Organism The nematode worm Ascaris lum-bricoides.

Clinical features The fertile egg, when swallowed, hatches in the stomach and the larva penetrates the intestinal mucosa to enter the blood stream, passing through the venous and pulmonary circulations to the lungs where it breaks through the alveolar wall to emerge in the bronchioles. Migrating up to a main bronchus, it ascends the trachea and is swallowed back into the gastro-intestinal tract. By the time it reaches the intestines, it has developed into an adult, the fertilized female laying eggs into the excrement (Fig. 10.1). This common intestinal parasite can occur in considerable numbers without causing any symptoms and is often found when a routine stool examination is performed. When the larvae pass through the lungs, pneumonitis and possible haemoptysis can occur, otherwise the sheer number of worms can cause intestinal obstruction or blocking of vital structures, such as the common bile duct. Where nutrition is marginal, the loss of nutrient can be sufficient to tip a child into malnutrition. It has been calculated that 25 worms can produce a loss of 4 g protein daily from a diet containing 40-50 g protein. Deficiency of vitamins A and C can also occur. Academic performance in school decreases with heavy Ascaris infections.

Diagnosis Direct smear examination of the stool is sufficient for diagnosis (Fig. 9.1). The egg has a strong outer coat, which is stained brown from bile pigments, thus differentiating it from hookworm eggs, which are of a similar size.

Transmission The eggs are not infective until they have undergone development in the soil for 1-2 weeks. They require warmth and moisture to develop and will remain viable in the soil for a considerable period of time awaiting the right conditions. The infective larva goes through stages of

Fig. 10.1. Ascaris lumbricoides life cycle.

development within the egg casing and if swallowed, infection occurs. Eggs are normally swallowed in polluted water, on vegetables that have been washed with polluted water or by swallowing earth directly. Eggs are passed during indiscriminate defecation.

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