4. DISCUSSION
The Ecuadorian network presents small world properties, strong community
structure and a steadily growing tendency over the years, contributing
to CSF transmission; these characteristics, as well as the spatial and
temporal trends, could be considered by the surveillance system to
improve their procedures and update the disease control and management
policy, enabling the application of targeted or risk-based surveillance.
These results provide a framework for future exploration, relevant to
the Veterinary service and stakeholders implementing prevention and
control strategies for CSF or any other infectious disease.
Although the Ecuadorian pig network is of medium size (83.000 nodes,
396,000 edges), compared to others, such as those in Slovenia (Knific et
al., 2020), Germany (Lentz et al., 2016) and Italy (Crescio et al.,
2020), network metrics such as graph density, diameter and small-world
properties match those observed in these countries. However, compared to
studies in Latin America, it is more similar to that of the state of
Rio-Grande-do Sul, Brazil (11,800 nodes, 445,000 edges) (Machado et al.,
2021) and larger than networks reported in Argentina (6,300 nodes,
22,000 edges) (Baron et al., 2020), Peru (500 nodes, 25,000 edges)
(Gomez-Vazquez et al., 2019), and the state of Santa Catarina, Brazil
(10,000 nodes, 50,000 edges) (Cespedes et al., 2021). Although each
network is completely different, their size shows the importance of this
sector for the Ecuadorian economy and the potential hazards for the
spread of diseases across the country or even the region.
More than half of the movements (57%) originated from or were destined
for markets, network analysis of cattle shares a similar proportion
(53%) (Vinueza et al., 2022), apparently markets have a central
influence in the organization of trade in both species, which could be
related to the Andean marketing structure (Benítez and Sánchez, 2001)
and the livestock population dynamics within the economy (Vernon and
Keeling, 2009). This emphasises the high risk of markets as potential
spreaders of any animal disease and the need to prioritise them in the
prevention and control policies, further analysis could be useful to
asses the risks of individual markets and the sales held (Robinson and
Christley, 2007), also the possibility of monitoring strategies in this
facilities.
Backyard producers constituted 89% of the premises in the network,
facing major technological constraints and a lack of biosecurity
practices, making them a population at higher risk of CSF infection
(Martínez-López et al., 2013). Network metrics such as degree could be
used to select those at higher risk and implement target surveillance.
Parishes with the top degree centrality contributing with 50% of pig
movements, could be prioritised by allocating more human and financial
resources, focusing movement supervision on the most likely places to
encounter disease (Cameron, 2012).
Higher seasonal movement activity was observed from May to August and
December; the last could be related to traditional celebrations, also
evidenced in Peru and related to Christmas and New-year festivities
(Gomez-Vazquez et al., 2019) when pork meat dishes are highly consumed;
in addition, lower seasonal movement activity was observed from January
to April, and September to November, with a general behaviour that is
repeated across the study years. Thus, the veterinary service could
prioritise its actions, modulating their activities according to
seasonal network activity rather than administrative or historic
decisions.
The communities computed could be important to implement the zoning
strategy considered in the Ecuadorian CSF eradication project and
implemented in the zoning strategy in Colombia (Pineda et al., 2020)
(ICA, 2016). The eradication strategy could be divided into steps,
focusing on one community at a time, analysing the risks of infection by
incoming movements (Grisi-Filho et al., 2013). The veterinary service
has a preponderant role in disease management, closing borders between
communities or differentiating requirements for intercommunity movements
could be valid strategies, without significant effects on trade
(Cardenas et al., 2019), rather than the traditional approach of
managing only administrative divisions (provinces).
Some communities showed spatially disconnected parishes due to
commercial activity, where producers buy piglets in distant markets and
sell them at higher prices than local ones after fattening; also
irregular movement schemes could be related.
The border provinces of Loja and El Oro contributed a quarter of the
origin and destination of national movements to abattoirs; this
proportion is higher than mass marketing centres such as Santo Domingo
and Cotopaxi and the major consumption centres in Guayaquil (largest
Ecuadorian city) and Quito (capital). We could attribute this result to
the underestimation of production, also informal trade along the
extensive borders (Terán et al., 2004).
Previous molecular findings from 2015 CSF outbreaks, showed a close
relation with strains isolated in 2010 in Peru (Garrido Haro et al.,
2018); moreover, the largest Peruvian pig trading community extended
close to the border with Ecuador, which is reported to be of higher risk
of CSF occurrence (Gomez-Vazquez et al., 2019). Further studies that
take these variables into account could be necessary to assess the risk
of regional disease spread in border areas.
The network of pig movements contributed to the CSF transmission
process; supporting the fact that movement networks are more important
than spatial proximity in the spread of CSF (VanderWaal et al., 2020),
(Lee et al., 2017), (Rosendal et al., 2014). Pig farming is generally
practised indoors (Rodríguez-Estévez et al., 2010), feral pigs could
mediate transmission, but in Ecuador they are not commonly seen and no
records are kept of them to analyse any relationship. Currently, traders
and markets provide the best conditions (direct contact) for CSF
transmission and should be prioritised in monitoring and control
activities, without underestimation of mechanical transmission and
irregular movements.
The lack of unique identification codes for premises in the outbreak
database, and the lack of geographic coordinates in the premises
database was a limitation in this study.