Paper: The problem of loiasis in West Africa with special reference to recent investigations at Kumba in the British Cameroons and at Sapele in Southern Nigeria

Abstract

1. (1) The problem of loiasis in British West Africa is discussed with special reference to investigations carried out by members of the Loiasis Research Scheme, at Kumba in the British Cameroons, and at Sapele in Southern Nigeria. 2. (2) The distribution of loiasis in West Africa is shown to correspond closely with the distribution of Chrysops silacea and Chrysops dimidiata, which are confined to the equatorial rain forest whereas in the south-western province of the Sudan, which fringes on this area, C. distinctipennis has been shown by other workers to be the main vector. These three species belong to the subgenus Kleineana. 3. (3) Although only adults of C. silacea and C. dimidiata were captured at Kumba, no fewer than five species of Chrysops (C. silacea, C. dimidiata, C. longicornis, C. langi and C. griseicollis) were bred from larvae collected at Kumba. Only C. silacea and C. dimidiata were found biting man at Kumba, and it is suggested that the remaining three species feed on hosts other than man, probably monkeys. 4. (4) Although both sexes of Chrysops were bred in normal proportions from pupae, not a single male was noted amongst more than 2,000 C. silacea and C. dimidiata captured at Kumba and at Sapele. It is pointed out that the site at which the males occur is a point of more than academic interest, since it probably determines the route taken by the recently emerged females and this, in its turn, may have a bearing on possible methods of control. 5. (5) It has been shown that C. silacea and C. dimidiata occur in large numbers in the high canopy of the forest surrounding habitations at Kumba, and that they descend to bite man at ground level only when clearings have been made in the forest. It is suggested that at Kumba the forest canopy is their normal habitat and that when man is not available, monkeys are their chief source of blood. On the rubber estates at Sapele where the tree canopy is low and where the undergrowth is not so dense as at Kumba, C. silacea and C. dimidiata can easily perceive man at ground level and readily descend from the canopy to feed on him. 6. (6) Monkeys appear to be rare on the rubber estates at Sapele, but at Kumba they are very numerous, and the canopy dwelling species were found to harbour filariae closely resembling L. loa, but the exact species has not yet been determined. 7. (7) It is shown that Chrysops is a “pool feeder” and that having lacerated the tissues it takes up not less than 10 to 20 c.mm. of blood, when feeding to repletion. The bearing of these observations on the proportion of flies found infected in nature is discussed. 8. (8) Evidence is produced which suggests that the infective larvae of L. loa when deposited by the feeding fly may not be able to penetrate unbroken skin. 9. (9) The life-cycle of L. loa in the vertebrate and in the invertebrate host is discussed. 10. (10) Some of the results of loiasis surveys carried out on the human population at Kumba are recorded. It is shown, by repeated examinations of relatively large quantities of blood (50 c.mm.), that in some persons the micro filariae of L. loa may be found constantly present in large numbers, whilst in others the number found is small and their appearance is intermittent. Although it would appear that the proportion of cases of loiasis without microfilariae in the peripheral blood can be greatly reduced by increasing the intensity of the examination, yet there always remains a residuum of cases amongst those living in a hyperendemic area of loiasis, in which prolonged examinations fail to reveal microfilariae. Various possible explanations of these phenomena are advanced. 11. (11) The reactions of the human host to the presence of the larval and adult stages of L. loa are discussed, with special reference to the occurrence of “Calabar swellings.” It is concluded that the seriousness of loiasis is usually underestimated. 12. (12) Various possible methods of controlling loiasis are discussed, and it is emphasized that at the present time the knowledge available concerning the parasite and its vector is insufficient to warrant the recommendation of any particular method of control on a large scale.