Burden and epidemiological characterisations of Streptococcus suis in Chiang Mai, Thailand
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The burden of Streptococcus suis infection in humans is increasing worldwide. In Thailand, S. suis is the second most commonly recorded zoonosis. The principal sources of human S. suis infection are pig and pork products. A detailed understanding of the epidemiological characteristics of S. suis and the burden of the disease may help improve prevention and control policy to reduce the burden of this bacterial infection. The work presented in this thesis focuses on human outbreaks of S. suis in Chiang Mai, Thailand, in humans and backyard pigs. This thesis examined the characteristics of previous outbreaks of S. suis in humans and calculated the incidence, disease burden and the associated economic burden of S. suis infection in Chiang Mai. The backyard pig system is important for S. suis transmission and this thesis examined the characteristics of the backyard pig production system in Chaing Mai and examined the prevalence and risk factor for S. suis infection in pigs. Finally, to examine transmission of S. suis, isolates collected during this study were identified and subject to molecular characterization. A retrospective analysis of surveillance data for S. suis cases in Chiang Mai between 2005 to 2014 highlighted the annual incidence rate over this ten year period of 15.52 per 1,000,000 population, 6.5 times higher than for the rest of Thailand (2.37 per 1,000,000 population). The case fatality rate was high at 10.12%. The impact on human health of S. suis infection was derived from surveillance data for the year 2013. The health burden measured in term of Disability Adjusted Life Years (DALYs) was estimated at 7.41 per 100,000 population. Most of the health burden (98.28%) was in adults aged 15-64 years. Males had 3.5 times the health burden of females. The consequences of hearing loss and deafness had significant impacts on affected individuals quality of life. The economic impact of S. suis outbreaks in Chiang Mai was between 2013 and 2014 was estimated from interview data. Most patients were covered for their health costs by the national health security scheme, with expenditure due to S. suis on average being 37,955 baht (£759) per patient. Out of pocket expenses for individuals and their families averaged 5,198 baht (£104) per patient. An epidemiological survey of backyard pig production facilities was undertaken in Chiang Mai province where there was a reported high incidence of S. suis cases in humans occurred each year. Most holdings had between one to five pigs and all holdings shared similar characteristics and management practices. The prevalence of S. suis was in pigs was 4.8% (95%CI=2.2-7.4%). Pigs living in larger spaces (≥ 1.2 m2) showed a lower risk for S. suis infection (OR = 4.35, 95% CI = 1.07-25.21). Examination of the isolates from this study revealed a diversity of serotypes. Only one isolate was identified as S. suis serotype 9. The rest did not match any common serotypes for S. suis (1, 2, 7 or 9) and known virulent strains were not identified. Twelve independent sequence profiles were determined by MLST, of which, 11 were novel. Backyard pigs were found to be commonly infected with a range of previously unidentified S. suis and may be a significant reservoir of human infection.