Academic journal article Bulletin of the World Health Organization

The Risk and Dynamics of Onchocerciasis Recrudescence after Cessation of Vector Control

Academic journal article Bulletin of the World Health Organization

The Risk and Dynamics of Onchocerciasis Recrudescence after Cessation of Vector Control

Article excerpt

Introduction

The objective of the Onchocerciasis Control Programme in West Africa (OCP) is to control onchocerciasis as a disease of public health and socioeconomic importance and to ensure that there will be no recrudescence thereafter [1]. The strategy has been to interrupt transmission by vector control through larviciding, and this has been very successful. After 8-10 years of control operations, onchocerciasis was no longer a public health problem in more than 90% of the original OCP area [2], and after 12-14 years the prevalence of infection had fallen to very low levels or zero in most of this area [3, 4]. Because of the major decline in the parasite reservoir and the high costs of aerial larviciding, it was important to determine how many years of successful vector control were needed before the operations could be discountinued, and the vector allowed to return, without running an unacceptable risk of onchocerciasis recrudescence thereafter.

The risk of recrudescence depends on the interaction between many factors. This made it necessary to use an epidemiological model to study the required duration of vector control. The computer simulation model of ONCHOSIM [5], which has been developed to analyze epidemiological trends and to evaluate prospectively alternative control strategies, simulations the transmission of onchocerciasis and the effects of vector control and chemotherapy [6, 7]. Using this model, we have investigated the risk and dynamics of recrudescence after different periods of vector control in an onchocerciasis focus. Reported is the impact of the major confounding model parameters on recrudescence and on the recommendations about the required duration of vector control.

Materials and methods

Simulation of recrudescence

The ONCHOSIM model uses the technique of microsimulation; this involves the simultaneous simulation of the life-histories of individual persons and of individual female and male parasites in the human cost [5]. Collectively, the simulated persons constitute the population of a hypothetical endemic focus. One of the most important outputs of the simulation is the microfilarial load in skin-snips for each member of the population. In order to facilitate detailed comparison with observed data, the results of the simulation are presented in the same statistical format that is used for the epidemiological evaluation of vector control in the OCP.

Fig. 1 illustrates the results obtained with ONCHOSIM for the simulation of a period of vector control and recrudescence thereafter. Soon after the start of control, the community microfilarial load (CMFL), i.e., the geometric mean number of microfilariae per skin-nip (mf/s) in adults [8], starts to decrease, followed by the mf prevalence. When vector control is interrupted after 11 years, new inoculated worms become productive after a delay of several years, and the trends reverse; the mf prevalence starts rising first, followed by the CMFL much later.

Confounding model parameters

The transmission and control of onchocerciasis are governed by many parameters, most of which also influence the risk and dynamics of recrudescence. The following parameters are very important in this respect: the pre- and postcontrol level of exposure to the bites of Simulium spp. and the exposure variation in a community; the life span of the Onchocerca volvulus worm and possible variations in this; and the infection level of biting flies as a function of the human skin mf density. Estimates for these important confounding parameters are discussed below. They are based on epidemiological data collected by the OCP, especially the frequency distributions of skin-snip counts obtained at various intervals since the start of control, and on experimental results.

Exposure level (relative biting rate) and

exposure heterogeneity

After a given period of vector control, communities with a high precontrol endemicity level, and hence a high biting rate per person, have a higher risk of recrudescence than communities in less endemic areas. …

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