Spatial models of helminth transmission and control
infection, helminth infections, helminths
ONCHOCERCIASIS, HELMINTH INFECTIONS, HOOKWORM, MAY, INFECTION, WORMS, LYMPHATIC FILARIASIS
Human helminth infections enter a human host as larval stages, reach maturity within the human host and reproduce sexually to release further transmission stages to infect either an intermediate host such as an insect or snail or a further human host. Their offspring leave the body as fertilized eggs or larval stages, and transmission between people occurs indirectly, either through environmental reservoirs of contaminated soil, water or via vector species [Models of helminth transmission differ from models of micro-parasites because the generation time of helminth species are far longer (typically between 1 and 10 years depending on the species) than that of bacteria and viruses, and the infectiousness of an infected individual is strongly dependent on their burden [One of the earliest models for helminth transmission was proposed by Anderson & May where the heterogeneity in burden is described by assuming worm burdens per person in a closed localized population have a negative binomial distribution [The population size for which such models are appropriate for, is dependent on how many people live within an area in which parasite transmission can be considered local. What constitutes ‘local’ for helminth transmission depends on the route of the exposure to infection. For example, if the route is contaminated water, as is the case for schistosome transmission, the locality is determined by the residing locations of people who use and concomitantly contaminate the source; or, for example, if the helminth is transmitted by an insect vector (e.g. lymphatic filariasis and onchocerciasis), the locality is defined by the range and habitat of the insect. To date, the majority of models of helminth transmission have focused on applications where all transmission is local or where non-local transmission is accounted for by assuming a constant rate of infectious material entering an infectious reservoir. This has been appropriate because generally the models are applied to transmission in villages in rural settings where it can be reasonably assumed the majority of cases arise from local transmission within a village.Increasingly, attention is being focused on transmission in settings where previously endemic areas have received intensive deworming through repeated rounds of mass chemotherapy. The past two decades have seen great progress in many countries with endemic helminth infections in reducing the prevalence of infection as a result of preventative chemotherapy targeted at either school-aged children in the case of soil-transmitted helminths and schistosome parasites, or the whole community in the case of the vector-transmitted filarial worms. As prevalence falls year by year, marked heterogeneity in infection levels is often observed owing to local differences in both the coverage and compliance to treatment, and environmental conditions. In these circumstances, it is be important to develop an understanding of what influences the chance of elimination of transmission in the long term in a defined locality, and what are the chances of infection resurgence post mass drug administration (MDA) cessation. Within these settings, owing to the patchy spatial pattern of prevalence left after deworming, it is possible that non-local transmission may become the dominating factor influencing the re-emergence of widescale transmission. Therefore, models that account for people movements linked to spatial parasite transmission over large scales are needed. The ground-breaking recent work of Touloupou and Retkute, which links geostatistical mapping, a helminth transmission model and a statistical importance sampling method to produce very large-scale predictive maps, does include an element of spatial transmission, although does not incorporate observed people movement patterns [In this paper, we create an explicitly spatial model for hookworm transmission, which incorporates realistic human movement patterns that facilitate the spatial spread of infection. Our spatial model is designed to mimic the transmission of soil-transmitted helminths with a focus on hookworm. However, it is easily adapted for other human helminth infections. We aim to address a number of questions. The first is: can our model predict the decline in hookworm prevalence observed in the TUMIKIA study, over the course of repeated rounds of MDA [