The majority of observers maintain that there is, especially among Europeans, no acclimatization-that is, no immunity to the fever after a long resort in fever regions. On the contrary, daily experience teaches that a person who has been once attacked shows an increased predisposition. If the fact is adduced for acclimatization that subsequent relapses become constantly weaker, the many cases in which the relapse is pernicious and fatal may be brought forward in opposition.
Kelsch and Kiener deny any acclimatization for Europeans and allow only a relative tolerance. This consists in a decrease of the symptoms after a long infection, or, in other words, a diminished reaction on the part of the organism. Yet this does not prevent, as both writers add, the average duration of life from standing below the normal, and the progressive depopulation of these regions. This is about the same thing as Laveran reports among the Arabians in Algiers. They react less to the infection and manifest less frequently grave pernicious forms, although cachexia is not uncommon.
Maurel asserts that in French Guiana he could trace no white family further than four generations, although for hundreds of years thousands have been immigrating there. He states this in support of the assumption that no immunity is acquired by the Caucasian race. It was here that Maurel made the observations mentioned before in relation to negroes being more resistant.
Koch believes in an acquired immunity, at least for non-Europeans. He writes : " The Indians who came recently to the East African coast are exceedingly susceptible to tropical malaria ; some of the worst cases that I saw were in Indians. Still, thousands of Indians may be found on the African coast who are apparently immune. The same seems to be true of the Arabs, and we have similar reports about the Chinese at Sumatra. The Chinese coolies just after immigration are very susceptible, and many die. After living some time in Sumatra they lose this susceptibility and are then more highly valued and better paid. In consideration of these facts there is no doubt in my mind that malarial immunity does exist."
In this relation I would like to say that we must distinguish between genuine immunity and what Kelsch and Kiener have designated "relative tolerance," or diminished reaction. We can characterize as immune only the individual in whose blood the parasites will not live, and not one whose blood harbors the parasites, with constant diminution of the hemoglobin, and who shows gradually progressing cachexia, even though manifesting no regular paroxysms. With Kelsch and Kiener we will designate this latter condition as "relative tolerance." This condition, is very well known, even among Europeans. Many investigators, myself among them, have observed in the blood of old malaria cases considerable numbers of parasites without any manifestation of fever. Livio Vincenzi mentions a series of old malarial patients who showed quartan parasites (with sporu lation forms), but no symptoms, and who would buy no more quinin. Van der Schwer reports the same with parasites of the second group, and he asks the question if this is not a species of immunity. The Arabs mentioned by Laveran, who exhibited no fever, but declined under the cachexia, probably possessed this tolerance.
If malarial parasites were like ordinary saprophytes, our distinction in regard to immunity would be of no practical importance. But this is not the case; on the contrary, their evolution is closely associated with the destruction of the red blood corpuscle, and the distinction, therefore, becomes decidedly practical.
In our opinion the question is whether it is possible for a person to become actually immune or only arrive at a stage where, without fever paroxysms and intervals of actual disease, he gradually succumbs to the cachexia. Among Europeans we believe only the latter to be possible. If it is different with other races, it must be left to the future to decide.