Volume 39, Issue 6
  • ISSN: 0002-9637
  • E-ISSN: 1476-1645



Transmission of at 4 locations with different prevalences of human onchocerciasis in the Atitlán region of Guatemala is described in relation to vector density and infection rates. The percentages of residents with skin biopsies positive for microfilariae of at these locations were 13.8%, 33.3%, 65.4%, and 89.6%. The following variables associated with transmission were calculated from our observations (the values are presented in an order that corresponds with the above prevalence rates): frequency of third-stage larvae (calculated on an annual basis) in parous , 0, 0.004, 0.005, and 0.004; estimated daily biting density of , 23, 24, 254, and 1,509 flies per day; and estimated annual infective biting density (based on ), 0, 18, 185, and 1,101 potentially infective bites per year. The frequencies of third-stage larvae are very small compared with those observed in Africa, and suggest that transmission of in Guatemala depends on high vector density. Locations with low, and perhaps tolerable, levels of onchocerciasis (<15% of female residents with skin biopsies positive for microfilariae) have mean daily biting densities for of ≤24 flies, and infected residents normally have mean microfilarial densities of ≤3 microfilariae per mg of skin. Stratification of prevalence rates by age group proved useful for assessing current transmission within a village.


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