The experience of many observers taught that malaria , unlike other infectious diseases, had no fixed period of incubation. In the very great majority of cases the incubation seems to be about six to fourteen days. Yet isolated cases have been reported in which the disease apparently broke out a few hours after exposure to the swamp miasm, and many in which the disease became manifest for the first time months or even years after removal from malarial regions. We are now in a position, however, to doubt the accuracy of these observations or to explain them satisfactorily on the basis of the mosquito malarial cycle.
The duration of the incubation can be deduced with certainty only in cases where persons visited but once and for a short time a focus of infection, as, for instance, ships' crews that went on land for a few hours and after return to the ship left the coast, or troops that camped one night in an infected region and moved away the next day.
We mention as an example Lind's ship, "Merlin." This remained six days on the coast of Senegambia. A portion of the crew was sent on land for the purpose of obtaining wood and water. Two days after the ship set out to sea those who cut the wood became ill; one day later, the others. Blaxall reports, in relation to a battleship which remained five days in the harbor of Port Louis, that two persons became ill after twelve and fourteen days respectively, another after forty eight, and still another after one hundred and sixty days (!).
Sorel reports that on August 8, 1881, 18 men were added to the post Takitount, which lay in a perfectly salubrious district. These were set on land on August 3, near Bougie, and started toward the above mentioned post on the sixth. On August 18, 12 fell ill with malaria ; on the twenty first another, and on the twenty fourth, still another. The infection resulted on the road through the unhealthy region between Bougie and Takitount. The incubation was, therefore, seven to nine days.
The determination of the incubation is much more uncertain when it concerns people who live continuously in a malarial district, since such cases give no clue to the time of infection. Yet if the person attacked visited, one or two weeks before, a particularly severe malarial region, went on a hunting expedition, or something similar, it becomes probable that the infection took place at that time.
The possibility of an incubation of a few hours was questioned by many writers. As a matter of fact, we could scarcely picture to ourselves, either by the air or mosquito theory, that the organism could be attacked within a few hours by so many parasites that they would be able to cause a fever without previous multiplication, and such a multiplication takes place in the most rapidly sporulating parasites only after the course of twenty four hours.
If we adhere to the principle that the paroxysms of fever occur only at the moment of sporulation of a generation, these cases become even more suspicious, since we can scarcely conceive that the parasites which have just gained entrance to the organism would sporulate a tempo.
Hertz observed in himself, a short time (one half hour) after he had visited a swampy region, ringing in the ears, faintness, burning in the throat, nausea, chilly sensations, and a few hours later a fever paroxysm. Plehn had a similar experience in West Africa, and though he found no parasites in the blood at the time of this first paroxysm, he found one ten days later, when new paroxysms took place. It is naturally possible only by a positive.,blood examination to decide if incubations of such short periods actually occur.
We could likewise with difficulty picture to ourselves the very long incubations, with a duration between one month and several years. We would sooner assume, with Thayer, that the parasites, though present, for some reason never increased to such an extent as to make the symptoms of the malarial infection manifest. After the course of several weeks or months symptoms due to the latent virus appearing, they are regarded as the first effect, on account of the absence of other symptoms. These would, therefore, resemble certain cases of syphilis in which the secondary symptoms appear without the primary sore having been remarked.
The great majority of cases (incubation from six to twenty days) are most readily explained by assuming that at the time of infection a number of parasites gain entrance to the organism, and these require a period of incubation in order to multiply before they are able to produce symptoms. From this it would also follow that the number of parasites gaining entrance would have a marked influence on the duration of the incubation. Moreover, the species of parasites would play a role in that the quotidian parasites, as a result of daily reproduction, would increase more rapidly in numbers than the tertian parasites, and these again more rapidly than the quartan.
Yet other individual factors, too, must have an influence on the incubation, since we see people who have been exposed to the same noxious influences manifesting symptoms not only not simultaneously, but often at considerable intervals.
The duration of the incubation in experimentally produced malaria corresponds in its variations with that of the spontaneously acquired disease. Table IV shows that the incubation of these cases, omitting the ones previously treated with serum, fluctuated between three and twenty one days.
Quartan fever shows in five cases incubation periods of from eleven to eighteen days; the average, therefore, is 13.4 days. Tertian fever shows in seven cases incubation of from six to twenty one days; the average is, therefore, eleven days. Seven cases with parasites of the second group (ameboid, with and without crescents) show fluctuations between three and fourteen days; the average is, therefore, 6.5 days. In two cases showing crescents without (probably with small numbers of) ameboid parasites, the incubation was thirteen and fifteen days; the average being, therefore, 14.