With the warmer weather we start to see more mosquitos. Mosquitos have a history of carrying diseases such as malaria, West Nile virus, and a host of other diseases. But they also carry diseases that can be severe to animals. Therefore, it is vital pet parents are aware of the damage and disease they can cause their pets.

Heartworm is one serious disease that results in severe lung damage, heart failure and other organ damage. Left untreated, it results in death in pets, mainly dogs, cats, and ferrets. When a mosquito bites an animal infected with heartworms, it sucks up the microscopic heartworm larvae. These larvae continue to develop in the mosquito, and the mosquito deposits the parasite into its next victim.

It takes about 6 to 7 months for the larvae to mature into adult heartworms. The adult heartworms mate and the females release their offspring into the dog’s bloodstream. Inside a dog, a heartworm’s lifespan is 5 to 7 years. Adult heartworms look like strands of cooked spaghetti. The male worms reach about 4 to 6 inches long and females reach about 10 to 12 inches long.

Heartworm Disease in Dogs

Symptoms of Heartworm Disease in a Dog

  • Mild persistent cough
  • Reluctance to exercise
  • Fatigue after moderate activity
  • Decreased appetite
  • Weight loss

The symptoms of heartworm disease can depend on how many heartworms are living in the dog. We call it the “worm burden”. The average worm burden in dogs is 15 worms, but that number can range from 1 to 250 worms. Symptoms can depend on how long the dog has been infected. Also, symptoms can depend on how the dog’s body is responding to the presence of the heartworms. The dog’s activity level plays a role in the severity of the disease and in when symptoms are first seen. Symptoms are not always obvious in dogs with low worm burdens, have been recently infected, or are not very active. Dogs with heavy worm burdens, have been infected for a long time, or are very active, often show obvious symptoms. There are four classes, or stages, of heartworm disease.

Stages of Heartworm

Stage 1

No symptoms or mild symptoms such as an occasional cough.

Stage 2

Mild to moderate symptoms such as an occasional cough and tiredness after moderate activity.

Stage 3

More severe symptoms such as a sickly appearance, a persistent cough, and tiredness after mild activity. Trouble breathing and signs of heart failure are common. Chest x-rays can usually show the heart.

Stage 4

There is a large mass of worms which physically blocks the blood flowing back to the heart. This stage is life-threatening and requires quick surgical removal of the heartworms. However, surgery is risky and even with surgery, most dogs in this stage die. Not all dogs with heartworm disease develop into stage 4. But if it is left untreated, heartworm disease will progress and damage the dog’s heart, lungs, liver, and kidneys, eventually causing death.

Treatment for Heartworm Disease

A veterinarian can give a dog an arsenic-containing FDA approved drug. There is also a topical treatment called Advantage Multi for Dogs, which is applied to the dog’s skin.

In any case, the treatment for heartworm disease is not easy on the dog. Treatment can be toxic to the dog’s body. It can cause serious complications, such as life-threatening blood clots to the dog’s lungs. Additionally, the treatment is expensive. It requires multiple visits to the veterinarian, blood tests, x-rays, hospitalization, and a series of injections.

The Best Treatment is Prevention!

The FDA has approved drugs to prevent heartworms in dogs. These drugs require a veterinarian’s prescription. Most products are a topical liquid applied on the skin or as an oral tablet once a month. Both chewable and non-chewable oral tablets are available. Also, a veterinarian can inject a product under the skin every 6 or 12 months. Some heartworm preventives contain other ingredients that are effective against roundworms, hookworms, fleas, ticks, and ear mites.

Testing a Dog for Heartworm

A veterinarian tests a dog’s blood to check a dog for heartworms. The test detects specific heartworm proteins. The adult female worms release these proteins into the dog’s bloodstream. In most cases, these tests can accurately detect infections with one or more adult female heartworms. The test detects heartworm proteins in a dog’s blood about 5 months after it is bitten by an infected mosquito.

Another test detects the heartworm larvae in a dog’s blood. Only adult heartworms can mate and produce larvae. Therefore, this test indicates whether or not the dog has adult heartworms. The test can detect the larvae about 6 months after infection from a mosquito. It takes about that long for the heartworms to develop from infective larvae into adults that mate and produce larvae.

Do not put your dog on any heartworm preventative medication if they have not been tested for heartworm. Heartworm preventives do not kill adult heartworms. Also, giving a heartworm preventive to a dog infected with adult heartworms may be harmful or deadly. The preventative medication may cause the larvae in the dog’s bloodstream to suddenly die. This can trigger a shock-like reaction and possibly death for the dog.

Heartworm Disease in Cats

Mosquitos can also infect cats with heartworm. However, cats are not natural hosts for heartworm. Therefore, cats are less prone to heartworm than dogs

Heartworm disease in cats is a bit different than in dogs. Heartworms do not live as long in cats as they do in dogs. The average lifespan is only 2 to 4 years. They also don’t grow as long, and fewer of them mature into adults. Usually a cat has only one or two worms. However, because of a cat’s small body size, a cat with only a few worms is still considered to be heavily infected.

In cats, it takes 7 to 8 months for infective larvae to mature into adult heartworms and produce larvae. This is about one month longer than in dogs. The presence of larvae in a cat’s bloodstream is uncommon. Only 20 percent of cats with heartworm disease have larvae in the bloodstream, compared to 80 to 90 percent of dogs with heartworm disease. Also, the presence of larvae in the bloodstream is inconsistent and short-lived in cats.

What are the Symptoms of Heartworm Disease in Cats?

  • Difficulty or labored breathing
  • Vomiting
  • Decreased activity
  • Loss of appetite
  • Weight loss
  • Convulsions.
  • Blindness.
  • Fluid in the lungs.

Not all cats with heartworm disease show symptoms. However, cats that show symptoms usually show signs of breathing difficulties due to the lung damage caused by the heartworms.

Cats typically show symptoms of heartworm disease at two different points. One is when the immature heartworms arrive in the arteries of the heart and lungs. The other is when the adult heartworms die. Cats with heartworm disease rarely show signs of heart failure. However, some infected cats die suddenly from heartworm disease without ever showing signs of being sick. And in some cases, cats have been able to rid themselves of heartworms without ever having any symptoms.

It is harder to detect heartworm infections in cats than in dogs. Veterinarians generally use two types of blood tests in combination to check a cat for heartworms. However, negative test results do not rule out heartworm infection. Positive test results may or may not mean that there is an active heartworm infection. 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.

Infection in Cats

After a mosquito bites a cat, the immature heartworms arrive in the heart and lung arteries in about 3 to 4 months. Many of these immature heartworms die, causing a strong inflammatory response in the cat’s lungs. We call this response Heartworm Associated Respiratory Disease (HARD). It refers to breathing difficulties, such as trouble breathing, increased breathing rate, and cough. It may be difficult to distinguish HARD from feline asthma or feline bronchitis.

When the adult heartworms die, they release toxins into the cat’s bloodstream which causes lung damage, leading to respiratory problems or sudden death. Even the death of one worm can be fatal for a cat.

There is no FDA-approved drug to treat heartworm disease in cats, although symptoms may be managed with medications. If a veterinarian can detect heartworms by ultrasound, surgical removal of adult heartworms may be an option. But surgery is risky. And if the heartworms are not removed intact, there can be potentially serious complications, such as shock and death.

Should Cats Be Tested for Heartworms?

We recommend that cats be tested for heartworms before starting heartworm prevention, although this pre-testing is less useful than in dogs. Talk to your veterinarian about testing your cat for heartworms.

Heartworm Disease in Ferrets

Ferrets can also get heartworms from the bite of an infected mosquito. They are extremely susceptible to heartworm disease and are at risk for the disease even if they are indoor pets. 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 low worm burdens. A test shows larvae are seen in the bloodstream in only 50 to 60 percent of ferrets with heartworm disease.

Symptoms of heartworm disease in ferrets

  • Decreased activity
  • Coughing
  • Trouble breathing
  • Overall weakness
  • Heart failure can occur in severe cases.

Based on a ferret’s symptoms, a veterinarian may perform chest x-rays and an ultrasound of the heart to determine if it has heartworm disease. Blood tests to detect heartworm infections in ferrets are generally unreliable.

Treatment for Ferrets

Unfortunately, no drugs are FDA-approved to treat heartworm disease in ferrets. And only one drug, Advantage Multi for Cats, is approved to prevent heartworms in ferrets. It is a topical solution that is applied monthly. Besides preventing heartworms, Advantage Multi for Cats also treats flea infestations on ferrets by killing adult fleas. We recommend year-round prevention medication for all ferrets.


Only a veterinarian can prescribe heartworm treatment and preventative treatment. Heartworm preventive medication will not kill existing adult worms. We cannot overstress that prevention is the best treatment for heartworm. It’s much easier (and less expensive) to prevent heartworm than to treat it after the fact. Contact us if you have any questions or need to schedule your pet for a heartworm test or preventative medication.

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