Is the movement of feeder pigs responsible for the spread of PRRS?
Although the critical role of pig movements in the spatial spread of porcine reproductive and respiratory syndrome virus (PRRSV) has been previously reported -, understanding the relative contribution of different types of pig movement to the spread and evolution of PRRS can help us focus disease management efforts during outbreaks. Using the viral sequence data, we can plot patterns of spatial spread of PRRS and analyze how potential factors in the spread of pathogens, such as different types of animal movements, shape the patterns of regional spread of the virus. .
Here, we applied evolutionary models to explain the geographic spread and evolution of a rapidly spreading strain of PRRSV type 2 (L1A subline), which is mainly correlated with type RFLP 1-7-4. This subline emerged in ~ 2014 and has since made up ~ 60% of the PRRSV sequences detected in the United States. In the model, we combined ORF5 PRRSV sequences collected from farms in a dense hog production region in the United States between 2014 and 2017 with animal movement data, spatial and environmental data. We grouped data on animal movements by age group of moved animals: movements of weaned pigs (3-4 weeks), fattening movements (8-25 weeks) and breeding movements (≥21 weeks) . The study area was then divided into 13 geographic areas, and travel data was summarized for all farms in each area.
We observed that the intersectoral spread of L1A was positively associated with three main factors: movement of feeder pigs, spatial contiguity of sectors, and farm density. Compared to movements of weaned and breeding pigs, transport of feeder pigs was much more strongly associated with higher rates of viral dispersal between sectors. This suggests that nurseries may act as enhancers for the viral population, possibly due to the decrease in maternal antibodies and the admixture of weaned pigs from multiple sources. In addition, unlike on livestock farms, biosecurity measures in nurseries (or the trucks that originate from them) tend to be more relaxed, which can influence the risk of further transmission of the virus. Production systems should consider stricter biosecurity measures in nurseries and improved truck hygiene to mitigate the risk of disease spread from nurseries.
Higher rates of intersectoral viral dispersal have also been documented between adjacent sectors, highlighting the importance of the spatial structure of host populations in shaping the distribution of the disease. L1A propagation between adjacent sectors could have been by mechanical propagation, especially in areas with shared service providers and infrastructure. Thus, for effective disease management, biosecurity threats in adjacent areas must be taken into account when designing response measures in neighboring areas. A high density of farms has been associated with the spread and maintenance of PRRS -. Therefore, our results, although not surprising, highlight the risk associated with a high density of farms.
In summary, the movements of fattening pigs played a more critical role in forming the spatial propagation patterns of PRRSV L1A, more than other types of movements. More research is needed to elucidate why these movements from nurseries may be riskier, but the ability to better identify critical pathways of disease spread is invaluable in making informed decisions to target and optimize measures for the disease. intervention. Taken together, these results suggest that the dynamics of PRRS and the transport of growing pigs may contribute disproportionately to regional spread and long-term persistence of PRRS.
This work is published here.
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