Contact patterns between enterprise types
Small world networks have similar average path lengths, but greater clustering than an equivalent random network. The LBM network exhibited small world network properties, implying that particular members of this network were well-connected and linked, so that in the event of HPAI outbreaks, local disease spread would likely take place to poultry enterprises connected with the LBM. These markets should be targeted for surveillance and control. The collector yard network had the highest betweenness score and had a high number of daily operations. This implies that they acted as important connectors within the poultry trade system. Our results indicated that there was unrestricted flow of poultry movements between collector yards. HPAI control measures could be targeted at these collector yards to prevent disease transmission to commercial farms and LBMs.
Commercial farms sold poultry within the existing trade network as well as to markets as distant as Jakarta, hence the commercial farm network had the largest number of nodes and spanned 10 Districts, including long-range movements. Wibawa et al. (2018) investigated the rate of contacts between production Sectors. They found that most enterprises were not closed, and that visits to and from other poultry farms were common. Hence, transmission of HPAIV between sectors was possible. While we were unable to investigate the nomadic duck contact network, other investigators have consistently shown that presence of nomadic duck flocks is an important risk factor for highly pathogenic avian influenza outbreaks. For example, Henning et al. (2016) described the movement of flocks and characterised the network in Districts neighbouring Purbalingga. They found that transporters of nomadic duck flocks, hatcheries and rice paddy owners were likely to play an important role in the spread of the HPAI virus. Meyer et al. (2017) performed a similar study describing the long-distance free-grazing duck production system in South Vietnam, and showed that both direct and indirect contacts between free-grazing duck flocks were frequent and diverse.
The information generated by the different elements of the study were generally consistent with the scientific literature and the mixed methods approach was useful to draw together the evidence into a coherent narrative. Although the study design and the data generated by the study could not easily be applied for statistical models, the information and data can be utilised to inform exploratory transmission models.