In 1897, Ronald Ross discovered malarial parasites in mosquitoes. Mary Gibson examines the events surrounding this seminal discovery.

Sir Ronald Ross was born in India - at the hill station of Almora - on 13 May 1857 to a captain in the Bengal Army and his wife. His childhood appears to have been similar to that of most British children in India of the time, which entailed his being sent home to England at the age of eight for his health and education.

He did not return to India until 1881, by which time he was medically qualified (MRCS and LSA) and had been commissioned into the Madras branch of the Indian Medical Service.

In 1883, after various temporary postings, he was sent as Acting Garrison-Surgeon to Bangalore, which he considered "probably the best station in southern India". It was in Bangalore that Ross first became interested in the breeding habits of mosquitoes. He discovered the ones that regularly fed off him while he shared a bungalow with the adjutant were breeding in the water butt under his window, and he conducted his first attempt at mosquito control by overturning the tub.

When he suggested to the adjutant that life in the mess would be a good deal more pleasant if there were no water containers in which mosquitoes could breed, his suggestion was treated with derision.

For the next few years, Ross led a peripatetic life, holding temporary appointments in various stations in the Madras Presidency. During his first home leave from 1887 to 1888, he met and married Rosa Bessie Bloxam and acquired the Diploma in Public Health. During the second, in 1894, he met Patrick Manson, who showed him 'Laveran's bodies' (malaria parasites), and convinced him of the possibility that mosquitoes carried malaria. Ross returned to India fired with the intention of proving this theory.

Ross rejoined his regiment at Secunderabad, where he bred mosquitoes for experimental purposes and fed them on malaria patients by putting the patient under a mosquito net and releasing the insects into it. Manson advised him to 'follow the flagella', and suggested that malaria was carried through infected drinking water. Ross pursued the flagella and also tested the enteric theory by paying a man called Lutchman and two others to drink water in which mosquitoes had died. Lutchman developed a fever, but recovered three days later, and Ross could not find any malaria parasites in his blood; the other men remained healthy.

At the beginning of September 1895, Ross was posted to Bangalore to deal with an epidemic of cholera and to report on the sanitary condition of the town. He remained at Bangalore until May 1896 and earned a glowing tribute from the British Resident.

Despite the pressure of his sanitary work Ross was still occupying his spare time with malaria, and at the end of May 1896 he made an observation which, with the benefit of hindsight, was very important. He wrote to Manson: "The belief is growing on me that the disease is communicated by the bite of the mosquito... She always injects a small quantity of fluid with her bite - what if the parasites get into the system in this manner." Unfortunately, as he was using Culex mosquitoes, which do not transmit malaria, experiments to test this theory came to nothing.

Success in sight

After his return to Secunderabad, Ross began to wonder if he were using malaria-bearing mosquitoes and decided to continue his investigations with species from a highly malarious area. With some difficulty he obtained leave and went to the notoriously malarious valley of Sigur Ghat. Three days later he went down with malaria, despite having slept under a mosquito net and behind losed windows. When he returned to the Ghat, his attention was drawn to mosquitoes of a species he had not seen before.

The monsoon was late in 1897: the heat was appalling and Ross could not bring himself to look through his microscope for a month. "Well do I remember that dark hot little office in the hospital at Begumpett," he recalled, "with the necessary gleam of light coming in from under the eaves of the veranda. I did not allow the punka to be used because it blew about my dissected mosquitoes, which were partly examined without a cover-glass; and the result was that swarms of flies and of 'eye-flies' - minute little insects which try to get into one's ears and eyelids - tormented me at their pleas-ure, while an occasional Stegomyia revenged herself on me for the death of her friends. The screws of my microscope were rusted with sweat from my forehead and hands, and its last remaining eye-piece was cracked!"

In mid-August 1897 one of his assistants brought him some larvae that he had not seen before and the following day the Hospital Assistant pointed out a mosquito similar to the ones that Ross had found in Sigur Ghat. Ross dissected it and found nothing out of the ordinary but the Hospital Assistant rushed in to the laboratory to tell Ross that the unfamiliar larvae brought in the previous day had hatched into similar mosquitoes. They were fed on a patient called Husein Khan but nothing was found when they were dissected.

More mosquitoes hatched out and were fed, and by 20 August 1897 Ross was down to his last two mosquitoes. He dissected one and found nothing - until he got to the stomach, "when I saw a clear and almost perfectly circular outline before me of about 12 microns in diameter. The outline was much too sharp, the cell too small to be an ordinary stomach-cell of a mosquito. I looked a little further. Here was another, and another exactly similar cell." The following day Ross killed his last mosquito and found similar but much larger cells. He wrote to Manson with his exciting news: "Now prick up your ears because the hunt is up again."

Ross took ten days' leave to write a paper, 'On some peculiar pigmented cells found in two mosquitoes fed on malarial blood', and was cautious enough to have his work verified by a colleague, Surgeon-Major John Smyth. He sent this off immediately to the British Medical Journal, which took three months to publish it.

By that time Ross had been transferred on another temporary appointment to a small and isolated station called Kherwara in Rajastan which was conspicuously free of malaria, and it was not until the following February that he was put on official duty to investigate the disease. He travelled to Calcutta, which endured its coldest winter for years, leaving hardly any cases of malaria to study. He was obliged to study 'bird malaria' instead, and it was by using birds that in July 1898 he was able to prove that mosquitoes carried bird malaria through their bite.

He had already written a report 'On the cultivation of Proteosoma Labbe 1898' for the Director General of the Indian Medical Service and had sent a copy to Patrick Manson. Manson was in Edinburgh for the Annual Meeting of the British Medical Association and was able not only to report on Ross's work on the oocysts in mosquitoes' stomach but also, as Ross had telegraphed him, to announce that the parasite was transferred through the vector's bite.

Ross and his family sailed for home in February 1899. On their arrival in England, Manson encouraged Ross to apply for a post at the Liverpool School of Tropical Medicine. His application was accepted and he remained at Liverpool until the end of 1912 in a sometimes stormy relationship with the School's authorities. In 1902 he was awarded the Nobel Prize for Medicine, the first Briton to win it.

Mary E Gibson

Adapted from 'Wellcome News', issue 12.