The Making of Arguments

by J. H. Gardiner

It is important to note that in the three cases in which the injection of the blood brought about an attack of yellow fever, careful culture from the same blood, taken immediately after injection, failed to show the presence of Sanarelli's bacillus.

Having demonstrated the fact that yellow fever is propagated by mosquitoes, Dr. Reed and his associates have endeavored to ascertain whether it may also be propagated, as has been commonly supposed, by clothing, bedding, and other articles which have been in use by those sick with this disease. With reference to the experiments made for the solution of this question I cannot do better than to quote in extensa from Dr. Reed's paper read at the Pan-American Medical Congress in Havana.

[This extract from Dr. Reed's paper describes in careful scientific detail the experiments which finally established the fact that the contagion came through mosquitoes, and in no other way. Into a small house, thoroughly air-proof, were brought bedclothes, clothing, and other articles which had been contaminated by yellow fever patients. Then for twenty days men who were nonimmune to the fever slept in this building, with no evil effects. This experiment was repeated several times. Then in another building similar, except that it was ventilated by mosquito-proof windows, and had been thoroughly disinfected, another volunteer was bitten by mosquitoes which had first bitten patients suffering with yellow fever; and he developed the disease. The last paragraph of the extract is as follows:]

"Thus at Camp Lazear, of seven nonimmunes whom we attempted to infect by means of the bites of contaminated mosquitoes, we have succeeded in conveying the disease to six, or 85.71 per cent. On the other hand, of seven nonimmunes whom we tried to infect by means of fomites [cloth and other material generally capable of carrying germs] under particularly favorable circumstances, we did not succeed in a single instance."

It is evident that in view of our present knowledge relating to the mode of transmission of yellow fever, the preventive measures which have heretofore been considered most important, that is, isolation of the sick, disinfection of clothing and bedding, and municipal sanitation, are either of no avail or of comparatively little value. It is true that yellow fever epidemics have resulted, as a rule, from the introduction to a previously healthy locality of one or more persons suffering from the disease. But we now know that its extension did not depend upon the direct contact of the sick with the nonimmune individuals and that isolation of the sick from such contact is unnecessary and without avail. On the other hand, complete isolation from the agent which is responsible for the propagation of the disease is all-important. In the absence of a yellow fever patient from which to draw blood the mosquito is harmless, and in the absence of the mosquito the yellow fever patient is harmless—as the experimental evidence now stands. Yellow fever epidemics are terminated by cold weather because the mosquitoes die or become torpid. The sanitary condition of our southern seaport cities is no better in winter than in summer, and if the infection attached to clothing and bedding it is difficult to understand why the first frosts of autumn should arrest the progress of an epidemic. But all this is explained now that the mode of transmission has been demonstrated.

Insanitary local conditions may, however, have a certain influence in the propagation of the disease, for it has been ascertained that the species of mosquito which serves as an intermediate host for the yellow fever germ may breed in cesspools and sewers, as well as in stagnant pools of water. If, therefore, the streets of a city are unpaved and ungraded and there are open spaces where water may accumulate in pools, as well as open cesspools to serve as breeding places for Culex fasciatus, the city will present conditions more favorable for the propagation of yellow fever than it would if well paved and drained and sewered.

The question whether yellow fever may be transmitted by any other species of mosquito than Culex fasciatus has not been determined. Facts relating to the propagation of the disease indicate that the mosquito which serves as an intermediate host for the yellow fever germ has a somewhat restricted geographical range and is to be found especially upon the seacoast and the margins of rivers in the so-called "yellow fever zone." While occasional epidemics have occurred upon the southwest coast of the Iberian Peninsula, the disease, as an epidemic, is unknown elsewhere in Europe, and there is no evidence that it has ever invaded the great and populous continent of Asia. In Africa it is limited to the west coast. In North America, although it has occasionally prevailed as an epidemic in every one of our seaport cities as far north as Boston, and in the Mississippi Valley as far north as St. Louis, it has never established itself as an epidemic disease within the limits of the United States. Vera Cruz, and probably other points on the Gulf coast of Mexico, are, however, at the present time, endemic foci of the disease. In South America it has prevailed as an epidemic at all of the seaports on the Gulf and Atlantic coasts, as far south as Montevideo and Buenos Aires, and on the Pacific along the coast of Peru.

The region in which the disease has had the greatest and most frequent prevalence is bounded by the shores of the Gulf of Mexico, and includes the West India islands. Within the past few years yellow fever has been carried to the west coast of North America, and has prevailed as an epidemic as far north as the Mexican port of Guaymas, on the Gulf of California.

It must be supposed that Culex fasciatus is only found where yellow fever prevails. The propagation of the disease depends upon the introduction of an infected individual to a locality where this mosquito is found, at a season of the year when it is active. Owing to the short period of incubation (five days or less), the brief duration of the disease and especially of the period during which the infectious agent (germ) is found in the blood, it is evident that ships sailing from infected ports, upon which cases of yellow fever develop, are not likely to introduce the disease to distant seaports. The continuance of an epidemic on shipboard, as on the land, must depend upon the presence of infected mosquitoes and of nonimmune individuals. Under these conditions we can readily understand why the disease should not be carried from the West Indies or from South America to the Mediterranean, to the east coast of Africa, or to Asiatic seaport cities. On the other hand, if the disease could be transmitted by infected clothing, bedding, etc., there seems no good reason why it should not have been carried to these distant localities long ago.

The restriction as regards altitude, however, probably depends upon the fact that the mosquito which serves as an intermediate host is a coast species, which does not live in elevated regions. It is a well-established fact that yellow fever has never prevailed in the City of Mexico, although the city has constant and unrestricted intercourse with the infected seaport, Vera Cruz. Persons who have been exposed in Vera Cruz during the epidemic season frequently fall sick after their arrival in the City of Mexico, but they do not communicate the disease to those in attendance upon them or to others in the vicinity. Evidently some factor essential for the propagation of the disease is absent, although we have the sick man, his clothing and bedding, and the insanitary local conditions which have been supposed to constitute an essential factor. I am not aware that any observations have been made with reference to the presence or absence of Culex fasciatus in high altitudes, but the inference that it is not to be found in such localities as the City of Mexico seems justified by the established facts already referred to.