Hookworms

Organism Ancylostoma duodenale and Necator americanus cause the two common hookworm infections of humans.

Clinical features The infective (filariform) larvae directly penetrate the skin and migrate to a blood vessel or lymphatic vessel from where they are carried in the circulation to the lungs. In the lungs, they break out of the alveoli, find their way up the trachea and enter the gastro-intestinal tract. The adult stage is finally reached in the duodenum or jejunum, where the male and female worms mate and produce eggs (Fig. 10.2).

Despite its extensive journey through the human body, like Ascaris, the hookworms are very well adapted to their host and only produce symptoms when heavy infections occur. The passage through the skin can result in a transient urticaria (ground itch), while that through the lungs pneumonitis and haemoptysis. Occasionally, the haemoptysis can be sufficient to suggest a diagnosis of tuberculosis. The main effect results from the adult worms attaching to the intestinal wall where they invaginate a piece of mucosa, extracting blood and nutrients. Anaemia results from frank blood loss and depletion of iron reserves. The degree of anaemia produced depends upon the worm load and one estimate calculates that 60-120 worms (measured by 30 worms excreting 1000 eggs/g faeces) will result in slight anaemia, whereas over 300 worms (10,000 eggs/g faeces) will cause severe anaemia. The newly established worm may produce several bleeding points and if the sexes are unbalanced, the search for a mate can result in increased activity. These effects will naturally be most profound in the growing child and the pregnant woman. It is the combination of malaria, malnutrition and other intercurrent infections, in combination with hookworms, that accentuate the seriousness of this infection.

Diagnosis Eggs are found in faecal examination. They are oval and have colourless thin walls differentiating them from Ascaris, which has a thick brown exterior (Fig. 9.1). The eggs of the two species are identical and only the adults can be differentiated, mainly from their characteristic mouthparts (Fig. 10.2).

Transmission The eggs are passed in the faeces and hatch within 24-48 h to liberate an intermediate (rhabditiform) larva. After some days, it moults to produce the infective filariform larva. In suitable conditions of moist, warm but shaded soil (30°C for N. americanus and 25°C for A. duodenale), this stage of the larva can live for several months awaiting the opportunity to penetrate through the skin of a new host. (The ingested third stage larvae of Ancylostoma can also produce infection.) The larva commonly penetrates the foot of the unshod person and intense infection can occur where areas of beach or bush are demarcated for defecation purposes. Non-human hookworms can also penetrate the skin and produce cutaneous larva migrans (see Section 17.4).

Necator americanus

Necator americanus

Filariform larva

Fig. 10.2. The hookworm life cycle and identification.

Filariform larva

Fig. 10.2. The hookworm life cycle and identification.

Incubation period 8-10 weeks.

Period of communicability From about 2 months after infection to up to 5 years; generally 1 year.

Occurrence and distribution N. americanus, despite its name, is the more widely distributed, being found extensively throughout the tropical belt and well north of the Tropic of Cancer in America and the Far East. A. duodenale is found in the Far East, the Mediterranean and the Andean part of South America (Fig. 10.3). It has been suggested that N. americanus was carried from Africa to the Americas as a

result of the slave trade. Altogether, some 720 million of the world population have hookworms.

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