Middle East DVEP - Armed Forces Pest Management Board

latitudes of 13 o N and 10 o S. Many foci are scattered throughout eastern and centralAfrica. About a dozen small foci have been identified in Central and South America,where onchocerciasis was presumably introduced through transportation of infectedslaves. In the Middle East, onchocerciasis occurs in southwestern Saudi Arabia and thelength of Yemen; it may also occur in Oman (Figure 12). It is prevalent across the RedSea in Ethiopia. In Yemen, onchocerciasis occurs along streams flowing into the Gulf ofAden and the Red Sea. It is known to be endemic at elevations of 300 to 1,200 m in allpermanent western-flowing wadis between the northern Wadi Surdud (Al HudaydahGovernorate) and the southern Wadi Ghayl (Ta'izz Governorate). Cases have beenreported from Al Hudaydah to Ta'izz, mostly in Al Barh between Mokha and Ta'izz. Aunique form of this disease, called Sowda or the "black disease," occurs in Yemen andSaudi Arabia. It is characterized by a low titer of microfilariae in the peripheralcirculation of humans and hyperpigmented, pruritic lesions, usually affecting just one leg.Transmission Cycle(s). Man is the definitive host in which O. volvulus multiplies.Microfilariae in human skin are ingested by vector black flies when they suck blood. Inthe Middle East, vectors are members of the Simulium damnosum complex. Themicrofilariae transform within the black fly to an infective stage that enters the humanhost when the fly takes subsequent blood meals. This period of development requires 7to 14 days. Man is also the reservoir host. Onchocerciasis is not considered a zoonosis,although natural infections have been found in a spider monkey in Guatemala and agorilla in the Congo. Chimpanzees can be infected in the laboratory.Vector Ecology Profiles.Members of the S. damnosum complex are the primary vectors. Simulium(Edwardsellum) rasyani is a member of this complex, to date reported only from Yemen.This species may also occur in the Asir District of Saudi Arabia, which is adjacent to theYemeni foci. Identification of species within the S. damnosum complex requireschromosomal analysis, since they are morphologically identical. Simulium dentulosum isconsidered a possible vector because of its anthropophilic biting habits.After a bloodmeal, female black flies lay eggs on emergent vegetation along streams, oron logs and rocks that are splashed with water. Several masses of 150 to 500 eggs maybe laid over a life span of 3 to 4 weeks. Eggs hatch in 2 to 3 days at temperatures of 25 to30° C. Using caudal suckers and silken threads, black fly larvae attach to rocks in swiftflowingstreams, generally in mountainous areas of 300 to 1,200 m. They requirerelatively clean streams with high oxygen content. Larvae feed on small crustaceans,protozoa, algae, bacteria, and decaying bits of plants and animals suspended in the water.They progress through 6 to 9 (often 7) instars, and pupate 7 to 12 days after hatching,depending on temperature. Pupae are found in streams for about 1 to 2 weeks prior toemergence of adults.Simulium damnosum complex vectors are fierce biters that emerge in large numbersduring the rainy season. Many generations can be produced (probably 5 to 10 per year inthe Middle East) as long as streams are flowing. Females often circle in swarms aroundthe lower extremities of human hosts. They are persistent biters that feed primarily122