Discussion
fits, infection
INFECTION, EVENTS, HOOKWORM INFECTION
In this study, we have developed the first explicitly spatial model of soil-transmitted helminth transmission, and applied it to hookworm infection using survey data from the TUMIKIA trial. Our model, with and without human mobility, fits well to the baseline data, reproducing the spatial prevalence pattern, although it is less accurate at recovering the prevalence within areas of low intensity infection. In future studies, a better fit may be possible by adapting the nonlinear function in equation (2.2) for The spatial model is able to reproduce the pattern of the decline in prevalence as MDA is administered, although our model underestimates the overall reduction in prevalence after the fourth round of MDA. It may be possible to achieve a higher accuracy, by including the first round of MDA in the fitting methodology, or by including other parameters into the fitting methodology, such as The most important public health insight generated by our spatially explicit transmission model is that widespread bounce-back of infection is possible after a hypothetical intensive MDA scenario. In the absence of movement between villages, the intensive MDA would have eliminated transmission through the region. This result is important, since all locations in the simulated scenario reached very low prevalence levels, hence it would appear unlikely that a very small number of infected people are able to cause resurgence in their pixel and also take trips to repeatedly seed external pixels. We believe the observed bounce-back occurs because the movement model creates a small amount of positive feedback, and the large number of individuals simulated makes relatively rare events on an individual level, highly probable when summed over the entire population.The spatial model contains many assumptions and caution should be exercised over the generalization of the results. More simulations are required to explore how different movement patterns and scales for each pixel (3 km × 3 km in the present calculations) influence the outcomes of differing MDA programmes in terms of coverage, frequency of delivery and compliance patterns. For example, consider two particular issues. First, the predictions rely on the gravity model based on data captured via mobile phones from Namibia chosen to represent human movement in Kwale, Kenya. This cannot capture precisely the real movement patterns of the population in Kwale sub-county so caution should therefore be applied before placing too great a weight on these findings. Also, it is not clear to what extent these results are generalizable to other regions. More rural, or urban, social geographies will change the volume and nature of movements between villages, and level of endemicity of the helminth parasite (underlying Second, in this study we have used a spatial grid resolution of 3 km × 3 km, with the assumption that each human host in the pixel (i.e. mirroring a village) is in contact with the same infectious reservoir. Hookworm infectivity can be very focal owing to both environmental conditions and behavioural habits, and it is possible that this scale assumption is poor in low transmission settings. This can be explored using the described model framework by using higher grid resolutions, although careful consideration is needed about whether movement patterns at a fine spatial scale be accurately captured, as the gravity model used in this study may not be appropriate on all scales. These are topics for future exploration. We also assume in this study that the majority of an individual's trips of duration of less than a day, for example, commuting to work or travelling to school, occur within the individual's 3 km × 3 km pixel. This may be a weak assumption that underestimates the magnitude of spatial transmission, and therefore data, or more realistic models, describing travelling habits of specific populations will improve the model's accuracy [In conclusion, we have demonstrated in this paper the potential role that human movement has on sustaining helminth parasite transmission across linked communities. This in turn is important for the design of public health interventions and associated monitoring and evaluation programmes with respect to deciding what is a sensible spatial scale over which to administer control impact and measure outcomes. These results are pessimistic in the sense that MDA coverage levels that are predicted to permit transmission elimination without extensive people movements are invalidated when much movement occurs between communities. They bring into question the long-term effectiveness of mass deworming as a standalone method of control, and suggest that lasting elimination will require MDA interventions to be accompanied by large-scale improvements to sanitation and hygiene. These must also occur on a large spatial scale such that the effective reproductive numbers for the parasite at each spatial location are reduced to below unity in value.