There occurs in sheep, in districts where louping ill is prevalent, a disease of unknown etiology, characterized by the appearance of an acute
febrile reaction in the affected sheep. The causal agent of the disease is transmitted by nymphal and female ticks, The larva appears to be unable to transmit the causal agent. This disease has been called tick-borne fever. It bears no immunological relationship to louping ill of sheep.
The virus of louping Ill has been recovered from ticks which fed on sheep reacting to the disease, and from nymphal ticks which resulted from
larvae gorged on in experimentally induced case of Louring Ill.
Louping Ill has been produced in three sheep by the agency of the tick. In one of these sheep, a thermal reaction developed, followed by trembling, partial paralysis, and prostration. The virus of leaping Ill was recovered from its tissues. Another sheep developed a thermal reaction
followed by death, without the appearance of clinical symptoms. The virus was recovered from its tissues. The third developed a thermal reaction
and recovered. The virus was recovered from its blood, and it was shown on recovery, to be immune to louping Ill infection.
In another sheep a thermal reaction was produced by the agency of ticks, and the evidence suggests that this reaction was due to a louping Ill infection. Three sheep developed a thermal reaction when infested with ticks, but the relation of this reaction to louping Ill was not determined.
Of the reactions assumed to be due to louping Ill infection, the two which were followed by death were caused by nymphs, the two, from which the
sheep recovered, by females in the one case, and by females and larvae in the other. The three reactions of an undetermined nature were caused by larvae.
The possibility of biological transmission of louping Ill by the tick may therefore be regarded as proven.
In a number of experiments: ,with nymphs and larvae which were presumed to be infected, no reactions were obtained in the experimental sheep. The possible explanations of the negative nature of these results are discussed.
From the evidence obtained, a tentative hypothesis is put forward of the role of the tick in the transmission of the disease in nature.