Epidemiology of STH Infections and Schistosomiasis
The most striking epidemiological features of human helminth infections are aggregated distributions in human communities, predisposition of individuals to heavy (or light) infection, rapid reinfection following chemotherapy, and age-intensity profiles that are typically convex (with the exception of hookworm).
For all the major human STH and schistosome infections studied to date, worm burdens exhibit a highly aggregated (overdispersed) distribution so that most individuals harbor just a few worms in their intestines, although a few hosts harbor disproportionately large worm burdens (Anderson and May 1991). As a rule, 20 percent of the host population harbors approximately 80 percent of the worm population. This overdispersion has many consequences, both with regard to the population biology of the helminths and the public health consequence for the host, because heavily infected individuals are simultaneously at highest risk of disease and the major source of environmental contamination. One feature that may help explain overdispersion is that individuals tend to be predisposed to heavy (or light) infections. Predisposition has been demonstrated for all four major STHs and the schistosomes. The underlying cause of such predisposition remains poorly understood. However, a combination of heterogeneity in exposure to infection or differences in susceptibility to infection and the ability to mount effective immunity (genetic and nutritional factors) is likely to be important.
People of all ages rapidly reacquire infection following treatment, but in schistosomiasis, older people reacquire infection at slower rates than younger ones (Kabatereine and others 1999). The rate of reinfection is specific to certain species of helminths and depends on the life expectancy of that species (short-lived helminths reinfect more rapidly), on the intensity of transmission within a given community, and on the treatment efficacy and coverage. The basic reproductive rate (Ro)describes the transmission potential of a parasite (and thus its ability to reinfect the host). It defines the average number of female offspring produced during the life span of the parasite that survive to reproductive maturity in the absence of density dependence.Ro is determined by parasite immigration anddeath rates as well as by host density (and, in schistosomiasis, also snail density). A parasite will fail to become established unless Ro is greater than unity (Anderson and May 1991). Adult worms usually survive between one and four years, whereas eggs can sometimes remain viable for several more years in the environment. Therefore, reinfection rates will remain high until adults are removed with chemotherapy and until infective stages, through time, become uninfective. In reality, density-dependent processes regulate parasite populations; at endemic equilibrium, the effective reproductive ratio equals unity (that is, each female replaces herself). Control programs rely on reducing the effective reproductive ratio long enough for the parasite population to be driven to local elimination. Theoretically,Ro provides useful insights, and it is helpful to think of control programs attempting to break the transmission cycle by reducing to less than unity. Therefore,Ro estimates can be made about how long and how many rounds of chemotherapy are required to treat intestinal helminths. For example, A. lumbricoides with an of three and a life Roexpectancy of one year will need to be treated annually with a drug that is 95 percent efficacious and with coverage of more than 91 percent of the population. Where Ro is five—that is,in areas where transmission is higher—treatment must be given more frequently than once a year (Anderson and May 1991).
The age-dependent patterns of infection prevalence are generally similar among the major helminth species, exhibiting a rise in childhood to a relatively stable asymptote in adulthood (figure 24.1). Maximum prevalence of A. lumbricoides and T. trichiura is usually attained before five years of age, and the maximum prevalence of hookworm and schistosome infections is usually attained in adolescence or in early adulthood. The nonlinear relationship between prevalence and intensity has the consequence that the observed age-prevalence profiles provide little indication of the underlying profiles of age intensity (age in relation to worm burden). Because intensity is linked to morbidity, the age-intensity profiles provide a clearer understanding of which populations are vulnerable to the different helminths (figure 24.1). For A. lumbricoides and T. trichiura infections, the age-intensity profiles are typically convex in form, with the highest intensities in children 5 to 15 years of age (Bundy 1995). For schistosomiasis, a convex pattern is also observed, with a similar peak but with a plateau in adolescents and young adults 15 to 29 years of age (Kabatereine and others 1999). In contrast, the age-intensity profile for hookworm exhibits considerable variation, although intensity typically increases with age until adulthood and then plateaus (Brooker, Bethony, and Hotez 2004). In East Asia it is also common to find the highest intensities among the elderly. However, more generally, children and young adults are at higher risk of both harboring higher levels of infection (thus greater levels of morbidity) and becoming reinfected more quickly. Both may occur at vital stages in a child's intellectual and physical development.
Both host-specific and environmental factors have been identified that may affect the risk of acquiring or harboring heavy-intensity helminth infections.
No genes that control for human helminth infection have yet been identified. However, recent genome scans have identified a locus possibly responsible for controlling S. mansoni infection intensity on chromosome 5q31-33 and loci controlling A. lumbricoides intensity on chromosomes 1 and 13. There is also evidence for genetic control of pathology attributable to S. mansoni, with linkage reported to a region containing the gene for the interferon gamma receptor 1 subunit (Quinnell 2003).
Behavior, Household Clustering, and Occupation
Specific occupations, household clustering, and behaviors influence the prevalence and intensity of helminth infections (Bethony and others 2001), particularly for hookworm, in which the highest intensities occur among adults (Brooker, Bethony, and Hotez 2004). Engagement in agricultural pursuits, for example, remains a common denominator for hookworm infection. Behavioral and occupational factors, through their effect on water contact, interact with environmental factors to produce variation in the epidemiology of schistosomiasis.
Poverty, Sanitation, and Urbanization
STH and schistosomiasis depend for transmission on environments contaminated with egg-carrying feces. Consequently, helminths are intimately associated with poverty, poor sanitation, and lack of clean water. The provision of safe water and improved sanitation are essential for the control of helminth infection. Although the STH and schistosome infections are neglected diseases that occur predominantly in rural areas, the social and environmental conditions in many unplanned slums and squatter settlements of developing countries are ideal for the persistence of A. lumbricoides (Crompton and Savioli 1993). Schistosomiasis transmission can also occur in urban areas.
Climate, Water, and Season
Adequate warmth and moisture are key features for each of the STHs. Wetter areas exhibit increased transmission, and in some endemic areas, both STH and schistosome infections exhibit marked seasonality (Brooker and Michael 2000). Recent use of geographical information systems and remote sensing has identified the distributional limits of STH and schistosomes on the basis of temperature and rainfall patterns (Brooker and Michael 2000). For schistosomiasis, specific snail intermediate hosts prefer certain types of aquatic environments. Construction of dams is known to extend the range of snail habitats, thereby promoting the reemergence of schistosomiasis.