INSECTS, PROPAGATION OF DISEASE BY. Care ful observation and patient investigation have resulted in establishing the fact that the trans mission of disease, in many instances hitherto unexplained, has been due to the agency of insects. The most momentous medical discovery during the year 1899 was that of the cause of malaria in human beings. There are several scientists to whom credit is due in the matter; but the lion's share of it belongs to Patrick Manson, of Dublin, and Major Ross, a surgeon in the British Army. stationed at Calcutta. During several years of laborious work they forged the links that completed the discovery. As long ago as 1807. Crawford, an American physician, sug gested the possibility of the transmission of malaria to man by the mosquito. Again, in 1848, Nott, of New Orleans, referred to the part played by insects in propagating malarial fever. In 1883 A. F. A. King (q.v.), of Washington, D. C., reiterated the theory at. some length. Laveran (q.v.), who, in 1880, discovered the plasmodium, malaria', or lurmosporidium, the parasite of the disease, declared in 1891 his adherence to the mosquito theory, as did also Fliigge in the same year. In 1892, Pfeiffer showed that certain pro tozoa, called coceidia, which are found as para sites in the rabbit. are capable of two cycles of development, one being exogenous. He mentioned that Koch (q.v.) had suggested that a similar condition might bold good for the parasite of malaria, and that exogenous malarial spores might be conveyed to man by the agency of blood sucking insects. In 1894 Manson, of Dublin, ap peared as a vigorous supporter of the mosquito theory as best calculated to explain the various conditions of the problem. He drew a parallel between the malarial parasite and the filaria Bancrofti, which he had investigated very thor oughly. He suggested that the female mosquito fills herself with infected blood, deposits her eggs, and dies beside them. The water in which she lies becomes contaminated with the spores developing in her body, and is then drunk by men, or the spores are inhaled with dust from dried puddles, or the larvae after being hatched feed on the dead body of the mother, and thus become carriers of infection, or the ground may become infected by the bodies of mosquitoes that die and fall upon it. Ross's work, in his studies of malarial organisms in birds' blood is really monumental. He discovered that after a special variety of gray mosquito on the blood of birds containing a certain mature organism called proteoso»ta, the stomach wall of the insect always contained certain pigmented eoreidia two days later. After other changes had occurred
on the eighth or ninth day, the eoeeidia ruptured and set free innumerable thread-like bodies which are distributed by the blood-current of the mos quito through her tissues. Eventually these bodies are found in certain glands in the thorax of the insect, whose duets open at such a point as to furnish secretion that lubricates the lancets of the mosquito. When she punctures the skin of her victim, this secretion, containing the thread-like bodies, is injected into the bottom of the wound. Manson and his pupil, Daniels, proved that these thread-like bodies, or germinal spores, develop into mature proteosomata in a bird bitten by the infected mosquito, thus com pleting the cycle. While Manson was conducting his experiments, Grassi, the distinguished Milan ese, was completing his studies on the mosquitoes of Italy. In IS9S he determined that Anopheles claviger was the variety that carried malarial infection. A man who had never been subjected to malarial infection was exposed to the bites of the mosquito Anopheles clariger, and in a short time began to suffer from malarial fever. Qui nine cured him of the attack. The Italian scien tists Bigrrami, Bastianelli. and Grassi, working in conjunction, arrived at the conclusion that the malarial hemisporidia (which consist of small reproductive cells produced within a cyst) run through a cycle in man which is characterized by a long amceboid stage and an absence of the en capsulated phase. reproducing themselves a great number of times during the completion of this cycle, and also giving rise to forms which remain sterile in man, known as Grassi's gameti. These latter forms, taken into the stomach of the Ano pheles with the blood she draws from an in fected man, develop into sporozoa. and these in turn form sporozoites, delicate filaments which find their way to the salivary glands of the The saliva anointing the lancet of the anopheles carries the infection to the next vic tim bitten. In East and Central Africa experi ments were made by Daniels which led him to believe that Anopheles funestus (Giles) is the chief agent in distributing malaria in that coun try. and that man is the only intermediate host of the malarial parasites.