Human leptospirosis in Seychelles: A prospective study confirms the heavy burden of the disease but suggests that rats are not the main reservoir

Abstract

Background: Leptospirosis is a bacterial zoonosis caused by pathogenic Leptospira for which rats are considered as the main reservoir. Disease incidence is higher in tropical countries, especially in insular ecosystems. Our objectives were to determine the current burden of leptospirosis in Seychelles, a country ranking first worldwide according to historical data, to establish epidemiological links between animal reservoirs and human disease, and to identify drivers of transmission.

Methods: A total of 223 patients with acute febrile symptoms of unknown origin were enrolled in a 12-months prospective study and tested for leptospirosis through real-time PCR, IgM ELISA and MAT. In addition, 739 rats trapped throughout the main island were investigated for Leptospira renal carriage. All molecularly confirmed positive samples were further genotyped.

Results: A total of 51 patients fulfilled the biological criteria of acute leptospirosis, corresponding to an annual incidence of 54.6 (95% CI 40.7-71.8) per 100,000 inhabitants. Leptospira carriage in Rattus spp. was overall low (7.7%) but dramatically higher in Rattus norvegicus (52.9%) than in Rattus rattus (4.4%). Leptospira interrogans was the only detected species in both humans and rats, and was represented by three distinct Sequence Types (STs). Two were novel STs identified in two thirds of acute human cases while noteworthily absent from rats.

Conclusions: This study shows that human leptospirosis still represents a heavy disease burden in Seychelles. Genotype data suggests that rats are actually not the main reservoir for human disease. We highlight a rather limited efficacy of preventive measures so far implemented in Seychelles. This could result from ineffective control measures of excreting animal populations, possibly due to a misidentification of the main contaminating reservoir(s). Altogether, presented data stimulate the exploration of alternative reservoir animal hosts.

Fig 1. Diagnostic flow chart of tests done, number of enrolled patients and diagnostic results.

Fig 2. Leptospirosis total positive cases over a one-year period (1^st December 2014 to 30^th November 2015) in relation to rainfall data (mm) obtained from Seychelles airport and including three months before and after the study period.

Fig 3. Minimum-spanning tree of Leptospira interrogans sequence types (STs) based on the MLST scheme #3 (http://www.pubmlst.org/leptospira/).

Sequence Types from humans (in blue) and rats (in yellow) from Seychelles were included into a network constructed with previously published STs reported from various hosts worldwide and shown in grey circles. Group founders are shown in purple circles. The circle sizes of ST02, ST142 and ST143 reflect the relative abundance of each ST in the data set acquired from human and rat samples. The numbers indicated on branches represent the number of mutations between each ST.

Fig 4. Distribution of sampled rats on Mahé Island, with infection status.

A. The quadrants show the distribution of Leptospira-infected Rattus norvegicus and R. rattus plotted by Wet Season (Northwest monsoon, February-March 2014, n = 464) and Dry Season (Southeast monsoon, June-July 2013, n = 275). Circle sizes represent the relative number of rats captured at each site with a representation of the positives (in red) and negatives (in blue). B. Sampling sites are plotted with urban and rural habitats appearing in orange and green, respectively (see S1 Table for details including GPS coordinates). Maps were produced using QGIS, and the Mahé Island shape file obtained from OpenStreetMap (https://www.openstreetmap.org).