Heartworm Disease

What is heartworm disease?

Heartworm disease, or dirofilariasis, is a serious and potentially fatal disease. It is caused by a blood-borne parasite known as Dirofilaria immitis or as we know heartworms. Adult heartworms are found in the heart, pulmonary artery, and adjacent large blood vessels of infected dogs. Rarely, worms may be found in other parts of the circulatory system. Female adult heartworms are 6 – 14″ long (15 – 36 cm) and 1/8″ wide (3 mm). Males are about half the size of females. One dog may have as many as 300 worms present when diagnosed.

Adult heartworms may live up to five years. During this time, females produce millions of offspring called microfilaria. These microfilariae live mainly in the small vessels of the bloodstream. Since transmission requires the mosquito as an intermediate host, the disease is not spread directly from dog to dog. Spread of the disease therefore coincides with mosquito season, which can last year-round in many parts of the United States. The number of dogs infected and the length of the mosquito season are directly correlated with the incidence of heartworm disease in any given area.

Canine heartworm disease occurs all over the world. In the United States, it was once limited to the south and southeast regions. The highest numbers of reported cases are still within 150 miles of the Gulf of Mexico and the Atlantic Ocean coastlines and along the Mississippi River and its tributaries. However, the disease is spreading and is now found in most regions of the United States, including California, Oregon, and Washington. Factors that affect the prevalence of heartworm infection include the species of mosquitoes, the climate, and the presence of reservoir animals. The risk of infection is greatest when mosquitoes are actively feeding. This typically requires temperatures over 50°F (10°C).

What are the symptoms of heartworms?

In the early stages of the disease, many dogs show few symptoms or no symptoms at all. The longer the infection persists, the more likely symptoms will develop. Active dogs, dogs heavily infected with heartworms, or those with other health problems often show pronounced clinical signs. Signs of heartworm disease may include a mild persistent cough, reluctance to exercise, fatigue after moderate activity, decreased appetite, and weight loss. As heartworm disease progresses, pets may develop heart failure and the appearance of a swollen belly due to excess fluid in the abdomen. Dogs with large numbers of heartworms can develop sudden blockages of blood flow within the heart leading to a life-threatening form of cardiovascular collapse.

How is heartworm disease diagnosed?

In most cases, one or more simple blood tests will diagnose heartworm disease. Further diagnostic tests are often required in heartworm-positive dogs to determine if the dog can safely undergo heartworm disease treatment. Some or all of the following diagnostic procedures are recommended before treatment is started:

Serological test for antigens to adult heartworms (antigen test, ELISA). This test is performed on a blood sample. Radiographs are often recommended in dogs with heartworm disease, to assess the extent of heart and lung damage present prior to beginning treatment.

Bloodwork (complete blood cell count, serum biochemistry). Blood tests may also be recommended prior to the treatment of heartworm disease, in order to assess for the presence of heartworm-associated organ damage.

Treatment

Once your dog is diagnosed with heartworm disease, your veterinarian may recommend a course of antibiotics, heartworm preventives and steroids before beginning the actual adult worm treatment. Many veterinarians begin treatment by prescribing doxycycline, based on promising results from published studies of doxycycline use in dogs infected with heartworms.

Since the heartworm treatment only kills adult worms, veterinarians may prescribe a monthly heartworm preventive to kill the smaller larvae before initiating adult heartworm treatment.

The administration of corticosteroids at the same time as the antibiotics and heartworm preventive also helps reduce inflammation.

Once your dog has completed the course of steroids, heartworm preventive and antibiotics, he should be ready to start the actual adult heartworm treatment. The treatment for heartworm disease takes at least 60 days to complete and consists of a series of drug injections that kills the worms. There is only one drug approved by the FDA to kill adult heartworms in dogs, an organic arsenical compound that is injected into the dog’s lumbar, or back, muscles.

On the days injections are given, your dog must stay in the hospital for observation to make sure he doesn’t have any serious reactions to the treatment. Your veterinarian may also prescribe a tapering dose of steroids for a period of time following the injections.

But, wait, there’s more! Your dog should be retested after treatment and six months later to ensure that all of the larvae, microfilariae and adult worms are dead. Dogs who remain heartworm positive six months after treatment may need to repeat treatment to kill the remaining worms.

During treatment and for several weeks afterward, your dog’s activity level must be limited. Fragments of dead worms can block blood flow through pulmonary vessels and worsen the inflammatory response; too much exercise increases blood flow to blocked areas, causing capillaries to rupture as the body tries to pump blood through the blocked vessels. This heightens the likelihood of complications, such as coughing, breathing difficulties and sudden death. To reduce the risk of complications, it’s essential that you restrict exercise for the entire time your dog is undergoing treatment and for a period of time afterward. Depending on the severity of the damage caused by the heartworms, this can mean complete confinement in a crate except for potty walks on leash or minimal activity in the home, with only brief walks on leash and crating when no one is there to monitor his activity and make sure he remains calm.

Your veterinarian will let you know when your dog can resume normal activity levels.

What is the response to treatment and the prognosis post-treatment?

Dog owners are usually surprised at the improvement in their dog following treatment for heartworms, especially if the dog had been demonstrating clinical signs of heartworm disease. Many dogs display renewed vigor and vitality, improved appetite, and weight gain.

How do monthly preventives work?

Whether the preventive you choose is given as a pill, a spot-on topical medication or as an injection, all approved heartworm medications work by eliminating the immature (larval) stages of the heartworm parasite. This includes the infective heartworm larvae deposited by the mosquito as well as the following larval stage that develops inside the animal. Unfortunately, in as little as 51 days, immature heartworm larvae can molt into an adult stage, which cannot be effectively eliminated by preventives. Because heartworms must be eliminated before they reach this adult stage, it is extremely important that heartworm preventives be administered strictly on schedule (monthly for oral and topical products and every 6 months for the injectable). Administering prevention late can allow immature larvae to molt into the adult stage, which is poorly prevented.

You can prevent your dog from getting heartworms by using a heartworm preventive. When a dog has been successfully treated for heartworms, it is essential to begin a heartworm prevention program to prevent future recurrence. With the safe and affordable heartworm preventives available today, no pet should ever have to endure this dreaded disease. Consult with your veterinarian to determine which heartworm preventive program is best for your dog.