The epidemiology of an African horse sickness outbreak in the Western Cape Province of South Africa in 2004

Abstract

Historically African Horsesickness (AHS) outbreaks are rare occurrences in the Western Cape Province. The 2004 outbreak was particularly troubling since it followed only five years after the previous outbreak and even before any cases were reported further inland, which is traditionally the source of infection for the southern (non-endemic) parts of the country. Following confirmation of the diagnosis, control measures were immediately instituted and an epidemiological investigation was initiated. The investigation revealed, inter alia, that serological profiles of case horses were inconsistent. A case was subsequently defined as a horse showing typical symptoms of AHS and from which virus could be isolated. The disease pattern for both the 2004 and 1999 outbreaks can be classified as sporadic epidemics. This type of epidemic pattern is to be expected in a vector borne disease and it is typical in a disease situation where some of the animals are immune. The temporal pattern revealed that the level of immunity in the equine population of the affected area was higher during the 2004 outbreak than during the 1999 outbreak. In addition, it showed a clustering of cases during the initial stages of both the 2004 and 1999 outbreaks. This illustrated the efficacy of the control measures (including movement and vector control), which was instituted immediately after the diagnosis of the first case. The analysis of the spatial pattern during both the 2004 and 1999 outbreaks identified the Eerste-river-valley as a high-risk area for the outbreak of AHS in the surveillance zone. The population pattern during the 2004 outbreak illustrated that the risk of dying of AHS was higher in horses of 5 years and younger (p<0.10). It was shown that vaccination and stabling offers the best protection against the risk of dying as a result of AHS infection in an exposed population (p<0.05). A questionnaire survey was conducted as part of the epidemiological investigation and it revealed that only 12.4% of equine holdings in the affected area practiced vector control, while a high percentage of horses (69.6%) were protected by means of vaccination, which impacts negatively on the purpose of a surveillance zone. The number of Culicoides imicola midges in the area where the outbreak was detected was extremely high, constituting 94.6% of the Culicoides midge population. This is comparable with the 1999 outbreak when 96.0% of the midges collected were identified as C. imicola. However, during a similar survey in 1996, C. imicola comprised only 11.3% of the population (Neville et al. 1988, Venter, G., personal communication 2004). Furthermore, the outbreak was detected even before significant rainfall was recorded in the region and transmission occurred at average minimum temperatures below 15 °C. The virus responsible for the 2004 outbreak was typed as AHS serotype 1, while AHS serotype 7 was identified as the cause of the 1999 outbreak. The source of the infection in the 1999 outbreak was the illegal movement of two horses from the Free State Province in the infected zone into the surveillance zone. Although no absolute proof could be obtained, there is strong evidence that the source of the 2004 outbreak was again the movement of horses, this time from Namibia, accentuating that horse movements constitutes the highest risk to the integrity of the free zone. Since the ability to control an outbreak successfully is directly dependant on rapid detection and given the large number of vaccinated horses as a result of the outbreaks and the AHS movement control policy, amendments to the export policy and legislation are recommended. AHS outbreaks in the control area of South Africa cause substantial financial loss to the horse industry and the controlling authorities.