Discovery of the mosquito transmission of parasites
It was now known that malaria was caused by a parasite rather than by vapours, bacteria, or other such airborne agents. The question arising from this discovery, then, was how does the parasite get from one person to the next? Again, Giovanni Maria Lancisi had a theory. “Mosquitoes”, he proposed, “always inject their salivary juices into the small wounds on the surface of the body.” And “…no controversy can arise concerning the harmful effects which [they], by mixing their injurious juices with the saliva…inflict upon us” (quoted in Lehrer, 1979).  His idea, presented in 1717, was not significantly  elaborated upon until the wonderfully named Albert Freeman Africanus King proposed a similar theory in a lecture to the Philosophical Society of Washington in 1882 (Cook and Webb, 2000). King went further than Lancisi, stating that the mosquito was the source of malaria, and that eradication of the disease could be achieved by preventing mosquitoes from biting people. He elaborated on this idea by suggesting that Washington city could be covered by a giant mosquito net as tall as the Washington Monument in order to rid the city of the disease. This last comment proved too much for his scientific audience, and somewhat undermined his fundamental ideas, which were, with the benefit of hindsight, extremely prescient.

It was a Scotsman, Patrick Manson, who became the most vigorous exponent of the mosquito vector theory of malaria transmission, when in the 1880s, inspired by Laveran’s publication of the parasitic nature of the disease, he began to wonder if it could be transmitted by a mosquito in the same way that he had recently shown for the filarial nematodes (Wuchereria bancrofti) that cause elephantiasis. He was further inspired by the work of Smith and Kilbourne, who in 1893, proved that Texas Cattle Fever (bovine babesiosis) was transmitted by the tick Boophilus bovis, and of David Bruce who proved the following year that the tsetse fly, Glossina morsitans, was the vector of nagana (Gubler, 2001). In the light of these findings Manson postulated that the flagellated forms of malaria parasites were taken up by mosquitoes during feeding, and these would then migrate into the tissues where they would become forms that were infectious to another human. He went on to theorise, however, that as mosquitoes were, at the time, thought to die straight after laying their eggs, the parasites escaped from the dead body of the flies into the marshy waters and infected people who drank them, thus returning to the ideas of Hippocrates.

Manson clearly saw the urgency of experimental work to prove this theory, and initially applied for funding to travel to malarious areas to undertake such endeavours himself. He was refused, however, but managed to convince Surgeon-Major Ronald Ross to investigate his theories while serving in the Indian Medical Service. Ross quickly applied his considerable enthusiasm and dedication to the attempt to find evidence of malaria parasites in mosquitoes. After much laborious dissection of countless mosquitoes of various different genera, Ross eventually found what he was looking for – on August 20th 1897 he saw the pigmented parasites first described by Laveran in the mid-gut of an Anopheles mosquito known to have fed on a malarious patient. He immediately reported this discovery to Manson, and published a description of his work in the British Medical Journal. He had planned to prove that infected mosquitoes were able to transmit the parasites to an uninfected host, but his was interrupted when he was posted to a non-malarious area of India. Manson, however, arranged for him to continue his work the following year when he embarked upon a study of the avian malaria parasite Plasmodium relictum, with which he demonstrated the presence of sporozoites (which he named “germinal rods”) in the salivary glands of a mosquito that had fed on an infected lark.

Manson presented the results of Ross’ work at the Edinburgh meeting of the British Medical Association on the 28th of July 1898;  the transmission of malaria by mosquitoes had been proven (Cook, 1997). It remained to be shown, however, that human malaria could be induced in man by the bite of an infected mosquito. This was finally proven by Amigo Bignami and Giovanni Grassi, who successfully infected Abele Sola (a volunteer with no history of malaria) with malaria from an infected mosquito in October 1898 (Ascenzi A, 1999).
References

Lehrer,S. (1979). The search for Microbes. In Lehrer,S. (Ed.), Explorers of the Body, . Doubleday, New York.

Cook,G.C. and Webb,A.J. (2000). Perceptions of malaria transmission before Ross' discovery in 1897. Postgraduate Medical Journal, 76, 738-740.

Gubler,D.J. (2001). Prevention and control of tropical diseases in the 21st century: Back to the field. American Journal of Tropical Medicine and Hygiene, 65, V-XI.

Cook,G.C. (1997). Ronald Ross (1857-1932): 100 years since the demonstration of mosquito transmission of Plasmodium spp. on 20 August 1897. Transactions of the Royal Society of Tropical Medicine and Hygiene, 91, 487-488.

Ascenzi A (1999). Malaria diseases and parasites. Parassitologia, 41, 33-38.