The human hookworms

 

introduction 

   The hookworms belong to the Order Strongylida, a large order, and of great interest as it contains many important pathogens of man and domesticated animals. This order is further subdivided into three Superfamilies, the Strongyloidea (the hookworms in man), and two related groups, the Superfamily ‘Trichostrongyloidea’, intestinal nematodes which are of veterinary importance in many domesticated animals (e.g. Haemonchus contortus in cattle and Nippostrongylus brasiliensis in rodents) and members of the Superfamily ‘Metastrongyloidea’ (the lungworms, in domesticated animals).


  In man, there are two species capable of causing intestinal infections, Ancylostoma duodenale native to parts of Southern Europe, North Africa and Northern Asia parts of Western South America, and Necator Americans in Central and Southern Africa, Southern Asia, Australia and the Pacific Islands. These are very important human pathogens. It has been estimated that there are 1200 million cases of hookworm infection in man annually, of which about 100 million of which are symptomatic infections with accompanying anaemia. Also, the larvae of several species of hookworms infecting domesticated animals may penetrate human skin, causing pathology even though they do not develop to the adult parasites in man.

Morphology

    The adult parasites are small cylindrical worms, 0.5-1.5mm long (Ancylostoma duodenale being slightly larger than Necator americanus ). The posterior end of the male worm is equipped with a normal copulatory bursa, used to hold the female nematode in place during mating. The females themselves have a vulva situated near the centre of the body, slightly anterior in Necator and slightly posterior in Ancylostoma.

  The anterior end of the parasites are formed into a buccal  Scanning electron micrograph of the mouth capsule of Ancylostoma duodenale (left), note the presence of four "teeth," two on each side and Necator americanus (right) capsule, absent in members of the other Strongylida superfamilies, by which the different genera and species within the group may be differentiated. For example, members of the genus, Necator have capsules equipped with cutting plates on the ventral margins and within the tablet itself small dorsal teeth. In contrast members of the family, Ancylostoma has pairs of teeth on the ventral margin of the capsule. The number of teeth will vary between different species of Ancylostoma but is usually between one and four pairs.

The eggs are bluntly rounded, thin-shelled, and are almost indistinguishable between the different species, measuring approximately 60 by 40 µm, the eggs of Ancylostoma being slightly larger than those of Necator.

Life cycle

   The life cycles of all the hookworms are very similar. The eggs are passed in the faeces, once exposed to air they mature rapidly if conditions are right, with both moisture and warmth essential for development. When developed, they hatch to liberate a rhabditiform (i.e. having an oesophagus where a thick anterior region is connected via a neckline region with a posterior bulb) L1 larvae after a few days. These larval nematodes feed on bacteria and organic material in the soil, where they live and grow for about two days before undergoing the first moult.

  After about five days more growth they moult again, to produce a much more slender L3 larvae. The L3 larvae have a much shorter oesophagus, is a non-feeding form, and is the infective form of the parasite. Infection takes place by penetration of the skin, for example, when walking with bare feet over contaminated damp soil, followed by entry into the circulatory system.

    Here they are carried to the heart, and then lungs. Once in the lungs, they are too large to pass through the capillary bed there. Instead, they become trapped, and the burrow through the capillary epithelium, entering the air spaces. They then migrate up through bronchi and trachea and are then swallowed. Once swallowed, they pass into the intestine and bury themselves between the intestinal villi. Here they moult to form the L4 larvae, equipped with a buccal capsule allowing adherence to the gut wall. After about thirteen days post-infection they moult for the final time, producing immature adult worms.

 These mature over three to four weeks (i.e. five to six weeks after infection), then mate and commence egg-laying to complete the life cycle. These parasites show a very high fecundity, female Necator americanus  producing up to 10,000 eggs daily, while female Ancylostoma duodenale produces up to 20,000 eggs daily