Polymerase chain reaction monitoring of transmission of Onchocerca volvulus in two endemic states in Mexico.

To investigate the impact of mass ivermectin treatments in Mexico on Onchocerca volvulus transmission, entomologic surveys were carried out in the two endemic states of Oaxaca and Chiapas. The data suggest that substantial progress towards the goal of elimination has been made. A comparison pre- and post-ivermectin data from a community in Southern Chiapas showed a 97% decrease in seasonal transmission potential, but some level of polymerase chain reaction positivity was still detectable. In other communities from northern Chiapas and Oaxaca where there are no baseline data, there was an absence or near absence of infective flies. Residual transmission was not evenly distributed because differences were seen in the infection and infective rates from different catch points. These findings suggest that while substantial progress towards elimination has been made in Mexico, it may be necessary to modify ivermectin distribution strategies to eliminate transmission in areas where transmission persists.

Entomological evaluation by pool screen polymerase chain reaction of Onchocerca volvulus transmission in Ecuador following mass Mectizan distribution.

The prevalence of infected and infective black flies was estimated by pool screen polymerase chain reaction (PCR) amplification in the three river basins in Ecuador endemic for onchocerciasis. Mass distribution of ivermectin (Mectizan) resulted in dramatic declines in the prevalence of infected and infective flies. In the Rio Santiago river basin, no infections were detected, suggesting that transmission had ceased. The ratio of infected to infective flies in Simulium exiguum was 10-fold lower than the corresponding ratio for Simulium quadrivittatum, suggesting that S. exiguum is a more-competent vector for Onchocerca volvulus than S. quadrivittatum. However, the prevalence of infective flies in the two species was not different, suggesting that S. quadrivittatum may play an important vectorial role where it is the dominant human-biting species. The data demonstrate that pool screen PCR is an efficient way to monitor transmission in areas subject to control, and to certify an area as free of O. volvulus transmission.

Simultaneous detection of three mosquito-borne encephalitis viruses (eastern equine, La Crosse, and St. Louis) with a single-tube multiplex reverse transcriptase polymerase chain reaction assay.

Three mosquito-borne human encephalitis viruses (eastern equine encephalitis virus [EEE], St. Louis encephalitis virus [SLE], and La Crosse encephalitis virus [LAC]) are sympatric in the southeastern United States. However, little is known concerning the temporal and spatial pattern of the distribution of these viruses in this area. As part of surveillance activities to detect the transmission of these 3 viruses in the Tennessee Valley area, we developed a single-tube multiplex reverse transcriptase polymerase chain reaction (RT-PCR) assay capable of detecting these 3 mosquito-borne viruses in a single reaction. Three viruses were differentiated by size of amplified products. Sensitivities of the multiplex RT-PCR assay for SLE, EEE, and LAC were 1-3 log median tissue culture infective doses per pool, roughly comparable to the reported sensitivity of PCR detection assays for the individual viruses, and 1 log more sensitive than antigen-capture assays for SLE and EEE. The sensitivity of the multiplex PCR was not changed significantly when carried out in the presence of extracts prepared from 50 uninfected mosquitoes. The cost of the assay is estimated at $2.98 per test, similar to the cost of other RT-PCR-based assays for viruses. However, adaptation of the RT-PCR to a multiplex format adds less than $0.01 to the per-unit cost of an RT-PCR assay targeting a single virus species. Analysis of these data suggests that the single-tube multiplex RT-PCR assay represents a sensitive, specific, cost-effective, and rapid method for monitoring activities of the 3 endemic mosquito-borne human encephalitis viruses in mosquito populations in the southeastern United States.