Interruption of infection transmission in the onchocerciasis focus of Ecuador leading to the cessation of ivermectin distribution

Abstract

Introduction: A clinically significant endemic focus of onchocerciasis existing in Esmeraldas Province, coastal Ecuador has been under an ivermectin mass drug administration program since 1991. The main transmitting vector in this area is the voracious blackfly, Simulium exiguum. This paper describes the assessments made that support the decision to cease mass treatment.

Methodology and principle findings: Thirty-five rounds of ivermectin treatment occurred between 1991-2009 with 29 of these carrying >85% coverage. Following the guidelines set by WHO for ceasing ivermectin distribution the impact on parasite transmission was measured in the two vector species by an O-150 PCR technique standard for assessing for the presence of Onchocerca volvulus. Up to seven collection sites in three major river systems were tested on four occasions between 1995 and 2008. The infectivity rates of 65.0 (CI 39-101) and 72.7 (CI 42-116) in 1995 dropped to zero at all seven collection sites by 2008. Assessment for the presence of antibodies against O. volvulus was made in 2001, 2006, 2007 and 2008 using standard ELISA assays for detecting anti-Ov16 antibodies. None of total of 1810 children aged 1-15 years (between 82 and 98% of children present in the surveyed villages) tested in the above years were found to be carrying antibodies to this antigen. These findings were the basis for the cessation of mass drug treatment with ivermectin in 2009.

Significance: This fulfillment of the criteria for cessation of mass distribution of ivermectin in the only known endemic zone of onchocerciasis in Ecuador moves the country into the surveillance phase of official verification for national elimination of transmission of infection. These findings indicate that ivermectin given twice a year with greater than 85% of the community can move a program to the final stages of verification of transmission interruption.

Figure 1. Map of the endemic area and locations of the entomological and serological surveys carried out between 1995 and 2008.

1- Playa de Oro, 2 - Angostura, 3 -Guayabal, 4 -Agua Salada, 5 -Pichiyacu, 6 - Playa Grande, 7- Herradura, 8 -San Miguel, 9- El Tigre, 10-Agua Blanca, 11- Callemansa, 12 - Corriente Grande, 13 – Colón de Onzole, 14 - Calpulí, 15 - Las Pavas, 16- Naranjal.

Figure 2. Drug distribution coverage for the Onchocerciasis Program.

Percentage (%) of eligible people treated every 6 months from 1991–2009. Line indicates a 85% coverage level.