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

Abstract

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.

Aggregation and distribution of strains in microparasites

“…In practice, heterogeneity in host^vector contact (and no doubt in many other aspects of the host^parasite system) is the rule rather than the exception (Woolhouse et al 1997) and this implies that there is an expectation of aggregated distributions of vector-borne microparasites among hosts. This may swamp the e¡ects of any positive interactions between strains or types (but these may be detected in di¡erent ways, as described above) and may override the e¡ects of any negative interactions.…”

“…To prevent maternal antibodies from confounding the analyses, all foals less than ¢ve months of age and four foals aged ¢ve months which tested positive to eight or more serotypes were excluded from the analysis (Lord et al 1997). Final sample sizes for each age class are given in table 1.…”

“…Much of the analysis has already been reported (Lord et al 1997); the full details can be found there. Table 1 summarizes the results.…”

“…The degree of aggregation will a¡ect whether targeted vaccination or intervention programmes are more e¤cient than mass treatment (Woolhouse et al 1997). Aggregation may also a¡ect the prevalence of a parasite in either the host or vector population, with consequent e¡ects on control programmes.…”

Opinion of the Scientific Panel on Animal Health and Welfare (AHAW) on request from the Commission on bluetongue vectors and vaccines

“…These models have been widely used to evaluate the effect of different strategies such as vector control and the use of vaccines on the transmission dynamics of malaria. The same approach was used with some modifications by Lord et al (1996aLord et al ( , 1996bLord et al ( , 1997aLord et al ( , 1997bLord et al ( , 1998aLord et al ( , 1998b to understand the epidemiology of African horse sickness transmission. Although this approach was used to generate and evaluate specific underlying hypotheses, the approach is suitable to assess various options for controlling and managing a vector born disease such as BT.…”