If we can explain why it is that relapsing fever has no local lesion, why the fever recurs, and why one attack of the disease confers no immunity from a second, we shall probably find in that explanation a clue to the explanation of similar phenomena in the malarial fevers.
These peculiarities.of relapsing fever are all due to the fact that the spirilla which give rise to it find the material necessary to vivification and reproduction in the blood. The blood is their nidus; and the spirilla are found abundantly in the blood for the same reason that the poison of small-pox is most abundant in the matter of the pustules, the poison of typhoid fever in the discharges from the sloughing intestinal glands, and the poison of scarlet fever in the peel ing cuticle and in the discharges from the throat.
The materials requisite to the organic growth of the poisons of the eruptive fevers (nitrogen and water) exist all over the body and are practically unlimited. The special material essential to their vivification and reproduction exists only in the nidus and therefore only in limited quantity. So long as any of it remains the poison continues to be propagated and the febrile symptoms are kept up. When it is exhausted the necessary nidus no longer exists, the propa gation of the poison ceases, and the fever comes to an end. That it is the propagation of the poison in the system, and not its mere pres ence there, that causes the febrile symptoms proved by the fact that these symptoms come to an end and the patient enters on con valescence while the system still contains much of the poison, as is evidenced by the facts of infection. What brings the fever to an end, then, is not the extinction or elimination of the poison, but the ex haustion of the nidus. Were this nidus to be renewed before the whole of the poison was eliminated, the poison would be reproduced and the febrile symptoms would recur. Such a thing is not likely to happen in the eruptive fevers because the tissues which constitute the nidus are not capable of speedy reproduction; cuticle and intestinal glands are not likely to be formed anew. It is different in relapsing; fever. Its nidus is the blood, and the blood is an ever-changing fluid containing nothing which is not likely, when used up, to be sooner or later replaced. As it is the localization of the nidus in a special locality that gives to each of the eruptive fevers its distinctive and characteristic feature, so it is the localization of its nidus in the blood, that gives to relapsing fever its distinctive feature—the relapse. It
relapses because the nidus is renewed before there has been time for the whole of the poison to be eliminated from the system.
If such be the explanation of the relapse it is evident that cases may occur in which, either from more rapid elimination of the poison or more tardy renewal of the nidus, the poison is all eliminated before the nidus is renewed; in such circumstances there would be no re lapse, the attack being completed in one seizure. That such cases do occur is an established fact in the history of relapsing fever. Of 2,425 cases collected by Murchison 724 had no relapse; of 100 con secutive cases under Murchison's own care 4 were completed by one pyrexial attack; of 400 recorded by Litten 6 had no relapse.
On the other hand it is evident that the process might be repeated more than once, that a third seizure Might be caused in the same way as the second, and a fourth in the same way as a third; and so it is found to be. Of 1,500 cases collected by Murchison a second relapse occurred in 109, or in 1 in 14; a third in 9, or in 1 out of 166, and a fourth relapse, i.e., 5 pyrexial attacks, iu 1 of the 1,500. The dura tion of the attacks gets shorter as they go on. In Litton's cases the mean duration of the first attack was 6.6 days; mean duration of the second attack was 4.9 days ; mean duration of the third attack was 3.1 days; mean duration of the fourth attack was 3.1 days; while that of the fifth attack was only 2.3 days.
The longer the attack the more poison is produced; so that with each successive seizure there is less poison to be eliminated and a correspondingly greater chance of its elimination being completed before the nidus is renewed. The readiness with which the nidus is renewed probably diminishes also with each successive attack. Thus it very soon happens that the renewal of the nidus is sufficiently de layed to permit of the complete elimination of the poison prior to its renewal, and to the establishment of permanent convalescence. It is in time sure to be reproduced, however, and the blood restored to its former state. One attack of relapsing fever, therefore, confers no im munity from a second.