What does soil-transmitted helminth elimination look like? Results from a targeted molecular detection survey in Japan

Abstract

Background: Japan is one of the few countries believed to have eliminated soil-transmitted helminths (STHs). In 1949, the national prevalence of Ascaris lumbricoides was 62.9%, which decreased to 0.6% in 1973 due to improvements in infrastructure, socioeconomic status, and the implementation of national STH control measures. The Parasitosis Prevention Law ended in 1994 and population-level screening ceased in Japan; therefore, current transmission status of STH in Japan is not well characterized. Sporadic cases of STH infections continue to be reported, raising the possibility of a larger-scale recrudescence of STH infections. Given that traditional microscopic detection methods are not sensitive to low-intensity STH infections, we conducted targeted prevalence surveys using sensitive PCR-based assays to evaluate the current STH-transmission status and to describe epidemiological characteristics of areas of Japan believed to have achieved historical elimination of STHs.

Methods: Stool samples were collected from 682 preschool- and school-aged children from six localities of Japan with previously high prevalence of STH. Caregivers of participants completed a questionnaire to ascertain access to water, sanitation and hygiene (WASH), and potential exposures to environmental contamination. For fecal testing, multi-parallel real-time PCR assays were used to detect infections of Ascaris lumbricoides, Necator americanus, Ancylostoma duodenale and Trichuris trichiura.

Results: Among the 682 children, no positive samples were identified, and participants reported high standards of WASH.

Conclusions: To our knowledge, this is the first STH-surveillance study in Japan to use sensitive molecular techniques for STH detection. The results suggest that recrudescence of STH infections has not occurred, and that declines in prevalence have been sustained in the sampled areas. These findings suggest that reductions in prevalence below the elimination thresholds, suggestive of transmission interruption, are possible. Additionally, this study provides circumstantial evidence that multi-parallel real-time PCR methods are applicable for evaluating elimination status in areas where STH prevalence is extremely low.

Keywords: Ancylostoma duodenale; Ascaris lumbricoides; Multi-parallel real-time PCR; Necator americanus; STH; Soil-transmitted helminth; Targeted prevalence survey; Trichuris trichiura; WASH.

Fig. 1

Historical trend map of Ascaris egg-positive percentage among elementary school children. Ascaris egg-positive percentages in 1960, 1966 and 1973 are represented in gray scale. These maps were created using statistical data from the Ministry of Education, Culture, Sports, Science and Technology, Japan [–20] and the website Hakuchizu nurinuri (https://n.freemap.jp), which provides copyright-free maps. The maps were then edited with Adobe Photoshop version CS 5.1

Fig. 2

Sampling flowchart. Of the 1940 children recruited from elementary schools, kindergartens and a nursery, 649 submitted fecal samples. Additionally, family members meeting the inclusion criteria also voluntarily provided samples, of which 33 were eligible for analysis. In total, 682 samples were analyzed in this study