Laboratory infection of Anopheles pharoensis with Wuchereria bancrofti

BRIEF COMMUNICATIONS Bulletin of the World Health Organization, 55 (6): 765-766 (1977) Laboratory infection of Anopheles pharoensis with Wuchereria bancrofti F. W. MOSHA 1 & S. A. MAGAYUKA 2 Abstract A. pharoensis was infected from a donor with a high count of W. bancrofti microfilariae. Of the freshly dissected mosquitos, 81.6 % were found to have ingested microfilariae, with an average of 12 ± 2.2 microfilariae per mosquito. An infectivity rate of 41.9 % was observed in mosquitos dissected between the eleventh and fifteenth days after feeding. A mean of 7.6 ± 1.2 third-stage larvae was found in infective mosquitos. Although A. pharoensis has not yet been found naturally infected with third-stage larvae of W. bancrofti, these studies suggest that it is a poten- tial vector of Bancroftian filariasis. Anopheles pharoensis is not known to be a natural vector of Bancroftian filariasis. However, since this species readily feeds on man as well as on animals and will frequently come indoors to feed, it can be considered a potential vector of this disease. The present study was designed to see whether an East African strain of A. pharoensis could support the full development of Wuchereria bancrofti under labora- tory conditions. Materials and methods Some 575 wild A. pharoensis, collected from the Lake Jipe area of Kenya, were allowed to feed between 21 h 30 and 22 h 00 on a human donor with an average density of 850 microfilariae per 0.1 ml of blood. Seventeen percent of the mosquitos were dissected immediately; the remainder were placed in paper cups, in groups of ten, and held in an insectary maintained at 26°C and 85% RH. Some cotton wool soaked in a weak sugar solution was placed in each cup. Dead mosquitos were removed from the cups each morning and dissected for filarial worms. Those that remained alive were dissected between the thirteenth and fifteenth days after feeding. 1 Research Officer, East African Institute of Malaria and Vector-Borne Diseases, P.O. Box 4, Amani, United Repub- lic of Tanzania. 2Senior Technician, WHO/MRC Helminthiasis Research Unit, P.O. Box 950, Tanga, United Republic of Tanzania. Results As shown in Table 1, 81.6% of the mosquitos ingested microfilariae. The mean number of micro- filariae per mosquito was 12 ± 2.2 (range 1-81). The infectivity rate of the mosquitos dissected be- tween day 11 and day 15 was 41.9%. The overall infectivity rate was 15.1 %. The mean number of infective larvae per mosquito was 7.6 + 1.2 (range 1-38). Some 39.6% of the mosquitos survived until the tenth day, when infective larvae first appeared in the mouthparts. The cumulative mortality was 60.4% by day 10 and 78.8% by day 13. In nonin- fected mosquitos (control), the cumulative mortality rates were 31.9% and 45.6% on days 10 and 13, respectively. Discussion The infectivity rate (41.9%) in A. pharoensis was similar to that observed for Culex pipiens fatigans (58.6%) that had fed simultaneously on the same donor and had been maintained in the same insec- tary. However, the cumulative mortality rate was only 33.4% in C. p. fatigans on day 10, much lower than the 78.8% observed in A. pharoensis. The high mortality in A. pharoensis may partly account for its poor vectorial capacity. Smith (1) also noted that wild A. pharoensis in East Africa did not live long enough for worms to attain full development. Natural infections of developing larvae have been reported in A. pharoensis (1-5). Infective-stage larvae of Brugia sp. (Bushrod, F., personal communica- tion, 1977) and Dirofilaria spp. (6) have been dis- sected from wild-caught A. pharoensis along the East African coast. Brengues (7), in West Africa, found that experimentally infected A. pharoensis supported full development of W. bancrofti. Although A. pharoensis has not yet been found naturally infected with third-stage larvae of W. ban- crofti, these studies suggest that it is a potential vector of Bancroftian filariasis. ACKNOWLEDGEMENTS The authors wish to thank Ms C. Wakoli, Ms J. Elineema, Ms H. Kombo, and Ms A. Mtango for their 3663 -765- 766 BRIEF COMMUNICATIONS Table 1. W. bancrofti larvae dissected from A. pharoensis each day following an infective blood meal a Days Total Mosquitos with Mosquitos with Mosquitos with after mosquitos microfilariae sausage larvae infective larvae feeding dissected No. % No. % No. % 0 98 80 81.6 0 0 0 0 1 23 16 69.6 0 0 0 0 2 18 14 77.8 0 0 0 0 3 23 12 52.1 4 17.4 0 0 4 15 2 13.2 9 60.0 0 0 5 45 1 2.2 28 62.2 0 0 6 42 2 4.8 1 3 31.0 0 0 7 52 0 0 21 40.4 0 0 8 35 0 0 16 45.7 0 0 9 35 0 0 1 8 51.4 1 2.9 10 22 0 0 13 59.1 1 13.6 11 38 0 0 5 13.2 9 23.7 12 28 0 0 8 28.6 11 39.3 13 40 0 0 3 7.5 15 37.5 14 37 0 0 1 2.7 25 67.6 15 24 0 0 2 8.3 10 41.7 Total 575 127 141 72 a Only those worms at the latest stage of development were counted in mosquitos that had different filarial stages. technical assistance; and Mr P. Wegesa, Dr J. McMahon, Dr J. Hitchcock, Ms F. Bushrod, and Mr N. Kolstrup for their useful suggestions. REFERENCES 1. SMITH, A. Bulletin of entomological research, 46: 505- 515 (1955). 2. TAYLOR, A. W. Annals of tropical medicine and parasitology, 24: 425-435 (1930). 3. NEREV-LAMAIRE, M. Annales de parasitologie humaine et comparee, 11: 370-402 (1933). 4. SENEVET, G. Encyclopedie entomologique, 19: 361 (1935). 5. GRJEBINE, A. & BRYGOO, E. R. Memoires de l'Institut scientifique de Madagascar, s&rie E, 9: 291-306 (1958). 6. NELSON, G. S. Journal of helminthology, 33: 233-256 (1959). 7. BRENGUES, J. Memoires O.R.S. T.O.M., No. 79, pp. 264-267 (1975).