Vector competence for Onchocerca volvulus in the Simulium (Notolepria) exiguum complex: Cytoforms or density-dependence?

Wetten, Sally; Collins, Richard C.; Vieira, Juan Carlos; Marshall, Clare; Shelley, A. J.; Basáñez, María‐Gloria

doi:10.1016/j.actatropica.2007.05.009

Cited by 9 publications

(4 citation statements)

References 39 publications

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“…This success was achieved in an endemic focus with special challenges for disease control and elimination. First, the dominant vector, S. exiguum , was a particularly voracious and effective transmitter of the parasite [ 8 – 11 , 21 , 22 ], and secondly, the onchocercal skin and eye disease in this focus was arguably the most severe form in the Americas [ 13 , 14 , 18 , 24 ].…”

Section: Discussionmentioning

confidence: 99%

Elimination of onchocerciasis in Ecuador: findings of post-treatment surveillance

Guevara

Lovato²,

Proaño

et al. 2018

Parasites Vectors

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BackgroundThe Esmeraldas focus of onchocerciasis in Ecuador expanded geographically during the 1980s and was associated with severe ocular and skin disease. Mass drug administration (MDA) with ivermectin started in 1991, initially once but later twice a year, in the principle endemic focus followed by all satellite foci. Treatment was stopped in 2009 when entomological assessments determined that transmission of Onchocerca volvulus had been interrupted.MethodsThree years after the cessation of ivermectin treatment in 2012, as defined by the WHO guidelines for onchocerciasis elimination, blackfly collections were done in four sentinel sites in former hyperendemic areas. The presence of infective larvae in local vectors, Simulium exiguum and Simulum quadrivittatum, was assessed by detection of O. volvulus DNA by PCR. Additional flies captured in four extra-sentinel sites located in former hyper- and mesoendemic dispersed isolated areas were also assessed.ResultsThe results from 68,310 captured blackflies, 40,114 from four sentinel villages in the previously hyperendemic areas (Corriente Grande, El Tigre, San Miguel on Río Cayapas and Naranjal on Río Canandé) and 28,197 from extra-sentinel locations, were all negative for the presence of O. volvulus. These extra-sentinel sites (Hualpí on Río Hoja Blanca, Capulí on Río Onzole, La Ceiba on Río Tululví and Medianía on Río Verde) were included to provide additional evidence of the impact of MDA on the transmission of O. volvulus in isolated endemic areas.ConclusionsOur data indicate that transmission of O. volvulus has been stopped in all endemic areas in Ecuador, including all satellite foci outside the main focus. These findings indicate that a strategy of ivermectin distribution twice a year to over 85% of the treatment-eligible population was effective in eliminating the infection from Ecuador in a focus with a highly competent primary vector, S. exiguum, and where the infection rates were equal to or greater than observed in many onchocerciasis foci in Africa.

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Section: Discussionmentioning

confidence: 99%

Elimination of onchocerciasis in Ecuador: findings of post-treatment surveillance

Guevara

Lovato²,

Proaño

et al. 2018

Parasites Vectors

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“…By applying such methods, previous studies have adjusted for the effects of measurement error in microfilarial load when exploring how microfilarial load relates to: a) the uptake of microfilariae by the black fly ( Simulium ) vector [64], [65], and b) the development of larvae within the vector [66]. Standard methods of adjustment were not valid for the analysis presented here because of the overdispersion evident in the response variable (the number of deaths per stratum) and the non-linearity of the hyperbolic and sigmoid dose-response functions.…”

Section: Discussionmentioning

confidence: 99%

Density-Dependent Mortality of the Human Host in Onchocerciasis: Relationships between Microfilarial Load and Excess Mortality

Walker

Little

Wagner³

et al. 2012

PLoS Negl Trop Dis

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BackgroundThe parasite Onchocerca volvulus has, until recently, been regarded as the cause of a chronic yet non-fatal condition. Recent analyses, however, have indicated that in addition to blindness, the parasite can also be directly associated with human mortality. Such analyses also suggested that the relationship between microfilarial load and excess mortality might be non-linear. Determining the functional form of such relationship would contribute to quantify the population impact of mass microfilaricidal treatment.Methodology/Principal FindingsData from the Onchocerciasis Control Programme in West Africa (OCP) collected from 1974 through 2001 were used to determine functional relationships between microfilarial load and excess mortality of the human host. The goodness-of-fit of three candidate functional forms (a (log-) linear model and two saturating functions) were explored and a saturating (log-) sigmoid function was deemed to be statistically the best fit. The excess mortality associated with microfilarial load was also found to be greater in younger hosts. The attributable mortality risk due to onchocerciasis was estimated to be 5.9%.Conclusions/SignificanceIncorporation of this non-linear functional relationship between microfilarial load and excess mortality into mathematical models for the transmission and control of onchocerciasis will have important implications for our understanding of the population biology of O. volvulus, its impact on human populations, the global burden of disease due to onchocerciasis, and the projected benefits of control programmes in both human and economic terms.

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“…In many host-parasite systems, 20% of hosts are responsible for 80% of new infections [16,17]. In vector-borne diseases, parasite overdispersion has been observed both in vertebrate hosts and in vector populations [18][19][20][21][22]. Despite this, studies have mainly focused on the epidemiological consequences of parasite overdispersion for the host, rather than for the vectors [17,23,24].…”

Section: Introductionmentioning

confidence: 99%

Last-come, best served? Mosquito biting order and Plasmodium transmission

et al. 2020

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A pervasive characteristic of parasite infections is their tendency to be overdispersed. Understanding the mechanisms underlying this overdispersed distribution is of key importance as it may impact the transmission dynamics of the pathogen. Although multiple factors ranging from environmental stochasticity to inter-individual heterogeneity may explain parasite overdispersion, parasite infection is also overdispersed in an inbred host population maintained under laboratory conditions, suggesting that other mechanisms are at play. Here, we show that the aggregated distribution of malaria parasites within mosquito vectors is partially explained by a temporal heterogeneity in parasite infectivity triggered by the bites of mosquitoes. Parasite transmission tripled between the mosquito's first and last blood feed in a period of only 3 h. Surprisingly, the increase in transmission is not associated with an increase in parasite investment in production of the transmissible stage. Overall, we highlight that Plasmodium is capable of responding to the bites of mosquitoes to increase its own transmission at a much faster pace than initially thought and that this is partly responsible for overdispersed distribution of infection. We discuss the underlying mechanisms as well as the broader implications of this plastic response for the epidemiology of malaria.

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Vector competence for Onchocerca volvulus in the Simulium (Notolepria) exiguum complex: Cytoforms or density-dependence?

Cited by 9 publications

References 39 publications

Elimination of onchocerciasis in Ecuador: findings of post-treatment surveillance

Elimination of onchocerciasis in Ecuador: findings of post-treatment surveillance

Density-Dependent Mortality of the Human Host in Onchocerciasis: Relationships between Microfilarial Load and Excess Mortality

Last-come, best served? Mosquito biting order and Plasmodium transmission

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