The newly arrived Spaniard, as soon as he had had yellow fever and could present a certificate of immunity, could command double the wages that he could get before he had the disease. So that when Dr. Reed proposed to some of these men that they should go out to his camp, have a mild case of yellow fever, be well cared for and when recovered be given by him a certificate of immunity, he found no difficulty in getting volunteers, and when, in addition to that, he promised each man who had the disease a bonus of two hundred and fifty dollars, the service became exceedingly popular.
Dr. Reed had prepared, as I have above mentioned, a very comfortable camp at an isolated point near Camp Columbia, well separated from all other dwellings. This camp was kept under military guard, so that no one could come and go without Dr. Reed’s knowledge. Here he placed his non-immune volunteer Spaniards whom he had gotten from Havana, and kept them under observation for two weeks, taking their temperatures every day so as to be sure that they had not contracted yellow fever before they went out to the camp.
At this point he made another important discovery in the mode of yellow-fever propagation. He found that the mosquito herself had to wait from ten to fifteen days after she had bitten a man sick with yellow fever before she herself conveyed the disease. He found that the mosquito for the first week or ten days after she had bitten the yellow-fever patient was entirely harmless though she fed freely on non-immunes. But after the twelfth or fourteenth day she would give the disease to every non-immune whom she bit. I have often seen the non-immune doctors and nurses at Las Animas Hospital put their hands in the jars where infected mosquitoes were kept during the first seven days of their infection and allow them to draw their fill of blood, for the purpose of feeding them, but they would not think of doing this after the seventh or eighth day. Two of these nurses afterwards contracted yellow fever from allowing mosquitoes to bite them after the twelfth day, and one of them, Miss Mass, died from the disease so contracted.
Dr. Carlos Finlay, in his many experiments on the human being, was unaware of these two facts with regard to the transmission of yellow fever: first, that the mosquito could only become infected by biting a human being within the first three days of his disease; and second, that she could only become infectious, that is, transmit the disease, when some twelve or fourteen days had passed since she had bitten the sick man. Dr. Finlay put a great many of his mosquitoes to the sick man after the third day, and in no case did he apply his mosquito to the non-immune twelve or fourteen days after she had bitten the infected person.
Dr. Henry R. Carter had published a paper on certain observations of his made during the epidemic of 1898 in the neighborhood of Jackson, Mississippi. It had long been known to men practically familiar with yellow fever that, in general, when you took a patient suffering from yellow fever into a house where yellow fever had not before existed, the people in that house did not at once develop the fever. We explained this by stating that it was due to the fact that the germs of yellow fever went from the patient to favorable grounds for development about the house, and there underwent some development which enabled them to produce the disease in non-immune man. We thought that the dirtier and more unhygienic were the conditions of the house, the more favorable were the conditions for the further development of the germs. Dr. Carter recorded a number of cases where the houses were isolated and the conditions favorable for making the observations, and found that the average time from the introduction of a yellow-fever patient into a house until the first case of yellow fever was contracted in that house, was about seventeen days. These observations were published to the world.
Dr. Reed was greatly impressed by this publication of Dr. Carter’s. He reasoned that if it were the mosquito which transmitted the disease, this period of extrinsic incubation must be due to a period of incubation in the mosquito. He says:
We were also much impressed by the valuable observations made at Orwood and Taylor, Mississippi, during the year 1898 by Surgeon Henry R. Carter, U. S. Marine Hospital Service, “A Note on the Interval between Infecting and Secondary Cases of Yellow Fever, etc.” (Reprint from New Orleans Medical Journal, May, 1900.) We do not believe that sufficient importance has been accorded these painstaking and valuable data. We observe that the members of the yellow-fever commission of the Liverpool School of Tropical Medicine, Doctors Surham and Meyers, to whom we had the pleasure of submitting Carter’s observations, have been equally impressed by their importance. (British Medical Journal, Sept. 8, 1900, pp. 656-70.)
The circumstances under which Carter worked were favorable for recording with considerable accuracy the interval between the time of arrival of infecting cases in isolated farmhouses and the occurrence of secondary cases in these houses. According to Carter, “the period from the first (infecting) case to the first group of cases infected at these houses, is generally from two to three weeks.”
The house having now become infected, susceptible individuals thereafter visiting the houses for a few hours, fall sick with the disease in the usual period of incubation, one to seven days.
Other observations made by us since our arrival confirmed Carter’s conclusions, thus pointing, as it seemed to us, to the presence of an intermediate host, such as the mosquito, which having taken the parasite into the stomach, soon after the entrance of the patient into the non-infected house, was able, after a certain interval, to reconvey the infecting agent to other individuals, thereby converting a non-infected house into an “infected” house. This interval would appear to be from nine to sixteen days (allowing for the period of incubation) which agrees fairly closely with the time required for the passage of the malarial parasite from the stomach of the mosquito to its salivary glands.