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.