There has been much discussion concerning the relationship Of the spirochætes. Formerly, they were regarded as bacteria closely related to the forms grouped in the genus Spirillum. The results of the detailed study which the spirochætes have received in recent years, have led most of the workers to consider them as belonging to the protozoa. The merits of the discussion we are not concerned with here, but rather with the fact that a number of diseases caused by spirochætes are arthropod-borne. The better known of these we shall discuss.
African Relapsing Fever of Man—It has long been known to the natives of Africa and to travelers in that country, that the bite of a certain tick, Ornithodoros moubata, may be followed by severe or even fatal fever of the relapsing type. Until recent years, it was supposed that the effect was due to some special virulence of the tick, just as nagana of cattle was attributed to the direct effect of the bite of the tsetse-fly. The disease is commonly known as "tick-fever" or by the various native names of the tick.
In 1904, Ross and Milne, in Uganda, and Dutton and Todd on the Congo, discovered that the cause of the disease is a spirochæte which is transmitted by the tick. This organism has been designated by Novy and Knapp as Spirochæta duttoni.
Ornithodoros moubata ([fig. 142]), the carrier of African relapsing fever, or "tick-fever," is widely distributed in tropical Africa, and occurs in great numbers in the huts of natives, in the dust, cracks and crevices of the dirt floors, or the walls. It feeds voraciously on man as well as upon birds and mammals. Like others of the Argasidæ, it resembles the bed-bug in its habit of feeding primarily at night. Dutton and Todd observed that the larval stage is undergone in the egg and that the first free stage is that of the octopod nymph.
The evidence that the fever is transmitted by this tick is conclusive. Koch found that from five per cent to fifteen per cent, and in some places, fifty per cent of the ticks captured, harbored the spirochæte. The disease is readily transmitted to monkeys, rats, mice and other animals and the earlier experiments along these lines have been confirmed by many workers.
Not only are the ticks which have fed on infected individuals capable of conveying the disease to healthy animals but they transmit the causative organism to their progeny. Thus Möllers (1907), working in Berlin, repeatedly infected monkeys through the bites of nymphs which had been bred in the laboratory from infected ticks. Still more astonishing was his discovery that ticks of the third generation were infective. In other words, if the progeny of infected ticks were fed throughout life on healthy animals, and on maturity deposited eggs, the nymphs which hatched from these eggs would still be capable of carrying the infection.
The developmental cycle of the spirochæte within the tick has not been fully worked out, though the general conclusions of Leishman (1910) have been supported by the recent works of Balfour (1911 and 1912), and Hindle (1912), on the life cycle of spirochætes affecting fowls.