Keep the Worms Out of Your Pets Heart! The Facts about Heartworm Disease | FDA

Video Heartworm what is it


The health of your pet is important. Learn the facts about heartworm disease so you can keep your pet healthy and free of heartworms.




heartworm disease: what is it and what causes it?

Heartworm disease is a serious disease that causes severe lung disease, heart failure, other organ damage, and death in pets, primarily dogs, cats, and ferrets. It is caused by a parasitic worm called dirofilaria immitis. the worms are spread through the bite of a mosquito. the dog is the definitive host, meaning that the worms mature into adults, mate, and produce offspring while living inside a dog. the mosquito is the intermediate host, meaning that the worms live inside a mosquito for a brief transition period to become infectious (capable of causing heartworm disease). the worms are called “heartworms” because the adults live in the heart, lungs, and associated blood vessels of an infected animal.

in the united states, heartworm disease is most common along the atlantic and gulf coasts, from the gulf of mexico to new jersey, and along the mississippi river and its major tributaries, but it is has been reported in dogs in all 50 states.

the life cycle of the heartworm in dogs

Copper-colored Dog

In an infected dog, adult female heartworms release their young, called microfilariae, into the dog’s bloodstream. when a mosquito bites an infected dog, the mosquito becomes infected with the microfilariae. over the next 10 to 14 days and under the right environmental conditions, the microfilariae develop into infective larvae while living inside the mosquito. microfilariae must pass through a mosquito to develop into infective larvae. When the infected mosquito bites another dog, the mosquito spreads the infective larvae to the dog through the bite wound. in the newly infected dog, it takes 6 to 7 months for the infective larvae to mature into adult heartworms. the adult heartworms mate and the females release their young into the dog’s bloodstream, completing the life cycle. See a chart of the life cycle of heartworms in dogs.

Heartworm disease is not contagious, which means a dog cannot get the disease from being around an infected dog. Heartworm disease is only spread through a mosquito bite.

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Inside a dog, the life expectancy of a heartworm is 5 to 7 years. Adult heartworms look like strands of cooked spaghetti, with males 4-6 inches long and females 10-12 inches long. the number of worms living inside an infected dog is called the worm load. the average worm load in dogs is 15 worms, but that number can range from 1 to 250 worms.

how do you test a dog for heartworms?

A veterinarian uses blood tests to check a dog for heartworms. An antigen test detects heartworm-specific proteins, called antigens, that adult female heartworms release into the dog’s bloodstream. In most cases, antigen tests can accurately detect infections with one or more adult female heartworms. The earliest heartworm proteins can be detected in a dog’s bloodstream is about 5 months after being bitten by an infected mosquito.

Another test detects microfilariae in a dog’s bloodstream. microfilariae in the bloodstream indicate that the dog is infected with adult heartworms (because only adult heartworms can mate and produce microfilariae). The earliest microfilariae can be detected in a dog’s bloodstream is about 6 months after being bitten by an infected mosquito (because it takes that long for heartworms to develop from infective larvae into adults that mate and produce microfilariae) .

When should a dog be tested for heartworms?

The timing and frequency of heartworm testing depends on many factors. Some of these factors include:

  • the age of the dog when heartworm prevention is started;
  • whether the owner forgot to administer heartworm prevention and for how long;
  • if the dog is switched from one type of heartworm prevention to another;
  • if the dog recently traveled to an area where heartworm disease is more common; and
  • the length of heartworm season in the region where the dog lives.
  • Dogs 7 months of age and older should be tested for heartworm before starting heartworm prevention. A dog may appear healthy on the outside, but inside, heartworms may be alive and thriving. If a heartworm-positive dog is not tested before starting preventive treatment, the dog will remain infected with adult heartworms until she becomes sick enough to show symptoms. Heartworm preventatives do not kill adult heartworms. furthermore, giving a heartworm preventative to a dog infected with adult heartworms can be harmful or fatal. if the microfilariae are in the dog’s bloodstream, the preventative can cause the microfilariae to die suddenly, causing a shock-like reaction and possibly death.

    Annual testing of all dogs for heartworm prevention is recommended. Talk to your dog’s veterinarian about the best time for your dog’s annual heartworm test.

    what are the symptoms of heartworm disease in a dog?

    The severity of heartworm disease is related to the number of worms living inside the dog (the worm load), how long the dog has been infected, and how the dog’s body responds to the presence of the worms from the heart. The dog’s activity level also influences the severity of the disease and when the first symptoms are observed. Symptoms of heartworm disease may not be apparent in dogs that have a low worm load, have been recently infected, or are not very active. Dogs that have a heavy worm load, have been infected for a long time, or are very active often show obvious symptoms of heartworm disease.

    There are four classes or stages of heartworm disease. the higher the class, the worse the disease and the more obvious the symptoms.

    • class 1: no symptoms or mild symptoms such as occasional cough.
    • Class 2: Mild to moderate symptoms, including occasional cough and tiredness after moderate activity.
    • Class 3: More severe symptoms, such as sickly appearance, persistent cough, and tiredness after mild activity. shortness of breath and signs of heart failure are common. for heartworm class 2 and 3, changes in the heart and lungs are usually seen on chest x-rays.
    • class 4: also called cava syndrome. there is such a heavy load of worms that the blood returning to the heart is physically blocked by a large mass of worms. cava syndrome is life-threatening and prompt surgical removal of heartworms is the only treatment option. surgery is risky, and even with surgery, most dogs with cava syndrome die.
    • Not all dogs with heartworm develop cava syndrome. however, if left untreated, heartworm disease will progress and damage the dog’s heart, lungs, liver, and kidneys, eventually leading to death.

      Is there a treatment for heartworm disease in dogs?

      Melarsomine dihydrochloride (available under the trade names Immiticide and Diroban) is an arsenic-containing drug approved by the FDA to kill adult heartworms in dogs. It is given by injection deep into the back muscles to treat dogs with stabilized class 1, 2, and 3 heartworm disease. Another drug, Advantage Multi for Dogs (imidacloprid and moxidectin), is FDA-approved to kill microfilariae in the dog’s bloodstream advantage multi for dogs is a topical solution that is applied to the dog’s skin.

      the treatment of dirofilariosis is not easy neither for the dog nor for the pocket of the owner. the treatment can be potentially toxic to the dog’s body and can cause serious complications, including life-threatening blood clots in the dog’s lungs. Treatment is expensive because it requires multiple visits to the vet, blood tests, X-rays, hospitalization, and a series of injections.

      The best treatment is prevention!

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      Many products are FDA-approved to prevent heartworms in dogs. all require a veterinary prescription. most products are administered monthly, either as a topical liquid applied to the skin or as an oral tablet. Chewable and non-chewable oral tablets are available. a product is injected under the skin every 6 to 12 months, and only a veterinarian can administer the injection. Some heartworm preventives contain other ingredients that are effective against certain intestinal worms (such as roundworms and hookworms) and other parasites (such as fleas, ticks, and ear mites).

      year-round prevention is best! talk to your dog’s veterinarian to decide which preventative is best for your dog.

      The American Heartworm Society advocates “think 12”. give dogs 12 months of heartworm prevention and have them screened for heartworms every 12months.

      orange colored cat

      can cats get heartworm disease?

      Cats can also get heartworms after being bitten by an infected mosquito, although they are not as susceptible to infection as dogs. a cat is not a natural host for heartworms because the worms do not thrive as well inside a cat’s body. Both indoor and outdoor cats are at risk for heartworm disease.

      Is heartworm disease different in cats?

      Heartworm disease in cats is a little different than in dogs. Heartworms in cats don’t live as long (average lifespan is only 2-4 years) or grow as large, and fewer of them mature into adults. the worm burden is lower in cats than in dogs. a cat usually has only one or two worms. however, due to its relatively small body size, a cat with only a few worms is considered severely infected.

      In cats, it takes 7 to 8 months for infective larvae to mature into adult heartworms and produce microfilariae. this is about a month longer than in dogs. the presence of microfilariae in a cat’s bloodstream is rare. only 20 percent of cats with heartworms have microfilariae in their bloodstream, compared to 80 to 90 percent of dogs with heartworms. furthermore, the presence of microfilariae in the bloodstream is inconsistent and short-lived in cats.

      It is more difficult to detect heartworm infections in cats than in dogs. Veterinarians typically use two types of blood tests in combination to check a cat for heartworms. however, negative test results do not rule out heartworm infection, and positive results may or may not mean that an active heartworm infection is present. A veterinarian uses the results of both blood tests, along with the cat’s symptoms and the results of other tests, such as X-rays and an ultrasound of the heart, to determine if a cat has heartworm disease.

      what are the symptoms of heartworm disease in cats?

      Not all cats with heartworm show symptoms. some cats can spontaneously get rid of heartworms without having any symptoms. however, some infected cats die suddenly from heartworm disease without showing any signs of being ill. cats with heartworm disease can have very nonspecific symptoms that mimic many other feline diseases. These nonspecific symptoms include vomiting, decreased activity and appetite, and weight loss. cats with heartworm disease rarely show signs of heart failure.

      In cats showing symptoms of heartworm, respiratory symptoms are most apparent due to lung damage caused by heartworm. Cats typically show symptoms of heartworm disease at two times: when the immature heartworms reach the arteries of the heart and lungs, and when the adult heartworms die.

      Immature heartworms reach the arteries of the heart and lungs about 3 to 4 months after an infected mosquito bites a cat. many of these immature heartworms die, triggering a strong inflammatory response in the cat’s lungs. This response is called heartworm-associated respiratory disease (HARD) because respiratory signs, such as shortness of breath, increased respiratory rate, and cough, are the most obvious. can be difficult to distinguish from feline asthma or feline bronchitis.

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      When adult heartworms die, they release toxins into the cat’s bloodstream that cause lung damage, leading to respiratory problems or sudden death. even the death of a worm can be fatal to a cat.

      There is no fda-approved drug to treat heartworm disease in cats, although symptoms can be controlled with medication. Surgical removal of adult heartworms may be a treatment option if the heartworms can be seen on ultrasound. But surgery is risky, and if the heartworms are not removed intact, there can be potentially serious complications, including shock and death.

      once again, prevention is the best treatment!

      Several products are FDA-approved to prevent heartworms in cats. There are topical and oral products for cats, all of which are administered monthly and require a veterinary prescription. Some heartworm preventatives contain other ingredients that are effective against certain intestinal worms (such as roundworms and hookworms) and other parasites (such as fleas, ticks, and ear mites).

      again, year-round prevention is best! Talk to your cat’s veterinarian to decide which preventative is best for your cat.

      Should cats be tested for heartworms?

      It is recommended that cats be tested for heartworm prior to initiating heartworm prevention, although this pre-testing is less useful than in dogs. Talk to your cat’s veterinarian about testing your cat for heartworms.

      what about heartworm disease in ferrets?

      Ferrets can also get heartworms from the bite of an infected mosquito. Ferrets are similar to dogs in their susceptibility to heartworm infections, but their symptoms are more similar to those seen in cats.

      Infected ferrets typically have a low worm burden, and microfilariae are seen in the bloodstream in only 50 to 60 percent of ferrets with heartworm disease. Symptoms of heartworm disease in ferrets include decreased activity level, cough, shortness of breath, and general weakness. heart failure can occur in severe cases. Depending on a ferret’s symptoms, a vet may perform chest x-rays and an ultrasound of the heart to determine if it has heartworm. blood tests for heartworm infections in ferrets are generally unreliable.

      There are no fda-approved medications to treat heartworm disease in ferrets. And only one drug, Advantage Multi for Cats (imidacloprid and moxidectin), is approved to prevent heartworm disease in ferrets. Available only by veterinary prescription, it is a topical solution that is applied monthly. In addition to preventing heartworms, Advantage Multi for Cats also treats flea infestations in ferrets by killing adult fleas.

      again, prevention is the best treatment! Year-round prevention is recommended for all ferrets. Talk to your ferret’s vet about preventing heartworm disease in your furry friend.

      Can people get heartworms from their pets?

      People cannot get heartworms from their pets. Heartworms are only spread through the bite of an infected mosquito. In rare cases, people can get heartworms after being bitten by an infected mosquito. But because people are not natural hosts for heartworms, the larvae often migrate to the arteries of the heart and lungs and die before developing into adult worms.

      How can I learn more about heartworm disease?

      • talk to your pet’s vet
      • contact the fda center for veterinary medicine at 240-402-7002 or [email protected]
      • read prevent heartworms in dogs, cats and ferrets throughout the year
      • Visit the American Heartworm Society website at
      • visit
      • *special thanks to david a. crum, dvm, mph for sharing his experience with ferrets.

Content Creator Zaid Butt joined Silsala-e-Azeemia in 2004 as student of spirituality. Mr. Zahid Butt is an IT professional, his expertise include “Web/Graphic Designer, GUI, Visualizer and Web Developer” PH: +92-3217244554

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