Livestock, pathogens, vectors, and their environment: a causal inference-based approach to estimating the pathway-specific effect of livestock on human African trypanosomiasis risk

Abstract

AbstractLivestock are important reservoirs for many diseases, and investigation of such zoonoses has long been the focus of One Health research. However, the effects of livestock on human and environmental health extend well beyond direct disease transmission.In this retrospective ecological cohort study we use pre-existing data and methods derived from causal inference and spatial epidemiology to estimate three hypothesized mechanisms by which livestock can come to bear on human African trypanosomiasis (HAT) risk: the reservoir effect, by which infected cattle and pigs are a source of infection to humans; the zooprophylactic effect, by which preference for livestock hosts exhibited by the tsetse fly vector of HAT means that their presence protects humans from infection; and the environmental change effect, by which livestock keeping activities modify the environment in such a way that habitat suitability for tsetse flies, and in turn human infection risk, is reduced. We conducted this study in four high burden countries: at the point level in Uganda, Malawi, and Democratic Republic of Congo (DRC), and at the county-level in South Sudan.Our results indicate cattle and pigs play an important reservoir role for the rhodesiense form (rHAT) in Uganda, however zooprophylaxis outweighs this effect for rHAT in Malawi. For the gambiense form (gHAT) we found evidence that pigs may be a competent reservoir, however dominance of the reservoir versus zooprophylactic pathway for cattle varied across countries. We did not find compelling evidence of an environmental change effect.Author summaryOne Health research is most commonly interested in livestock as reservoirs of zoonotic diseases (i.e., infectious diseases transmissible from animals to humans), however livestock also exert environmental effects on a range of scales. At a local level, grazing and brush-clearing activities related to livestock keeping can reduce vegetation and increase temperature, in turn reducing habitat for and density of disease reservoirs and vectors. Furthermore, many arthropod vectors of human (zoonotic and non-zoonotic) diseases exhibit host species preference; when livestock hosts are preferred, the presence of these animals may reduce the risk of human infection. When all three of these effects act in concert, their relative strength governs whether the overall effect of livestock on human disease risk is positive (harmful) or negative (protective), a balance which is likely focus-specific.Using pre-existing data and methods drawn from causal inference and spatial epidemiology, we estimate the contribution of these three pathways to the effect of cattle and pigs on human African trypanosomiasis (HAT) risk in Uganda, Malawi, Democratic Republic of Congo, and South Sudan. We find little evidence of an environmental change effect, however cattle and pigs appear to play both reservoir and zooprophylactic roles in the epidemiology of this disease.