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Preventing Heartworms In Dogs

Most dog owners are aware that their pets may be susceptible to heartworms, but it's less clear to many people exactly what heartworms are, the problems they cause and the best ways to prevent them. Heartworm disease, however, merits attention as a serious threat that can be painful to your dog and is both risky and expensive to treat. In the worst cases, the disease can be deadly.

Happily, heartworm disease is almost 100-percent preventable. The best advice? Reduce your dog's risk by seeing your veterinarian for recommendations on regular preventive treatments.

What are heartworms?

Heartworms, or Dirofilaria immitis, are parasites that affect dogs, cats and ferrets, and some wild animals. Heartworms are transmitted via blood through mosquitoes. When a mosquito bites an infected animal, the blood they take in can contain the immature worms, which are also known as microfilariae. Within the mosquito, the microfilariae then develop into heartworm larvae. When an infected mosquito bites another susceptible animal, the larvae are transmitted into this new host.

Once established in a new host, the larvae may mature into adults. As they persist, heartworms move to the blood vessels of the lungs and heart, eventually taking up primary residence in the pulmonary artery, the large blood vessel that carries blood from the heart to the lungs. Within six months, adults can grow up to 14 inches long.

The adult worms can live up to seven years in a dog. Within a host, adult worms may mate and produce new microfilariae. When a mosquito bites this animal, it takes in the microfilariae, and if it bites another animal, the lifecycle begins again in a new host.

The risk of heartworm infection begins to rise in the spring, when mosquitoes become more prevalent and immature worms have had a chance to incubate in the mosquitoes' bodies.

Once the dog begins to show signs of illness from heartworms, the infection is classified as heartworm disease. If you see symptoms that you suspect are caused by heartworm disease, see your veterinarian right away.

Signs of heartworm disease include:

  • Chronic coughing
  • Difficulty breathing
  • Fatigue
  • Loss of appetite
  • Rough hair coat
  • Fainting

The disease can be deadly. The worms cause inflammation that damages arteries, and if enough worms are present, the heart has to work harder to pump blood through the clogged-up arteries. The heart can become enlarged, and the increased strain can cause heart failure.

How do you prevent heartworm disease?

It's much easier on your pet, and more economical, to prevent heartworms rather than treat them after they've already become established.

Rescue heartworm treatments are often effective for dogs, but they can be risky, and they're also expensive. Most treatments administer medicines that are designed to kill the heartworms without harming the pet, but the treatments can be harsh and have negative side effects. Rescue treatments also require a dog to be confined to a quiet place for long periods during the recovery period to prevent adverse reactions from the dog's immune system and the dead worms from causing complications in the lungs. Occasionally, severe cases can even require major surgery.

Regular preventive treatments are effective and highly recommended by veterinarians. They'll save you the cost of rescue treatments, and more importantly, could help your pet avoid a lot of pain. Most of these preventive treatments kill any immature worms that enter the body, before they have a chance to grow and cause serious damage. In addition, most monthly heartworm preventives have activity against intestinal parasites, according to the American Heartworm Society (AHS). Many of these intestinal parasites can also infect people—another good reason to use preventive treatments.

There are a number of FDA-approved heartworm preventives on the market, in a variety of different formulations. These include daily and monthly tablets and chewables and monthly topical (skin) treatments. If you have trouble remembering a monthly treatment, ask your vet about injections that provide protection up to six months. Your veterinarian can help select the best preventive treatment and method of administration for your dog.

Before placing your dog on heartworm preventive, your veterinarian will probably conduct a simple blood screening to make sure your dog isn't already infected. Using preventive treatments with infected dogs won't kill existing adult heartworms and can cause severe complications. Your veterinarian may schedule regular tests in the future to ensure your dog remains heartworm free.

It only takes one bite from an infected mosquito to transmit heartworms, and so both indoor and outdoor dogs should be treated. In addition, AHS now recommends that all breeds in all regions of the country should take a preventive year-round. Typically associated with warmer areas in the past, veterinarians have reported finding heartworm disease in all areas of the United States.

Note: This information on heartworms is provided for educational purposes only and is not intended to replace the advice of your veterinarian. For further information about the best ways to prevent heartworm disease, consult your veterinarian.

Read preventing heartworm in cats.

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