The nematode Dirofilaria immitis is the cause of heartworm disease in dogs. These are long, threadlike worms with small mouths. These worms do not lay eggs, but instead release prelarval stages called microfilariae. These circulate in the blood and tissue fluids and are available for ingestion by a blood-feeding mosquito. While in the mosquito, the microfilariae develop into infective larvae. They gain access to another host through mosquito bites. The larvae develop in the subcutaneous tissues for about 2 months. Eventually, they enter the bloodstream and lodge in the pulmonary arteries.
By 6 months, the worms have grown to maturity in the larger branches of the pulmonary arteries and begin producing microfilariae. Adult heartworms have a life span of up to 5 years.
Once the adult worms are in place, the body launches an immunologic assault against them. The more vigorous the attack and the greater the number of worms present, the more severe the resulting disease. Some dogs actually appear to be allergic to the worms and develop a severe illness even in the presence of a small number of worms.
Dogs with large numbers of worms or dogs that are highly allergic develop clinical signs of heartworm, including:
Treatment of heartworms is a four step process involving:
Annual examination for heartworms can be valuable in finding infection before they become clinical. In areas where heartworm infestation is high, typically during mosquito season, preventative medicine may be indicated.