A multidisciplinary assessment of the distribution of African horse sickness in Namibia

Abstract

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. Keywords: anthropogenic effects, Culicoides, humidity, precipitation, temperature, qualitative risk matrix.