Transmission intensity and the patterns of Onchocerca volvulus infection in human communities.

Abstract

Abstract.We focus on possible constraints upon Onchocerca volvulus establishment in humans in relation to exposure rates to infective larvae (L3) as measured by the annual transmission potential (ATP). We use mathematical and statistical modeling of pre-control west African (savanna), Mexican, and Guatemalan data to explore two hypotheses relating human infection to transmission intensity: microfilarial (mf) loads either saturate with increasing ATP or become (asymptotically) proportional to the ATP. The estimated proportion of L3 developing into adult worms ranged from 7% to 0.3% (low and high intensity areas, respectively). Relationships between mf prevalence and both mf and transmission intensity were nonlinear and statistically similar between west Africa (Simulium damnosum s.l.) and Meso America (S. ochraceum s.l.). This similarity extended to the relationship between mf intensity and ATP. The critical biting rates for onchocerciasis introduction and persistence (which depended on vector competence and host preference), were approximately 10-fold higher in settings where onchocerciasis is transmitted by S. ochraceum than in those where the vector is S. damnosum. A role for focal vector control in Mexico and Guatemala, in addition to nodulectomy and ivermectin, is suggested. INTRODUCTIONAt the start of the 21st century onchocerciasis or "river blindness" still poses a threat to public health in many tropical regions of Africa and more focally in Latin America despite a concerted research effort and high expenditure on control programs. Although much research has been conducted on the biology and epidemiology of the host-parasite interaction, relatively little is known about the population dynamics of the parasite in terms of the key regulatory constraints on population growth. In an attempt to further understanding of such processes, a number of researchers have explored the properties of simple and complex mathematical models of the transmission dynamics of human onchocerciasis.1 Considerable progress has been made concerning the regulatory constraints operating within the vector, 2-6 with some progress being recently made in investigating those acting within the human host.

Human infection patterns and heterogeneous exposure in river blindness

“…Preintervention entomological indices (biting and infection rates) have been collated for Simulium ochraceum s.l. in the Guatemalan fincas (29) and reported for Simulium guianense s.l. and Simulium incrustatum in mesoendemic and hyperendemic Amazonian areas (30)(31)(32).…”

“…We have used a structured and full-lifecycle transmission model to explain such profiles across continents. Most models focus on onchocerciasis in West Africa (13)(14)(15)(16)(17), with some models focusing on Latin America (19), but few attempt to bring epidemiological commonalities and differences into a single and coherent mathematical framework (29). Age-specific exposure to vector bites (13)(14)(15)(16), operation of parasite-related human mortality (13,15), and parasite-induced immunosuppression (12,17,26) are among the mechanisms put forward to explain observed departures Observations of ABR and L3 load are independent from the data to which the model has been fitted.…”

“…Regulatory constraints (nonlinearities) are present in both humans and vectors via ⌸ H , ⌸ V and L (22,29). At endemic equilibrium, an explicit solution is obtained (and used for efficient estimation) by ignoring density-dependent parasite uptake by vectors, i.e., with ⌸ V constant and parasite abundance within flies regulated solely through parasite-induced vector mortality.…”

“…For simplicity and tractability, we included a reduction in parasite establishment in humans with exposure to L3 larvae (22,29) and parasite-induced vector mortality but not density-dependent parasite uptake by vectors (22). The various density-dependent mechanisms, by acting on different processes in the parasite's life cycle, may differentially influence rates of reinfection after treatment (Churcher, T. S., J.A.N.F.…”

The impact of Ivermectin mass treatment on the prevalence of onchocerciasis by rapid epidemiological assessment in Galadimawa, Kaduna state, Nigeria

“…Onchocerciasis (river blindness) still poses a threat to public health in many tropical regions of Africa and Latin America, despite the concerted effort and high expenditure on control programmes (Basanez et al 2002). So far, the control measures that have been implemented were aimed at interrupting transmission of the Onchocerca volvulus (Hougard et al 1997) parasite either by eradicating the vector or the parasite.…”