Plains zebras in the controlled area of South Africa’s Western Cape Province, vital to the country’s export horse trade, are unlikely to represent a long-term source of African Horse Sickness (AHS).
That is the finding of researchers Thibaud Porphyre and John Grewar, who undertook modelling to assess the potential of plains zebras to maintain the life-threatening disease in the province.
AHS, a disease spread by biting midges, is a major barrier to the trade of horses from all affected countries in sub-Saharan Africa.
The disease is endemic in South Africa except for a tightly controlled area in the Western Cape Province, which allows the nation to export horses from that area under strict conditions.
This controlled area, at the very south-western tip of South Africa, consists of an inner AHS free zone, a surveillance zone and a protection zone, which differ in their risk profile and the nature of the equine population in each zone.
However, sporadic outbreaks have occurred there in the past 20 years.
Porphyre and Grewar, writing in the open-access journal PLOS ONE, note that zebras are thought to be the natural reservoir hosts for the disease.
AHS has not been reported in zebras in the province since at least 1993.
However, there is the potential that the presence of zebra populations in Western Cape Province could maintain virus circulation in the area and act as a year-round source of infection for horses.
However, it remains unclear whether the conditions present in the province would enable persistent circulation of AHS in the local zebra populations.
The pair conducted modelling to understand more about the risk.
In South Africa, it is mandatory to register properties that keep zebras in the AHS controlled area.
Overall, 2498 plains zebras are understood to be in the province, spread across 212 holdings. Most of the populations are small, with a median population size of seven and a maximum of 168.
Using their model, they assessed 40 independent incursions of AHS in different scenarios. Incursion events were assumed to result from the introduction of a single infected (but not infectious) adult female into a given plains zebra population.
The pair showed that populations of plains zebras present in the province are not large enough for AHS introduction events to become endemic.
They found that coastal populations of zebras need to be more than 2500 individuals for disease to persist for more than two years, even if zebras are infectious for more than 50 days.
“African horse sickness cannot become endemic in the coastal population of the Western Cape Province unless the zebra population involves at least 50,000 individuals,” they reported.
“Finally, inland populations of plains zebra in the Western Cape Province may represent a risk for AHS to persist but would require populations of at least 500 zebras or show unrealistic duration of infectiousness for AHS introduction events to become endemic.
“Our results provide evidence that the risk of AHS persistence from a single introduction event in a given plains zebra population in the Western Cape Province is extremely low and it is unlikely to represent a long-term source of infection for local horses.”
They continued: “While there is still much to be established regarding the host, vector, environmental and viral components of the disease in wild equids, we show that, in the current population structure (both herd and region-level), it is unlikely that zebra are in populations large enough to maintain a persistent AHS infection in and around the South African AHS controlled area.”
However, they did find potential for within-region differences in risk of persistence of the virus, and further study is required to confirm this potential.
Porphyre is with the Roslin Institute at the University of Edinburgh, and Grewar is with South African Equine Health and Protocols NPC.
Porphyre T, Grewar JD (2019) Assessing the potential of plains zebra to maintain African horse sickness in the Western Cape Province, South Africa. PLoS ONE 14(10): e0222366. https://doi.org/10.1371/journal.pone.0222366