Seasonal Variation in the Incidence ofTrypanosoma vivaxinGlossina palpalis(R.-D.)

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Abstract
Over 3,000 examples ofG. palpaliswere age-grouped and dissected forT. vivaxover a period of a year. The incidence of infection throughout was highest in Age-group 3 and lowest in Age-group 1. There was a marked seasonal fluctuation, the incidence being highest during the months of heavy rainfall,i.e., August to October, and lowest in the driest.It has been suggested that the rise in the rate of infection during the rains is merely due to the increasing average age of the fly population, but by the method of age grouping it is shown that this is not the case.The degree of infection inG. palpalisin Sierra Leone is comparable with that ofG. tachinoidesin Nigeria.It is not thought that the seasonal variation in the incidence of infection can be ascribed to changes in the type and abundance of food. The true cause of the fluctuating incidence of infection like the source of infection is unknown.The failure to recognize seasonal differences in the rate of infection has led to some important practical misconceptions.