In natural grazing grounds, bovines affected with East Coast fever remain ambulatory until a few days or even a few hours before death, and larval and nymphal Rhipicephalus appendwulatus and R. evertsi, which have fed and engorged, will infest the pasture over a wide area. Subsequently, when susceptible cattle graze over this area small numbers of infected instars may attach at intervals and begin to feed. In this experiment an attempt was made to simulate these conditions as closely as possible.
In group A wnere one infected tick was fed on the 1st day, two on the 3rd day, and three on the 21st day, two out of six calves developed an unmistakable East Coast fever infection, with Koch's bodies present in gland smears. In one calf (no. 5724) the reaction was most severe and was undoubtedly due to the three ticks which attached on the 1st and 3rd days, while in another calf (no. 115) the reaction was less severe and was most probably due to the three ticks which attached on the 21st day.
In group B where one infected tick was fed on the 1st day, two on the 5th day, and three on the 21st day; five out of six calves gave a definite unmistakable East Coast fever reaction with Koch's bodies present in gland smears, four of the reactions being undoubtedly due to the three ticks which attached on the 1st and 5th days. There is as yet no reasonable explanation as to why the number of definite reactors should differ so markedly in the two groups.
It is noteworthy that all these reactions were mild and that recovery in each case was uneventful. The only death to occur was control calf no 5729, on which ten infected ticks had fed at one time, but from the results of the other controls it may be assumed that the strain of Theileria parva had by some means become attenuated or that the calves had increased tolerance or resistance.
It is difficult to understand why calf no. 109 failed to react in the first experiment but showed some susceptibility when challenged 96 days later, and why calves nos. 5711, 116, 5642 and 39 gave doubtful reactions. No explanation can be given at this stage, but it raises two practical questions. First, it is difficult to assess the minimum intensity of a reaction which is sufficient to give a durable immunity. The fact that calf no. 114, after what appeared to be a definite T. parva reaction, reacted a second time on exposure to infection 110 days later, proved that the immunity was not prolonged. It is true that the first reaction was only mild, but practically the same remarks could be applied to calves nos. 117 and 5725, yet these calves had not lost their immunity on challenge. Secondly, it is difficult to ensure, by observation of daily temperatures alone, that all calves are really fully susceptible before being used on a critical East Coast fever experiment, for if only a few infected ticks are accidentally introduced into stables or grazing paddocks, it may be assumed that mild subclinical infections will result. These calves may then show varying degrees of resistance to subsequent infections, and results such as those experienced in these experiments would be a normal sequelae.
The intensity of the leucopaenia appeared to bear no relationship to the number of days Koch's bodies were present in gland smears. It was present before the first rise in temperature in all the calves except no. 114, the only calf to show a second reaction on challenge.
