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Occurrence of the savanna species of die Simulium damnosum complex in Liberia
518 TRANSACTIONSOF THE ROYALSATIETYOF TROPICAL MEDICINEAND HYGIENE(1987) 81, 518
1 Short
I
Occurrence
Report
1
4
of the savanna species of the Simulium
damnosumcomplex in Liberia R. GARMS Bernhard-Nocht-Institut, D-2000 Hamburg 4, German Federal Republic The savanna species of the Simuiium damnosum complex are the most important vectors of blinding onchocerciasis. Of the 3 savanna speciesknown from West Africa, S. damnosums.str. and S. sirbanum are the principal targets of the Onchocerciasis Control Programme (OCP). Their capacity to fly long distances has caused considerable problems. During each rainy season,invasions of large numbers of adult flies from uncontrolled breeding sites outside the programme areahave necessitatedthe extension of the area in order to control them (WALSH et al., 1981; BALDRY et al., 1985). S. damnosums.str. is known to occur in Liberia (GARMS & VAJIME, 1975). Information on the distribution of the savanna vectors in this country, which is adjacent to the south-western boundaries of the programme, is thus clearly of importance in both the OCP and the epidemiology of Liberian onchocerciasis. Members of the S. damnosum complex can be reliably identified only by examination of the larval polytene chromosomes. S. damnosum s.str. larvae were identified cytotaxonomically from 2 sites on the St John River: Duo Town (6”56’N, 9’09’W) and Soko Town (7”03’N, 9”09’W) on 1 March 1971; and from one site on the Cestos River: Bah Town (6”27’N, 8’38’W) on 16 June 1971 (GARMS & VAJIME, 1975). S. sirbanumlarvae were collected on 26 February 1986 from Yalamba (8”31’N, 1O”ll’W) on the Makona River, which forms the frontier between Liberia and Guinea (R. Baker, personal communication). Adult female savanna flies can be distinguished from the forest speciesrecorded in Liberia (S. yahense and S. sanctipauli s.1.) by their pale wing tufts, postcranial and abdominal setae and pale, short, compacted antennae. Biting savanna llies were collected from the following sites:Cestos River at Bah Town on 16 June 1971 (22 females), 16-18 May 1984 (96) and 23-24 April 1985 (98); in the Bong Range on the St Paul River at Gengema (6”54’N, lo”22’W) on 18 January 1985 (l), on the Marvor Creek (6”52’N, lO”24’W) on 1 March 1985 (l), on the Men Creek at Menla (6”56’N, 10°15’W) on l-8 April 1985 (2), on the Lofa River at Kessely Town (8”13’N, 9”44’W) in February 1974 (51) and on 5-8 March 1985 (5), at Dabu Town (8”00’N, 9’55’W) on 7-8 March 1985 (14), and on the Makona River at Maa (8”31’N, lO”9’W) on 23 October 1986 (1). R. Baker (personal communication) found that 54% of 216 flies caught biting man at Yalamba, which is near Maa, in March
1986 were savanna flies. Nulliparous flies indicating vector breeding in the vicinity were taken at Bah Town, Marvor Creek, Menla, Kessely Town, Dabu Town. The morphological identification of some flies from the Cestos River was confirmed by analysis of cuticular hydrocarbons (18 S. damnosums.str.. 1 S. sirbanum; A. Phillips, personal communication). All the records of savannaspecieswere made in the dry season or at the beginning of the rains. These species disappeared at the height of the rams when only high biting densities of S. yahense and S. sanctipauli s.1. were found. Presumably there is a retraction northwards of the southern limits of the savanna species in the rainy season. However, although our knowledge of their distribution is still incomplete, in the dry seasonthey apparently cover a large area. The presenceof savannaflies in the Bong Range is especially interesting since it shows that these flies can penetrate deep into the forest zone during the dry season, presumably aided by the prevailing northeasterly harmattan winds. The biting densities recorded in this areawere too low to be epidemlologically significant. However, if the ecological changesand habitat destruction due to human activities in the rain forest zone continue, the savanna speciesmay establish themselves in larger populations. In contrast, substantial numbers of biting females were caught on the upper Cestos River, also in the forest, at Bah Town. Out of 194 flies which were identified and dissected, 122 (63%) were parous, 6 (4.9%) of the parous females harboured 1st stage larvae and 4 (3.3%) had a total of 7 third stagelarvae indistinguishable from Onchocercavolvulus. Three Ilies had infective larvae in the head. This clearly showed that the savanna speciesof the S. damnosumcomplex can be vectors of onchocerciasis in Liberia. The excellent technical assistancein the field and in the laboratory of Mrs M. Kerner is gratefully acknowledged. References
Baldry, D. A. T., Zerbo, D. G., Baker, R. H. A., Walsh, J. F. & Pleszak, F. C. (1985). Measures aimed at controlling the invasion of Simulivm damnosum Theobald s.1. (Diptera: Simuhidae) into the Onchocerciasis Control Programme area. I. Experimental aerial larviciding in the Uuner SassandraBasin of south-eastern Guinea in 1985. Tb$cal
Pest Management, 31, 255-263.
Ganns, R. & Vajime, C. G. (1975). On the ecology and distribution of the species of the Simulium damnosum complex in different bioclimatic zones of Liberia and Guinea. Tropenmedizin und Parasitologic, 26, 375-380. Walsh, J. F., Davies, J. B. St Garms, R. (1981). Further studies on the reinvasion of the Onchocerciasis Control Programme by Simulium damnosum s.1.: the effects of an extension of control activities into southern Ivory Coast chng 1979. Tropenmedizin und Parasitologic, 32, 269-
Acceptedfor publication .5 January 1987
1 Short
I
Occurrence
Report
1
4
of the savanna species of the Simulium
damnosumcomplex in Liberia R. GARMS Bernhard-Nocht-Institut, D-2000 Hamburg 4, German Federal Republic The savanna species of the Simuiium damnosum complex are the most important vectors of blinding onchocerciasis. Of the 3 savanna speciesknown from West Africa, S. damnosums.str. and S. sirbanum are the principal targets of the Onchocerciasis Control Programme (OCP). Their capacity to fly long distances has caused considerable problems. During each rainy season,invasions of large numbers of adult flies from uncontrolled breeding sites outside the programme areahave necessitatedthe extension of the area in order to control them (WALSH et al., 1981; BALDRY et al., 1985). S. damnosums.str. is known to occur in Liberia (GARMS & VAJIME, 1975). Information on the distribution of the savanna vectors in this country, which is adjacent to the south-western boundaries of the programme, is thus clearly of importance in both the OCP and the epidemiology of Liberian onchocerciasis. Members of the S. damnosum complex can be reliably identified only by examination of the larval polytene chromosomes. S. damnosum s.str. larvae were identified cytotaxonomically from 2 sites on the St John River: Duo Town (6”56’N, 9’09’W) and Soko Town (7”03’N, 9”09’W) on 1 March 1971; and from one site on the Cestos River: Bah Town (6”27’N, 8’38’W) on 16 June 1971 (GARMS & VAJIME, 1975). S. sirbanumlarvae were collected on 26 February 1986 from Yalamba (8”31’N, 1O”ll’W) on the Makona River, which forms the frontier between Liberia and Guinea (R. Baker, personal communication). Adult female savanna flies can be distinguished from the forest speciesrecorded in Liberia (S. yahense and S. sanctipauli s.1.) by their pale wing tufts, postcranial and abdominal setae and pale, short, compacted antennae. Biting savanna llies were collected from the following sites:Cestos River at Bah Town on 16 June 1971 (22 females), 16-18 May 1984 (96) and 23-24 April 1985 (98); in the Bong Range on the St Paul River at Gengema (6”54’N, lo”22’W) on 18 January 1985 (l), on the Marvor Creek (6”52’N, lO”24’W) on 1 March 1985 (l), on the Men Creek at Menla (6”56’N, 10°15’W) on l-8 April 1985 (2), on the Lofa River at Kessely Town (8”13’N, 9”44’W) in February 1974 (51) and on 5-8 March 1985 (5), at Dabu Town (8”00’N, 9’55’W) on 7-8 March 1985 (14), and on the Makona River at Maa (8”31’N, lO”9’W) on 23 October 1986 (1). R. Baker (personal communication) found that 54% of 216 flies caught biting man at Yalamba, which is near Maa, in March
1986 were savanna flies. Nulliparous flies indicating vector breeding in the vicinity were taken at Bah Town, Marvor Creek, Menla, Kessely Town, Dabu Town. The morphological identification of some flies from the Cestos River was confirmed by analysis of cuticular hydrocarbons (18 S. damnosums.str.. 1 S. sirbanum; A. Phillips, personal communication). All the records of savannaspecieswere made in the dry season or at the beginning of the rains. These species disappeared at the height of the rams when only high biting densities of S. yahense and S. sanctipauli s.1. were found. Presumably there is a retraction northwards of the southern limits of the savanna species in the rainy season. However, although our knowledge of their distribution is still incomplete, in the dry seasonthey apparently cover a large area. The presenceof savannaflies in the Bong Range is especially interesting since it shows that these flies can penetrate deep into the forest zone during the dry season, presumably aided by the prevailing northeasterly harmattan winds. The biting densities recorded in this areawere too low to be epidemlologically significant. However, if the ecological changesand habitat destruction due to human activities in the rain forest zone continue, the savanna speciesmay establish themselves in larger populations. In contrast, substantial numbers of biting females were caught on the upper Cestos River, also in the forest, at Bah Town. Out of 194 flies which were identified and dissected, 122 (63%) were parous, 6 (4.9%) of the parous females harboured 1st stage larvae and 4 (3.3%) had a total of 7 third stagelarvae indistinguishable from Onchocercavolvulus. Three Ilies had infective larvae in the head. This clearly showed that the savanna speciesof the S. damnosumcomplex can be vectors of onchocerciasis in Liberia. The excellent technical assistancein the field and in the laboratory of Mrs M. Kerner is gratefully acknowledged. References
Baldry, D. A. T., Zerbo, D. G., Baker, R. H. A., Walsh, J. F. & Pleszak, F. C. (1985). Measures aimed at controlling the invasion of Simulivm damnosum Theobald s.1. (Diptera: Simuhidae) into the Onchocerciasis Control Programme area. I. Experimental aerial larviciding in the Uuner SassandraBasin of south-eastern Guinea in 1985. Tb$cal
Pest Management, 31, 255-263.
Ganns, R. & Vajime, C. G. (1975). On the ecology and distribution of the species of the Simulium damnosum complex in different bioclimatic zones of Liberia and Guinea. Tropenmedizin und Parasitologic, 26, 375-380. Walsh, J. F., Davies, J. B. St Garms, R. (1981). Further studies on the reinvasion of the Onchocerciasis Control Programme by Simulium damnosum s.1.: the effects of an extension of control activities into southern Ivory Coast chng 1979. Tropenmedizin und Parasitologic, 32, 269-
Acceptedfor publication .5 January 1987