Dirofilaria immitis - Dog heartworm

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Dirofilaria immitis

Taxonomy, Common Name, Disease

 

* CLASS: SECERNENTEA

* SUBCLASS: SPIRURIA

* ORDER: SPIRURIDA

* SUPERFAMILY: FILARIOIDEA

* FAMILY: ONCHOCERCIDAE

Scientific name - Dirofilaria immitis

Common name - Dog heartworm

 

Hosts

dogs, foxes, wolves, coyotes, cats, ferrets.

Distribution

Worldwide but most common in mild and warm climates. In the U.S. they

are prevalent along the Atlantic and Gulf Coasts.

Life Cycle

Adults are parasites mainly of the chambers of the right side of the

heart and pulmonary artery. Individual eggs developing in the uterus

are enclosed in a thin vitelline membrane. As the embryo elongates,

the surrounding membrane stretches to conform as an enclosing sheath.

At birth, the membrane is lost and the embryo appears in the blood as

a sheathless microfilaria. Microfilariae are deposited in the blood of

the chambers of the right side of the heart and pulmonary artery. They

are carried through the lungs, into the left chambers of the heart,

and into the systemic circulation. There is marked nocturnal

periodicity in the peripheral blood. Mosquitoes serve as vectors.

After microfilariae are ingested durding bloodfeeding, they migrate

from the intestine within 24 to 36 hours into the Malpighian tubules,

where further development and a molt from the first to the second

stage juvenile occurs. After 9 days they enter the abdominal hemocoel,

where the second molt occurs. Third stage larvae are about 900 um long

and appear 10 to 20 days after entering the mosquito. The third stage

juveniles migrate to the mouthparts of the moquito. Infection of dogs

occurs while mosquitoes are feeding. Third stage juveniles escape onto

the skin and enter it through the feeding site. For about 80 days,

juveniles are in the subcutaneous tissues and muscles where the third

molt takes place 9 to 12 days after entry. In the tissues, the fourth

stage juveniles attain lengths up to 25 mm. They begin entering the

right side of the heart shortly after the fourth molt 60 to 70 days

after entering the dog. Development to maturity with males 14 to 19 cm

long and females 23 to 31 cm takes 174 to 223 days, at which time

microfilariae appear in the blood. The reproductive period exceeds 2

years and may extend to 5.

Symptoms-Pathogenicity

Symptoms in dogs appear when more than 25 worms are present. Up to 60

worms cause circulatory difficulty. With 100 or more worms, there is

blockage of the pulmonary artery and right side of the heart,

accompanied by interference with the heart valves. The right side of

the heart becomes dilated and enlarged. Blood backs up in the liver

and other parts of the body, causing general congestion and

degeneration. Dogs fatigue easily, cough, and appear unthrifty. Based

on a survey of 37,000 asymptomatic dogs living in California, Theis

has correlated infection with living in geographical areas with an

elevation of 152-1,000 meters and a 30 year mean annual rainfall of 65

cm or more.

Management

Prevention by drug therapy. Daily oral administration of

diethylcarbamazine or monthly administration of Ivermectin. Diagnosis

is by finding of microfilariae in the blood.

Importance

This is the most important filarioid parasite of domestic animals in

North America.

Characteristics

Adults are long, white, thread-like worms. Males measure 12 to 16 cm

long with the tail spirally coiled. It bears narrow alae and three

pairs of large caudal papillae, one of which is postanal and three

pairs of small ones near the tip of the tail. The left spicule is 324

to 375 um long and the right 90 to 229 um. Females are 25 to 30 cm

long with the vulva opening just behind the posterior end of the

esophagus. Microfilariae are sheathless, 218 to 329 um long, and have

a long pointed tail.