ANALYSIS OF MIGRATION SUCCESS OF ONCHOCERCA-LIENALIS MICROFILARIAE INTHE HAEMOCOEL OF SIMULIUM-VITTATUM

Migration success (i.e. the proportion of worms that reach the thorax) of Onchocerca lienalis microfilariae (mf) in the haemocoel of Simuliu m vittatum was studied by inoculating mf into the posterior abdomen, a nd recording their distribution in the blackfly body at predetermined time points. Mf arrive into the thorax by active locomotion rather tha n by drifting in haemolymph currents. Migration into the thorax was co mpleted by 12 h post inoculation (pi) but was not continuous throughou t this period. Migration proceeded in two phases; the first occurred 0 -2 h pi and the second at 6-12 h pi. Overall, migration success 12-24 h pi was only 36%, indicating that a substantial number of mf failed t o reach the thorax, either because they were eliminated by the fly's d efensive response or because they remained in the abdomen. Migration s uccess was density independent. Mf that arrive into the thorax within 2 h pi did not differ in their migration potential from mf that remain ed in the abdomen at this time. In flies where more mf migrated succes sfully there was lower mf loss, indicating that migration success was linked to mf loss. Moreover, the proportion of mf in the thorax was no t correlated with mf loss, suggesting that mf loss affected the number of mf that migrated successfully, rather than the reverse causal rela tionship.