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 1, A).
Fig1. 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.)
PATHOGENESIS AND SPECTRUM OF DISEASE
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
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 1). 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 2 and 3). 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.
Fig2. Hookworm egg, iodine stain.
Fig3. 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 (see clinical trials.gov).
