Elimination of Onchocerciasis from Mexico

Abstract

Background: Mexico is one of the six countries formerly endemic for onchocerciasis in Latin America. Transmission has been interrupted in the three endemic foci of that country and mass drug distribution has ceased. Three years after mass drug distribution ended, post-treatment surveillance (PTS) surveys were undertaken which employed entomological indicators to check for transmission recrudescence.

Methodology/principal findings: In-depth entomologic assessments were performed in 18 communities in the three endemic foci of Mexico. None of the 108,212 Simulium ochraceum s.l. collected from the three foci were found to contain parasite DNA when tested by polymerase chain reaction-enzyme-linked immunosorbent assay (PCR-ELISA), resulting in a maximum upper bound of the 95% confidence interval (95%-ULCI) of the infective rate in the vectors of 0.035/2,000 flies examined. This is an order of magnitude below the threshold of a 95%-ULCI of less than one infective fly per 2,000 flies tested, the current entomological criterion for interruption of transmission developed by the international community. The point estimate of seasonal transmission potential (STP) was zero, and the upper bound of the 95% confidence interval for the STP ranged from 1.2 to 1.7 L3/person/season in the different foci. This value is below all previous estimates for the minimum transmission potential required to maintain the parasite population.

Conclusions/significance: The results from the in-depth entomological post treatment surveillance surveys strongly suggest that transmission has not resumed in the three foci of Mexico during the three years since the last distribution of ivermectin occurred; it was concluded that transmission remains undetectable without intervention, and Onchocerca volvulus has been eliminated from Mexico.

Fig 1. Location, number of cases, and population at risk when ivermectin distribution began at the study sites: Panel A: Map of the Southern Mexico states showing the three endemic foci for onchocerciasis. Panels B-D: the eighteen sentinel and extra-sentinel communities in Oaxaca (Panel B; prevalence data from 1995), Northern Chiapas (Panel C; prevalence data from 1999), and Southern Chiapas (Panel D; prevalence data from 1995).

Fig 2. The onchocerciasis communities under post-treatment surveillance (PTS) phase in Mexico: Panel A: Map of the Southern Mexico states showing the two endemic States for onchocerciasis. A total of 98 communities were under PTS phase in the Oaxaca focus (Panel B). In addition, 13 and 559 communities were also under PTS phase in the Northern (Panel C) and Southern Chiapas foci, respectively (Panel D).

Fig 3. Mass drug administration (MDA) with ivermectin in two endemic foci: Coverage rate, expressed in percent, of the eligible population.

The eligible population excluded pregnant and lactating women and children under 5 years of age. These groups represented 9.5% of the total population during the last year of MDA in the Southern Chiapas focus in 2011. Panel A: Semi-annual treatment regimen was employed in Northern Chiapas from 2000 through 2007 when MDA ceased. Panels B-C. Semi-annual regimen was employed in Southern Chiapas from 1995 through 2011 (Panel B; Figure taken from Rodriguez-Perez et al., 2013 [18]); in addition, quarterly treatment was employed in 50 communities from 2003 through 2008 and in 163 communities from 2009 through 2011 when MDA ceased (Panel C). The semi-annual treatment regimen was employed in Oaxaca from 1996 through 2008 when MDA ceased (see Fig 2 in Rodriguez-Perez et al., 2010 [20]). The line at 85% indicates the coverage needed to be maintained in order to interrupt transmission.

Fig 4. Number of new clinical cases in two endemic foci: The number of new clinical cases, (individuals diagnosed for the first time as positive by Mazzotti reaction, nodules, or skin biopsies) in the Oaxaca focus (Panel A) in the Southern Chiapas focus (Panel B; Figure taken from Rodriguez-Perez et al., 2013 [18]).

The number of new clinical cases in the Northern Chiapas focus can be found in Fig 2 of Rodriguez-Perez et al., 2010 [19].