“Fever,” i.e., malaria, is responsible for so large a share of the sickness peculiar to tropical countries that the subject of its prevention requires especial and separate consideration. Up to a few years ago the causation of malaria was a complete mystery. We had known for some score of years or more that the disease was due to the presence of certain minute animalcules in the blood, but as to how they got there, or the manner in which they passed from man to man, we had not the remotest idea. There was a general opinion that the seeds of the disease were carried by the air in the form of what was pretentiously spoken of as a miasma—a formidable word which served well enough to hide from the profane vulgar the fact that no one could define, or in fact had the vaguest notion as to what a miasma might be. It was also popularly known in many parts of the world that miasmata found great difficulty in getting through a mosquito net; but by the majority of the profession these traditions were looked upon as laic fables, unworthy of scientific attention, though there were not wanting observant practitioners of tropical medicine who were willing to admit the efficacy of the protection afforded by a mosquito net, and who even attempted to account for the fact by all sorts of lame physical explanations, barring only the simple one that a mosquito net serves very fairly the purpose for which it is designed, viz., of keeping out mosquitoes. The condensed moisture of the dew on the fluff of the meshes in some way attracting the germs or dissolving some assumedly gaseous emanation, was a favourite so-called explanation, and was, I believe, that adopted by the late Prof. Maclean, of Netley, who in his lectures was always careful to impress upon us the protection afforded by mosquito nets, as a well-established, though ill-understood, fact.
This was before the date at which the French military surgeon, Laveran, discovered the fact that malaria was due to the presence in the blood of certain animal parasites (Protozoa), and although this discovery was made in 1880, it was many years before its truth was accepted by the general body of the medical profession, who have somehow always exhibited a curious reluctance to admit the harmfulness of animal parasites.
Some years later Sir Patrick Manson, F.R.S., then a hard-working doctor in practice in China, made the remarkable discovery that the blood-worm disease, which is very common in those parts, was conveyed from man to man by the agency of mosquitoes; and as the parasitic origin of malarial fever became more and more firmly established, the idea suggested itself to him that this disease, too, might very well be transmitted by the intervention of the same insects.
At this time Sir Patrick had left China, to work harder than ever in London, so that he was unable to personally test the truth of his surmise, which he, however, communicated to Major Ronald Ross, I.M.S., who after prolonged work, was able to establish the truth of Manson’s suggestion, which was a remarkable instance of the value of imagination in science. Almost immediately after Rossi’s work was confirmed and amplified by Prof. Grassi, of Rome, and by several other naturalists, and the fact that malarial diseases are communicated by the agency of mosquitoes, and can be carried from man to man in no other manner, is now absolutely established. Medical science has commonly to accept, as a working theory, whatever hypothesis of the causation of disease may appear most tenable, but in the present case there is no room for doubt, and, like the protective power of vaccination, the carriage of malaria by mosquitoes only, may be taken as one of the few absolutely proven facts of medicine.
In saying as much, it is not implied that the reader may not on his travels meet with medical men sceptical or hostile to this theory, but this is because the training required to appreciate the cogency of the facts adduced in proof is that, not of a medical man, but of a naturalist, and though the profession of medicine numbers in its ranks many distinguished naturalists, it is quite possible to gain the highest qualifications without acquiring any knowledge of zoology sufficient to render the student capable of really forming an opinion on such a point; for, as a matter of fact, the five years of medical training are so overburdened with absolutely necessary medical subjects, that any critical knowledge of the associated subjects of chemistry, physics, and biology must needs be left to the after years of those to whom good fortune affords sufficient leisure to admit of their attacking the fringe of these great subjects when they are no longer in statu pupilaris. Hence, especially among the older hands, there are numbers of medical men who would regard the above statements as premature, but the reader will find it difficult to find any naturalist who entertains any doubts on the subject. Many details undoubtedly remain to be worked out, but the broad data may be taken as absolute facts, of a character that future investigations can only amplify.
These facts may be shortly stated as follows:—
(1) Malarial fevers are caused by the presence in the blood of minute animal parasites. There are several species of these, corresponding to the various types of fever; but the life-history of all is broadly the same.
(2) These animalcules multiply in the blood, and when they have become sufficiently numerous, determine an attack of fever, but while in this stage cannot pass from the blood of one human being to another, except by the somewhat difficult vivisectional experiment of injecting the living blood of an infected person into the vessels of a healthy subject, a process which cannot occur in Nature.
(3) Large numbers of the malaria animalcules are destroyed by what may be called the vital powers of the patient’s blood, and the question whether an untreated case of malaria dies or recovers, depends on the outcome of the struggle between the parasites and the vital forces of their host.
(4) The process of multiplication of the parasites within the human body is by simple division, or non-sexual breeding of the single cell of which each parasite consists; but by a well-known law of the life-history of this class of animalcules this method of multiplication cannot continue indefinitely without the intervention of a period of sexual multiplication; and this can not occur within the human subject under any circumstances, but only in the bodies of certain species of mosquitoes; so that the disease always tends to wear itself out, provided the strength of the patient holds out sufficiently long; and the parasites are unable to find their way into the blood of other human beings by natural processes, unless mosquitoes of certain special species be present.