A genus of filaria (family Onchocercidae, superfamily Filarioidea). Dirofilaria species are usually found in mammals other than man, but rare examples of human infection are known, as by Dirofilaria immitis.

ICD-9-CM: 125.6.

Dirofilaria immitis, the dog heart worm, causes a common zoonotic filarial infection. It is endemic to tropical, subtropical, and warm temperate regions of the world.

Adult worms reside in the right ventricle and pulmonary artery of the dog while the unsheathed microfilariae circulate in the blood. Once ingested by the appropriate mosquito vector the microfilariae undergo development into infective larvae which can be transmitted to both dogs and humans. Development of the larvae into adult worms takes about 180 days in dogs. The worms never reach full maturity in humans.

Most humans infected with D. immitis are asymptomatic. Coin sized lesions can be found in the lungs during radiological examinations. These nodules result from an immune response to the dead or dying worms which are either necrotic or in the processes of being calcified.

The dog heartworm causes pulmonary dirofilariosis in humans when immature worms accidentally infect humans and begin developing in nodules in the lungs or other subcutaneous tissue. In heavy infections of dogs, cats and other wildlife, the worms can cause circulatory distress, interference with functions of the heart valves and cause congestion of the right side of the heart. Cirrhosis of the liver and endarteritis can become apparent after 9-10 months if no treatment is administered. Initially in dogs the clinical signs include decreased exercise endurance, chronic cough and collapse after excercise.


Symptoms: In humans the clinical symptoms are less apparent including lack of stamina.

Diagnosis: Diagnosis in animals includes testing the blood for microfilariae and performing a thoracic radiography. Administering a lung biopsy and chest x-ray is the primary diagnostic tool in humans.

Treatment Options:

In order to temporarily control this disease in humans it is important keep dogs indoors during peak mosquito biting hours, which is at night. This will control the infection of the reservoir host. Also, spraying to abate mosquitoes would control the disease by decreasing the vector population. The adaptability of some vector mosquito species particularly in the genus Culex, plays an important role in the spread of infection. To control the transmission of D. immitis in domestic animals, preventative techniques administered by regular veterinary visits can drastically reduce the risk of infection. For example, Selamectin is a liquid applied topically on a dog or cat that safely prevents the transmission of the adult heartworm. Dogs living in open air and poor conditions seem to be more prone to becoming infected.

Related article: Parafilaria multipapillosa

DISCLAIMER: This information should not substitute for seeking responsible, professional medical care.


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