Patterns of detection of Strongyloides stercoralis in stool specimens: implications for diagnosis and clinical trials

Abstract

Reported efficacies of drugs used to treat Strongyloides stercoralis infection vary widely. Because diagnostic methods are insensitive, therapeutic trials generally require multiple negative posttreatment stool specimens as evidence of drug efficacy. However, only a single positive stool specimen is usually required for study enrollment. To determine the reproducibility of detection of S. stercoralis larvae in the stool, 108 asymptomatic infected men submitted 25 g of fresh stool once a week for eight consecutive weeks for examination by the Baermann technique. During the 8-week study, 239 (27.7%) of 864 stool specimens were positive for S. stercoralis. Rates of detection of larvae in the stool specimens ranged from eight of eight specimens in 3 (2.8%) men to none of eight specimens in 36 (33.3%) men. Of 43 men for whom S. stercoralis was detected in at least two of the first four stool specimens, only 1 (2.3%) man tested negative on all of the next four specimens. In comparison, of 29 men who had detectable larvae in only one of the first four specimens, 22 (75.9%) tested negative on all of the next four samples. Thus, if these 29 men had been enrolled in a therapeutic trial between the first and second sets of four specimens, the efficacy of a drug with no activity against this parasite would have been estimated to be 76%. These data suggest that patterns of S. stercoralis detection vary widely among infected persons and that intermittent larval shedding can lead to inflated estimates of drug efficacy. Before a patient is entered in a clinical trial of drug efficacy, four consecutive stool specimens should be examined for S. stercoralis; only persons with two or more positive specimens should be enrolled.

Efficacy of parasitological methods for the diagnosis of Strongyloides stercoralis and hookworm in faecal specimens

“…However, it is worth noting that 2 samples identified as positive by the Baermann-Moraes were found to be negative by the APC method (data not shown), suggesting the need for the concurrent use of both methods. Moreover, APC was more accurate than the spontaneous sedimentation and Baermann-Moraes methods for the diagnosis of hookworm, as demonstrated in others studies (Dreyer et al, 1996;Knopp et al, 2008). It is important to mention that 8 stool samples with hookworm larvae were identified by Baermann-Moraes.…”

“…The parasitological diagnosis of S. stercoralis infection usually relies on the detection of larvae in stool samples. However, the majority of cases involve a chronic infection, with an intermittent and small larvae load in the faeces, decreasing the sensitivity of parasitological tests (Dreyer et al, 1996;Uparanukraw et al, 1999). Several coprological methods have been used to detect larvae in stool samples, including direct faecal smears, the Baermann-Moraes method, spontaneous sedi-mentation, formalin-ether concentration, filter paper culture and agar plate culture (Arakaki et al, 1990;Kobayashi et al, 1996;Siddiqui and Berk, 2001).…”

“…Studies have demonstrated the high efficiency of the agar plate culture (APC) method, which can detect more than 90% of positive cases of strongyloidiasis, even when only a few worms are present (Arakaki et al, 1990;Koga et al, 1992;Dreyer et al, 1996;Hirata et al, 2007). Moreover, for parasitological methods dependent on the movement/migration of live rhabditoid larvae, the use of fresh stool samples that have not been refrigerated is recommended, although no systematic studies have been performed to support this recommendation.…”

Prevalence of parasitic infections among recent immigrants to Chicago

“…[8, 49, 61, 62] Further, the O&P exam has been shown to have serious limitations, in part because some parasitic infections are characterized by intermittent shedding in the stool. [70] The lack of adequate diagnostic tests for parasitic infections was also evident in the fact that, for many of the parasites screened for in this study, the primary method of diagnosis is serologic testing. Serologic tests can have significant limitations as they often have cross-reactivity among multiple parasites and these tests cannot distinguish between prior and current infections.…”

Strongyloidiasis in patients at a comprehensive cancer center in the United States

“…It is recommended that patients without cancer residing in endemic regions who have unexplained meningitis, a recurrent bloodstream infection, or pneumonia due to enteric gram‐negative bacteria should be evaluated for intestinal parasitic infestation (i.e., for S. stercoralis infection) 2, 12, 14. Because of the risk of concurrent S. stercoralis infection in patients with Ascaris lumbricoides or Schistosoma mansoni , a careful stool analysis is needed to identify S. stercoralis ova that are shed in small numbers, often intermittently 5, 14, 26. Patients from regions that are endemic for S. stercoralis who are undergoing high‐risk antineoplastic therapy or SCT may be screened for latent infestation by a combination of stool examination and serologic analysis.…”