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Mechanical Transmission in Trypanosomiasis
GENERAL ARTICLES.
MECHANICAL TRANSMISSION IN TRYPANOSOMIASIS.
By GEORGE E. OWEN, M.R.C.V.S., Northern Nigeria. THE object of this article is to suggest that this method of transmission, given suitable opportunities, can become serious, rather than to lay stress on its frequency and importance. The facts referred to here occurred in the Barotse reserve of Northern Rhodesia during the years 1908-1913. I shall omit the names of places and small details which would convey meaning only t6 those knowing the district, and would not appeal to most readers. From 1908 to 1913 some 2500 to 3000 head of cattle died from trypanosomiasis in this district, -although it is free from glossina morsitans. A point of particular interest is that the mortality began each year about the beginning of Febrllary and practically ceased after June. December to March are the wet months and the season when flies are most numerous. This is significant in view of the period when heavy losses in cattle occur. Another important fact is that there were some dozen horses in this part (some bred every year), and numerous dogs, sheep, and goats, but nobody has diagnosed or known of one natural case amongst them. One would suppose, therefore, that this was a strain of trypanosome pathogenic to cattle only. This, however, was notth~case, for I found horses, sheep, goats, and dogs all easily infected by inoculation, particularly dogs. Transport riders with dogs lost many of their oxen but never any dogs. One trader who lost at least rooo head of cattle bred horses on this very spot where his cattle died in numbers, yet he had no losses (except from South African horse-sickness), and looked upon it as a good place for horses. In spite of this fact, horses were susceptible. We are faced, therefore, with the onus of finding some reason for this, and a satisfactory one. In seeking for an explanation of these facts, the first point requiring notice is that trypanosomiasis was definitely confined to certain herds, others grazing over the same ground being free, and in every infected native herd there was a history of exchanging cattle with, or temporary hiring to, traders or transport drivers. Again, in 1908 a cattle owner had come into the district with sick cattle and he lost most of them in three years from trypanosomiasis. With his advent the disease became very noticeable. In 1912 I was sent up to investigate, and at once resorted to isolation. The result was that I held over 800 head of cattle without a loss in 1913 in the area which was the centre of disease in 1912. These 800 were the animals remaining out of a lot of 1100. They were carefully watched for about six months after the fly season,
260
GEN ~:RAL ARTICLES.
all infected and suspected animals being removed before the flies again became numerous. During April and May in 1912 I saw some 200 cases, whereas during February, March, April, and May in I913 I saw only five cases, everyone of which occurred in a suspected herd. Nobody will question the fact that had.a cyclical transmitter been present isolation would have been useless-the disease could not have been confined to certain herds. Similarly, had ticks played the part of intermediary host, the disease would not have been confined to certain herds while others on the same veld remained unaffected. Tabanid
MECHANICAL TRANSMISSION IN TRYPANOSOMIASIS.
By GEORGE E. OWEN, M.R.C.V.S., Northern Nigeria. THE object of this article is to suggest that this method of transmission, given suitable opportunities, can become serious, rather than to lay stress on its frequency and importance. The facts referred to here occurred in the Barotse reserve of Northern Rhodesia during the years 1908-1913. I shall omit the names of places and small details which would convey meaning only t6 those knowing the district, and would not appeal to most readers. From 1908 to 1913 some 2500 to 3000 head of cattle died from trypanosomiasis in this district, -although it is free from glossina morsitans. A point of particular interest is that the mortality began each year about the beginning of Febrllary and practically ceased after June. December to March are the wet months and the season when flies are most numerous. This is significant in view of the period when heavy losses in cattle occur. Another important fact is that there were some dozen horses in this part (some bred every year), and numerous dogs, sheep, and goats, but nobody has diagnosed or known of one natural case amongst them. One would suppose, therefore, that this was a strain of trypanosome pathogenic to cattle only. This, however, was notth~case, for I found horses, sheep, goats, and dogs all easily infected by inoculation, particularly dogs. Transport riders with dogs lost many of their oxen but never any dogs. One trader who lost at least rooo head of cattle bred horses on this very spot where his cattle died in numbers, yet he had no losses (except from South African horse-sickness), and looked upon it as a good place for horses. In spite of this fact, horses were susceptible. We are faced, therefore, with the onus of finding some reason for this, and a satisfactory one. In seeking for an explanation of these facts, the first point requiring notice is that trypanosomiasis was definitely confined to certain herds, others grazing over the same ground being free, and in every infected native herd there was a history of exchanging cattle with, or temporary hiring to, traders or transport drivers. Again, in 1908 a cattle owner had come into the district with sick cattle and he lost most of them in three years from trypanosomiasis. With his advent the disease became very noticeable. In 1912 I was sent up to investigate, and at once resorted to isolation. The result was that I held over 800 head of cattle without a loss in 1913 in the area which was the centre of disease in 1912. These 800 were the animals remaining out of a lot of 1100. They were carefully watched for about six months after the fly season,
260
GEN ~:RAL ARTICLES.
all infected and suspected animals being removed before the flies again became numerous. During April and May in 1912 I saw some 200 cases, whereas during February, March, April, and May in I913 I saw only five cases, everyone of which occurred in a suspected herd. Nobody will question the fact that had.a cyclical transmitter been present isolation would have been useless-the disease could not have been confined to certain herds. Similarly, had ticks played the part of intermediary host, the disease would not have been confined to certain herds while others on the same veld remained unaffected. Tabanid