Dog Heartworm Treatment

Heartworm disease occurs when a dog becomes infested with a roundworm called Dirofilaria immitis. It was first identified in the United States in 1856 and occurred most frequently on the seacoast in the southeastern United States. Heartworms (Dirofilaria immitis) are parasitic worms, which inhabit the heart muscle and lungs of infected dogs. Heartworm disease can cause serious health problems including death due to heart failure or pneumonia. Dogs are considered the definitive host for heartworms (Dirofilaria immitis). However, heartworms may infect more than 30 species of animals (e.g., coyotes, foxes, wolves and other wild canids, domestic cats and wild felids, ferrets, sea lions, etc.) and humans as well.

Canine heartworm infection is widely distributed throughout the United States. Heartworm infection has been found in dogs native to all 50 states. All dogs regardless of their age, sex, or habitat are susceptible to heartworm infection. The highest infection rates (up to 45%) in dogs (not maintained on heartworm preventive) are observed within 150 miles of the Atlantic and Gulf coasts from the Gulf of Mexico to New Jersey and along the Mississippi River and its major tributaries. Other areas of the United States may have lower incidence rates (5% or less) of canine heartworm disease, while some regions have environmental, mosquito, and dog population factors that allow a higher incidence of heartworm infection.

Heartworms belong to the same class of worms as roundworms. In fact, they look a bit like roundworms, but that is where the similarity ends. When a mosquito carrying infective heartworm larvae bites a dog and transmits the infection, the larvae grow, develop, and migrate in the body over a period of several months to become sexually mature male and female worms. These reside in the heart, lungs, and associated blood vessels. Even as immature adults, the worms mate and the females release their offspring (microfilariae) into the blood stream. The time elapsed from when the larvae enter the dog until the minute offspring can be detected in the blood (pre-patent period) is about six to seven months. The male heartworms (four to six inches in length) and the females (10-12 inches) become fully grown about one year after infection, and their life span in dogs appears to be at least five to seven years.

The adult worms can obstruct the different chambers of the heart and the various large blood vessels leading from the heart to the lungs. First, the right pumping chamber (ventricle) of the heart and the large pulmonary artery leading from it to the lungs becomes enlarged as the worms take up space. If worms die, they may follow the flow of blood into smaller vessels in the lung and obstruct those vessels. In severe infections, the worms can also block the large vein (vena cava) bringing blood to the right side of the heart. As the blood backs up, the liver becomes enlarged and damaged.

Clinical Signs

The severity and onset of clinical signs relate to 3 things: the number of worms, the duration of the infection and the dog’s response. Many dogs infected with heartworms may not show any evidence of the infection in the early stages of the disease. Signs can eventually appear and will worsen with increased numbers of worms and with lengthy infections, making early detection and prevention programs an important part of managing your pet’s health.

Initially dogs tend to develop a cough and experience difficulty breathing. They may also exhibit a decreased tolerance to exercise, develop fainting episodes or suffer severe weight loss. Since the heart and lungs are affected, many of the clinical signs seen or observed are typical of heart failure, pneumonia or other debilitating diseases.