral Chewable Tablets
  • Dogs should be started on heartworm prevention prior to 6 months of age. If they are older than 6 months of age when starting, they should be tested for heartworm disease with the 4dx test prior to starting the preventative.

    Common Questions:

    Why do we recommend year round prevention if the mosquito population disappears in the cold weather?
    There are several reasons why we recommend year round prevention. First, almost all of the heartworm preventatives are also effective gastro-intestinal dewormers. Dogs are commonly exposed to gastro-intestinal parasites in the environment, such as roundworms, hookworms, and whipworms, especially if they graze on grass or eat feces. Monthly prevention keeps these common exposures from preventing serious disease. Second, many of our clients travel with their pets to the south Atlantic coast in the fall and winter. This area is a year-round hotbed of heartworm disease, so it is critically important to make sure dogs visiting these areas are on a monthly preventative. Third, because of the recent global warming trends, the winter range of mosquitoes may be spreading northward. Since heartworm infection can cause such severe disease, it is best to play it safe and keep dogs on preventative year round.

    Why do we recommend annual screening for heartworm if dogs are on year-round prevention?
    First, no preventative is 100% effective. There have been some recent reports in Texas suggesting that the heartworm parasite may be developing some resistance to the preventatives we have been using for decades. In the rare incident of prevention failure, we want to be sure we catch heartworm disease early so treatment will carry less risk. Second, as mentioned previously, the 4dx screening test also screens for tick diseases. It is helpful for us to know if your dog has been exposed to Lyme, Ehrlichia, or Anaplasma, so we can recommend the proper course of action.

    More information on heartworm disease is available at:
    Note: Some of the information and language on Heartworm Disease above has been obtained from the website of Dr. Lisa Bennett, DVM- Beaver Lake Animal Hospital, Issaquah, WA.

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