Hookworm Disease Workup

Updated: Jul 31, 2018
  • Author: David R Haburchak, MD, FACP; Chief Editor: Pranatharthi Haran Chandrasekar, MBBS, MD  more...
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Workup

Approach Considerations

Laboratory findings (eg, complete blood count [CBC]) may be consistent with iron-deficiency anemia. A differential count may reveal eosinophilia (1000-4000 cells/µL). Serologic tests (eg, tests for A caninum) are usually available only in research laboratories.

Stool should be examined for ova and parasites. The examination may demonstrate significant number of hookworm eggs. However, because egg laying may be delayed, stool examination should not be considered a sensitive test for identifying hookworm infection. Stool examinations may have to be repeated.

Progress is being made in polymerase chain reaction (PCR)-based methods for the specific diagnosis of hookworm infection. [37, 38]

Other causes of iron loss and blood loss should be excluded.

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Blood Studies

Anemia is confirmed by CBC and peripheral blood smear results that demonstrate signs typical of iron-deficiency anemia. Microscopy reveals hypochromic, microcytic red blood cells (RBCs).

Upon initial infection, eosinophilia is usually present during the migratory phase before stool findings are positive. [4] Eosinophilia is surprisingly persistent and may be due to attachment of the adult worms to the intestinal mucosa. Peak eosinophil counts are 1350-3828 cells/µL at 5-9 weeks after experimental human exposure to 45-50 infective larvae. Eosinophilia can be a clue to hookworm, as well as Strongyloides infestation, in chronically infected patients.

Eosinophilia (along with raised serum immunoglobulin E [IgE] levels) is uncommon in cases of cutaneous larva migrans but is almost universally present in cases of eosinophilic enteritis. [39]

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Stool Examination

The diagnosis is confirmed with direct microscopic analysis of fecal samples to verify the presence of hookworm eggs. The specimen is fixed in formalin and prepared as a wet mount. [4, 15]

During early infection, results of stool studies may be normal; in rare cases, the worm or larvae may be present in the fecal sample. In patients with mature infection, eggs may be seen during stool examination. If eggs are not seen, the likelihood of clinically significant infection is very low. When infection is suspected, stool should be evaluated promptly because eggs hatch into infective larvae within 24 hours.

Direct microscopic stool examination usually reveals ovoid eggs with thin colorless shells, measuring approximately 60 ´ 40 µm (see the image below). Under basic light microscopy, the eggs of N americanus and those of A duodenale appear morphologically similar. Larvae and adult worms can be distinguished by rearing filariform larvae in a fecal smear on a moist filter paper strip for 5-7 days (ie, Harada-Mori filter paper strip culture).

Hookworm eggs examined on wet mount. Eggs of Ancyl Hookworm eggs examined on wet mount. Eggs of Ancylostoma duodenale and Necator americanus cannot be distinguished morphologically. Image courtesy of Division of Parasitic Diseases, Centers for Disease Control and Prevention (CDC).

Distinguishing between N americanus and those of A duodenale is not critically important for choosing the type of anthelmintic drug to use, except that arrested larvae of A duodenale can enter breast milk and cause vertical transmission; these arrested larvae can also reactivate after initial treatment and again cause intestinal disease without reinfection.

Although hookworm eggs are easily distinguished from the eggs of other helminths, rhabditiform larvae are occasionally seen in old stool specimens (see the image below). Differentiating the larvae of Necator or Ancylostoma organisms from those of Strongyloides organisms requires attention to the unique morphologic features, particularly the relatively short buccal cavity and prominent genital primordium of Strongyloides larvae.

Hookworm rhabditiform larva (wet preparation). Ima Hookworm rhabditiform larva (wet preparation). Image courtesy of Division of Parasitic Diseases, Centers for Disease Control and Prevention (CDC).

Various methods (eg, ether concentration or Kato-Katz thick smear) can be used for quantitative assessments. The worm burden may be estimated by counting the number of eggs per gram of stool, multiplying by the daily stool weight, and dividing the result by 25,000. A worm burden of less than 25 is generally subclinical; a burden of more than 500 worms is clinically significant.

In most cases, stool-concentration techniques are unnecessary, because most individuals with clinically significant infection excrete a large number of eggs. Eggs are easily detectable in unconcentrated specimens at levels of 1200/mL or higher. However, such techniques may be helpful for diagnosis of minimal infections expected during control programs. Because the intensity and prevalence of hookworm infection decrease with public health intervention, newer concentration methods (eg, FLOTAC) have some advantages over older methods. [40]

In cases of cutaneous larva migrans, stool examination is not indicated, because the diagnosis can be made clinically and because the larvae remain confined to the skin in almost all cases. In cases of eosinophilic enteritis, no eggs are found, because adult A caninum worms do not produce eggs in human hosts.

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Other Studies

In cases of severe infection, chest radiography may show diffuse alveolar infiltrates during the migration of the larvae through the lung. Once infection is established in the gut, imaging studies are not helpful.

In cases of eosinophilic enteritis, colonoscopy may reveal ileal and colonic ulceration and, occasionally, adult hookworms.

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