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Laboratory Diagnosis

The laboratory diagnosis of clinical ancylostomiasis is usually made by counting the number of eggs in a direct saline smear of a fresh sample of feces, or by a stool concentration method. Twenty eggs is considered the dividing point between subclinical and clinical infection. Although differentiating the various hookworm species by egg size is very difficult microscopically, a species-specific diagnosis is attainable if infective larvae can be reared in coprocultures. Stools older than 24-hours may contain hookworm larvae which will have to be distinguished from those of Strongyloides stercoralis. Occasionally, the eggs, larvae or adults of hookworms will be identified in tissues. A skin test antigen derived from N. americanus has been used for screening populations. Several serologic tests are available, but have not proven useful.

Using the radioisotope chromium-51, it has been determined that N. americanus produces a daily blood loss of 0.03 ml per worm, although others consider the loss to be 0.06 to 0.1 ml. For A. duodenale the figure is 0.15 to 0.26 ml per day. This translates to a loss of approximately 3 ml of blood per day in light infections and 100-ml per day with heavy infestations by these habitual blood-sucking parasites. Adult hookworms possess anticoagulants, including the potent factor Xa inhibitor, that allow them to ingest blood. The anemia seen in hookworm disease is hypochromic and microcytic and, when profound, may be accompanied by markedly hyperplastic bone marrow with increased normoblasts as well as myeloid metaplasia of the liver and spleen. Serum iron is decreased and its utilization as measured with tagged radioisotopes is increased. The principal theories concerning the causation of hookworm anemia are chronic blood loss and depletion of iron stores with deficient iron intake and toxic factors. It has been elucidated by Gilles et al that the degree of anemia depends on the iron content of the patient's diet, the state of the iron reserves, and the duration and severity of the infection. Where the iron content of the diet is low (eg, Mauritius), a small worm burden will be serious, whereas in Nigeria a state of hemostasis can be sustained even with 800 worms present in the intestinal tract, since food there contains much iron. Peripheral eosinophilia is often present in the blood in hookworm infections.

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