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Heartworm Disease FAQs


Common questions about heartworms

How does heartworm disease affect dogs and cats?

Infected mosquitoes inject tiny immature worms called microfilariae into our pets. The microfilariae swim through the blood vessels and create turbulence that damages blood cells and vessel walls. As the worms mature, the damage increases causing clotting, scarring and narrowing of blood vessels. When matured, adult heartworms can measure up to 12 inches in length.

Heartworms increase the blood pressure, forcing the heart to pump harder. This increased demand can cause damage to your pet's heart. The more worms invading the blood vessels, the faster the damage occurs. Worst of all, heartworms can live inside your pet for as long as 5 years!

Pets with advanced heartworm disease may show signs of heart failure including frequent coughing, tiring easily, abdominal swelling, decreased appetite, weight loss, fainting and blood clotting problems.

Symptoms are caused because heartworms prefer to live in the heart and main pulmonary artery in the lungs. If the pulmonary artery or smaller arteries weaken, heartworms enter the air passages and can be coughed up. Sometimes a pet that's coughing up worms will appear to be vomiting, but the worms are actually coming from the lungs.

Heartworm disease is nearly 100% preventable if your pet is given a monthly heartworm preventative

Top vet-recommended heartworm preventatives


When does my pet need heartworm protection?

The American Heartworm Society recommends that pets remain on heartworm protection year-round because when the medication isn't given consistently, pets are left vulnerable to becoming infected. Heartworm disease has been reported in all 50 states at all times of the year. The American Heartworm Society also feels it is easier for most pet owners to remember to give a medication every single month than to give it a few months of the year.

Are heartworms able to transmit between pets?

Heartworm infections do not go directly from one pet to another. However, if a pet has a heartworm infection and is bitten by a mosquito, the mosquito can pick up the heartworm microfilariae and transmit the disease to other pets in the area.

Can humans get heartworms from mosquitoes?

Yes, humans can get heartworms from mosquitoes. However, heartworms don't usually circulate to the heart in humans, as they do in pets. Instead, the larvae encyst in the lungs or eyes. Fortunately, heartworm infections in humans are rare, but it's one more reason to make sure your pet doesn't harbor them.

Why do heartworm preventatives require a prescription?

Federal law requires a prescription for heartworm preventatives because giving this medication to pets that already have heartworm infections can cause serious illness or death. For your pet's safety, your veterinarian needs to assess heartworm status before prescribing medication.

Are heartworm preventatives harmful to some dog breeds?

The normal dose should not be a problem, but giving 10-20 times the normal heartworm disease prevention medication dose is dangerous for dogs with Collie genetics (Australian Shepherds, Shelties, Collies, Border Collies). The normal dose that kills the developing heartworm larvae is safe. Higher doses of these medications, such as those used in cattle and horses to kill intestinal parasites, can harm dogs with Collie breeding. Latest research shows that all drugs of the macrolide family (ivermectin, milbemycin, moxidectin, doramectin, selamectin) can cause toxicosis in Collie breeds.

What should I do if I miss a dose of heartworm prevention medication?

If there has been a lapse in heartworm protection, you should consult with your veterinarian as soon as possible. Your vet may perform a heartworm test on your pet, or may instruct you to re-start your pet on heartworm prevention medication. Your veterinarian may want to re-test your pet 6 months later as well, since heartworm infection cannot be detected for approximately 6-7 months following initial infection.

Why does my pet require a blood test before being given heartworm preventatives?

Before starting heartworm prevention, veterinarians test to make sure the pet doesn't already have a heartworm infection. Giving heartworm disease prevention medications to pets that already have infections can cause severe illness or death.

Pets younger than 7 months old can begin heartworm prevention without testing, but the vet may recommend testing 4-7 months after starting the medication. This is because it takes up to 7 months after being infected for microfilariae to mature to the point they produce a positive test.

After a pet is bitten by a heartworm-infected mosquito, how long does it take before the blood test is positive?

It takes up to 7 months for the heartworm microfilariae to mature to a stage where infection can be detected and the test is positive.

Are heartworm prevention medications harmful to my pet?

Heartworm prevention medications have been tested and confirmed safe by the FDA.

Can pets be allergic to heartworm preventatives?

Yes. Pets can be allergic to medications for two reasons. First, they may be allergic to the medication itself, just like humans can be to penicillin. Second, they can be allergic to flavoring agents, such as beef, pork, corn, wheat, milk products, soy, brewers yeast, etc. If a pet has allergies to these, consult with your veterinarian for an alternative.

How can I encourage my pet to take the heartworm prevention medication?

When giving your pet oral heartworm prevention medication, use something moist and tasty to get your pet's saliva flowing. For example, offer a bit of cheese, liver sausage, or hamburger.

Is heartworm prevention medicine safe if I already use a monthly flea preventative?

Be sure to tell your veterinarian about any medications you are already giving your pet prior to starting heartworm prevention. Recent innovative heartworm and flea prevention combination products like Trifexis may be easier to give your pet, with the added benefits of protection against multiple parasites in a single dose.

Will switching heartworm preventatives be harmful to my pet?

Pets can switch from one heartworm medication to another only with their veterinarian's approval, and following a negative heartworm test.

If my pet is heartworm-positive, should I still administer a monthly heartworm preventative?

See your veterinarian who will assess your pet and determine an appropriate treatment plan.


Max's Tip: Innovations in heartworm and flea prevention can make it easier than ever to protect your pet against multiple parasites at once! Read our article covering combination heartworm & flea preventatives.  
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