| Summary: | A statistically derived disease reaction index, based on daily parasitological, clinical and haematological measurements observed in 440 5-9 month old Boran cattle following laboratory challenge with Theileria parva in clinical trials of a sub-unit vaccine against East Coast fever, is described. Principal component analysis was applied to 13 variables including first appearance of schizonts, first appearance of piroplasms and first occurrence of pyrexia, together with duration and severity of these symptoms, and white blood cell count. The first principal component, which accounted for over 80% of the total variation expressed by the 13 variables, provided the definition for the disease reaction index, defined on a scale of 0-10. The extension of the method to 133 cattle exposed to natural tick challenge, for which incubation periods are unknown and white blood cell count is impracticable to measure, is also described. A correlation of 0.98 was found between the laboratory and field reaction indices.
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