THE SPATIAL AND SEASONAL DISTRIBUTION OF AFRICAN HORSE SICKNESS AND ITS POTENTIAL CULICOIDES VECTORS IN MOROCCO

African horse sickness (AHS) is a vector-borne, infectious disease of equines that is caused by African horse sickness virus (AHSV). The onl y proven field vector is the biting midge Culicoides imicola, although C. obsoletus and C. pulicaris are suspected vectors. There was a rece nt epizootic of AHS in Iberia (1987-90) and Morocco (1989-91). In 1994 -45 a total of 3887 light trap samples were taken from twenty-two site s distributed over most of Morocco. Culicoides imicola was found to be very widely dispersed, with the greatest catches in the low-lying nor thwestern areas (between Tangier and Rabat) and at Marrakech. Culicoid es imicola was absent at one site only, near Settat. Culicoides imicol a was found at altitudes ranging from 4 to 1275m and in climatic condi tions ranging from subhumid to saharan. In general, the catch of C, im icola peaked in late summer and autumn, with a smaller peak in spring. In areas where the insect appears most abundant at least one adult C. imicola per night may be caught in a light trap at all times of year, thus providing a possible means of viral overwintering. Culicoides ob soletus and C. pulicaris are also widely distributed in Morocco but tr ap catches were much lower than for C. imicola. Peak catches occurred in spring, and late summer and autumn. Other frequently caught species were C. circumscriptus, C. newsteadi, C. puncticollis and members of the odibilis subgenus. In general, the findings for C. imicola corresp ond well with the distribution of disease outbreaks during the epizoot ic. Although disease outbreaks were widespread in the country, the gre atest number of reported cases was in the northwest (1989-90); in 1991 there were also significant numbers in Marrakech province. No cases w ere reported in a large area to the west of the Atlas mountains (inclu ding Settat) despite the presence of a large equine population. It is likely that during the epizootic the virus overwintered in the northwe st (1989) and in Marrakech province (1990). Disease outbreaks occurred from July to December, with a peak from September to November. An une xplained phenomenon is the large number of reported cases of AHS in mu les in Chefchaouen province in 1990, despite the apparent low abundanc e of C. imicola at a site at Chefchaouen. It is argued that C. obsolet us and C. pulicaris were probably of little significance to the epidem iology of AHS in Morocco in 1989-91.