Introduction
Corridor disease is an acute, usually fatal disease of cattle resembling East Coast fever and is caused by infection with buffalo- derived Theileria parva strains transmitted by ticks from African buffaloe (Syncerus caffer) (Figure 30.1). The disease was first recognized in 1934 in Zimbabwe as a form of pathogenic theilerial infection distinguishable from East Coast fever on clinical, pathological, parasitological and epidemiological grounds.11 Previous occurrences in the region may well have been obscured by the widespread prevalence of East Coast fever. Twenty years later the disease was recognized in South Africa18 and the causal organism was then identified as a new species, Theileria lawrencei, by Neitz16 in 1955. It was later proposed as a subspecies of T. parva, namely T. parva lawrencei.25 Subsequent investigation has revealed that the parasite is T. parva and is the same as that causing East Coast fever and Zimbabwe theileriosis, although the distinction between these disease forms has been retained and remains useful for descriptive purposes.27 The disease wasdescribed by Neitz.17
The disease was named buffalo disease, for obvious reasons, and Corridor disease, because the first outbreak in South Africa occurred in the corridor between the Hluhluwe and Umfolozi game reserves in KwaZulu-Natal. It occurs sporadically throughout eastern, central and southern Africa wherever there is contact between cattle and infected African buffalo in the presence of the ticks, Rhipicephalus appendiculatus, R. zambeziensis and R. duttoni (see Vectors: Ticks and East Coast fever: Figure 29.1). In the countries north of the Zambezi River, Corridor disease is not recognized as such in East Coast fever endemic areas and falls into the East Coast fever complex together with the disease caused by the cattle-derived parasite. After foot-and-mouth disease it is the most important disease transmitted from African buffalo to cattle and is a major constraint to the acceptance of the presence of buffalo in cattle-raising areas for purposes of conservation or recreation.
Aetiology and life cycle
Buffalo-derived T. parva is morphologically and serologically indistinguishable from cattle-derived T. parva and has an identical life cycle (see East Coast fever: Figure 29.2).
It is mainly transmitted by R. appendiculatus, although R. zambeziensis replaces R. appendiculatus as the vector in the more arid areas of southern Africa.12, 23 Epidemiological evidence suggests that in Angola the disease is transmitted by R. duttoni.6
Buffalo-derived T. parva can be considered to be a group of T. parva strains which are adapted to tick transmission within the African buffalo population, including the red dwarf buffalo (Syncerus caffer nanus) of Angola.6 It is universally distributed in wild buffalo throughout eastern, central and southern Africa, except in the Addo Elephant National Park in South Africa. It is usually non-pathogenic in this species although calves reared in captivity can be susceptible to T. parva infection and show clinical signs 9; fatal disease can also occur following experimental infection.2 The parasite persists indefinitely in infected buffalo in both schizont and piroplasm form and can be recovered consistently by tick transmission or culture of lymphocytes from such animals.31
Buffalo-derived T. parva is not well adapted to the ox and when transmission from buffalo to cattle occurs it usually fails to complete its development; many cattle die before sufficient time has elapsed for development of piroplasms, and in those that survive, piroplasms are very scanty and evidence of their presence by PCR disappears within two months 14, 36. Recovered animals are usually not infective for ticks but are immune to reinfection. In southern Africa the disease is considered to be self-limiting within a population of cattle once it is removed from the source of infection, because the seasonality of the tick does not permit acquisition of infection within the period of persistence of piroplasms; onward transmission from cattle has only been demonstrated experimentally. In eastern Africa repeated passage of infection in cattle has resulted in a change of the character of the organism on a number of occasions, the parasite behaviour and the disease that it causes becoming indistinguishable from classical East Coast fever.1, 13 This change is attributed to a ‘transformation’ of the parasite as it adapts to the ox, but an alternative explanation that the phenomenon represents selection of a T. parva strain from a mixed population in the buffalo must also be considered. There is no evidence that ‘transformation’ of buffalo-derived T. parva occurs in the region south...
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