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Laboratory Diagnosis

Recovery and identification of rhabditiform larvae and adult worms in fresh stool specimens (Fig. 13.2B,C), or adult rhabditiform worms or filariform larvae in older stools or after stool culture, is the preferred method for establishing the diagnosis. This may require the examination of multiple stool specimens since it is well known that the presence of larvae in the stools is quite variable on a daily basis. Estimates of recovery of larvae from the stool range from 27-57%. Concentration techniques should be used for stool specimens (especially the Baermann technique for Strongyloides), since larvae may be scarce and large amounts of stool are usually needed to recover a few larvae. The larvae of Strongyloides and hookworms can be distinguished from one another on stool exams by their tail morphology. The filariform larvae of Strongyloides have a notch in their tail, whereas the hookworm filariform larvae have a pointed tail.

Strongyloides larvae can be identified also in duodenal aspirates; the accuracy of diagnosis by this modality ranges from 36-91% in infected patients. Duodenal or jejunal biopsy is too sophisticated a procedure for routine use but will confirm the diagnosis in most patients. Occasionally larvae may be recovered from the sputum during the pulmonary phase of strongyloidiasis.

It is rare to find eggs of S. stercoralis in stool specimens. Eggs may be obtained from duodenal aspirates ("Entero-test"), but they are sometimes difficult to distinguish from those of other helminths, such as hookworm or Trichostrongylus (Fig. 13.2B)

There is a complement fixation test which is positive in 75% of infected individuals, but it is too nonspecific to be of diagnostic help. The ELISA test is also sensitive but nonspecific.

The characteristic hematological abnormality is eosinophilia; this may reach 10,000-12,000 cells per mm³ and will persist at a slightly lower level while the infection lasts. The eosinophil count will be very high in the migratory larval stage of strongyloidiasis and the patient often has pulmonary symptoms as seen in Löffler's syndrome, so that the illness may resemble tropical pulmonary eosinophilia. A substantial fall in the eosinophil level during a severe Strongyloides infection has prognostic significance because it suggests a lowered immunity. In most patients there will be a polymorphonuclear leukocytosis during the early stages of infection, which will decrease or become a neutropenia as the condition becomes chronic.

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