Estimating The Burden Of Parasitic Disease

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Parasitic infections are amongst the most ubiquitous of infections of humans. The first estimates of the global extent of infection suggested that more than a quarter of the world's population was infected with one or more important pathogen at any one time (Stoll, 1947) and the latest estimates suggest the same conclusion (Chan et al, 1994a; Michael et al., 1996; Bundy, 1998; WHO, 1997). In Table 2.2 we show the most current estimates of some of the major parasitic infections of humans.

But simply measuring prevalence only tells part of the story. The important variable from a human and public health perspective is not the mere presence of infection, but the burden of death and disability with which it is associated. In order to estimate this, a recent study sponsored by the World Bank has led to the development of a technique that combines information regarding both morbidity and mortality from a specific disease into a single measure (World Bank, 1993). This health indicator is termed the 'disability-adjusted life year' (DALY) and has gained usage not only to provide standardized assessments of the health burden of specific diseases but also to allow comparisons of the impact on public health (both morbidity and mortality) of the various packages of health

Fig 2.4 Simulated results from a mathematical model of helminth vaccination programs. The upper two figures show predicted age profiles of worm burden generated by assuming that contact with infection is greatest in the early teenage years. Mass vaccination at different ages is modelled. Vaccine protection is assumed to be complete initially but to decay at a rate specified by the inverse of the average vaccine duration. The effect of vaccination is to reduce the rate of infection. The upper figure shows vaccination in 1 year-old children. The top (bold) line shows the age profile in the absence of vaccination and the light lines show vaccination with average vaccine duration of 2, 5, 10, 15, 20 and 30 years, respectively, from top to bottom. The middle figure is identical, except that vaccination occurs at age 5. The lower figure shows the percentage reduction in mean worm burden achieved by the programs as a function of the average vaccine duration with vaccination at age 1 and 5 (modified from Bundy et al., 1995)

Table 2.2 Estimated global prevalence of the major human helminth infections (in millions)

Helminth

Sub-Saharan Africa

Other Asia and Islands

India

China

Latin Americaa

Middle Eastern crescent

Total

Ascaris lumbricoides

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