Pet Talk: Heartworm disease in dogs

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Many dog owners have heard of heartworm disease but may not fully understand how the disease develops. Some owners may even question if an annual test for heartworm disease is necessary. Dr. Sonya Wesselowski, clinical assistant professor of cardiology at the Texas A&M College of Veterinary Medicine & Biomedical Sciences, said testing for heartworm disease and administering regular monthly heartworm medication is crucial.

“All dogs should be tested for heartworm disease every year at their annual wellness visit,” Wesselowski said. “Additionally, patients that have not previously been on heartworm prevention or those that have had a lapse in their heartworm prevention should be tested immediately, then again in six months, and annually thereafter.”

To help dog owners understand why preventing heartworm disease is so important, Wesselowski explained how the disease develops. “Heartworm disease is caused by a long, thin worm known as Dirofilaria immitis,” she said. “Heartworms live within the heart, lungs, and blood vessels and can cause damage in these areas. This damage can lead to lung disease, elevated blood pressure inside the lungs, and even heart failure. In some cases, other organs in the body are affected as well.”

“The first step in the heartworm lifecycle occurs when a mosquito bites an infected dog,” she continued. “The mosquito ingests the microscopic offspring of the adult heartworm known as microfilaria. These microfilaria then mature inside the mosquito over ten to 14 days and become infective larvae that can be deposited onto the skin of another animal when the mosquito bites again. The larvae enter their new host through the bite wound left by the mosquito and develop into adult heartworms in about six months. The adult heartworms can then live for five to seven years in the infected dog.”

Signs of heartworm disease in dogs range from a mild cough and decreased exercise capacity to heart failure and the accumulation of abdominal fluid. In severe cases, heartworms can cause caval syndrome, a rapidly progressing fatal disease that blocks blood flow within the heart. Caval syndrome causes symptoms such as labored breathing, collapse, and dark-colored urine.

The thought of heartworms infecting Fido may sound like a nightmare. Thankfully, treatment is available. However, according to Wesselowski, resolution of a heartworm infection is not a quick and easy process. “If a dog tests positive for heartworms, the first step is to confirm the diagnosis with a follow-up test,” she said. “Once the diagnosis is confirmed, additional laboratory and imaging tests will help your veterinarian stage the severity of the disease and develop an appropriate treatment plan. Treatment involves multiple oral medications for at least one month, followed by a set of several injections to kill the adult heartworms. Strict exercise restriction and kennel confinement is essential throughout the entire course of treatment to reduce the risk of serious complications that can be associated with resolving the heartworm infection.”

Wesselowski also stressed that the treatment of heartworm disease is expensive for pet owners and taxing for affected pets. The heartworms can also cause long-term damage to the heart and lungs that remains even after the heartworm infection is successfully resolved. This means that when it comes to heartworm disease, prevention is key.

“Administration of regular monthly heartworm preventatives is crucial to prevent heartworm disease and to avoid the stress, expense, and potential complications that can be associated with heartworm treatment in our beloved family pets,” Wesselowski concluded.

Pet Talk is a service of the College of Veterinary Medicine & Biomedical Sciences, Texas A&M University.

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