There are frequent statements of malarial fever relapsing after years; yet so far as I have gone into the literature I have found no case in which this was determined with the necessary certainty. In my opinion only the examination of the blood, together with a critical consideration of all external factors (especially the epidemiologic conditions), could decide such a case. I must confess that to me such an occurrence appears very doubtful. We are naturally not justified in stamping a fever as a malarial relapse simply because the person affected suffered some years before from malaria , even when the objective examination shows for the moment no other intelligent cause. It is only necessary to recall how frequently we see feverish infections to which we can give no name. A positive blood finding alone can exclude reinfection.
The type of the relapse is most frequently intermittent, sometimes subcontinued. Even cases which began as severe continued or subcontinued often manifest in the relapse simple quotidian, tertian, or irregular intermittent fever. The relapse consists sometimes of one single paroxysm; again of several consecutive ones. With successive relapses from the same case the type may change.
The other symptoms are usually less severe than in the primary disease, yet severe relapses do occur and it is not rare to find them showing the same pernicious symptoms as the original disease, or even others.
During the apyretic interval a few ameboid parasites, isolated melaniferous leukocytes, often a considerable number of crescents, a,re usually found in the peripheral blood, though the blood examination may be absolutely negative for days.
Frequently some psychic or physical circumstance appears to be responsible for the outbreak of a relapse, as pleasurable and intellectual excitement, change of residence, the performance of some unaccustomed work, Bacchanalian or venereal excesses, colds, dreams, indigestion, etc.
Reinfection is very common. Different from so many other infectious diseases which induce a certain amount of immunity, malaria seems rather to increase the predisposition to a new infection. Although certain races show less susceptibility than Europeans (for instance, the negroes), this seems to depend more on a congenitally low predisposition than on an acquired immunity.
The reinfection may be caused by the same parasites as the first infection or by others.