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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. The damage causes clotting, scarring, and narrowing of blood vessels. The blood pressure goes up. As the pressure goes up, the heart pumps harder to get the blood through the vessels. This causes the heart to fail. The more worms damaging the blood vessels, the faster the damage occurs. Heartworms can live from 3-5 years. That's a lot of damage.

The pet's 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 look like they are vomiting, but the worms are coming from the lungs. Pets with advanced heartworm disease have signs of heart failure including frequent coughing, tiring easily, abdominal swelling, decreased appetite, weight loss, fainting, and blood clotting problems.

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

What's the Scientific Name for Heartworm Disease?

Dirofilaria immitis.

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 some pets are developing heartworm disease. The Society 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 a mosquito bites that dog, the mosquito can pick up the heartworm microfilariae, which it can transmit to other pets.

Can Humans Get Heartworms From Mosquitoes?

Yes humans can get heartworms from mosquitoes, but the heartworms don't usually circulate to the heart, 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 the dose is missed it should be taken as soon as remembered unless it's almost time for the next regularly scheduled dose.

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 less 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 6-7 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 7 months for the heartworm microfilariae to mature so that an infection is 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 2 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 medication, use something moist and tasty to get your pet's saliva flowing. For example, offer a bit of cheese, liver sausage, or hamburger. Try following these steps:
  • Offer a treat
  • Offer the medication wrapped in treat. Let your pet see you have another larger treat waiting
  • Offer the largest treat
  • Provide water
  • Examine your pet's mouth to make sure it's empty

Why Does Some Heartworm Prevention Medication Need to Be Chewed?

An example of a heartworm preventative that must be chewed is Heartgard. If giving your pet a chewable medication, don't encourage the pet to swallow it. Encourage your pet to chew it. If your pet isn't chewing the medication, there are alternatives:
  • Switch to a a heartworm preventative that can be swallowed (Sentinel or Iverhart)
  • Switch to a topical heartworm disease prevention product, such as Revolution

Will Switching Heartworm Preventatives Be Harmful To My Pet?

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

If My Pet Is Heartworm-Positive, Should I Still Feed a Monthly Heartworm Preventative?

Perhaps. See your veterinarian who will assess your pet's heart and lungs and decide on therapy. Your veterinarian may advise a treatment that kills adult heartworms so that your pet's heart and lungs can return to normal. Or, your veterinarian may advise using a heartworm disease prevention medication continuously for 2 years to kill microfilariae and allow the adult worms to die gradually.

Heartgard provides a slower, safer kill of adult worms, but is not FDA-approved for use in heartworm-positive dogs. Revolution is approved for use in heartworm-positive dogs. Your veterinarian will decide which treatment is best.

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