African horse sickness (AHS) is the most lethal infectious, non-contagious, vector-borne
disease of equids and accordingly has been declared notifiable by the OIE – World Animal
Health Organisation. African horse sickness virus (AHSV) is transmitted via Culicoides midges
and the disease has a seasonal occurrence that is influenced by environmental conditions that
favour the breeding of Culicoides midges. Studies on the interactions between the virus, its
vector and host require knowledge of the epidemiology of AHSV, the environment as well as
anthropogenic factors influencing its occurrence. In an effort to manage AHS, this study
addresses the need for a multidisciplinary assessment of criteria in order to characterise the
distribution of AHS for the development of a risk assessment tool in Namibia. Contrary to
expectations that the arid conditions of Namibia would limit the outbreaks of AHS, on-going and
escalating outbreaks caused a renewed interest in the vectors and the distribution of the
disease. The first part of the study investigated the historical perspectives on the prevalence
and distribution of AHS in southern Africa. The most important observations made during this
investigation were the underreporting of AHS in Namibia, as well as the distribution across the
districts. The importance of the effects of AHS on historical events is highlighted, with the limited
movement of horses during the AHS seasons being an imperative historical precaution. The
Culicoides species composition and environmental factors influencing AHS occurrence were
measured for two years at three sites in Namibia. A total of 79142 Culicoides individuals were
identified with 48 different species collected. The dominance of the proven AHSV vector varied
from 42.7% in Okahandja (high incidence) to 6.8% in Aus (low incidence). A precipitation event
is one of the most important environmental parameters, with a significant increase in the
number of Culicoides collected the week after an event. When comparing the effect of modelled
climatic variables on the distribution of AHS in South Africa and Namibia, precipitation was
found to have the most significant effect in Namibia and temperature in South Africa. The
pattern of AHS occurrence has always been thought to coincide in Namibia and South Africa.
However, this seems not to be the case. It was found that although the same climatic
parameters in both countries are the drivers for the disease, the combination of the parameters
had a different effect on the occurrence of AHS in the respective countries. A social survey was
conducted across Namibia and South Africa to assess the relationship between social
parameters and the occurrence of AHS outbreaks. Movement of horses was indicated as a
major factor in AHS distribution. Areas with higher movement correlated with higher AHS
incidence. It was also evident that the process of reporting was unknown to horse-owners and
that traditional precautionary measures such as stabling during dawn and dusk was the most
popular. Integrating the results obtained during this study, the following parameters were
classified according to their importance as drivers of AHS: precipitation > movement status >
temperature and humidity relationship > Normalised Difference Vegetation Index (NDVI) > soil
type. The last section of the thesis comprises the application of a risk analysis and the
development of a qualitative risk tool from which the AHS risk of a site can be estimated. With
the application of the risk matrix, Luderitz was found to be the appropriate area to apply for AHS
recognition status as a possible equine export station in Namibia. Ultimately, determining the
distribution of AHS is a complex process that should involve a variety of scientific fields for a
combination of techniques and/or approaches to achieve a comprehensive and applicable risk
assessment tool. Significant contributions made by this investigation include the identification of
parameters critical for AHS distribution and the development of a risk matrix tool to estimate the
risk of the occurrence of AHS outbreaks in Namibia.