Onchocerca ochengi transmission dynamics and the correlation of O-ochengi microfilaria density in cattle with the transmission potential

The intensity of natural transmission of Onchocerca ochengi and Onchocerca volvulus by anthropo-boophilic Simulium damnosum s.l. was studied longitudi nally in two cattle watering sites of a cattle ranch within a predominantly cattle populated area of the Guinea savanna of Cameroon and related to cat tle O. ochengi skin microfilaria abundance. During the 12 months study peri od, a total of 4 696 flies was individually dissected to examine the monthl y transmission potential (MTP) of O. ochengi and O. volvulus. The estimated Simulium damnosum s.l. annual biting rates (ABR) on human baits were 47 52 9 flies at the bank of the Vina "du sud" river. The ABR at the lake, which was situated at about 2 km upland from the perennial river, was 8 579. The monthly parous rate was highly correlated with monthly biting rate. The ann ual transmission potentials (ATP) of O. ochengi were calculated to be 7 732 and 1 669 at the riverbank and the lake, respectively. Transmission occurr ed mainly in the dry season, peaking in the months of January to mid-March when dermal microfilaria density in the animals was also the highest. The O . ochengi microfilaria uptake by the fly vectors was host microfilaria dens ity-dependent. The MTP of O. ochengi was positively correlated with dermal microfilaria density. The mean number of microfilariae per fly taken up dur ing a blood meal was high during the dry season as was the mean number of i nfective larvae per fly but declined significantly with the onset of the ea rly rains. A similar seasonality of transmission was also observed for O. v olvulus that was concurrently transmitted by the same vector flies, but its ATP was comparatively much lower: 1 332 infective larvae per man per year at the riverbank and 107 around the lake. The population dynamics of cattle microfilariae therefore plays an important role in the regulation of O. oc hengi transmission.