The epidemiology and ecology of infectious diseases in Ethiopian village chickens and the role of co-infection in infection risk

The scavenging village chicken is important to millions of smallholders in Ethiopia, as in other less-economically developed countries, for its contribution to the economic, nutritional and social well-being of farmers, especially women and children. Infectious diseases are frequently cited as the greatest constraint to village chicken production, and in Ethiopia, most mortality is attributed to seasonal outbreaks of Newcastle disease (ND). This study conducted four cross-sectional surveys over an 18-month period in two geographically distinct regions of Ethiopia, to examine a range of bacterial, viral and parasitic infections in randomly-selected village chickens, and to look at their 6-month survival rate. The two chicken populations of the different regions were found to be different in terms of their population dynamics and phenotypic characteristics, and these may be driven by farmer demands, which are dictated by the local economic and cultural value placed on specific qualities in the chickens. Over the course of the study, no large outbreaks were observed in the eight villages which took part in the study, and only 9 out of 1280 birds (0.7%) were found to be serologically positive for ND. However, even in the absence of large outbreaks, around 20% of the birds in the study were reported to have died of disease within the 6-month follow-up period, and a further 13% lost to predation. Both location and seasonal variation influenced a bird’s fate, as did farmer decisions, such as choosing birds with specific characteristics to sell or eat. Rather than large outbreaks, the rainy season appeared to be associated with increased small-scale losses, and a variety of signs were described, suggesting several pathogens may be involved. No single infection measured at the time of sampling was a good predictor of subsequent death from disease; instead different pathogens appeared to be important in each region, and reduced the probability of survival through a variety of mechanisms. Positive correlations between Pasteurella and Salmonella, and between Marek’s disease and parasitic diseases were identified, but fewer birds than expected were identified with pathogens from both these groups, perhaps suggesting a decreased chance of survival for co-infected birds. Strong seasonal variation in prevalence was not observed for any of the infections in the study, suggesting that seasonal rises in disease mortality are unlikely to be attributable to a single infection, but other factors may play a role, including an increased probability of co-infection. This makes it difficult to prioritise control strategies for individual diseases; instead development programmes may find broad-based strategies, such as improving hygiene and chick management may be more beneficial to minimise the small-scale losses. Programmes also need to be tailored to local needs rather than assuming a blanket strategy will work equally well for all farmers or regions. Any development strategies to control single diseases should consider potential impacts on non-target infections, due to the existence of multiple interactions between pathogens in this system.