The spatial and seasonal distribution of African horse sickness and its potential Culicoides vectors in Morocco

Abstract

African horse sickness (AHS) is a vector-borne, infectious disease of equines that is caused by African horse sickness virus (AHSV). The only proven field vector is the biting midge Culicoides imicola, although C. obsoletus and C. pulicaris are suspected vectors. There was a recent 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 sites distributed over most of Morocco. Culicoides imicola was found to be very widely dispersed, with the greatest catches in the low-lying northwestern areas (between Tangier and Rabat) and at Marrakech. Culicoides imicola was absent at one site only, near Settat. Culicoides imicola was found at altitudes ranging from 4 to 1275 m and in climatic conditions ranging from subhumid to saharan. In general, the catch of C.imicola 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 obsoletus and C.pulicaris are also widely distributed in Morocco but trap 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 correspond well with the distribution of disease outbreaks during the epizootic. Although disease outbreaks were widespread in the country, the greatest number of reported cases was in the northwest (1989-90); in 1991 there were also significant numbers in Marrakech province. No cases were reported in a large area to the west of the Atlas mountains (including Settat) despite the presence of a large equine population. It is likely that during the epizootic the virus overwintered in the northwest (1989) and in Marrakech province (1990). Disease outbreaks occurred from July to December, with a peak from September to November. An unexplained phenomenon is the large number of reported cases of AHS in mules in Chefchaouen province in 1990, despite the apparent low abundance of C.imicola at a site at Chefchaouen. It is argued that C.obsoletus and C.pulicaris were probably of little significance to the epidemiology of AHS in Morocco in 1989-91.