Sylvan yellow fever in Central America

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The mosquitos of the Kaimosi Forest, Kenya Colony, with special reference to yellow fever

1. The danger of the establishment of yellow fever in certain of the forests of Kenya is pointed out. Kenya has had two recognised cases of the disease in recent years.

2. The Kaimosi forest was selected as a suitable area for investigation and the general environmental conditions are described.

3. A mosquito survey was undertaken over a period of two years and particular attention was paid to the question of “acrodendrophily” i.e., the preference shown by certain species of mosquitos for life in the tree-tops.

4. Five species of mosquitos—Culex nebulosus, Aëdes capensis, A. fraseri, A. deboeri subsp. demeilloni and A. longipalpis breed in high tree-holes; the last, almost entirely so. The many characteristically low breeders include A. aegypti and A. africanus. There is an interesting distinction between the two commoner Megarhines; M. brevipalpis breeds high, M. barbipes, low.

5. Four species of mosquitos—A. apicoargenteus, A. fraseri, A. africanus and A. longipalpis were found biting in the tree-tops; the last two being rare in ground catches.

6. Forest with a broken canopy proved best for adult catching and the morning was a better time than the (usually overcast) afternoon. Night searches gave negative results. Catches on high tree platforms appeared to be more prolific than those oa the ground.

7. The following points of special interest in regard to individual species of mosquitos are summarised below: —

(i) Megarhine larvae consume about 20 fourth-stage aegypti larvae a night. Pupal stage lasts on an average for nine days.

(ii) Aëdes aegypti breeds commonly in two sites (a) recently felled trees at forest edge; (b) rock pools in rivers in drought periods. Larvae are found a mile or more inside the forest. Breeding of this species has been discovered as high as 6,500 feet.

(iii) Aëdes apicoargenteus was the commonest adult. Some specimens survived 55 days in captivity.

(iv) Aëdes africanus shows a seasonal incidence, being commonest at the end of the “long rains”. Its favoured breeding places are the buttresses of the tree, Craibia elliottii.

(v) In the larval stage, Aëdes capensis is the commonest mosquito.

(vi) The characteristic breeding place of Theobaldia fraseri contained 79 parts of oxidisable matter per 100,000 of water.

8. Probably at least two trees per acre have rot holes with larvae and about 14 per cent, of the Dracaena plants are infested. Holes in shaded granite boulders are another common source of sylvan mosquitos.

9. Surveys of adjacent forest areas at different altitudes revealed striking changes in the mosquito fauna. Aëdes pulchrithorax is the prevalent species at the 6,500 foot level; A. kapretwae at the 8,500 foot; whilst in general, A. africanus, A. aegypti and A. bambusae appear to be widespread.

10. A study of the lists of mammals, trees and mosquitos indicates that the relict forest of Kaimosi contains numerous “western” species which are not found elsewhere so far to the east.

11. Immunity tests performed at the Yellow Fever Research Institute, Entebbe, demonstrated the absence of yellow fever in man and monkeys at Kaimosi. The monkeys show, however, immunity to Semliki forest virus and either Japanese B or St. Louis encephalitis virus.

12. The human inhabitants suffer from malaria and onchocerciasis (the latter carried by Simulium neavei) and the monkeys and fruit bats are infected with species of Plasmodium.

13. The Kaimosi forest from every standpoint (including meteorological) is suitable for the establishment of jungle yellow fever and possible measures of control ate briefly suggested.

14. Short descriptions of new species, etc., of mosquitos are given in the Appendix