People, patches and parasites: New paper explores the socio-ecology of disease in Africa
Just out in Human Ecology is a new paper – People, patches and parasites: the case of trypanosomiasis in Zimbabwe. It’s open access, so do have a look!
It presents the results of a project looking at the socio-ecology of disease in Africa – part of the Dynamic Drivers of Disease in Africa Consortium – which has had a number of other recent outputs, linking social and natural sciences in the investigations of disease dynamics in Africa.
The Zimbabwe work started with a puzzle. Why was it that after nearly a century of control efforts, tsetse flies and the disease they carry – trypanosomiasis (sleeping sickness in humans) – still persists. Indeed human cases seem to be on the rise, although reporting is poor. The battle against the tsetse fly has been intense. Everything has been thrown at them: habitats have been cleared with bulldozers, intermediate wildlife hosts have been exterminated, chemicals have been sprayed from the air and from the ground, millions of sterile male flies released, traps have been set. And still, despite all this, the fly is still there, and the disease still a threat – mostly to poor people living in places like the Zambezi valley, but also those who visit, including hunters and tourists.
For sure the extent of tsetse infestation has changed over time. The paper includes some rough estimates of the ‘tsetse front’ at different periods since the near extermination of the fly following the rinderpest pandemic of the late nineteenth century, when wildlife were all but wiped out. There is no longer a tsetse ‘belt’, as there once was, as flies have retreated into particular habitat patches.
A new science: rethinking methods
This suggests a different type of science too. Much conventional science and associated monitoring and control efforts directed at the dreaded tsetse fly have assumed uniform distributions. Sampling approaches, in particular, have not caught up with the more complex spatial reality and the new geography of the disease.
Standard transect survey techniques cut straight lines through landscapes, remote sensing imagery reveals only coarse patterns and sampling of livestock or households in a random way may miss spatial variability and more illegal, surreptitious activity (e.g. livestock theft or smuggling). In unravelling the puzzle we confronted, we had to think hard about sampling in particular.
Our study certainly made a few mistakes along the way. We used existing data and standard techniques as the starting point, hoping perhaps that by combining them we would find the answer, falling into a safe, classic multidisciplinary approach. The team had different ‘work packages’ and proceeded with approaches they knew. The social scientists did a survey, the epidemiologists sampled blood from cattle, the entomologists trapped along transects, the geographic information systems (GIS) specialist analysed their images. But individual pictures did not add up until we started to think together as a team; and most crucially with villagers who knew this disease landscape and its history.
It was some very informal discussions in the field on a transect walk with villagers, and in subsequent participatory mapping that the ‘ah ha’ moments really happened. Villagers pointed to particular patches of land – small in size, with certain biophysical and social-cultural characteristics, distinct from the wider landscape. These were the places where they’d seen flies (which were not appearing in vast numbers in our traps, except near the escarpment), and where they were convinced their cattle, and sometimes people, got sick. These patches were sometimes just small bits of vegetation along a stream or around a pool, or more dramatically the deep Mushagashe gorge that cut through the study area. These were the areas too where late dry season grazing could be found, where fruits could be collected and wild animals hunted.
We had to think again about our data, and the biases of our statistically-rigorous, but ultimately misleading, random sampling. So we looked harder at our available sampled data, differentiating the villages, looking at the links between sampling stations and patches, and extended our interviews to other areas to test our emerging hypotheses. We set up new traps in these patches and started to find more flies, and we explored the trypanosomiasis presence in certain villages and found it to be broadly related to proximity to patches. Our GIS team members looked harder and deeper into the patterns of habitat fragmentation, exploring particular areas pointed out by villagers.
Increased risk of disease where domestic and wild animals interact
It seems that, just as the villagers suggested, flies are persisting in particular sites, and that certain people and animals are differentially exposed to the risk of infection as a result. That we did not have human trypansomiasis data meant we could not fully test the link, but the animal trypanosomiasis data, assumed to be a close correlate, suggests something is going on. Thankfully the disease risk is not massive, but is probably significantly underreported. Epidemiologically, we see the intersection of two cycles of infection – one domestic, the other ‘sylvatic’ (linked to wildlife). When these come into contact, as in these patches, the chance of heightened infection emerges. Our findings are not that novel, as others have suggested these dynamics before; although not necessarily investigated them in such depth in one area through so many different lenses.
At the same time, we have also pulled out some important learning from the research process, which can inform other researchers who are looking at complex environment and development challenges like these: We realise we should have started much earlier – as a team and with villagers – exploring the framing of the hypotheses to test, and not relying on standard approaches imported from elsewhere. We should have pushed earlier for the interaction of different spatial, process and participatory modelling approaches, with the participatory investigations leading the way for framing questions, and defining data collection protocols. We should have listened harder – to other team members and villagers themselves - and spent more time in the field. We look forward to applying these lessons learned in our future research and hope others can learn from them, too.
Read the full paper: People, patches and parasites: the case of trypanosomiasis in Zimbabwe.
Image credits: Terry Feuerborn (trees, Zimbabwe), Derek Keats (zebra, Zimbabwe), Andrew Ashton (river, Zimbabwe)