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Onchocerca gutturosaandO. lienalisinflections in British cattle were studied by examination of cattle post-mortem originating from North Wales and Cheshire (north west England). In 463 adult animals, the microfilarial (mf) prevalence was 28·5%. In 95·3% of the inflected animals, gravid worms could not be found at either the ligamentum nuchae or the gastro-splenic omentum. Dermal mf at the head were identified asO. gutturosaon the basis of their highly significant assocation with the presence of gravidO. gutturosaat the ligamentum nuchae, which were found in only 3·2% of cattle. Mfs were isolated from different skin sites and from adult worms and a minimum of 10 mfs from each isolate were examined for width and acid phosphatase (AP) staining pattern. The width ofO. gutturosadermal mf was <4 μm (4 isolations), narrower than of putativeO. lienalismf isolated from umbilical skin of cattle without evidence ofO. gutturosa, which were in 20/22 isolations >4 μm wide. The dermal mf were also distinguished on the basis of different AP staining patterns which, for each species, correlated closely with that of hatched intrauterine mf from their respective adult female worms. Based on the criteria of morphology and AP staining patterns the mf species prevalences in the survey population were estimated asO. lienalis24·1% andO. gutturosa2·2%, with a further 2·2% of cattle inflected with both species. The results indicate that the predilection site of adultO. lienalisis not the gastro-splenic omentum. In North Wales, the distribution of the two species was different;O. lienaliswas widely distributed in all cattle rearing areas both lowland and upland, whereasO. gutturosawas largely restricted to valleys close to major rivers.

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