What Are the Main Symptoms of Dogs Having Heartworm Disease?

Your vet may detect abnormal lung sounds if your dog has heartworms.

An old adage states that an ounce of prevention is worth a pound of cure, and in the case of heartworms, the sage advice holds especially true. Even if your pet spends the bulk of his time in your house or yard, he can't get away from mosquitoes that carry the disease. Monthly preventive treatment combined with annual testing is the best way to keep the parasites from taking hold in your dog's heart, and knowing the symptoms is essential to getting prompt veterinary treatment.

In the Early Stages

Dogs of any age contract heartworms, but most cases occur in dogs more than 2 or 3 years of age. Mosquitoes ingest the larval form of the heartworms while feeding on an infected host, spreading the microfilariae when they feed on an uninfected dog. The larvae take six months or more to develop into adults and usually do not cause symptoms until additional life cycles produce more worms.

That Nagging Cough

As the worms grow to their adult size of up to 12 inches long, they take up residence in the heart, lungs and nearby blood vessels. The first noticeable symptom of the worms' presence is soft coughing. In the early stages of heartworm disease, your dog otherwise seems healthy. Heartworm tests during the early part of the disease may come back negative if there are too few female worms to produce antigens in the blood.

Couch Potato

As the disease develops, your dog's symptoms become more noticeable, with a persistent cough and reduced energy. A severely affected dog may become increasingly reluctant when it's time to snap on the leash and go for a run. Once he's out the door, he'll be out of breath and ready to quit sooner than he used to and take longer to recover afterwards. If you notice your dog takes longer than 30 minutes to quit panting heavily or is less interested in exercise, take him to the veterinarian for a checkup.

Don't Ignore It

Left untreated, heartworms cause enlarged heart or cardiac failure due to the worms blocking blood flow. Reduced blood flow to the lungs, kidneys and liver often results in failure of these organs. While the disease is not directly transmissible from the dog to another dog or human, an untreated dog provides opportunities for mosquitoes to acquire the parasite and spread it to others. Humans are sometimes infected, resulting in a condition known as dirofilariasis where a dead worm causes a coin lesion in the lungs or other organs. The condition is usually asymptomatic and diagnosed during X-rays for other conditions.