Heartworm Prevention, Diagnosis and Treatment

The heartworm (Dirofilaria immitis) is now recognized as a major, global pest affecting dogs, cats, wolves, coyotes, foxes, and some other animals. Researchers have devised diagnostic tests, preventives and treatments, but the disease has spread to all 50 states. The highest infection rates occur in dogs (not maintained on heartworm preventive) within 150 miles of the Atlantic and Gulf coasts and along the Mississippi River and its major tributaries. Other areas with large mosquitoe populations also have a high rate of infestation.

While many cat and dog owners are happy to think that their pet will never be bitten by an infected mosquito, heartworm prevention is simple. It involves a blood draw to determine whether the parasite is present and regular dosing with preventive medication. Heartworm infestation is dangerous; untreated dogs die and treated dogs go through weeks of discomfort while the worms are killed and expelled from their bodies.

The Infection Process

The parasites go through several life stages before emergence as adults and often need at least two hosts to complete the cycle. In heartworms, a mosquito serves as the intermediate host for the larval stage of the worm, also known as the microfilariae. The microfilariae burrow into the dog and undergo several changes to reach adult form, then travel to the right side of the heart through a vein and await the opportunity to reproduce. Adult heartworms can reach 12 inches in length and can remain in the dog’s heart for several years.

Dogs can have some microfilariae in their blood and worms in their lungs without manifesting the disease. Once the number of worms exceeds a certain number based on the size and activity level of the dog, however, the adult worms move to the heart and symptoms begin to occur. Very active dogs may experience symptoms with lower numbers of worms than couch-potato dogs. The time lag between the initial infestation of microfilariae and reproduction by adult worms living in the heart is six-to-seven months in dogs.

The worms grow and multiply, infesting the chambers on the right side of the heart and the arteries in the lungs. They can also lodge in the veins of the liver and the veins entering the heart. The first sign of heartworm infestation may not manifest for a year after infection, and even then the soft cough that increases with exercise may be dismissed as no more than unusual. However, the cough worsens and the dog may actually faint from exertion; he tires easily, is weak and listless, loses weight and condition, and may cough up blood.

Breathing becomes more difficult as the disease progresses. The progression is traumatic: the dog’s quality of life diminishes drastically and he can no longer retrieve a Frisbee or take a long walk in the park without respiratory distress. Congestive heart failure ensues, and the once-active, outgoing pet is in grave danger.

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