Potential effects of warmer worms and vectors on onchocerciasis transmission in West Africa

Abstract

Development times of eggs, larvae and pupae of vectors of onchocerciasis (Simulium spp.) and of Onchocerca volvulus larvae within the adult females of the vectors decrease with increasing temperature. At and above 25°C, the parasite could reach its infective stage in less than 7 days when vectors could transmit after only two gonotrophic cycles. After incorporating exponential functions for vector development into a novel blackfly population model, it was predicted that fly numbers in Liberia and Ghana would peak at air temperatures of 29°C and 34°C, about 3°C and 7°C above current monthly averages, respectively; parous rates of forest flies (Liberia) would peak at 29°C and of savannah flies (Ghana) at 30°C. Small temperature increases (less than 2°C) might lead to changes in geographical distributions of different vector taxa. When the new model was linked to an existing framework for the population dynamics of onchocerciasis in humans and vectors, transmission rates and worm loads were projected to increase with temperature to at least 33°C. By contrast, analyses of field data on forest flies in Liberia and savannah flies in Ghana, in relation to regional climate change predictions, suggested, on the basis of simple regressions, that 13–41% decreases in fly numbers would be expected between the present and before 2040. Further research is needed to reconcile these conflicting conclusions.

Factors affecting onchocerciasis transmission: lessons for infection control

“…Thus, some of the most important factors in transmission are sidelined and the biggest threat to onchocerciasis control programmes, reinvasion of infected vectors from outside controlled zones, is ignored. It is expected that future modelling work will remedy some of these omissions by including vector dynamics, following initial work linking them to EPIONCHO [104].…”

Onchocerciasis transmission in Ghana: biting and parous rates of host-seeking sibling species of the Simulium damnosum complex

“…Models of O. volvulus transmission dynamics that investigate intervention outcomes are mostly parameterised using S. damnosum s.s./S. sirbanum [18][19][20][21][22][23][24][25][26], with the exception of the model by Davies (1993) [27], based on transmission of forest onchocerciasis by S. soubrense B sensu Post, some quantitative analyses on other S. damnosum complex species including S. leonense and S. squamosum B [28,29], and a recent modelling study of the effect of climate change on O. volvulus transmission in Ghana and Liberia, including S. soubrense [30]. This research gap will need addressing in areas where different species compositions exist in the human-biting blackfly population.…”

Climate Change and the Neglected Tropical Diseases

“…Reviews of blackfly ecology and the relation between vector and parasite are available in specific contexts (e.g. see Cheke et al, 2015 for a modelling perspective; Takaoka et al, 1982 for a review specific to Guatemala). Here I summarise work that has been conducted on the potential impact of climate change in particular.…”