The evidence as regards the game in the outbreak of human Trypanosomiasis near Mwanza

Summary 1. Origin of the Outbreak.—No fresh light has been thrown on the origin of the outbreak. The alternatives remain as they were in 1922, and it is at present not more probable that infection was derived from game than that it was introduced with a human infect. The evidence available is to the effect that the outbreak, as an appreciable outbreak, is of recent occurrence. 2. Method of Spread.—Cyclical infection appears likely to have been at play; but even here man is indicated as a probable infector of the fly. 3. Game Distribution.—The position in 1922 was exactly as it was described by me at the time (1923, p. 324), execpting that the arrow in my map, between Kilalo and Ngasamo, indicating a game movement and a good deal of game abundance, should, I find, have been extended unbrokenly to Mtukuza. Observations made outside the area, or at a date when it has been to all intents and purposes a game reserve for eighteen months, are irrelevant. The position described in 1922 and the fact that it was preceded by very heavy slaughter of game were matters of observation and investigation, and the facts have been confirmed by others. The belief that the oft-urged experiment in game destruction has been practically carried out in the area was a statement of opinion. It may be correct or it may be mistaken. 4. Source of Infections.—The importance of infected man as a source of infection of the fly has probably been very greatly under-estimated. Actual evidence against the game as a reservoir of Rhodesian sleeping sickness is still confined to the resemblance between the trypanosomes, and, as regards this theory, the laboratory analogy relied on by Dr. Duke . The game may be a reservoir, but, especially in view of Taute's experiments, that needs to be proved. 5. Inference from Discoveries of T. rhodesiense Infection.—The mere fact of the occurrence of an autbreak is not “A link in a chain of evidence that wherever game infected with the brucei-type of trypanosome, fly of the morsitans group and man are brought into sufficiently close contact” cases of human sleeping sickness will arise. Many outbreaks, perhaps most, possibly all, may be originated by infected human beings. The evidence must be closely sifted and the probabilities separately arrived at for every outbreak and case. There has been such continuous movement of people between infected and uninfected areas during and since the war, that it will be surprising if cases and outbreaks of man-introduced infection are not found to be ubiquitous. 6. Effect of the Measures Adopted.—Most of the figures of sick for 1923 came from country which had been unsearched or in which control was otherwise incomplete in 1922. The Sultanates most fully searched and brought under control in 1922 showed the figures 2, 0 and 0 in 1923. This, it is believed by those best in a position to judge, was a true indicant of the success of the measures taken. 7. The Effect of the Prevalence of the “One-family” Bush Village.—Owing to the small size of such villages the feral conditions essential to the presence of tsetse in numbers exist right up to them and to their very small fields, and the inhabitants, sick and well, are attended by the flies in all their occupations. A maximum number of flies are thus laid open to infection, and a maximum number of healthy people. Thus, with village to village intercourse also considerable, epidemic is likely to be facilitated by a prevalence of the small village.