Laveran, like Lancisi, expresses the opinion that mosquitos may inoculate the malarial virus into man. The manner in which this is done he does not indicate. Bignami also believed that mosquitos inoculate, by means of the proboscis, malarial parasites, which they have taken from the ground, and to demonstrate this he made several valuable experiments. He showed first, in connection with Bastianelli, that is was possible to produce infection by means of minimum quantities of malarial blood infected subcutaneously. In order to imitate the sting of a mosquito he introduced under the skin the needle of a Pravaz syringe moistened with malarial blood. In several cases this was sufficient to produce a severe malaria .
Bignami then instituted, in association with Dionisi, a series of direct experiments with mosquitos, though these resulted negatively. They collected, for instance, from a malarial focus, a large number of mosquitos ("zanzare"), brought them to Rome, placed them in a room of the San Spirito Hospital, and allowed a robust man who offered himself for the experiment to sleep there. The man remained unaffected, as likewise another individual on whom the experiment was done. Moreover, the microscopic examination of numerous mosquitos by Bignami was without result.
* Yet Lancisi, as mentioned in another place, also suggested the other means of infection as possible.
R. Koch was of the opinion that the mosquitos suck the blood of malarial patients and transfer the parasites to their brood, and that only this generation, or possibly the subsequent one, can produce malaria by stinging. So far as his communications extend Koch seems to have made no personal experiments.
There were many observations and facts which supported this view of a connection between mosquitos and malaria . First, it is well known that the majority of malarial places are rich in such insects, though this was apparently not true in every single case. Dug gan argues against Manson that there are in Sierra Leone but few mosquitos (an observation that we now know to be quite false), in spite of the fact that it is a severe malarial region. Ziemann mentions the same of Kamerun.
Anderson proposes against the mosquito theory that the mosquitos usually sting the new arrivals, while it is the old residents who usually suffer the most severely from fever. That there are many places in the world swarming with mosquitos without malaria manifesting itself is a fact, yet not one militating against the mosquito theory, since the mosquitos are only the carriers of the virus.
Laveran further insisted that the same regulations which bring about the improvement of malarial foci cause a retrenchment or disappearance of the mosquitos. The facts most readily explained by the mosquito theory were:
That malaria should not be carried by the wind, or at most but short distances, since it is well known that these insects do not leave their mother soil. The wind scarcely begins to blow before they conceal themselves under leaves, in grass, etc. It is likewise well known that mosquitos are least annoying in q, room through which there is a good draft; that sleeping on a malarial soil is so particularly dangerous since the sleeping person would be a great attraction for the insects; that children are attacked much more frequently by malaria than adults; that the danger is greater during the night, since mosquitos swarm by preference at night; that there is but slight extension of malaria upward. Not because the mosquitos can fly no higher than a few meters, but because they prefer the lower atmospheric layers near the ground.
Finally, that artificial malaria has so far been produced only by the inoculation of malarial blood, which would seem to show that the disease might be produced by nature in a similar way.
A further argument for the mosquito theory lies in the analogy with the other blood diseases of man and animals. It is well known that the Filaria sanguinis is conveyed by mosquitos, the tsetse fly disease (nagana) of horses and dogs in Zululand by the tsetse fly, and Texas fever of cattle by the tick. Moreover, it has been also conjectured that recurrent fever is conveyed by the bite of the bedbug.
While recognizing the strength of these analogies, certain differences had to be insisted on. Texas fever is transferred from animals by means of the tick. If the infected cattle gain entrance to a healthy herd, individuals of the latter will be attacked, yet this did not seem to occur with malaria . This at first seemed to be against the applicability of Manson and Koch's parallel. If the mosquitos, after sucking malarial blood, were in a condition to produce malaria, then we ought to have found the disease spread by patients coming into a mosquito plagued region, even though it was free from malaria.
[How this as a matter of fact does occur and how infection is spread by means of particular mosquitos we shall see later.-Ed.]
In favor of the mosquito theory, isolated experiences were adduced to show that infection can be avoided in severe malarial regions by the employment of mosquito nets. It is well known, for instance, that Emin Pasha never traveled without a mosquito net, and to this is attributed the fact that he never suffered from malaria . Still, the experiences in this direction were too scanty to enable us at that time to draw definite conclusions.
Moreover, it had long been recognized that a fire near at hand makes it less dangerous to sleep at night on a malarial soil. This might readily be explained by the fact that the insects are killed or rendered inactive by the smoke.
The mosquito theory was then eventually experimentally taken up from many sides, and the result has been an absolute and convincing proof of its correctness.