Evidence of negligible soil-transmitted helminth prevalence in the Bolivian Chaco triggers interruption of preventive chemotherapy and calls for strengthening of surveillance

INTRODUCTION: Soil-transmitted helminthiases (STH) are the most prevalent neglected tropical diseases worldwide. STH control programmes based on preventive chemotherapy (PC) are endorsed by the World Health Organization (WHO). In Bolivia, a program based on 6-months single-dose mebendazole delivery to school-aged children achieved a dramatic decrease in STHs prevalence from 1987 to 2013: hookworm from up to 50% to 0.4–1.3%, Ascaris lumbricoides from up to 19% to 0.9–1.5%, Trichuris trichiura from 19% to 0%. AIM: To further assess STH prevalence among school-aged children in the Chaco Region, in order to inform the Bolivian STH control program policy decision. METHODS: In September 2016, 548 school-aged children were enrolled in a cross-sectional study involving 9 sentinel sites, randomly selected among the rural communities of the Cordillera Province (Santa Cruz Department) and the Gran Chaco Province (Tarija Department). Stool sample for direct microscopy and Kato-Katz technique were collected. Haemoglobin levels (Hb) were assessed by HemoCue, as surrogate indicator of STH-related morbidity. RESULTS: Of 426 stool samples collected and analysed, only 3 cases of hookworms infection were detected, and none with A. lumbricoides or T. trichiura observed. The overall STH prevalence was 0.7 (95% CI 0.2–2.1). Conversely, the prevalence of intestinal protozoan infection was high: Blastocystis hominis (212/426, 49.8%), Entamoeba hartmanni (119/426, 27.9%), Entamoeba coli (113/426, 26.5%), Endolimax nana (111/426, 26.1%) and Giardia intestinalis (102/426, 23.9%). The mean Hb was 12.15 g/dl (95% C.I. 12,1-12,2). CONCLUSION: Given the STH prevalence <1%, PC delivery in the surveyed areas was no longer recommended and the Bolivian program decided to interrupt it. Monitoring through annual cross-sectional survey in sentinel sites to detect possible recrudescence of infections has been planned. No substantial reduction of protozoan infections nor improvement of Hb were observed with respect to previous surveys, conducted in the 1980s and 1990s.PC appears to have played a major role in STH transmission reduction, as otherwise poor hygienic and health conditions persist in the Bolivian Chaco. The contribute of other factors, such as climate or behaviour changes will be matter of further investigations.