What Happens if Heartworm Is Left Untreated in Dogs?

All dogs in the United States are at risk for heartworm if they remain untreated. No state is ̶0;safe̶1; from heartworm infestation; dogs testing positive for heartworm have been found in every state, including Alaska and Hawaii. Leaving heartworms untreated can debilitate or disable your dog and may even cause death. The best treatment for heartworm is the use of preventive medication.

  1. What Happens When No Prevention is Used

    • Newly hatched heartworms, called microfiliae, are present in the bloodstream of up to 80 percent of all heartworm-infected animals. These microfiliae are vital to the spread of heartworm infection, as they must pass through a mosquito to spread inside another dog or cat. When a mosquito bites a dog unprotected by heartworm treatment, these microfiliae infect the new host and begin to grow inside the dog̵7;s blood vessels. The immature worms take several months to mature sufficiently to produce microfiliae of their own, which may be passed to another dog through an infected mosquito bite. Immature microfiliae can live for up to two years in their host, meaning that the dog can spread heartworm infection even when no infection is present in that host.

    What Happens When a Dog is Infected with Heartworms

    • For the next five to six months after introduction into an untreated dog̵7;s bloodstream, the microfiliae will grow into adult worms, taking their places in the right side of the heart and the pulmonary vein. Because an antigen heartworm test only detects adult female worms, heartworm infection is undetectable before the worms achieve maturity or if the dog̵7;s system contains only male worms. This delay allows the worms to grow up to 16 inches long. The dog̵7;s heart and arteries can be infected with as many as 250 worms by the time the owner is aware of the infection.

    What Happens When Heartworm Treatment is Not Started

    • Growing heartworms will crowd the chambers of the dog̵7;s heart, slowing the blood flow and choking off the oxygen supply to the dog̵7;s body. Heartworms can live in the dog̵7;s vital organs for five to seven years, potentially causing permanent and irreversible damage to the dog̵7;s organs and other tissues during their lifespan. Your dog, if untreated, has a fatal disease that eventually will cause sudden, unexpected heart failure and death if the condition continues.

    Symptoms of Heartworm Infection

    • Heartworm infection has many different symptoms, depending on the severity of the infection. A previously active dog may begin to tire easily or become listless. Dogs with even average activity levels may display shortness of breath or coughing with minor activity. Early symptoms also may indicate other diseases or infections and, therefore, may remain unnoticed, including lack of appetite, weight loss and listlessness. A distended, fluid-filled abdomen, called ascites, is an indication of advanced heartworm infection. However, not all dogs show detectable symptoms of heartworm infection.