How to Prevent Your Dog from Getting Heartworms

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Dogs are the natural host for a serious, and potentially fatal, roundworm parasite, referred to as heartworms. Heartworms, scientific name Dirofilaria immitis, can be found throughout the United States and parts of Canada. These horrible critters are transmitted when immature larvae are deposited into your dog through a mosquito's bite. These larvae travel through your dog's muscle tissue, into their bloodstream, and take up residence in the blood vessels that lead from the heart to the lungs. Here the larvae mature. Once mature, the worms can move into the heart, or reside in the blood vessels. If they occur in large numbers, they create blockages that prevent the flow of blood between the heart and lungs, which results in impaired heart functioning for your dog and ultimately death, if left untreated.

Understanding Heartworm in Dogs

Symptoms may take several months to appear as heartworms mature and multiply. Symptoms of heartworm infections include:

  • Respiratory difficulties, cough, labored breathing, abnormal lung sounds

  • Lethargy, weakness, and intolerance to exercise

  • Loss of appetite, weight loss

  • Fluid accumulation and swelling in the abdomen

  • Enlargement of the liver

  • Acute heart failure, caval syndrome from blockages in the heart, breathing difficulty, pale gums, dark-colored urine

There are several tests vets use to determine the presence of heartworms, these include:

  • Blood tests for antigens produced by the worm

  • Microscopic examination of tissue sample for the presence of microfilaria (baby heartworms)

  • Radiography, including x rays and ultrasounds to visualize worms present

  • Blood tests, chemistry, count, electrolytes

  • Urinalysis

If your dog has heartworms they can be treated with antiparasitics to kill the worms, however, these medications contain toxic substances, like arsenic compounds, that can have serious side effects. Reaction to the medication, or to the presence of a large number of dead worms that can still cause blockages in the blood vessels, can occur in your dog and complications can be serious or even fatal. In some cases, where large numbers of worms are present in the heart and pulmonary vessels, surgical removal is necessary. Damage that has occurred to heart, lung and other organs is irreversible.

Due to the serious nature and effects of this parasite, and the difficulty resolving infestation, prevention of heartworms is considered the best option for pet owners and preventative methods are available.

Causes and Prevention of Heartworm Infestations

Heartworms are a parasitic roundworm that mosquitoes acquire when they bite an infected host and pass on to another host during their next blood meal. These nasty creatures can grow up to a foot long, as adults, and reside in blood vessels, the heart, and pulmonary vessels of your dog. Many animals can contract heartworms, including cats, coyotes, and foxes that live in close proximity to dogs. Although the symptoms in other species may differ in severity and consequence, heartworms’ natural host is the dog.

Mosquitoes are able to pass microfilariae, immature heartworms, they pick up from any infected host while feeding, develop the heartworms into to larval form, and then pass them on to their next meal. Heartworms in canine hosts travel through body tissues into the heart, lungs and associated blood vessels, where they develop into adult heartworms. The worms cause blockages and damage to blood vessels and organs, including the heart for which they are famously named. The cycle takes about 6 months. If they are not discovered and treated, heartworms will multiply in your dog, and when they occur in large numbers your dog will become very ill and develop cardiac, pulmonary, circulatory, and organ problems that will become fatal. Treatment with medications that can have toxic effects or can result in large worm die off can cause complications once infection has occurred, so preventing heartworm infestation is the best option for you and your dog.

Preventative Medication

The most commonly recommended form of prevention for heartworm in your dog is a monthly medication that can be administered orally or topically and absorbed into the skin.


Although the medication is very effective, there are always exceptions, and therefore it is also recommended that you have your dog tested yearly for heartworm infection so that heartworms can be treated at an early stage when few are present and they have not caused extensive damage to vessels and heart.

Reducing Risk

An additional preventative method for heartworms is to prevent your dog from being bitten by mosquitos. You can use a mosquito repellant especially designed for dogs to decrease mosquito bites, keep your dog inside as much as possible in the evening or during high mosquito activity times, remove standing water near your home where mosquitos breed or use a variety of mosquito deterrents such as bug lights or citronella plants or candles in your yard.

Importance of Preventing Heartworms

Heartworms are a serious and sometimes deadly parasite that can be difficult to treat once infestation has occurred. Preventative medications are widely available and easy to administer, and regular testing can reveal an infestation early so complications will be minimized. A side benefit of regular testing is that other parasites and health problems can be picked up as a result.

Because treatment is expensive, invasive surgery may be required, and complications can result from worm die-off or toxicity of the medications used, prevention is a better option than treatment.

Prevention is the Most Effective Strategy

The heartworm is a parasite that has serious repercussions for our dogs if infestation occurs and is left untreated. Untreated heartworm infestations result in large heartworms and large numbers of heartworms that cause damage to the blood vessels of the heart and lung and can migrate into these organs, causing permanent damage that is eventually fatal. Even when treated, complications can occur and damage done is irreversible. Prevention with regular preventative medications and routine testing is recommended for dogs in the U.S. and any areas where heartworms can occur.

Preventing mosquito bites can also prevent infestation in your dog as heartworms are transmitted through mosquito bites. This potentially fatal disease is easily preventable and treatment is more effective if obtained at an early stage. Pet owners should be aware of the danger to their dogs and discuss appropriate preventative strategies with their veterinarian so their dogs are not infected with heartworms.