Hookworms
المؤلف:
Patricia M. Tille, PhD, MLS(ASCP)
المصدر:
Bailey & Scotts Diagnostic Microbiology
الجزء والصفحة:
13th Edition , p663-665
2025-10-26
56
Hookworms are known to have a worldwide distribution with two species known to infect humans, Ancylostoma duodenale (Figure 1) and Necator americanus (Figure2). They are the second most common helmintic infection reported in humans. The eggs and rhabditi form larvae of the two species are indistinguishable. Differentiation of the species is based on the morphology of the buccal capsule and the adult male copulatory bursa (see Figure 2, B).
Fig1. A, Ancylostoma duodenale head. B, Tail; note the appearance of the pointed tail. (Courtesy Dr. Henry Travers, Sioux Falls, S.D.)
Fig2. A, Necator americanus head; note the clearly evident rounded cutting plates protruding from the head. B, Copulatory bursa. (Courtesy Dr. Henry Travers, Sioux Falls, S.D.)
The parasites, infective filariform larvae, penetrate the skin and enter the circulation where the larvae are capable of breaking through the capillaries and entering the lungs of the host. The larvae migrate up the bronchial tree, over the epiglottis, and are swallowed. Upon entering the digestive system, the hookworms attach to the mucosa of the small intestine. Here they secrete anti coagulants and ingest blood as their source of nourishment. The worms mature and eggs are passed in the feces and deposited in soil where they mature into rhabditi form larvae. The noninfective rhabditiform larvae will then mature into filariform.
EPIDEMIOLOGY
Hookworms are found in areas with moist, warm soil capable of supporting the life cycle of the parasite. Trans mission is generally through direct skin penetration by filariform larvae.
ANCYLOSTOMA DUODENALE
GENERAL CHARACTERISTICS
A. duodenale, Old World hookworm, is prevalent in south ern Europe, northern Africa, Southeast Asia, and South America. The adult male tends to be larger than the adult male of N. americanus. They attach to the intestinal mucosa by well-developed mouthparts, especially teeth (see Figure 1).
PATHOGENESIS AND SPECTRUM OF DISEASE
A. duodenale is capable of maturation within the intestine without migrating through the lungs of the host. See Table 1 for a summarized detail of associated diseases.
Table1. Pathogenesis and Spectrum of Associated Diseases
Table1. Pathogenesis and Spectrum of Disease—cont’d
NECATOR AMERICANUS
GENERAL CHARACTERISTICS
N. americanus, New World hookworm, is prevalent in Africa, Southeast Asia, and South and Central America as well as the southeastern United States. They attach to the intestinal mucosa by well-developed cutting plates (see Figure 2, A).
PATHOGENESIS AND SPECTRUM OF DISEASE
See Table 1 for a summarized detail of associated diseases.
LABORATORY DIAGNOSIS
Hookworms are typically diagnosed by the presence of eggs or rhabditiform larvae found in stool specimens. The eggs and larvae of the two species are indistinguishable. The eggs are oval and thin-shelled and contain a clearly visible four- to eight-cell stage embryo. There is a characteristic clear space between the shell and the developing embryo (see Figure 2). Recovery and identification of eggs on direct smear or from concentration methods is recommended. Eggs may appear distorted on permanently stained smears. The rhabditiform larvae are typically 250 to 300 µm with a long buccal capsule and an inconspicuous genital primoridum (Figures 3 and 4). The larger filariform larvae are approximately 500 µm, with a pointed tail and a esophageal to intestinal ratio of 1 : 4. Both the rhabditi form and filariform larvae must be differentiated from S. stercoralis.
Fig3. Hookworm egg, iodine stain.
Fig4. Hookworm rhabditiform larvae. (Courtesy Dr. Henry Travers, Sioux Falls, S.D.)
Fresh stool stored at room temperature may result in continued maturation and hatching of larvae. Larvae may be cultured according to the Harada-Mori method previously described within this chapter.
THERAPY
Anthelmintic agents including albendazole, mebendazole, and pyrantel pamoate are indicated. However, as a result of variation in species and geographic distribution, some agents may not be effective in a specific population of parasites, and regional recommendations should be followed because of potential drug tolerance or resistance.
PREVENTION
Avoid contaminated soil and beaches. Wear appropriate footwear such as enclosed shoes in potentially contaminated areas.
As a result of the immunosuppressive activity associated with the production of hookworm proteins, vaccination may only be partially effective. Currently no preventive vaccine exists. However, a protein, ASP-2, secreted by infective larvae of N. americanus is being investigated as a potential recombinant vaccine.
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