The Culicoides vectors of African horse sickness virus in Morocco: distribution and epidemiological implications.

African horse sickness (AHS) is a vector-borne, infectious disease of equids caused by African horse sickness virus. The only proven field vector of the virus is the biting midge Culicoides imicola, although C. obsoletus and C. pulicaris are suspected vectors. In 1994-5 a total of 3887 light trap samples were collected from 22 sites distributed over most of Morocco. Culicoides imicola was found to be very widely distributed with the greatest catches in the low-lying north-western areas (between Tangier and Rabat) and at Marrakech. Culicoides imicola was absent at one site only, near Settat. In general, the catch of C. imicola peaked in late summer and autumn, with a smaller peak in spring. Catches of C. obsoletus were greatest in the north-western provinces of Morocco and in the south, while catches of C. pulicaris were greatest in the north. Although both species were widely distributed, trap catches were much lower than those of C. imicola. Peak catches were in spring or late summer and autumn. In general, the findings for C. imicola correspond well with the seasonal and spatial distribution of disease outbreaks during the 1989-1991 epizootic of AHS in Morocco. It is suggested that C. obsoletus and C. pulicaris were probably of little significance in the epidemiology of AHS in Morocco in 1989-91.