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.
PCR-based methods for the sensitive and specific diagnosis of hookworm have been developed, but are not widely used. [39]
Other causes of iron loss and blood loss should be excluded.
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. [10] 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. [40]
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. [41]
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. 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 Ancylostoma duodenale and Necator americanus cannot be distinguished morphologically. 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. 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). 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. Importantly, the Kato-Katz method requires the use of fresh stool samples so that the eggs do not die – otherwise, the observed egg count will be artificially low. 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. Accepted intensity classifications based on eggs per gram of stool are as follows:
Table 1: WHO intensity classifications (Open Table in a new window)
Intensity |
Eggs per gram of feces |
Mild |
< 1,999 |
Moderate |
2,000-3,999 |
Heavy |
>4,000 |
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. [42]
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.
Other Studies
In cases of severe infection, chest radiography may show diffuse alveolar infiltrates during the migration of the larvae through the lung to the intestine. 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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Adult hookworm attached to duodenal mucosa.
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Hookworm filariform larva. Courtesy of Division of Parasitic Diseases, Centers for Disease Control and Prevention (CDC).
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Adult Ancylostoma duodenale worm. Anterior end with mouth parts visible. Courtesy of Patrick W Hickey, MD.
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Adult Necator americanus worm. Anterior end with mouth parts visible. Courtesy of Patrick W Hickey, MD.
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Hookworm eggs examined on wet mount. Eggs of Ancylostoma duodenale and Necator americanus cannot be distinguished morphologically. Courtesy of Division of Parasitic Diseases, Centers for Disease Control and Prevention (CDC).
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Hookworm rhabditiform larva (wet preparation). Courtesy of Division of Parasitic Diseases, Centers for Disease Control and Prevention (CDC).
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Life cycle of hookworm. Courtesy of the Centers for Disease Control and Prevention (CDC) [https://www.cdc.gov/parasites/hookworm/biology.html].
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Hookworm egg in an unstained wet mount at 400x magnification. Courtesy of the Centers for Disease Control and Prevention (CDC) [https://www.cdc.gov/dpdx/hookworm/].
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Hookworm rhabditiform larva. Courtesy of the Centers for Disease Control and Prevention (CDC) [https://www.cdc.gov/dpdx/hookworm/].
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Side-by-side image of the mouth parts of each species of human-infecting hookworm. Courtesy of the Centers for Disease Control and Prevention (CDC) [https://www.cdc.gov/parasites/hookworm/biology.html].
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Cutaneous larva migrans. Courtesy of Wikipedia [WeisSagung, https://en.wikipedia.org/wiki/Cutaneous_larva_migrans].
Tables
Intensity |
Eggs per gram of feces |
Mild |
< 1,999 |
Moderate |
2,000-3,999 |
Heavy |
>4,000 |
Drug |
Adults |
Children (>24 months) |
Albendazole (preferred)* |
|
|
Mebendazole |
|
|
Pyrantel pamoate |
|
|
Thiabendazole |
|
|
