Transmission and distribution of virus serotypes: African horse sickness in zebra

The prevalence of African horse sickness (AHS) serotypes in zebra foals from the Kruger National Park, South Africa was examined for possible associations between serotypes and to estimate the basic reproduction number, R0. The distributions of serotypes between zebra were not independent in the 6- and 7-8-month-old age classes (P < 0.005). This does not necessarily imply biological interactions between serotypes, as heterogeneity in host-vector transmission rates can generate non-independent distributions of serotypes. Both age and month of capture were significant factors in the number of serotypes infecting each zebra (P < 0.0001). Pairwise, positive associations between non-cross-reacting serotypes were found in the 7-8-month-old class only. For AHS overall, estimates of R0 ranged from 31-68. Assuming serotypes are transmitted independently, estimates of R0 for individual serotypes ranged from 10 for serotype 1 to 23 for serotype 6. The wide range of estimates emphasizes the need for a better understanding of serotype transmission and interactions in AHS.