Many dogs show the first visible indications of infection only after the disease has progressed to the point where treatment is no longer feasible. Active dogs generally show a tendency to tire easily. Shortness of breath or repeated coughing are usually positive signs of heartworm infection. Hunting dogs are often no longer able to keep up the rapid pace of the chase and frequently drop from exhaustion. Jaundice, convulsions, and disturbances of vision may occur in extreme cases.
Emaciation often precedes death.
All dogs with adult heartworms should show some microfilarial activity in the peripheral blood. A blood test made by a veterinarian is the only way to determine whether or not your dog has heartworm disease. Since microfilariae generally appear in the blood within 9 months after an infection occurs, mild cases of the disease can and should be detected and treated long before any symptoms are evident. It was originally thought that long-haired dogs were less susceptible to heartworm infection because of the improbability that mosquitoes would be able to penetrate through the hair for a blood meal. It has since been shown that this line of thought is erroneous.