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Heartworm is a potentially deadly parasite that is transmitted only by mosquitos, which pick up larval heartworms, called microfilaria, circulating in the bloodstream of infected animals.

Dogs and other canids, such as fox, wolves, and coyote, are considered the primary heartworm hosts, but these parasites can also affect other mammals, including cats, ferrets, and, in rare cases, humans.

There are treatments available for dogs, but prevention is the wisest approach to dealing with heartworms.

What Is Heartworm Disease?

The canine heartworm that’s prevalent in the U.S. goes by the scientific name Dirofilaria immitis or D. immitis. It does not spread from dog to dog, but requires an intermediary, the mosquito, to infect new hosts.

The worms enter their host through a mosquito bite when it is taking a blood meal. Residue on the mosquito’s mouthpiece carries immature worms called microfilaria (which are only about 1/100th of an inch long) from an infected animal to an uninfected one. The immature worms travel through the bloodstream and, after about two months, settle in the right side of the heart, where they begin to grow.

They mature after six months and can live in the dog’s body for seven years, each reaching a length of up to a foot, and constantly producing offspring. After about a year, a dog may harbor hundreds of these worms, although 15 is the average burden. The worms cause inflammation and damage the heart, arteries, and lungs.

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How Widespread Are Heartworms?

The first canine heartworms in the U.S. were discovered in 1856, in the southeast, according to Stanford University. It is most common in the Atlantic and Gulf coasts, but has been reported in all 50 states, according to the U.S. Food & Drug Administration. It has been diagnosed around the world.

There are few habitable places that are totally safe, especially if you travel with your dogs, and it’s almost impossible to assess risk based on location. The American Heartworm Society (AHS) conducts surveys every three years of the number of cases reported by veterinarians and shelters throughout the country.

Areas with large populations of wild or stray animals pose an enhanced risk, but even dogs who stay inside most of the time are not totally safe from a mosquito’s bite. Also, pets being shipped from state to state may introduce heartworm into regions where it was historically not a problem. This is especially notable these days due to the bringing in of shelter dogs from heartworm endemic states and from disaster relief efforts. After Hurricane Katrina in 2005, about a quarter of a million pets traveled from the New Orleans area to new homes around the country. Some brought heartworm with them. AHS estimates that more than a million pets in the U.S. are infected.

What Are the Symptoms of Heartworm Infection?

In the early stages, there will be no signs of disease. About six months after the mosquito bite, blood tests will reveal the presence of antigens or microfilaria, according to the AHS. Other blood tests can measure antigens secreted by the female heartworm.

As the worms grow and multiply, the following symptoms will become evident, increasing in severity as the disease progresses. There are four classes of infection:

  • Class one has no symptoms or just mild cough.
  • Class two is marked by mild exercise intolerance and persistent cough.
  • Class three will result in greater exercise intolerance, abnormal lung sounds, weak pulse, syncope (fainting caused by impaired blood flow to the brain), decreased appetite, weight loss, ascites (swollen belly due to heart failure).
  • Class four is known as caval syndrome, a life-threatening cardiovascular collapse, which is marked by labored breathing, pale gums, and dark coffee-colored urine, leading to complete organ failure and death.

Diagnosis and Treatment of Heartworm Disease

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If your dog is diagnosed with heartworm through a blood test, your veterinarian will use different tests as confirmation. These include:

  • Radiographs to pinpoint abnormalities in the right side of the heart and pulmonary arteries.
  • Ultrasounds to show abnormal organ shape as well as wriggling worms.
  • Echocardiogram to see inside heart chambers and visualize worms.

Once the diagnosis is confirmed, your vet will advise you to:

  • Restrict activity. Exertion will worsen heartworm damage. Crate confinement may be necessary.
  • Kill adult worms. One drug is available for this purpose in the U.S. — Immiticide, manufactured and marketed by Merial. It is administered by deep intramuscular injection into the dog’s lumbar region of the lower back and is recommended for disease that has not progressed past class three. In addition, other drugs such as heartworm preventives, antibiotics, and steroids may be part of your veterinarian’s protocol.
  • Surgery. In extreme cases, veterinarians will resort to surgery, physically pulling the worms out.
  • Follow up with heartworm tests. The first should take place six months after successful treatment.

Preventing Heartworm

Treatment for heartworm is very hard on the dog, which is why veterinarians are so adamant about yearly testing and administration of preventative compounds — what scientists call chemoprophylaxis. These medications are also effective against other parasites, such as roundworms, hookworms, fleas, and tapeworms. Different formulations are available for dogs and cats, and it is important that you do not mix them up.

Following is a list of some preventatives. Discuss with your veterinarian about which one best fits your dog’s needs:

Sources: American Heartworm Society; Merck Veterinary Manual; Centers for Disease Control and Prevention

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