Vectors of the 1969 yellow fever epidemic on the Jos Plateau, Nigeria

Bull. Org. mond. SantJ 1972, 46, 669-673 Bull. Wld Hithi Org. Vectors of the 1969 yellow fever epidemic on the Jos Plateau, Nigeria V. H. LEE & DOROTHY L. MOORE 1 Entomological investigations ofthe possible mosquito vectors of the yellowfever epidemic on the Jos Plateau, Nigeria, were carried out between 27 October and 15 November 1969. Of the 5 species of Aedes (Stegomyia) collected, Ae. luteocephalus was the most abundant in human-bait captures. Ae. aegypti, Ae. africanus, andAe. vittatus were collec- ted in low numbers. The aegypti larval index in the areas investigated was very low and the species was not considered to be the primary vector in the epidemic. Ae. simpsoni larvae were abundant, but no adults were collected on human bait. Six isolates ofS different arboviruses were obtained: yellow fever from damaged Stego- myia sp.; dengue 2 from damaged specimens, probably all Ae. luteocephalus; Zika (2 iso- lates) from Ae. luteocephalus; Bwamba from Anopheles funestus; and a Nyando-group virus from An. gambiae. Following reports of a widespread yellow fever epi- demic on the Jos Plateau, Nigeria (Carey et al., 1972), entomological investigations in that area were be- gun on 27 October 1969 and continued until 15 No- vember, the primary purpose being to evaluate the populations of possible mosquito vectors of yellow fever (YF) and to obtain as many specimens as possible for virological studies. As the epidemic was essentially rural in nature, the investigations were concentrated in such areas, only brief trips being made to the cities of Jos and Bukuru to establish the prevalence there of domestic Aedes aegypti. Of 6 rural localities investigated initially, those of Du, Vwang (Vom), and Tagwe (Miango) subsequently received extra attention because of YF cases among the residents and the detected presence of various Stegomyia populations. Ecological aspects of the Plateau are described in a companion paper (Lee, 1972). The rains, which in 1969 lasted nearly a month longer than normal, sub- sided about 1 week after entomological studies were begun, and thereafter drying of the soil and of ex- posed water containers took place rapidly. In the single published paper on the mosquito fauna of the Plateau, Boorman (1961) reports the presence of 5 Aedes (Stegomyia) spp. of particular interest for 1 Virus Research Laboratory, University of Ibadan, Nigeria. The Laboratory receives support from the Rocke- feller Foundation. YF studies-aegypti, africanus, luteocephalus, simp- soni, and vittatus-and comments that, as vectors of arthropod-borne viruses, luteocephalus, africanus, and vittatus are the three most likely to be involved. MATERIALS AND METHODS Mosquitos Initially, an attempt was made to establish the Ae. aegypti index in rural localities from which many cases of yellow fever had come. Larvae were searched for in containers of all kinds, as well as in the leaf axils of plants growing near compounds- in particular, coco yam (Colocasia sp.), wild and do- mestic banana, Dracaena sp., and a plant known lo- cally as " rwang dyam " (probably Crinum sp.). Only a few accessible rot holes in trees were found, most of them already dry. Ground pools were virtually absent, but many of the larger rock holes examined still contained water. At most of the sites studied, samples of any larvae and pupae found were taken and held until they matured into adults for species determination. Considerable time was expended in a search for Ae. aegypti adults harbouring inside houses and shelters; both aspirators and the spray-sheet tech- nique were used. When the results proved unreward- ing, human-bait captures were attempted at ground level in and around compounds, in nearby farm plots, and in the surrounding bush. 2845 -669- 9 670 V. H. LEE & D. L. MOORE Table 1. Summary of collections of immature and adult Aedes (Stegomyia) spp. in 6 areas of Jos Plateau, 1969 Ae. Iuteo- HouseAe. aegypti Ae. africanus cephaIus Ae. simpsoni Ae. vittatus inspections Locality Ima Ad b Im Ad Im Ad Im Ad Im Ad Pre- Sites Du + + -_ - + + + + + many 28 Forom - - - - - + + - + - 7 6 Tagwe (Miango) _ + - + - + + - + + no inspections Vwang (Vom) + _ _ + - + + + + 9 18 Bukuru 3- - 12- - Jos + + ? ~~~~~~~~~~~~~~~many118 a Im = immature stage. b Ad = adult. A team of 10-11 labourers plus 1 mosquito scout 1 was organized to make landing-biting mosquito captures. Because particular interest attached to the Stegomyia spp., which are predominantly diurnal and crepuscular, collections were usually started at about 07.00 hours, interrupted for a rest period at 12.00-14.00 hours, and then resumed until 20.00 hours. Darkness occurred at about 18.15 hours. Each collector was provided with test tubes stop- pered with cotton-wool and was instructed to collect any mosquito landing on or feeding on him. Flash- lights were distributed, each group of 3-4 collectors being given one, to facilitate collecting after dark. Captured mosquitos were gathered from the team periodically. Each night, females, after being immo- bilized, were placed in plastic capsules sealed with tape bearing an identifying number, and dropped into a flask of liquid nitrogen for storage and transport to the laboratory in Ibadan. On arrival in Ibadan, the specimens were transferred to a mecha- nical freezer (-60'C) to await final identification just prior to being tested for the presence of virus. Virus isolation and identification Mosquitos were pooled by species and stage of feeding cycle in numbers usually not exceeding 15. They were then processed by standard techniques utilizing intracerebral inoculation of 2-day-old white mice. Presumptive arbovirus isolates were screened and identified by standard complement-fixation tests using reference immune mouse ascitic fluids. 1 Kindly detailed to the senior author by Dr Bhure, M.O.H., Jos. RESULTS Mosquito collections Table 1 gives a summary of collections of larval and adult Aedes (Stegomyia) spp. in six areas of the Plateau (for the locations of the villages, see map in Carey et al., 1972), and Table 2 lists, by species, all the female mosquitos captured and subsequently exa- mined for the presence of virus. Larvae of Ae. aegypti were encountered only rare- ly, and only in rural localities: at Du and at Chigi (Vom), where several specimens were found in a metal pan in a compound yard and in an earthen water pot, respectively, and at Vom, where one larva was collected from a rain barrel at the staff quarters and many were collected from discarded bedpans on the hospital premises. Larvae of this species were not found in leaf axils of plants or rot holes of trees, nor in more than 130 water pots and numerous tires and battery cases examined in residential and com- mercial districts of Jos and Bukuru. Female Ae. aegypti were likewise scarce, the 10 listed all being taken at the village of Tagwe (Miango) on human bait between 16.30 and 18.30 hours. No Ae. africanus larvae were found and again only 10 females were captured, 9 at Tagwe (Miango) and the other at Vwang (Vom) on human bait between 16.30 and 18.30 hours. No larvae of Ae. luteocephalus were found, but the 354 females collected came from all localities sur- veyed except Bukuru, with Tagwe (Miango) account- ing for 317 of the specimens. In rural areas, adult Ae. luteocephalus were active throughout the day, but peak feeding activity occurred at between 17.00 and VECTORS OF YELLOW FEVER ON THE JOS PLATEAU, NIGERIA Table 2. Numbers and species of mosquitos collected on Jos Plateau, 1969, and examined for virus Species No. Totals Aedes (Stegomyia) aegypti 10 africanus 10 luteocephalus 354 simpsoni 1 vittatus 36 sp. (damaged) 35 446 Aedes sp. 5 Aedes (Aedimorphus) dentatus group 54 59 Anopheles funestus 823 gambiae 271 spp. 17 1 111 Culex duttoni 74 pipiens fatigans 44 nebulosus 84 tigripes 8 spp. 127 337 Mansonia (Mansonioides) uniformis 21 (Coquillettidia) spp. 49 70 total 2 023 (205 pools) 18.00 hours, taking place both within and at the edges of the small remaining forest, as well as near village compounds away from forest. Two adults were taken at night, one in a light-trap operated in a compound and the other (a damaged specimen but considered to be Ae. luteocephalus) in a lighted room in Jos as it came to feed. Immature stages of Ae. simpsoni were abundant in leaf axils of coco yams and Crinum sp. in all the localities studied, and up to about 20 larvae and/ or pupae were collected from nearly all the water- containing coco yam plants examined. Larvae were commonly found in association with larvae of Urano- taenia ornata. Leaf axils of banana plants and Dra- caena sp., most of them dry when examined, did not yield larvae of Ae. simpsoni. No adults of the species were captured on human bait near or at sites where larvae or pupae had been found; the only two adults taken were a female, aspirated while resting within an earthen pot outside a house, and a male swept from vegetation. Larvae of Ae. vittatus were abundant in large rock holes, most of which still contained water, and were commonly found in association with larvae of Ano- pheles gambiae and Culex duttoni. In the three areas in which adult Ae. vittatus were collected, nearby rock holes were numerous and there was a consider- able production of adults; yet only 36 females were taken on human bait. Late afternoon was the period of greatest activity of this mosquito. All mosquitos of the Ae. (Aedimorphus) dentatus group, including Ae. dentatus and Ae. quasiunivittatus were collected on human bait in the evening at a single rural locality, Barrakin Ladi. The great majority of adult Anopheles spp. collec- ted were An. funestus and An. gambiae, taken in large numbers inside bedrooms of village compounds by the spray-sheet technique. Very few Anopheles were collected on human bait or in light-traps. Adults of Culex duttoni, C. nebulosus, and C. ti- gripes were taken on human bait or when resting on vegetation, while those of C. pipiens fatigans were mostly obtained in spray-sheet collecting. The 127 specimens listed were all females, mostly C. decens. Of the adult Mansonia spp. collected, the 21 M. (Mansonioides) uniformis specimens were taken on human bait, and the 49 Mansonia (Coquillettidia) spp., including M. maculipennis and a few M. cristata, were obtained in resting captures. Virus isolations Of the 205 species pools (2 023 mosquitos) inocu- lated into mice (Table 2), 6 pools yielded isolates identified as 5 virus types: yellow fever, dengue 2, Zika, Bwamba, and a Nyando-group virus. Details are given in Table 3. DISCUSSION Of the 2 023 female mosquitos collected and tested for the presence of virus, only 25% were captured on human bait (in about 350 man-hours of collecting), the remaining 75% being obtained by the spray-sheet technique and from vegetation. The relatively low density of diurnal and crepuscular culicine mosquitos indicated by these numbers was probably a result of the changes in weather conditions that were taking place at the time of the investigations. Rainfall con- tinued until 3 November; thereafter, drying occurred rapidly at the high daytime temperatures, and, in 671 V. H. LEE & D. L. MOORE Table 3. Virus isolates from mosquitos collected on Jos Plateau, 1969 Virus Strain no. Mosquito source Place collected collected yellow fever IbAr 45244 Aedes (Stegomyla) sp.; 6, fragments a Tagwe (Miango) 6 Nov. dengue 2 IbAr 45294 Aedes (Stegomyia) sp.; 10, damaged b Tagwe (Miango) 7 Nov. Zika IbAr 45250 Ae. luteocephalus; 15, unfed Tagwe (Miango) 6 Nov. IbAr 45293 Ae. luteocephalus; 15, unfed Tagwe (Miango) 7 Nov. Bwamba IbAr 45235 Anopheles funestus; 18, gravid Forom 5 Nov. Nyando group IbAr 45043 An. gambiae; 9, engorged and engorged/gravid Chigi (Vom) 27 Oct. a Detached heads, thoraces, and abdomens thought to be from Ae. luteocephalus. b Considered most likely to be Ae. luteocephalus. addition, the nights became very cool soon after sunset. Possibly this combination of drying weather and long cool nights severely curtailed mosquito pro- duction and activity, as well as any extrinsic virus cycle in the mosquitos. The mosquitos surviving these conditions, how- ever, were most productive in terms of arboviruses isolated. The recovery of 3 different viruses from 446 Aedes (Stegomyia) mosquitos represents a rela- tively high prevalence of infection in wild mosquitos. Furthermore, the 4 isolates of these 3 virus types all came from 390 Stegomyia mosquitos captured in a small remnant of forest on a broken hillside near Tagwe (Miango). The sole isolate of yellow fever virus was recovered from fragments of mosquitos damaged during stor- age and transit in liquid nitrogen. These mosquitos were known to be a Stegomyia but were neither Ae. aegypti nor Ae. vittatus; they were considered most likely to be Ae. luteocephalus because they were col- lected in an area in which 90% of the Stegomyia mosquitos were Ae. luteocephalus. The very low Ae. aegypti larval index observed makes it doubtful whether the species could have been a domestic or peridomestic vector of yellow fever. Unfortunately, it was not possible to derive an index of immature Ae. aegypti in sylvan sites. The numbers of adults obtained in human-bait captures indicate that the species was present at a very low level, at least in terms of anthropophilic forms. Ae. africanus also showed a very low level of adult activity, as determined by human-bait captures. Again, no data on immature stages were obtained in sylvan sites. Ae. luteocephalus appears to offer interesting possi- bilities as a vector of yellow fever in terms of numbers of adults observed, its ability to harbour dengue and Zika viruses, and its avid feeding attacks on man. As with Ae. aegypti and Ae. africanus, no information was obtained on the development sites of immature stages. With Ae. simpsoni, the large numbers of larvae and pupae observed surely represented a large adult popu- lation near dwellings as well as away from them; yet the sole female obtained was collected from a rest- ing site. Presumably in the Plateau area this species is not usually anthropophilic. This type of non- anthropophilic population has been shown to occur in East Africa (Gillett, 1951); in West Africa, it has been suggested that the species is not anthropophilic (Boorman & Service, 1960). (The senior author ob- served that captured Ae. simpsoni will readily feed on man when he allowed a 1-day-old adult female an opportunity to feed on his arm.) Although immature Ae. vittatus were abundant, this species was taken relatively infrequently on hu- man bait, in contrast to Boorman's (1961) suggestion that it is a likely vector of arboviruses on the Plateau. Ae. vittatus was suspected of being a vector in the yellow fever outbreak in the Sudan in 1941 (Kirk, 1941). All Ae. aegypti collected on the Plateau were of the dark form, which has not adapted to any great extent to domestic habitats. Although Ae. aegypti was capable of thriving under such conditions, as illustrated by the colonization of discarded bedpans, that situation was a localized one, and no parallel examples were seen elsewhere on the Plateau. Earthen water pots were the domestic vessels most frequently encountered both in the cities and in the rural vil- 672 VECTORS OF YELLOW FEVER ON THE JOS PLATEAU, NIGERIA 673 lages; yet of over 200 examined, only 1, in a village area, was found to harbour even a few Ae. aegypti larvae. There appear to be two reasons why domestic sites have not become colonized to a greater extent. First, the dark form of Ae. aegypti, which is consi- dered to be a wild form, may prefer nondomestic sites on the Plateau, even though in other parts of Nigeria it has adapted very well to completely domes- tic situations. Second, its opportunity for colonizing domestic sites may be reduced by local customs and by the fact that, as well water is readily available throughout the year in rural areas, storage of water in quantities or for long periods is unnecessary. Unfortunately, little information was available about the Plateau's mosquito populations on a sea- sonal basis. Detailed studies during the following seasons (1970) provided useful data on oviposition sites and biting rates, and will be presented elsewhere. ACKNOWLEDGEMENTS The authors thank Mr Z. 0. Babarinde for technical assistance in carrying out laboratory procedures. RESUME VECTEURS DE L'EPIDEMIE DE FIEVRE JAUNE DE 1969 SUR LE PLATEAU DE JOS (NIGERIA) Une enquete entomologique a e menee sur le plateau de Jos en octobre-novembre 1969 apres l'epid6mie de fievre jaune qui y a sevi. Son objectif etait d'6valuer l'importance des populations des vecteurs eventuels et de recueillir du materiel pour une etude virologique. On a eu surtout recours aux captures sur hote humain ou par pulverisations. On n'a trouve qu'un tres petit nombre de larves d'Aedes aegypti dans les regions rurales et aucune en milieu urbain. Les adultes de cette espece etaient egale- ment tres rares: 10 femelles seulement ont e capturees. Aucune larve d'Ae. africanus n'a et decouverte, et les captures n'ont foumi que 10 adultes femelles. La recolte a ete plus fructueuse en ce qui conceme Ae. luteocephalus: 354 adultes femelles, mais aucune larve. On a decele de nombreux gites larvaires d'Ae. simpsoni, mais aucun adulte n'a ete capture sur piege humain. Trente-six Ae. vittatus femelles ont ete recoltes et on a trouve de nombreux gites de cette espece dans les creux de rocher. Les captures par pulverisations effectuees en milieu rural ont permis de recueillir 1111 femelles d'Anopheles spp., comprenant une forte proportion d'An. funestus et d'An. gambiae. Parmi les autres especes identifiees figu- raient Ae. dentatus, Ae. quasiunivittatus, Culex duttoni, C. pipiens fatigans, C. nebulosus, C. tigripes, Mansonia uniformis, M. maculipennis et M. cristata. Le mat6riel obtenu a partir des 2023 moustiques cap- tures, repartis en 205 lots, a ete inocule par voie intra- cerebrale au souriceau. Six souches d'arbovirus ont ete isolees: un virus amaril, a partir d'Ae. (Stegomyia) sp. endommages; un virus de la dengue 2, a partir d'Ae. (Ste- gomyia) sp. endommages mais appartenant probablement a l'espece Ae. luteocephalus; deux souches de virus Zika, a partir d'Ae. luteocephalus; un virus Bwamba, it partir d'An. funestus; et un virus du groupe Nyando, a partir d'An. gambiae. REFERENCES Boorman, J. P. T. (1961) Bull. ent. Res., 52, 709-725 Boorman, J. P. T. & Service, M. W. (1960) W. Afr. med. J., 9, 67-72 Carey, D. E., Kemp, G. E., Troup, J. M., White, H. A., Smith, E. A., Addy, R. F., Fom, A. L. M. D., Jones, E. J., Bres, P., & Shope, R. E. (1972) Bull. Wld Hlth Org., 46, 645 Gillett, J. D. (1951) Ann. trop. Med. Parasit., 45, 110-121 Kirk, R. (1941) Ann. trop. Med. Parasit., 35, 67-112 Lee, V. H. (1972) Bull. Wld Hlth Org., 46, 641