The 2011 outbreak of African horse sickness in the African horse sickness controlled area in South Africa

Abstract

African horse sickness (AHS) is a controlled animal disease in South Africa, and as a result of the high mortality rates experienced, outbreaks in the AHS controlled area in the Western Cape Province have a significant impact on affected properties as well as on the exportation of live horses from the AHS free zone in metropolitan Cape Town. An outbreak of AHS serotype 1 occurred in the surveillance zone of the AHS controlled area of the Western Cape during the summer of 2011. The epicentre of the outbreak was the town of Mamre in the magisterial district of Malmesbury and the outbreak was confined to a defined containment zone within this area by movement control of all equids and a blanket vaccination campaign. A total of 73 cases of AHS were confirmed during this outbreak, which included four confirmed subclinical cases. The morbidity rate for the outbreak was 16%with a mortality rate of 14%and a case fatality rate of 88%. Outbreak disease surveillance relied on agent identification using polymerase chain reaction (PCR)-based assays, which is novel for an AHS outbreak in South Africa. The source of this outbreak was never confirmed although it is believed to be associated with the illegal movement of an infected animal into the Mamre area. This detailed description of the outbreak provides a sound scientific basis to assist decision making in future AHS outbreaks in the AHS controlled area of South Africa and in countries where AHS is an exotic or emerging disease.

African Horse Sickness Caused by Genome Reassortment and Reversion to Virulence of Live, Attenuated Vaccine Viruses, South Africa, 2004–2014

“…Similarly, changes in the AHS case definition that only came into effect in 2008 (after the 2004 Stellenbosch outbreak) probably resulted in an underestimation of subclinical AHSV infections during that outbreak. Whereas during the Stellenbosch 2004 outbreak only clinically affected, deceased horses were classified as having confirmed cases ( 13 ), major advances in AHS diagnostic testing (e.g., rRT-PCR–based methods) have occurred during the past 10 years that likely substantially increased the detection of subclinical infections by the time of the 2014 outbreaks ( 15 , 32 ). …”

“…‡An additional 5 clinical cases and 2 deaths that met the criteria of the current OIE AHS case definition were not included based on the case definition in place at the time of this outbreak ( 13 ). §An additional 11 subclinical cases that met the criteria of the current OIE AHS case definition were not included based on the case definition in place at the time of this outbreak ( 15 ). …”

A field investigation of an African horse sickness outbreak in the controlled area of South Africa in 2016

“…Previous outbreaks of AHS in the AHS controlled area have been described (Grewar et al., ; Sinclair, Bührmann, & Gummow, ; Weyer et al., ), with recent evidence that the majority of these outbreaks have been due to AHS live attenuated vaccine strain reassortment and/or reversion to virulence (Weyer et al., ). We describe the environmental, host, vector and viral patterns and attributes of the recent outbreak of AHS which occurred in the AHS surveillance zone in the Western Cape Province in April and May 2016 as well as the control measures implemented.…”

Development of three triplex real-time reverse transcription PCR assays for the qualitative molecular typing of the nine serotypes of African horse sickness virus

“…Importantly, virus serotype was readily determined in samples that were positive by AHSV RT-qPCR but negative by VI. Use of both assays in tandem will be invaluable in expediting outbreak responses, such as those that have occurred in the AHS surveillance zone of the AHS Control Area of South Africa (Grewar et al, 2013).…”

“…Traditional VI and serotyping was used previously to identify the AHSV-1 serotype involved in an outbreak of AHS in the region in 2004 (Sinclair et al, 2006), whereas the AHSV-1 that was responsible for the outbreak in 2011 was determined by sequence analysis of the AHSV type-specific L2 (VP2) gene of the causative virus contained in the blood of an affected horse. This latter process took six days to complete before vaccination could be instituted (Grewar et al, 2013). AHSV type-specific real-time RT-PCR (TS RTqPCR) assays offer the potential for more rapid determination of the AHSV type involved in such outbreaks.…”