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In this article we will discuss about the life history of guinea worm (Dracunculus medinensis).
Dracunculus is digenetic i.e. it requires two hosts to complete its life cycle. One is the primary host, the man which harbours the adult parasite in the subcutaneous tissue. The other is the intermediate host, the Cyclops in which the embryos undergo certain developmental changes before they become infective to man.
The adult females of Dracunculus are usually found in the subcutaneous tissues, especially of the legs, arms and shoulders. The site of copulation inside the definitive host is unknown, but the males disappear rapidly after the process, and the females, which require about 12 months to reach maturity. Mature female contains numerous embryos in its enlarged uterus which fills up the body cavity.
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For egg laying female worm takes up its position just below the skin in such a part of the body which regularly comes in contact with cold water, like hands, legs, feet etc. Here the worm secretes a substance (?) which causes a blister on the skin. The blister bursts eventually and a small, shallow ulcer is formed.
On contact with water, the uterus of female worm is projected out of the ulcer cavity, and a cloud of milky white fluid is released. This fluid is found to contain hordes of tiny coiled larvae with a length of about 600 μm, one-third of which is occupied by the long filamentous tail. When the host leaves the water, the exposed end of the uterus dries and shrivels and so blocks the release of further larvae.
The food reserves of the released rhabditoid (tiny) larvae are small but sufficient to enable them to survive in water for several days. These larvae have to develop in a species of Cyclops to become infective for the final host.
Each Cyclops can ingest as many as 15-20 guinea-worm larvae without being inconvenienced in any way. The infected Cyclops usually die at the end of 6 weeks but with a heavy infection they do not live for more than 2 weeks (normal life-span of Cyclops is about 3 months).
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The larvae break through the soft mid-intestinal wall and come to lie in the body cavity of Cyclops where they undergo metamorphosis (moulting) for two times and increase in size (becoming about 1 mm in length). The larvae become infective in about three weeks. Under favourable conditions it takes about 2 weeks for the development to be completed (Fig. 10.5).
Entrance into man and development into adult worms:
Human infection is brought about by accidently taking in infected copepods in drinking water. On reaching the stomach of a human being the Cyclopses are digested by the gastric juice and the larvae are liberated. Under the stimulus of the gastric juice the larvae become active, penetrate the gut wall, migrate through the tissues, moult twice and come to lodge in the viscera or subcutaneous tissues where they grow to sexual maturity.
The larvae in man become adult males and females. Growth and development of the worm is slow and nearly a year is required to become sexually matured and the female worms are ready to migrate to the skin and release their larvae. The males die after fertilizing the females.
The union between males and females takes place early in the deeper connective tissues and not in the gut. It takes about another 6 months for the gravid female to select its site for discharging the larvae in water. Hence the gravid female migrates and selects only those parts of the skin liable to come in contact with water such as the arms of women who wash clothes at river’s bank or the legs and backs of water carriers.
On reaching the skin surface, the worm pierces the lower layers of the skin with the front end of her body and excretes a toxic substance that irritates the tissues and causes a blister to form over the injured spot. The blister bursts eventually and a small shallow ulcer is produced.
On contact with water, the uterus of gravid female is projected out of the ulcer cavity and a cloud of milky white secretion containing a large number of active coiled larvae, is released. Thus, the larvae are back again in water and the cycle is repeated.
