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The intensity of natural transmission of Onchocerca ochengi and Onchocerca volvulus by anthropo-boophilic Simulium damnosum s.l. was studied longitudinally in two cattle watering sites of a cattle ranch within a predominantly cattle populated area of the Guinea savanna of Cameroon and related to cattle O. ochengi skin microfilaria abundance. During the 12 months study period, a total of 4 696 flies was individually dissected to examine the monthly transmission potential (MTP) of O. ochengi and O. volvulus. The estimated Simulium damnosum s.l. annual biting rates (ABR) on human baits were 47 529 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 annual transmission potentials (ATP) of O. ochengi were calculated to be 7 732 and 1 669 at the riverbank and the lake, respectively. Transmission occurred 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 density-dependent. The MTP of O. ochengi was positively correlated with dermal microfilaria density. The mean number of microfilariae per fly taken up during a blood meal was high during the dry season as was the mean number of infective larvae per fly but declined significantly with the onset of the early rains. A similar seasonality of transmission was also observed for O. volvulus 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. ochengi transmission.

Abstract: In recent years, bovine Onchocerca species have been used as models for human onchocerciasis in drug screens. They have been suggested for immunology studies and evaluation of vaccine candidates. Therefore, mast cells and their association with other inflammatory cells were studied in five onchocercal species of cattle and deer using immunohistology. Intact mast cells occurred in large numbers in the capsule and septae of nodules, in fibrous tissue adjacent to nonnodular worms, and perivascularly. Inactive and, more frequently, activated and degranulating mast cells were observed within infiltrates in the nodule center or around nonnodular filariae. They were not detected in direct contact with the cuticle of adult worms or of microfilariae or among the macrophages, giant cells, and neutrophils forming the innermost layer around the worms. Eosinophils, but not mast cells, were obviously associated with microfilariae-producing females. The distribution, frequency, and activity of mast cells were similar for all five species and O. volvulus.