A persistent risk of urogenital schistosomiasis transmission linked to the Cavu River in Southern Corsica since 2013

Abstract

Introduction Urogenital schistosomiasis, a tropical snail-borne disease, re-emerged in Europe in 2013, with 106 cases linked to exposure to the Cavu River in Southern Corsica. The implicated parasite strain was a hybrid of a human-bovine Schistosoma parasite (S. haematobium–S. bovis) of Senegalese origin. The parasite's introduction to the river was presumed to have occurred via an infected individual. Subsequent to this outbreak, we developed a human and snail surveillance system to detect and control schistosomal transmission. Environmental research and phylogenetic analysis were performed to understand the continuing transmission of schistosomiasis in the river. Methods Autochthonous urogenital schistosomiasis became a notifiable disease in France from 2016. A case was defined as a person with serological or parasitological evidence of schistosomiasis, with no history of travel outside of Corsica in a known endemic area. Patient specimens were genotyped, and compared with the 2013 outbreak strain. Seasonal molecular surveillance of snails in the Cavu River was established for rapid detection of schistosomes. Bulinus truncatus snails, intermediates host of S. haematobium, are collected weekly from the Cavu River when conditions are optimal for schistosomal transmission (June–September). Seroprevalence studies in livestock in Southern Corsica and in wild rodents captured along the Cavu River, were conducted. Results Following the 2013 outbreak, seven new cases were identified: one in 2015, four in 2016 and two in 2017. Three cases reported exposure to the Cavu River in summer 2015, two cases in both summer 2015 and 2016 and two other cases in 2016 and 2017. For the last cases, the period between the 2017 exposure and the diagnosis of the symptomatic case is of four weeks whereas the average time for onset of symptoms after exposure is of 30 weeks. Therefore, this suggests that the infection occurred in 2016. Although no specimen was isolated from patients exposed only in 2015, specimens from a patient exposed in 2015/2016 and from another patient exposed in 2016/2017 were closely related to the 2013 strain. With between 1965 and 5364 snails analyzed each year, the surveillance did not detect any Schistosoma-infested snails in the river from 2015 to 2017. Twenty-one rodents were captured for testing along the river and 3,479 cows and goats in Southern Corsica. All seroprevalence tests were negative. Discussion/Conclusion Corsica possesses all the necessary conditions for permanent transmission of urogenital schistosomiasis. Further research on the survival of infested snails over the winter period, and on the presence of a rodent reservoir in the river area are fundamental to evaluate the risk of ongoing transmission of urogenital schistosomiasis in the Cavu River. The persistence of a human reservoir in vicinity of the Cavu River would increase this risk. Therefore, tourists and locals exposed to the river must be advised of the risk of recreational water use in the Cavu. Public health authorities must reinforce physicians’ awareness of screening recommendations, to promptly detect and treat new cases and to prevent further transmission in Corsica. The maintenance of schistosomes in Corsica serves as a warning to other regions in Europe with environmental conditions favorable to urogenital schistosomiasis transmission.