5. It is certain that the microscopic hæmatozoa may be readily transferred to the stomach of blood-sucking insects, and it has been further demonstrated that the digestive organs of the mosquito form a suitable territory for the further growth and metamorphosis of the larval Filariæ.
6. The character of the changes undergone by the microscopic Filariæ, and the ultimate form assumed by the larvæ whilst still within the body of the intermediate host (Culex mosquito), are amply sufficient to establish the genetic relationship as between the embryonal Filaria sanguinis hominis, the stomachal Filariæ of the mosquito, and the sexually-mature Filaria Bancrofti.
In the month of September, 1878, I received a letter from Dr da Silva Lima announcing the fact that Dr Araujo had verified the existence of the embryos of Filaria Bancrofti in mosquitoes, at Bahia. These mosquitoes had, I understood, attacked a French priest in whose blood Dr Araujo also detected Filariæ. Thus, it fell to the lot of Araujo, through his untiring zeal, to verify in Brazil all the separate discoveries of Bancroft, Manson, and Lewis.
In the October issue of the ‘Pathological Society’s Transactions’ for 1878 Dr Bancroft records numerous cases of filarious disease, and he gives a succinct account of the circumstances connected with his original discovery.
In a clinical lecture published October 12th, 1878, Dr Tilbury Fox seeks to diminish the value of these discoveries, characterising helminthological investigators as merely “recent writers.” Dr Fox denies that Filariæ are a cause of true elephantiasis, but admits the occurrence of “elephantoid inflammation and inflammations due to Filariæ.” Dr Fox’s statement that “Filariæ have not been found in uncomplicated elephantiasis, that is, in disease without chylous exudation,” seems to me to be directly at variance with Manson’s recorded experiences. I hold that Manson has confirmed the truth of Lewis’s views, and that he has thoroughly proved that (to use his own words) “varicose groin glands, lymph scrotum, elephantiasis, and chyluria are pathologically the same disease.” In the first instance I was myself led to conclude that some of the forms of elephantiasis might be due to other causes than obstruction of the lymphatics caused by the presence of Filariæ; but the explanations of Lewis, of Bancroft, and of Manson more especially, have almost entirely removed this doubt. Those who seek to explain away the connection between genuine elephantiasis and Filariæ will do well to study Manson’s last important memoir. He shows that “elephantiasis and allied diseases are much more frequently associated with the parasite than are other morbid conditions.” This fact is brought out very clearly in his table of 670 cases, from which it appears that 58 per cent. of cases of Filaria are associated with elephantoid disease.
When this opposition to Manson’s views is likely to cease (on the part of those who do not happen to have been in any way instrumental to the discoveries in question) it is not easy to say. In a brief communication which appeared in the last number of the ‘Medical Times and Gazette’ for 1878, Dr Manson successfully combats the doubts that have been entertained respecting the rôle of the mosquito. Because Lewis found that canine hæmatozoa were digested, and thus perished in the stomach of mosquitoes, it had been argued that human hæmatozoa must necessarily undergo similar processes, and consequently die. Those who oppose the views of helminthologists in respect of the intermediary host-function of insects on such grounds can have very little general, and still less special knowledge of the phenomena of parasitism. It is the old story. When any new discovery is made, it must always pass through the ordeals of denial and doubt before it can be generally accepted as true; and, as in the case of Jenner’s immortal discovery, there will always remain a certain number of peculiar people who show themselves hostile to every advance in science. Dr Manson may take comfort from this consideration, and rest assured that the value of his discovery is quite unaffected by the opposition referred to.
Since I communicated the results obtained by Manson, Lewis, myself, and others to the Linnean Society, an even more exhaustive summary of the facts has been published by Dr Bourel-Roncière, in the ‘Archives de Médecine Navale.’ The distinguished author does full justice to the writings of English helminthologists, and dwells, with emphasis, upon the finds and interpretations of Lewis, Manson, and Bancroft. Incidentally, also, he comments upon Sir Joseph Fayrer’s early recognition of the etiological identity of hæmato-chyluria and elephantiasis, on other than helminthic grounds. The frequent concurrence of the two affections had especially struck Sir J. Fayrer as pointing to a probable common origin. He had also surmised that the disorders might be due to parasites.
Dr Bourel-Roncière, alike with the caution, precision, and logical reasoning of a cultured savant, concludes his elaborate review in the following terms:—“There are the facts. Certainly, many points remain obscure, many problems await a solution, and the last word has not been said on the actual part which the parasite plays in the pathogenesis of the affections above enumerated—its mode of action, the importance of its rôle, the extent of its pathological domain, the habitat of its progenitors, their identity, and so forth. All these questions will only be elucidated by necroscopic researches, which at present remain absolutely wanting.”
“However, notwithstanding the doubts which hover over the future value of these curious discoveries, it is difficult not to recognise their importance in the study of certain tropical diseases—which up to the present time have been attributed to vague and undetermined causes—hæmato-chyluria and elephantoid affections principally. Apart from the interest which attaches to the natural history of the nematoids, they raise, in effect, etiological and prophylactic questions, the extreme importance of which we believe it would be needless to demonstrate. It is greatly to be desired that the researches should be taken up in other parts of the globe, where endemicity and perhaps greater facilities for necroscopic investigation would render them fruitful—Cochin-China, Tahiti, &c. Fresh observations are necessary to confirm the first and to fill up notable gaps. The way has been brilliantly opened by the English and Brazilian physicians. Let our colleagues in the French colonies put their shoulders to the wheel; they have before them a vast field of study to explore.”