Hookworm eggs are discharged into the environment but may also develop in the human intestine. Development occurs at 10-40°C, with an optimal

Fig. 21.2 Characteristic hookworm egg

developmental temperature at 20-30°C. A. duodenale eggs hatch sooner than N. americanus and the embryos die rapidly in direct sunlight and when the moisture of the soil falls below 9% (Smith, 1990; Smith and Schad, 1989).

A first-stage larva, Lj, emerges from the egg and is approximately 250-300 ^ in length, with a characteristic flask-shaped muscular oesophagus. This is also known as the rhabditiform larva. It feeds on organic debris and bacteria and after 3 days of growth undergoes a moult, with the emergence of the second-stage, L2, larva. In the L2 stage the open mouth closes after 5-8 days, and mouth closure is concurrent with the transformation into the non-feeding third-stage filariform larva (L3). The L3 stage is infective from man; it migrates along moisture and travels in the soil. It is naturally highly susceptible to desiccation and is able to invade through intact skin, where it migrates to the lungs, then breaks out into alveolar capillaries, up the trachea and into the pharynx, is swallowed into the small intestine, where it develops into an adult and mates (Smith, 1990).

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