It was subsequently found that transmission occurs when the louse is crushed against the victim's skin. A pressing issue at the time was the possibility of African sleeping sickness (trypanosomiasis) spreading to India via similar blood-sucking insect vectors. In September 1908, at the request of the Indian Government, Captain Mackie was attached to the Royal Society’s Sleeping Sickness Commission headed by Sir David Bruce F.R.S. Based at Mpumo, Uganda. Mackie was one of Bruce's youthful “Three Musketeers,” the others being Captains H.R. Bateman and Albert Ernest Hamerton. The accompanying photo shows (L to R) Mackie, Lady Bruce, Bruce, Bateman and Hamerton at their lab in Uganda, 1908. The work in Africa represented a huge step forward in the understanding of trypanosome-related diseases not only in humans (using monkeys as models) but in domestic cattle and wild animals, including elephants. Some 17 publications arose from this work in which Mackie was a co-author. Of the many species of "Trypanosoma" studied only "T. gambiense" (now "T. brucei gambiense") causes sleeping sickness in humans. The disease is spread by blood sucking insects of the genus "Glossina", which include tse-tse flies. The researchers studied the length of time tse-tse flies retained the parasite, how long it took to mature, how long the flies remained capable of transmitting the infection, what proportion of flies were infected, how many were required to cause sickness, whether transmission could occur "mechanically" by surface contact of the fly's mouthparts, and many other aspects. In a critical series of experiments it was found that the flies remained infective for up to 75 days after becoming infected themselves and that a tiny drop of fluid taken from the gut of the 75-day-old fly injected under the skin of a monkey gave rise to sleeping sickness after an incubation period of eight days. The Commission's work served to reassure the Indian Government that there was no risk of sleeping sickness spreading to India as it was clear that only "Glossina" could carry the infection and no species of this genus were native to India. Mackie was recalled to India in 1909 and appointed Special Research Officer (Government of India) for the investigation of kala-azar (visceral leishmaniasis). In 1914 he studied cases of kala-azar in Nowgong, Assam, where there were many cases of the disease among the local population.