Species Extinction, Infrastructure Development and Epidemics: The Changing Ecology of African Horsesickness in the Cape Colony, c.1653–1900

The virus that causes African horsesickness does not affect any indigenous species, but produces high mortality among horses, a species introduced by the Dutch East India Company in 1653. While the insect-borne disease did not occur in the immediate vicinity of the Cape Peninsula, horsesickness could have constituted an endemic disease barrier to the horse-based expansion of the colonial sphere into the hinterland, where it was seasonally prevalent. That it did so to only a limited extent is due to a substantial alteration of the ecology of the disease that largely resulted from inadvertent side effects of anthropogenic modifications of the environment concomitant to the socio-economic development of the colony. This epidemiological transition evolved in two phases that overlapped chronologically but were clearly distinct regionally. It had started in the south-west of the Cape Colony in the later part of the 18th century and, in correlation with, first, the progressive extinction of quagga and zebra populations and, second, the economic intensification of pastoral and agricultural production and accelerated horse travel, gradually shifted over the following century toward its northern and eastern boundaries. During the earlier phase of this process, the area in which horsesickness was seasonally prevalent contracted steadily. However, the subsequent intensification of the utilisation of horses in transport and farming facilitated the recurrence of ever more frequent and economically devastating large-scale epidemics of horsesickness. The history of African horsesickness in the Cape thus provides an instructive example illustrating the unexpected consequences of human modifications of the environment.