Epidemiology of yaws in the Solomon Islands and the impact of a trachoma control programme

Abstract

Yaws is a re-emerging endemic treponemal infection. The Pacific Islands are believed to be a major focus of yaws worldwide. WHO has recently developed a strategy for global yaws eradication based predominantly on community mass treatment with azithromycin at a dose of 30mg/kg (max 2g). Mass treatment with azithromycin is also key to the WHO strategy for trachoma elimination, although the dose used is lower (20mg/kg – max 1g). In areas where trachoma and yaws are co-endemic, mass treatment of populations as part of trachoma control programmes might aid yaws eradication efforts, but could also have negative consequences if drug resistance were to be encouraged. Prior to mass treatment with azithromycin, the prevalence of clinical and serological evidence of yaws in the Solomon Islands was found to be high. Household contact with a seropositive individual was a strong risk factor for infection, especially if the contact also had an active skin lesion. Village level seroprevalence was shown to be the strongest risk factor for infection. Haemophilus ducreyi was identified as the likely cause of a large proportion of ulcerative skin lesions amongst children, which were clinically indistinguishable from those of yaws. A single round of mass treatment with azithromycin at a dose of 20mg/kg significantly decreased the prevalence of both clinical signs of yaws and serological evidence of active infection. This effect was shown to extend to at least 18 months after mass treatment in the absence of any further intervention. Not receiving treatment with azithromycin was the major risk factor for seropositivity following MDA at both 6 and 18 months of follow-up. A rapid diagnostic test for syphilis was shown to also have potential value for use in yaws. The sensitivity of the test was strongly associated with the antibody titre on gold standard testing, suggesting the test may be most appropriate for testing individuals with suspected active yaws where antibody titres are higher. Mathematical modelling data were used to establish the minimum number of rounds and coverage that are likely to be required to interrupt transmission. Consistent with the findings of the post-MDA prevalence surveys, the model predicted that high coverage – ideally above 80% - is likely to be required to interrupt transmission. This PhD has addressed several key questions about the epidemiology of yaws. Even within endemic populations, the disease is highly focal. Integration of rapid diagnostic tests into routine surveillance may help improve data quality and guide yaws elimination efforts at a national level. Given the strong association between coverage of mass treatment and risk of infection, new strategies to increase the reach of yaws eradication strategies are needed. Mathematical modelling may be of use in informing the design of these interventions.