Demographic patterns of human antibody levels to Simulium damnosum s.l. saliva in onchocerciasis-endemic areas: An indicator of exposure to vector bites.

Laura Willen,Orin Courtenay,Philip Milton,Petr Volf,Jonathan I D Hamley,Maria-Gloria Basáñez,Martin Walker,Mike Y Osei-Atweneboana

doi:10.1371/journal.pntd.0010108

Abstract

In onchocerciasis endemic areas in Africa, heterogenous biting rates by blackfly vectors on humans are assumed to partially explain age- and sex-dependent infection patterns with Onchocerca volvulus. To underpin these assumptions and further improve predictions made by onchocerciasis transmission models, demographic patterns in antibody responses to salivary antigens of Simulium damnosum s.l. are evaluated as a measure of blackfly exposure. Recently developed IgG and IgM anti-saliva immunoassays for S. damnosum s.l. were applied to blood samples collected from residents in four onchocerciasis endemic villages in Ghana. Demographic patterns in antibody levels according to village, sex and age were explored by fitting generalized linear models. Antibody levels varied between villages but showed consistent patterns with age and sex. Both IgG and IgM responses declined with increasing age. IgG responses were generally lower in males than in females and exhibited a steeper decline in adult males than in adult females. No sex-specific difference was observed in IgM responses. The decline in age-specific antibody patterns suggested development of immunotolerance or desensitization to blackfly saliva antigen in response to persistent exposure. The variation between sexes, and between adults and youngsters may reflect differences in behaviour influencing cumulative exposure. These measures of antibody acquisition and decay could be incorporated into onchocerciasis transmission models towards informing onchocerciasis control, elimination, and surveillance.

Highlights

Female blackflies of the Simulium damnosum sensu lato (s.l.) complex are the predominant vectors of Onchocerca volvulus in Africa
Repeat exposure to bites of infective blackflies is a key driver of parasite acquisition, and high biting rates are important determinants of transmission intensity and resurgence following control interventions, those based on mass drug administration (MDA) of ivermectin [2,3]
human landing catches (HLCs) methods likely lead to biased estimates of ABRs as they are typically performed on a few adult males at the riverbank close to blackfly breeding sites, where biting rates are likely to be higher and vector collectors are maximally exposed [7]

Summary

Introduction

Female blackflies of the Simulium damnosum sensu lato (s.l.) complex are the predominant vectors of Onchocerca volvulus in Africa This filarial nematode causes human onchocerciasis, commonly known as river blindness and proposed for elimination of transmission (EOT) in 12 countries by 2030 [1]. In onchocerciasis endemic areas in Africa, heterogenous biting rates by blackfly vectors on humans are assumed to partially explain age- and sex-dependent infection patterns with Onchocerca volvulus. To underpin these assumptions and further improve predictions made by onchocerciasis transmission models, demographic patterns in antibody responses to salivary antigens of Simulium damnosum s.l. are evaluated as a measure of blackfly exposure.

Objectives

Methods

Results

Discussion

Conclusion