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Heartworm FAQs

Drs. Foster & Smith Educational Staff
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Heartworm FAQ's

How does my pet become infected with heartworm?
Mosquitoes become infected with tiny immature forms of the heartworm, called microfilariae, when they bite an infected animal. Inside the mosquito, the microfilariae develop into larvae. When an infected mosquito bites a new animal, these larvae enter into the new animal's body. The larvae continue to grow and within 65 days, they finish their migration to the heart of the animal. Here they grow into adult worms, sometimes reaching a length of 14 inches. Within 6-8 months after first entering an animal, heartworms are mating and producing new microfilariae, which are released into the bloodstream.
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What do heartworms do?
Adult heartworms obstruct the chambers of the heart and large blood vessels leading from the heart to the lungs. Eventually other organs, such as the lungs and kidneys, can also be damaged. If left untreated, heartworm disease usually causes death.
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What is a complete heartworm prevention program?
A complete heartworm prevention program consists of three parts:

  1. Testing your pet regularly - We suggest that all adult pets being started on a heartworm preventive for the first time be tested. After that, annual re-testing should be performed. Consult with your veterinarian to determine the appropriate testing schedule for your dog or cat.

  2. Giving your pet preventive medication - Heartworm preventive medications kill the larvae, thus preventing them from maturing into adult heartworms. There are several different kinds of preventives, including Heartgard and Heartgard Plus (commonly misspelled as "Heartguard" or "Heartguard Plus"), Tri-Heart Plus, Sentinel, and others. Most are given monthly. Consistency and proper use of these medications are required for success.

  3. Reducing your pet's exposure to mosquitoes - this is the most difficult aspect of managing the risk of heartworm disease. But decreasing your pet's chances of being bitten in the first place reduces the risk of being infected. There are mosquito repellents available for pets. But more importantly, strive to make your yard less hospitable to mosquitoes.

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At what time of year should I start giving my pet a heartworm preventive?
Many veterinarians recommend heartworm preventives be given to pets year round, because several of these medicines also treat and control certain intestinal parasites. Additionally, it's easier for many owners to simply continue to administer the medication consistently throughout the year instead of calculating the appropriate stop and start dates.

However, if you prefer to give your pet a preventive seasonally, pay close attention to your area's mosquito season. For oral monthly preventives, begin administering the preventive after the first mosquito appears. Consult with your veterinarian regarding how long to give the preventive to your pet, when to start the preventive, and to determine the need for routine testing prior to the first dose of the season.
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Does my cat need to be on heartworm preventive?
Absolutely. Heartworm can be extremely deadly to cats, and there are some important differences between how dogs and cats react to heartworm.

  • Usually a cat has fewer adult heartworms than a dog, but severe disease can still develop. Though respiratory signs are most common, vomiting may also occur. Migration of larvae to other tissues of the body are more common in cats and thus, can affect other body systems such as the nervous system.

  • It is more difficult to treat cats for heartworm disease, as they are very sensitive to the drugs used. In most cases, an infected cat is treated by controlling the symptoms, and then eventually the adult heartworm will die on its own in two or three years. Surgery to remove the worms can be performed, but it a very delicate procedure.
The best thing any cat owner can do is prevent the infestation by putting the cat on a preventive.
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When should I start my puppy or kitten on a heartworm prevention program?
Some heartworm preventives can be given to puppies as young as 4 weeks of age, and kittens at 6 weeks of age. Consult with your veterinarian and consider the preventive you choose. Pay close attention to the medication's label and follow the manufacturer's guidelines.
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Can my ferret get heartworm disease?
Yes. Once considered rare, heartworm infection in ferrets is on the rise. Like cats, treating ferrets with heartworm disease is difficult. Additionally, because of their small size, only one adult worm can cause death.

For this reason, we recommend any ferret that could potentially be exposed to mosquitoes be placed on a heartworm preventive. Consult with your veterinarian about a preventive medication for your ferret.
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What are the signs of adult heartworm infestation?
In most cases, until the infestation is considered severe, there are relatively few symptoms. Dogs with adult heartworms may appear listless, may lose weight, or lack endurance when exercising. A cough is often the first sign of an infection.

In cats, signs may include coughing, vomiting, difficult or rapid breathing, seizures, poor appetite, and weight loss.

Quick and careful medical attention from a veterinarian is needed.
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How is heartworm disease treated?
The arsenic-based drugs used to kill adult heartworms are administered under close supervision of the veterinarian. Usually this requires an in-hospital stay for dogs during the first few days of treatment. As the adult heartworms die, they may obstruct blood vessels to the lungs, causing a pulmonary embolism, which can be fatal. To help reduce the possibility of this condition, the animal must be kept extremely quiet (cage rest) for 4 weeks following treatment. In severe cases, surgery is performed to remove the worms.

Getting rid of adult heartworms can be stressful, but is avoidable by putting your pet on a heartworm prevention program.
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Can humans get heartworm disease?
Yes, but it is very rare. Usually in humans, the worms migrate to the lungs instead of the heart. Here they form a lesion that can be seen on X-rays. There are normally few symptoms noticed with human heartworm infestation. Surgical removal of the lung lesion may be performed, if necessary.
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Does my dog need a test before I start giving him heartworm preventives?
The American Heartworm Society advises that all adult dogs being started on a heartworm preventive for the first time be tested. After that annual re-testing should be performed. The frequency of testing should take into account variables such as:

  • The age of your pet (the testing schedule is different for puppies)
  • Prevalence of heartworm disease in the area
  • Missed or late doses of heartworm preventive
  • The dog's lifestyle
  • The type of preventive used
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Can Heartworms be passed on from one pet to another?
No. Heartworms cannot be passed directly from one pet to another. Heartworm disease is transmitted by the bite of an infected mosquito. When a mosquito bites an infected animal, it will pick up the microfilariae (immature heartworms) that are present in the infected animal's bloodstream. These microfilariae mature into larvae inside the mosquito and are then passed to another animal when the mosquito bites the new animal.
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How long does it take an animal to test heartworm positive after it is infected by a mosquito?
Animals will usually test positive for heartworm disease approximately 6-8 months after they were bitten by an infected mosquito. It is much more difficult to diagnose heartworm disease in cats than in dogs.
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Learn what every pet owner needs to know about heartworm prevention:
visit our Special Heartworm Disease Information Section!

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