The L2 and L3 stages last for approximately two months. They then enter the heart where they reach adult size approximately 3 months after infecting your pet. The mosquito is the only natural vector of transmission for canine heartworms, and about 70 species are capable of carrying the disease. As you might expect, heartworm infection is more common in areas where mosquitoes are numerous, and outdoor dogs constantly exposed to mosquitoes are the most frequent victims. Heartworm infection may be detected by examining the dog's blood for the presence of circulating microfilarae.
Some dogs have adult worms in the heart but have no circulating microfilariae.
To diagnose the disease in these dogs antigen tests are used. Chest x-rays are also helpful in making a diagnosis and may also give some indication of the amount of damage caused by the infection. Ultrasound is also very useful in determining the severity of infection. Signs of heartworm disease that dog owners are most likely to notice include frequent coughing, sluggishness, rapid tiring, and labored breathing. These signs are most easily noticed in hunting and working dogs.