Trypanosome infection rates inGlossina morsitans submorsitansNewst. in Northern Nigeria

Abstract

Previous records and new data on trypanosome infection rates inGlossina morsitans submorsitansNewst. in Northern Nigeria are presented, and discussed in relation to the hosts fed on by this tsetse fly. The new observations were carried out in three areas: at Mando and Gamagira, both lying in a fly-belt north of Kaduna, and in the Yankari Game Reserve in Bauchi Province.Infection rates were obtained by dissection of flies, and the trypanosomes were identified by their locus; the validity of this method of identification is discussed. All infections withTrypanosomaspp. were attributable either to thevivaxgroup or to theconrgolensegroup; nobrucen-group trypanosomes were identified. More than 260 flies from each area were examined during March 1962, at the end of the dry season, and similar numbers during October 1962, at the end of the wet season.There was no significant difference between dry-season and wet-season infection rates in any area. Taking the two seasons together, the rate at Yankari (12%) was significantly higher than the rates at Mando (5%) and Gamagira (3%), which did not differ significantly. These contrasting infection rates could be related to the host species principally fed on by flies in the different areas as shown by blood-meal determinations. The lowest infection rates occurred where Suidae furnished a high proportion of meals (Mando, 51%; Gamagira, 67%) and Bovidae a small proportion (Mando, 16%; Gamagira, 9%). The high infection rate at Yankari was associated with a high proportion of Bovid meals (53%), especially from buffalo (Syncerus nanus) and bushbuck (Tragelaphus scriptus), and a lower proportion of Suid meals (33%). For the three areas the relationship between infection rate and percentage of Bovid meals was statistically significant.The species groups of infecting trypanosomes showed further contrasts between the three areas which were unrelated to the total infection rates. At Gamagira, 81 per cent, of infections werecongolensegroup, and this proportion was significantly higher than that at Mando (45%) or Yankari (37%). The differences could be related to the types of host from which blood-meals were principally derived. The highest proportion ofcongolense-group infections occurred where the highest percentage of meals came from Suidae (Gamagira, 67%), and lower proportions occurred at Mando and Yankari where Suid feeds were relatively fewer (51% and 33%, respectively).These findings are discussed in the light of existing evidence on the factors governing infection rates inGlossina. It is concluded that, within the over-riding influence exerted by temperature through geographical latitude, infection rates are determined by the type of host that forms the principal source of food. In some populations of Glossina the influence of the nature of the main food source can be sufficient to obscure the general effect of temperature in determining the level of infection. Some of the reasons for these relationships are discussed.