Studies on Onchocerca gutturosa (Neumann, 1910) and its Development in Simulium ornatum (Meigen, 1818). II. Behaviour of S. ornatum in relation to the transmission of O. gutturosa

Abstract

1. Observations have been made on the behaviour of S. ornatum in relation to its role as a vector of O. gutturosa, a common parasite of cattle in Britain.2. Most of the S. ornatum biting cattle alight on the ventral surface of the animal and it has been shown that this behaviour is affected by the degree of illumination.3. The umbilicus is the preferred site for a blood meal by S. ornatum and it is this region where the microfilarial density is the highest.4. The biting behaviour of S. ornatum is affected by the arrangement of the hair in relation to the skin surface. The umbilicus is preferred because here the hairs are so arranged that the flies readily gain access to the skin. The teats are bitten usually when fly activity is very heavy and then the umbilicus is covered with feeding S. ornatum.5. The mean number of O. gutturosa microfilariae ingested by S. ornatum, but not the quantity of blood, increases with increased time spent feeding on an infected animal.6. There is no causal relationship between the loss of blood per anum during and after feeding and the number of microfilariae ingested by S. ornatum.7. Few microfilariae of O. gutturosa are lost per anum by S. ornatum during feeding despite the loss of blood by this route from the majority of flies.8. The infective larvae of O. gutturosa are deposited by S. ornatum in the skin of the umbilicus of the cow from where they migrate to the neck or gastro-splenic region. This suggests that the worms possess an inherent directional mechanism determining their final site of development and that the point of inoculation of the infective larvae may have less importance in determining the sites of final maturation of Onchocerca species than has hitherto been believed.