The epidemiology of dermal leishmansiasis in British Honduras Part III. The transmission of Leishmania mexicana to man by Phlebotomus pessoanus, with observations on the development of the parasite in different species of Phlebotomus

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Abstract
During 1960-62 a study on the epidemiology of dermal leishmaniasis due to Leishmania mexicana in British Honduras revealed 18 species of Phlebotomus in the enzootic forest areas. In addition, 7 other sandflies remain to be identified and may represent new species. Nine species of Phlebotomus were found commonly to bite maa All were predominantly nocturnal in their feeding habits and, as they were readily infected with L. mexicana, all must be regarded as potential vectors in nature. The infection rates for sandflies fed directly on hamster lesions, at the periphery, and on normal adjacent skin were 95%, 48% and 0% respectively. Maintenance of wild-caught Phlebotomns species is discussed. Best results were obtained by keeping single flies in corked tubes containing a fresh green leaf. The corks had a groove cut throughout their length, to allow air exchange, and all tubes were kept in constant light and at approximately 100% relative humidity. Three hundred and thirty-two sandflies, including all the known man-biting species from British Honduras, were fed on the lesions of hamsters and mice infected with both human and rodent strains of L. mexicana 52 flies were induced to re-feed on volunteers (8 fed a 2nd time and 1 a 3rd time), in all inflicting a total of 90 probes. Transmission of _L. mexicana to man was achieved, by Phlebotomus pessoanus, on one occasion. This insect had fed on the infected hamster only 3 days and 23 hr. previously. The development of _L. mexicana in the insect host has been followed by a study of the gut contents and sections of entire sandflies which were fixed at 3,6,12,24,36, and 48 hr. and 3-5 days after their infective feed. The development is to an anterior station, the leptomonads reaching the proboscis as early as 4 days after the infecting blood meal. A total of 439 wild-caught female sandflies were dissected in an attempt to find insects naturally infected with L. mexicana. Flagellates were seen in the hind-gut of one specimen of P. ovallesi and others in the crushed gut from a single P. cruciatus (exact position in the gut uncertain). In the absence of animal inoculation the nature of these flagellates remains uncertain, but they are not thought to be leptomonads of L. mexicana. Attempts to demonstrate mechanical transmission otlL. mexicana were made by interrupting sandflies feeding on hamster lesions and immediately allowing such insects to renew feeding on volunteers. No transmission was achieved and it is felt that such a mode of transfer plays little or no part in the transmission of the parasite in nature. On the contrary, the killing and squashing of infected sandflies feeding on human skin may well facilitate entry of the parasite. Although sandflies were successfully infected withL. mexicana after feeding on a newly acquired and non-ulcerative human lesion, man-to-man transmission is considered most unlikely in nature.

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