Some observations concerning the transmission of East Coast fever by ticks

Abstract

In the experiments it has been proved: (1) That brown tick imagines which as larvae had become infected with East Coast fever and which had transmitted the disease in their nymphal stage were no longer infective for susceptible cattle. Four batches of ticks proved their infectivity in the nymphal stage on eight animals, but in their adult stage failed to transmit the disease to two susceptible animals.

(2) Ticks belonging to the same batches which were feeding on two animals rendered immune to East Coast fever by inoculation in the nymphal stage did not transmit the disease in their adult stage to six animals, thus proving that the brown tick which has become infected in one stage cleans itself in the following stage by feeding on an immune or susceptible animal.

(3) Ticks which became infected with East Coast fever in their larval stage and which passed their nymphal stage on a rabbit did not prove to be infective in their adult stage for susceptible cattle. This conclusion bears out that given above (2): showing that a tick loses its infectivity the first time it feeds on an animal susceptible or immune to East Coast fever.

(4) Clean or infective ticks feeding on an animal which has recovered from an attack of East Coast fever do not transmit the disease in their next stage. This conclusion is in support of experiments undertaken eight years ago (vide Annual Report of the Government Veterinary Bacteriologist, 1904-05).

(5) It has been demonstrated that certain batches of ticks collected at the same time and which fed under similar conditions do not transmit the disease in their next stage, even when infested in great numbers and on numerous animals. Other batches of ticks reared in exactly the same way and under similar conditions only infected a few animals, whilst again other ticks proved infective in almost every instance even when a minimum number were used. It is difficult to give an explanation of this fact, but it is quite likely that outside conditions have some influence. The ticks which did not transmit the disease were bred during the coldest time of the year. This may perhaps explain the fact that during the winter months the infection in the field is not so active as during the summer months, remembering at the same time that during the winter months tick life is generally weaker than in the summer.