Preventing and Testing for Heartworms in Dogs

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See your veterinarian for treatment guidance if you suspect your dog has heartworms.

Symptoms may not appear until a full year has passed since infection. Because of this, heartworm disease is often mistaken for another problem.

The most persistent sign is a soft, deep cough. After exercise, the cough may be so severe that that the dog faints. Weight loss, discharge of bloody sputum, listlessness, and weakness are also common.

The Heartworm Lifecycle

Start with an infected dog. This dog has adult heartworms living in its pulmonary arteries (they crawl into the heart after the dog dies). Female worms mate with male worms and produce microfilaria (first-stage larva, L1, or a “baby” heartworm). The microfilaria enter the circulation of the dog. When this infected dog with circulating microfilaria is bitten by a mosquito, the mosquito will ingest 1–2 microfilariae. If the mosquito ingests more larvae than this, it will die.

In the mosquito, the microfilariae (L1) will molt twice, to the L2 and then the L3 stage. At the L3 stage, the larvae migrate to the mosquito’s mouthparts. Then when the mosquito bites a dog, the larvae are deposited on the dog’s skin and then crawl into the bite wound left by the feeding mosquito. If a mosquito with the L1 or L2 larval forms bites a dog, it will not be transmitting heartworms to the dog. Likewise, if the L1 forms are not removed from the dog’s circulation by a biting mosquito, they will die off. The L1 stage does not “mature” into adult worms in the dog.

So, the L3 larvae that crawl into a dog bitten by a mosquito will develop in the dog’s subcutaneous tissues to L4 and finally L5 life stages. These then enter the venous system and enter the heart. They travel to the pulmonary arteries and become full-fledged adult worms, ready to reproduce.

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Heartworm Testing

When a dog is tested for heartworms, a sample of blood is drawn. The blood cells are lysed, and the remaining sample is examined microscopically for the presence of microfilariae. (This is the Knott’s test, or Filter test, depending on how it’s done).

If no microfilariae are seen, the dog is diagnosed as being heartworm negative and you can restart medication. Because of the development that the larvae must go through before becoming adult worms and reproducing, it takes, on average, 6 months from the time a healthy dog is bitten and infected until the dog has circulating microfilariae. This means that a heartworm test done less than 6 months since a dog was bitten and infected will be negative.

Dogs who have been taking Heartgard present another problem in the detection of heartworms. Heartgard will cause adult female worms already present in the dog to become sterile, so the females will not produce any microfilaria. Heartgard will not kill any adult worms. The adult worms, not the microfilariae, cause heart problems with dogs who have heartworms. It is the adult worms that we are really trying to protect the dog from when we use preventive medication.

So if a dog is on Heartgard and is tested for heartworms using the Knott’s test, chances are the dog will test negative even if there are adult worms present.

There is a different test (heartworm antigen test) for dogs who may have sterile worms. It uses a blood sample to test for antigens produced by the adult heartworms. If the dog has heartworm antigen, it has a greater than 99 percent chance of having heartworms. This test should be used on any dogs who are on Heartgard because they will not have microfilariae in their bloodstream. Likewise, if there are only low numbers of circulating microfilariae, the antigen test will give a positive result where the direct Knott’s (Filter) test may be negative. Just like the standard Knott’s test, the antigen test will be negative if the dog was infected less than 6 months ago.

It is therefore important for those dogs on the monthly medication to be tested with the antigen test rather than the Knott’s.

The video below explains more about heartworm disease in dogs:


Can another dog get heartworm by coming in contact with an infected dog’s blood (transfusion, bite)?

No. If a dog was infected and had circulating microfilaria, and these microfilariae were transplanted into a healthy dog via a transfusion, the healthy dog would not get adult heartworms because the lifecycle could not be completed within the body of the dog. A mosquito is needed for development from the L1 to the L3 stage.

Could a pregnant dog with heartworms give them to her own puppies?

No. For the same reason as above, you need the mosquito for the intermediate stages between microfilarae and adult worms. Although the placental barrier will keep the microfilarae out, even if this barrier broke down (which can happen), the pups will not be infested.

Preventive Medications

The monthly medications include Heartgard and Interceptor. Heartgard is ivermectin, and Interceptor is milbemycin oxime.

These medications work by killing any larvae that have entered the dog up to 45 days ago. They kill L3s, 4s, and 5s. These drugs are given monthly (30 days) for the convenience of giving on the same day each month and also to give you a safety margin. If you forget to give your dog their heartworm medication, you have about 15 days to remember to give it and the dog will still be protected. With the daily medication, forgetting for more than a day may result in your dog becoming infected.

The most common ways a dog will contract heartworms while on medication include:

  • Not being given medication regularly (for example, completely missed dosages)
  • Traveling from a winter environment to a summer environment, such as Florida, without giving the dog heartworm medication
  • Not weighing the dog while on the medication, so the dog outgrows the dosage
  • The dog vomits or has severe diarrhea after being given the medication

Treating Heartworm Disease

See your veterinarian right away for treatment guidance if you suspect your dog has heartworms.

Kristin Thommes contributed to this article.

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