Interruption of Onchocerca volvulus transmission in Northern Venezuela

Abstract

Background: Onchocerciasis is caused by Onchocerca volvulus and transmitted by Simulium species (black flies). In the Americas, the infection has been previously described in 13 discrete regional foci distributed among six countries (Brazil, Colombia, Ecuador, Guatemala, Mexico and Venezuela) where more than 370,000 people are currently considered at risk. Since 2001, disease control in Venezuela has relied on the mass drug administration to the at-risk communities. This report provides empirical evidence of interruption of Onchocerca volvulus transmission by Simulium metallicum in 510 endemic communities from two Northern foci of Venezuela, after 10-12 years of 6-monthly Mectizan (ivermectin) treatment to all the eligible residents.

Methods: In-depth entomologic and epidemiologic surveys were serially conducted from 2001-2012 in selected (sentinel and extra-sentinel) communities from the North-central (NC) and North-east (NE) onchocerciasis foci of Venezuela in order to monitor the impact of ivermectin treatment.

Results: From 2007-2009, entomological indicators in both foci confirmed that 0 out of 112,637 S. metallicum females examined by PCR contained L3 infection in insect heads. The upper bound of the 95% confidence intervals of the infective rate of the vector reached values below 1% by 2009 (NC) and 2012 (NE). Additionally, after 14 (NC) and 22 (NE) rounds of treatment, the seasonal transmission potential (±UL CIs) of S. metallicum was under the critical threshold of 20 L3 per person per season. Serological analysis in school children < 15 years-old demonstrated that 0 out of 6,590 individuals were harboring antibodies to Ov-16. Finally, epidemiological surveys made during 2010 (NC) and 2012 (NE) showed no evidence of microfilariae in the skin and eyes of the population.

Conclusions: These results meet the WHO criteria for absence of parasite transmission and disease morbidity in these endemic areas which represent 91% of the population previously at-risk in the country. Consequently, the two Northern foci are currently under post-treatment onchocerciasis surveillance status in Venezuela.

Figure 1

Map of Venezuela. Map of Venezuela (including topographic layer) showing the nine administrative boundaries (States) where the onchocerciasis has been endemic in the northern region of the country. Red areas over the map represent the two endemic foci extent. Red points over the enlarged States correspond to those communities (sentinel and extra-sentinel) were regular in-depth epidemiological evaluations (EEPs) were conducted in order to monitor the impact of treatment on parasite transmission in both foci.

Figure 2

Coverage rate with ivermectin, northern Venezuela. Coverage rate (percent) with ivermectin of the eligible population of the North-central (a) and North-east (b) foci, Venezuela, 2001–2012. The line at 85% indicates the coverage needed in a sustained way to interrupt transmission.