Helminthic infections

Hookworm

Occurrence:

Tropics and subtropical areas of

Africa, South America and Asia

Organisms:

Necator americanus, Ankylostoma

duodenale

Reservoir.

Humans

Transmission:

Contact with contaminated soil

Oral (A duodenale)

Control:

Chemotherapy

Correction of anaemia

Sanitary disposal of faeces

This is an important intestinal parasite which occurs commonly in warm climates, especially in communities with poor environmental sanitation. Anaemia secondary to blood loss is the most important clinical feature of hookworm infection. Although light to moderate loads of infection may be tolerated in well-nourished persons who have an adequate intake of iron, heavy infection usually leads to iron-deficiency anaemia and occasionally to severe protein depletion. Thus, the occurrence of disease in hookworm infection depends on the interaction of the load of infection, the state of the iron stores and the diet of the host.

PARASITOLOGY

The two main species which infect humans are Ankylostoma duodenale and Necator americanus. (Fig. 5.3) illustrates their life cycle.

Ankylostoma duodenale (unlike Necator americanus) may also be contracted by the faeco-oral route. The hookworms of cats and dogs, Ankylostoma braziliense and A. caninum, fail to achieve full maturity in humans but may cause a serpiginous skin rash -cutaneous larva migrans.

LABORATORY DIAGNOSIS

Hookworm infection is diagnosed by the identification of the eggs in the stool; concentration methods are used for detecting light infections. Quantitative assessment of the load can be made by using the Kato-Katz method, or other quantitative techniques. The larvae can be cultured from the stool on moist filter paper at room temperature (25°C). The species of worm can be identified from the larval stage or from expelled adults.

EPIDEMIOLOGY

Hookworm is endemic in the tropics and subtrop-ics; it is receding from the more developed areas being most prevalent in the rural areas of the moist tropics. It occurs in various parts of tropical Africa, southeastern USA, Mediterranean countries, Asia and the Caribbean (Fig. 5.4). In parts of West Africa, Central and South America, and elsewhere, mixed infections of both species occur. A. ceylan-icum has occasionally also been reported to cause disease in humans.

Reservoir

Humans are the only important source of human hookworm infection. The epidemiology of the disease is dependent upon the interaction of three factors:

■ suitability of the environment for eggs or larvae;

■ mode and extent of faecal pollution of the soil;

■ mode and extent of contact between infected soil and skin.

Survival of hookworm larvae is favoured in a damp, sandy or friable soil with decaying vegetation, and a temperature of 24-32°C. A. duodenale eggs resist desiccation more than those of Necator. The development of hookworm larvae in the eggs and subsequent hatching can be retarded in the absence of oxygen.

Transmission

Insanitary disposal of faeces or the use of human faeces as fertilizer are the chief sources of human infection in countries where individuals are barefooted. Thus, it is to be expected that hookworm infection will have a higher prevalence in agricultural than in town workers and that in many tropical countries it is an occupational disease of the farming community.

Oral route

It has been shown that although Afccofor direction is acquired almost exclusively by the percutaneous route, Ankylostoma infection may be contracted either percutaneously or orally. The latter mode of entry gives special significance to the reports of contamination of vegetables by these larvae and

Larvae of A. duodenale and N. americanus penetrate unbroken skin, enter blood stream, are carried to lungs, up trachea down oesophagus, to small intestine

Eg| ;s of A duodenale swallowed by man and develop into adult worms in small intestine

Eg| ;s of A duodenale swallowed by man and develop into adult worms in small intestine

Larvae of A. duodenale and N. americanus penetrate unbroken skin, enter blood stream, are carried to lungs, up trachea down oesophagus, to small intestine

Larvae emerge from eggs

Figure 5.3: Life cycles of Ankylostoma duodenale and Necator americanus.

Larvae emerge from eggs

Figure 5.3: Life cycles of Ankylostoma duodenale and Necator americanus.

underlines the biological differences between the two species.

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