Epidemiology

The life-cycle of Dracunculus is seemingly rather bizarre but is actually well adapted for trans mission of a parasite that utilises an aquatic intermediate host but occurs in arid or semi-arid areas. When an infected individual with a blister or ulcer places the affected portion of the body into a pond, or until recently the large open step wells found in India, thousands of first-stage larvae are released into the water from the uterus of the female worm. Not all of the 3 million larvae contained in the uterus are released at once and the now flaccid end of the worm usually dries up and more larvae are released on subsequent re-immersion. In water the larvae thrash actively and can live for a few days. If they are ingested by suitable species of predatory microcrustacean, commonly known as cyclops (previously regarded as all belonging to the one genus, Cyclops, but recently split into numerous genera, of which species of Tropocyclops, Meso-cyclops and Metacyclops are important as intermediate hosts), the larvae penetrate from the stomach into the body cavity (haemocoel) and develop into the infective third stage in 14 days at a temperature above 21°C. Human infection occurs when drinking water is obtained from ponds (Figure 20.3) and large open step

Fig. 20.3 (A) Large man-made pond in Eastern Nigeria, source of infection for dracunculiasis, schistosomiasis and malaria. (B) Domestic rain-water storage cistern

wells. After ingestion of infected cyclops, larvae are released in the stomach, penetrate through the abdominal wall, and about 3 months later males and females, measuring 3-4 cm long, mate in the subcutaneous tissues of the thorax region. The males remain in the tissues and die, while the females continue to grow and slowly move through the muscle planes to the final site of emergence (in about 80% of cases this is the lower limbs) which occurs around 1 year after infection. In most cases only one worm emerges, but there can be many. Twenty years ago, infection was widespread in rural communities in India, Pakistan, Yemen and most countries of sub-Saharan Africa, from Senegal to Ethiopia. It was estimated then that about 10 million people were infected world-wide each year.

Everywhere it occurs, dracunculiasis is seasonal: transmission is principally in the wet season in the semidesert (Sahel) areas of Africa, such as Burkina Faso, Mali, Niger and northern Ghana, Nigeria and Sudan; in humid (Guinea) areas of West Africa with rainfall exceeding 150 cm/year, such as Benin, Cameroon, Ivory Coast, Togo and southern Ghana, Nigeria and Sudan, there is less transmission during the rainy season, as many ponds turn into streams and there are few infected cyclops in the abundant, turbid water. In these areas, maximum transmission (and thus infection) occurs in the latter part of the dry season extending to the first rains, the main planting period. A high percentage of the inhabitants of a village may be infected each year (particularly in the 14-40 year-old working population). However, there is no carry-over of infection from one transmission season to the next.

Infection was eliminated from the Bokhara area in the 1930s and from southern Iran in the 1970s.

Dracunculus can infect various mammals, particularly dogs, but there is no evidence that they act as reservoir hosts of human disease.

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