If the malarial virus was suspended in the air, it would be impossible to understand why it would not be carried by the wind, so as to display its effect at a distance from its breeding place. Yet we could not, at this period, say that such transferences through the air had not taken place.
It was observed in many regions that the malarial cases increase when the wind blows from the direction of certain swamps. The idea that the malarial virus might be disseminated by the wind was first expressed by Lancisi, though the example that he chose to prove it was not very appropriate. Lancisi was of the opinion that the Roman Campagna was infected by the clearing of the forests under Gregory XIII, which opened up a path for the winds blowing over the Campagna from the Pontine marshes. He suggested that woods formed a sort of filter for the infected air. But Lancisi overlooked several points-for instance, on the way from the marshes toward Rome there are several places (Velletri, Genzano, Ariccia, Albano, etc.) which are entirely free from malaria , although they should come in contact with those winds sooner than the more northerly parts of the Campagna. Lancisi was correct in so far as there were numerous examples which apparently supported his idea. From recent times we may mention the observations of Jilek in regard to Pola. Jilek found that besides "the amount of rain and the heat, there were other factors governing the malarial mortality in Pola, namely, the winds, and that the east and southeast winds were the dangerous ones. In connection with this we must add that the two swamps, Prato Grande and Piccolo, lie in the southwestern part of the city.
Nielly observed malaria break out simultaneously in 27 men on board the "Recherche" forty eight hours after the ship had lain at anchor for one night in the Roads of Sainte Marie Bathurst. None of the infected had had fever previously.
Daville reports from the New Hebrides that as long as southeast breezes blow malaria is not seen, while with the appearance of southwest or western breezes it takes on the character of an epidemic. Maurel writes in relation to French Guiana: "As long as the air is still or the wind blows from a salubrious region it is possible to live within a few kilometers of a swamp without danger. Cayenne is an example of this. Yet when the wind turns and sweeps over the swamps for a few days, the places previously free begin to show numerous cases."
In almost all malarial places ceteris paribus endemicity apparently stood in relation to the dominating winds. Still it might have been objected that this connection was not due alone to a transference of the virus by the wind, but was dependent, too, on changes of temperature and moisture brought about by different winds. It appeared to be certain that the wind may be the medium of transference of malaria , though the distance to which this is possible is at most slight.
On this question Hirsch expresses himself as follows :"To measure the distance in figures to which malaria may be carried by the moving air is scarcely possible, yet it is highly probable that this is short. At least all the observations that have been made on dry land would so indicate, and even more decisively, the observations in relation to the spread of the disease from the land to ships. All experience in this regard goes to show that the crews of ships lying closely enough to the coast to be surely affected by the land breezes almost always remain free from the disease as long as they keep away from the infected land itself. And this is true of the severest malarial foci."
It was, therefore, very difficult to understand why, if the virus were in the air, it would not be carried great distances. How far mineral and vegetable impurities may be conveyed was well known. Bignami, to whom we are indebted for a thorough study of this question, added another embarrassment by pointing out that infections occur most frequently when the ground is moist, not when it is dry and dusty. A further fact, difficult of explanation by the air theory, was that the hours after sunset and before sunrise are the most dangerous, and that sleeping on a malarial soil is almost as sure as inoculation.
The wide spread opinion as to the presence of the malarial virus in the air brought with it the idea of investigating the air of malarial regions for microbes. Lancisi began these investigations, and among others we may mention Hammond, Lemaire, Maurel, Vogl, Grassi, and Calandruccio. The majority of these worked with Bouchet's aeroscope and Lemaire's condensator.
Vogl "found the air of Pola remarkably contaminated, and especially rich in certain organisms which probably stand in a genetic relation to the malaria occurring there" (quoted from Jilek). Maurel found in malarial air amebse which he failed to find in the air of healthy places. He likewise found amebse in the nasal mucus, a direct proof that protozoa may be introduced with respiration. Similar observations were made by Grassi and Calandruccio. These investigators found amebic cysts in the nasal mucus of doves which they had exposed for several nights to the emanations of swamps or malarial earth. Moreover, they succeeded, even though seldom, in demonstrating amebse in dew. I tried similar experiments with the earth of malarial regions and convinced myself of the presence of innumerable amebse in the water of condensation. These observations and experiments are now mainly, if not entirely, of historic interest only.
Lancisi was the first to express the opinion that the insects living in the swamps, especially the mosquitos, might, by their sting, be the agents of transference of the baneful swamp virus to man.* This theory was also taken up by Laveran as the most probable one, and a number of authoritative writers, like Manson, Bignami, and R. Koch, then followed him. Manson, supported by the previously mentioned experiment of Ross, presented the matter in this way: The mosquitos suck the blood of malarial patients, thereby taking up the parasites. They transfer the parasites in some changed form to their young, which are later deposited in the external world, especially in water,, This last, therefore (in the way of drinking), should be regarded as the only carrier of infection. It is the same process that Manson determined for the filaria.