Dracunculus medinensis

Phylum Nematoda

    Class Secernentea

        Order Spirurida

            Family Dracunculidae

                Dracunculus medinensis

Common Names:

Guinea worm, medina worm, or serpent worm

Geographic Range:

Africa, remote villages in Rajastan desert and Yemen

Definitive Host:

Humans

Intermediate Host:

Cyclopod crustacean (Cyclops vernalis) or water flea

Morphology:

One of the largest nematodes known, females can reach the length of 800 mm while males may only be up to  40mm.  Both sexes have a small, triangular mouth surrounded by a quadrangular, sclerotized plate but no lips.   Females have anterior and posterior branches to their uterus.  When gravid, the female intestine becomes       squashed and nonfunctional.

Life Cycle:

The female worm, engorged with juveniles, makes her way to subcutaneous tissue in the extremities, with 90% of the worms in the feet or legs.  Some juveniles break out either through the body wall of the female or out through  her esophagus.  This produces an antigenic response in the host resulting in a large, painful blister which           eventually bursts.  The female protrudes from this opening to expel her young.  This is done through contractions  caused by exposure to cool water that force the J1 out in periodic bursts of up to 500,000 at one time.  The J1  must directly enter the water in order to survive.  They must be ingested by the copepod intermediate within three days.  Within the copepod hemocoel, the J1 develop to the J3 in 12-14 days at 25 degrees Celcius.  The            copepod must then be ingested by the human host for the life cycle to continue.  After the copepod is digested,  the sheathed larvae penetrate the intestine and migrate to the muscle or subcutaneous tissue.   There they mature  and mate.   The male dies, is encysted, and degenerates.  The female migrates to the skin between the 8th and  10th month after infection.  The blister appears 10 to 14 months after the human host is infected.

Pathology and Clinical Signs:

Three major disease conditions can occur from Dracunculus infection:

        1. Emergent adult females cause hot and painful blisters

        2. Secondary bacterial infections

        3. Nonemergent worms that die under skin cause allergic reactions that could lead to death

Epidemiology:

Dracunculus depends on water for transfer, yet thrives in desert areas and in times of drought.  It is endemic in areas where drinking water sources are limited and used by many people.  It is often found in areas where  natives must actually enter the water pool to retrieve the water, as in the case of step wells.   More sanitary  methods of collecting water would greatly help to control infection.  The World Health Organization, in an effort to eradicate this parasite, has successfully reduced the number of cases from 3.5 million in 1986 to only 130,000 in 1995.  That is less than 4% of the 1986 figure.

Treatment:

Winding the protruding worm on a stick has been a successful treatment since antiquity.  Because the worm   protrudes only a few centimeters per exposure to water, this procedure takes, on average, three months to completely remove the worm.  The worm can also be removed surgically, and some drugs are used, but the  evidence for their effectiveness is dubious.

 

This page was last modified on January 26, 2004
Send questions and comments to Kim Bates