A relapsing fever of Persia is transmitted by a tick of the genus Ornithodoros, which is also true for the relapsing fever of Panama. There is great variation in the description of the different spirochaetes, and frequently measurements are given for short forms and long forms. They also vary from wave-like lines to corkscrew spirals. Again, different species have different types and different activities of movement. As a rule they are about 20 × 0.4 microns. The spirochaetes of the relapsing fever of Panama varied in length from 4.6µ to 22µ. Of these the most common were those measuring 13µ. Transverse division of the longest forms was seen in dark-field preparations. The monkey is susceptible to all species of relapsing fever spirochaetes. White mice and white rats are readily infected by S. duttoni and the Panama spirochaete but are refractory to S. recurrentis except after passage through monkeys. S. carteri causes only a transient infection in these small rodents.
Epidemiology.—With tick fever the epidemiology rests upon the life history of the tick O. moubata. This tick infests the rest houses along the route of travel, hiding in the crevices of floors and walls during the day and coming out at night to bite the sleeping inmates. The feeding occupies a long time, more than an hour. Both sexes bite man. The female lays about 100 eggs, from which nymphs emerge in about twenty days. The larval stage takes place in the egg. Shortly after emerging the nymphs suck blood. An important fact is that the female transmits the spirochaete to its ova, so that the ticks from such ova may transmit the disease.
Fig. 39.—Siphunculata and Rhynchota. 1. Pediculus capitis. 2. Pediculus vestimenti. 2a. Protruded rostrum of Pediculus. 3. Phthirius pubis. 4. Acanthia lectularia. 5. A. rotundata. 6. Conorhinus megistus.
Natives seem to suffer severely from tick fever in childhood but in adult life possess a sufficient degree of immunity so that the disease shows itself in a very mild form in those harboring spirochaetes.
The immunity conferred by an attack is not lasting and a second infection may occur within a year. Such second attacks, however, do not present the relapses so important in a clinical diagnosis. As a matter of fact there may be no symptoms and such cases with spirochaetes in their blood make ideal carriers for the infection of ticks or lice. Ticks can be infected by these carriers. In some of the rest houses 50% of the ticks may be infected. While the tick does not tend to leave its habitation it may be transported in the bundles of native porters. The transmitting agent of the North African relapsing fever and probably of the Indian type is the louse. The body louse deposits about 75 eggs in the clothes of the host, which hatch out in about four days and become adults in about two weeks. The head louse deposits its eggs or nits on the hair of the host’s head. Hagler has noted that, in Servia, typhus fever disappeared when lice were gotten rid of but relapsing fever continued to prevail until they also exterminated the bedbugs.
The Relapsing Fever of Panama.—Bates, Dunn and St. John have demonstrated that a tick, Ornithodoros talaje, transmits the relapsing fever of Panama. This tick seems to prefer the crevices and open joints of bamboo beds, laying its eggs and moulting in the hiding places. It comes out at night to feed on the occupants of the bed. Two white rats were inoculated with an emulsion made from ticks taken from a suspected bed. Both rats developed a spirochaete infection. A man inoculated with blood of one of these rats developed relapsing fever on the sixth day. A second man was inoculated subcutaneously with an emulsion of ticks collected from a bed and showed spirochaetes in his blood on the eleventh day.
Another man was bitten by ticks from the same source and was instructed not to scratch the bite. The ticks engorged in about 15 minutes, some of them secreting coxal fluid which mixed with blood from the bite made a scab. This man was positive for spirochaetes on the fifteenth day.
Pathology and Morbid Anatomy
The spirochaetes disappear from the peripheral circulation during the apyrexial period, notwithstanding which such spirochaete-free blood, when injected into monkeys, may bring about infection. Either a granule stage or an invisible stage of the parasite may be present. The relapse is probably due to the existence of resistant strains which are not destroyed by the lytic substances, developed during the attack.