Identifying age cohorts responsible forPeste des petits ruminantsvirus transmission among sheep, goats, and cattle in northern Tanzania
Authors: Herzog C. M. , de Glanville W. A. , Willett B. J. , Cattadori I. M. , Kapur V. , Hudson P. J. , Buza J. , Swai E. S. , Cleaveland S. , Bjørnstad O. N. 
Published: December 2019

Peste des petits ruminantsvirus (PPRV) causes a contagious disease of high morbidity and mortality in global sheep and goat populations and leads to approximately $2 billion USD in global annual losses. PPRV is currently targeted by the Food and Agricultural Organization and World Animal Health Organization for global eradication by 2030. To better control this disease and inform eradication strategies, an improved understanding of how PPRV risk varies by age is needed. Our study used a piece-wise catalytic model to estimate the age-specific force of infection (FOI, per capita infection rate of susceptible hosts) among sheep, goats, and cattle from a cross-sectional serosurvey dataset collected in 2016 in Tanzania. Apparent seroprevalence rose with age, as would be expected if PPRV is a fully-immunizing infection, reaching 53.6%, 46.8%, and 11.6% (true seroprevalence: 52.7%, 52.8%, 39.2%) for sheep, goats, and cattle, respectively. Seroprevalence was significantly higher among pastoral animals than agropastoral animals across all ages, with pastoral sheep and goat seroprevalence approaching 70% and 80%, respectively, suggesting endemicity in pastoral settings. The best fitting piece-wise catalytic models included merged age groups: two age groups for sheep, three age groups for goats, and four age groups for cattle. However, the signal of these age heterogeneities was weak, with overlapping confidence intervals around force of infection estimates from most models with the exception of a significant FOI peak among 2.5-3.5 year old pastoral cattle. Pastoral animals had a higher force of infection overall, and across a wider range of ages than agropastoral animals. The subtle age-specific force of infection heterogeneities identified in this study among sheep, goats, and cattle suggest that targeting control efforts by age may not be as effective as targeting by other risk factors, such as management system type. Further research should investigate how specific husbandry practices affect PPRV transmission.

Author Summary

Age differences in transmission are important for many infections, and can help target control programs. We used an age-structured serosurvey of Tanzanian sheep, goats, and cattle to explorepeste des petits ruminantsvirus transmission. We estimated rate at which susceptibles acquire infection (force of infection) to determine which age group(s) had the highest transmission rates. We hypothesized that an age-varying model with multiple age groups would better fit the data than an age constant model and that the highest transmission rates would appear in the youngest age groups. Furthermore, we hypothesized evidence of immunity would increase with age. The data supported our hypothesis at the species level and the best fitting models merged age groups: two, three, and four age group models were best for sheep, goats, and cattle, respectively. The highest rates occurred among younger age groups and evidence of immunity rose with age for all species. In most models, confidence interval estimates overlapped, but there was a significant FOI peak among 2.3-3.5 year old pastoral cattle. Importantly, these data indicate that there is not sufficient evidence to support targeted control by age group, and that targeted control based on production system should be more effective.