The diagnosis of chronic infections by the geohelminth S. stercoralis
has been traditionally difficult and further complicated by the need for laborious techniques using fresh stools without preservative, requiring strong clinical suspicion for its diagnosis. As opposed to hyperinfection syndromes, where diagnosis is relatively straightforward, chronic, usually asymptomatic, infections are a diagnostic challenge (25
). The analysis shown here, in a polyparasitized population and from diverse geographic regions, confirms that standard stool-based techniques are relatively insensitive in the chronic stage of this infection and that serologic assays provide a useful tool for the management of patients at risk for this infection and for the estimation of disease prevalence in a given population (11
). In this study, we confirmed the results of previous studies that the recombinant-antigen-based LIPS assay accurately differentiates between sera from patients with parasitologically proven S. stercoralis
infection and healthy, uninfected controls, compared to the current best available serologic test (21
). LIPS assays have been successfully applied to the characterization of antibody responses to multiple infections, including Pneumocystis jirovecii
, HIV, hepatitis virus, and Loa loa
and, more recently, to the diagnosis of infections due to S. stercoralis
). As has been shown previously, the LIPS format was additionally found to be superior to an ELISA-based format using the same NIE recombinant antigen (21
). The superior specificity of LIPS assays compared to the traditional ELISA can be explained, in part, by the use of mammalian COS1 cells, which produce antigens free of contaminating, cross-reacting bacterial proteins, as well as by the use of a solution-phase immunoprecipitation assay that allows detection of a large number of conformational epitopes. In addition, the LIPS assay (QLIPS method) can be performed in relatively little time (15 min) compared to a standard ELISA and requires only small amounts (1 μl) of human serum.
Cross-reactivity with other parasitic infections is a major concern in the development of any S. stercoralis serologic assay. In our study population, we found no association between seropositivity for S. stercoralis and infections with A. lumbricoides, hookworms, or H. nana. Since no other helminths were found in this population, this conclusion is limited to the mentioned STHs. The specificity of the serologic assays is also highlighted by the demonstration that the presence of other helminths in the stools did not affect the strongyloides-specific serology.
The highly accurate predictive values obtained with the cutoff values calculated for each serologic method would allow the use of both LIPS-based methods or the CrAg-ELISA at various clinical settings. In the case of community-based interventions in areas with high prevalence, each positive test could be trusted as positive with absolute confidence and negative tests would have predictive values above 97% for any prevalence level ≤20. It should be noted that with prevalence levels ≥20%, as in the population discussed here, false-negative values are of lesser clinical importance since mass treatments would be recommended regardless of the infection status and overtreatment rarely results in adverse events (19
). An exception to this situation would be treatment of pre-school-age children (<2 years old), a group with safety aspects of anthelminthic therapy still unresolved (1
). In the case of the management of individual cases in areas with low prevalence levels, the proposed cutoff values offer negative predictive values >99.5% with any of the four serologic methods.
The LIPS methodology used in this study differed from that used previously in that the input of Ruc antigen lysates was lower in this study (1 million LU rather than 10 million LU) (21
). The use of a lower input of lysate has been suggested to further lower background signals, although it may result in a lower dynamic range (5
). The rapid QLIPS format has not previously been applied to the diagnosis of S. stercoralis
infection, although it performed well in this study.
The relatively low sensitivity of the stool-based methods compared to the serologic methods is underscored by the high proportion of seropositive samples among the negative stools from Argentina. The significant difference in median titers between stool positive and stool negative samples (Fig. ), which remained significant when only the seropositive values were considered, could be secondary to the differential worm burden present in these groups, a concept that might explain the low sensitivity of the stool techniques in this population. Alternatively, seropositivity among stool negative individuals might represent past infections cleared through anthelmintic therapy or spontaneously (unlikely because of autoinfection). The time for “serologic cure” has been calculated to be 6 months for 90% of the cases but with wide variability and longer periods in other studies using larval-extract-based serologies (14
). The study population in Argentina had not been treated with anthelmintics for a period of at least 6 months prior to the survey. Our data are in agreement with those from a survey in Peru in which an ELISA was more sensitive than a comprehensive single stool evaluation and in which sedimentation concentration is more sensitive than Baermann and agar plate tests (29
), which have been reported as the most sensitive in several reports (7
The disadvantage of using a single stool specimen was in part controlled by the use of multiple techniques, which increased the overall sensitivity (3
) but still could explain the presence of a significant number of seropositive samples among stool samples negative for S. stercoralis
. This fact, despite not overcoming the presumed intermittent larval production by S. stercoralis
, also underlines the constraint posed by the collection of multiple samples from each individual in a community program. The difficulty of this analysis coupled with the logistic impossibility of performing all stool tests in every sample due to scarce stool volume further highlights the limitations of these tools in pediatric populations.
Thus, the present study highlights the advantages and improved sensitivity of rapid, highly specific serologic assays for community-based assessment of S. stercoralis infection. Because of the potential of this infection to cause hyperinfection syndrome along with its general detrimental health effects on infected individuals, the identification of at-risk individuals rapidly and inexpensively becomes of paramount importance, along with the ability to look at the efficacy of treatment using serologic surrogates.