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.
Mosquitoes usually have difficulty penetrating even short hair.
For this reason most feeding takes place on the underpart of the abdominal region. Since even long-haired dogs have relatively little hair on this area, mosquitoes have no difficulty feeding upon them. Limited feeding also occurs on the ears or muzzle area where the hair is matted down. Hunting breeds or breeds frequently run at field trials may often appear to be more susceptible to heartworm infection. Because these dogs spend a great deal of time in the field, they are probably fed upon by mosquitoes to a greater extent than are some other breeds.