Heartworm disease in dogs

Mar 21, 2022

Heartworm in dogs is a serious disease that can kill your dog (and cat, too). Heartworm disease causes permanent damage to your dog’s heart, lungs and arteries. This damage is so long-lasting that your dog’s health and quality of life may be diminished long after the parasites are gone.

Where do heartworms come from?

In short, they come from mosquitoes. Mosquitoes breed in standing water and they are everywhere. As a result, heartworm disease in dogs has been diagnosed in all 50 states. Host animals including stray and neglected dogs, wolves, foxes and coyotes often carry the worm. (Ferrets and sea lions are also potential hosts, but this is rare.) 

Some very high-risk areas include the Atlantic and Gulf coasts and along river tributaries. But don’t fall into the trap of thinking that living in snow country keeps your pet safe from mosquitoes and heartworm disease! Mosquitoes are tricky and highly adaptable — some mosquito species even successfully survive winter indoors. Even in Minnesota!

Living in the city versus the country is no guarantee that your dog will escape the disease either, especially with the increasing temperatures in cities due to heat islands. Even keeping your pet indoors is not a 100 percent guarantee of prevention, since blood-hungry mosquitoes do manage to find their way inside your home. 

How heartworm in dogs is transmitted

The way the mosquito transmits heartworm in dogs begins with a blood meal, meaning that the insect feeds on a host animal which has already been infested by the heartworm. There are at least one (and sometimes up to several hundred) foot-long adult worms already living inside the host animal. 

Happily ensconced in the host’s heart and arteries, adult female heartworms produce microscopic baby worms called microfilaria that circulate in the host’s bloodstream. When the mosquito bites and sucks up its blood meal, these tiny parasites tag along. And when the insect lands on its next source for a blood meal, namely your pet, the larvae are passed into the new host as the mosquito feeds. Over the next 6 months, the microfilaria grow into larvae and migrate to your dog’s arteries and heart, where they grow into adult worms and continue to multiply.

What are the first signs of heartworm in dogs?

  • Mild, persistent cough
  • Less playful, less interest in taking walks
  • Fatigue
  • Loss of appetite
  • Weight loss
  • Often, there are no signs of the disease until it becomes advanced

What are the signs of advanced heartworm?

  • Swollen belly, due to excess fluid in the abdomen
  • Labored breathing
  • Pale gums
  • Dark or bloody urine

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What can I do to prevent heartworm in my dog?

Make prevention of heartworm a key priority for your dog!  The American Heartworm Society recommends that you “Think 12” which means: (1) get your dog tested every 12 months for heartworm and (2) give your dog preventive treatment year-round, 12 months a year.

A variety of heartworm preventive medications are available by prescription from your veterinarian including: chewable, topical, and injectable. They all work by killing the microfilaria from mosquitoes before they can grow into adult worms inside your dog. The injectable heartworm preventives are available for use once or twice a year and are only administered in veterinary clinics. The oral and topical preventives for heartworm are generally administered at home monthly, and as your dog’s dedicated human, please take this process seriously. Talk to one of our vets at Airvet to learn which preventive is best for your dog.

Guidelines for preventing heartworm in dogs

  • Start your puppy (under 7 months of age) on heartworm prevention medication, even before your puppy can take its first heartworm test. This is wise, since it takes at least 6 months for a dog to test positive for heartworm after being bitten by a carrier mosquito. 
  • Have your puppy tested 6 months after your initial vet visit, and then again after another 6 months. After that, have your dog tested annually. If your dog tests positive, your veterinarian will recommend a treatment plan. The treatment for heartworm disease is expensive and poses risks. Again, it is much better to prevent the disease than to allow worms to form in your dog’s heart. 
  • For an adult dog over 7 months of age that hasn’t been receiving preventive medication, start with a blood test. If the test is negative, your veterinarian will prescribe heartworm prevention. Take your dog back to your vet in 6 months for a second test. After that, have your dog checked annually. Of course, if your dog tests positive, your veterinarian will recommend a treatment plan immediately.

Annual testing is necessary, even when your pup receives preventive heartworm medication year-round. If you miss just one dose, or give the prevention to your dog late (more than a month between pills), or if your dog vomits or spits out the pill, your dog is considered unprotected and at risk for heartworm.

Take care to use all heartworm medications before the expiration date listed on the package, to ensure maximum effectiveness.

Bonus! Many heartworm prevention products also prevent your pooch from getting intestinal parasites, including roundworms, that can pose serious health risks for humans, especially children.