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Mapping the Geographical Spread of PRRSv in the Netherlands

The Porcine Reproductive and Respiratory Syndrome virus (PRRSv) presents a major economic and welfare challenge to the global swine industry. In one retrospective study from the Netherlands, the economic losses during an outbreak varied between €59 and €379 per sow for an 18-week period, depending on the type of farm. Due to the prolonged nature and high prevalence within a farm, PRRSv has a significant impact on animal welfare. The risk of introducing a new PRRSv strain into a farm is not limited to PRRSv-negative farms; the impact of PRRSv introductions on both PRRSv-negative and PRRSv-positive farms can vary significantly, ranging from mild, asymptomatic cases to severe disease outbreaks, depending on the virulence of the strain and the crossreactivity of the existing immunity.

Currently, there is a lack of data on the relative contribution of direct and indirect routes to the transmission of PRRSv onto farms. Whereas direct transmission is mostly attributed to trade and transport of infected pigs, indirect routes include people, semen, manure, domestic/feral animals, rodents, insects, aerosol, animal feed, water and fomites. In our analysis, it was aimed to compare the prevalence of Dutch PRRSv-positive farms in areas with high and low pig densities, respectively. Furthermore, the relative contribution of trade contacts to PRRSv introduction on farms was investigated, contrasting with other possible other risk factors for PRRS introduction such as proximity between farms and airborne transmission of the virus.

Materials and Methods

Between 2022 and 2023, a total of 49 farms were included: 38 farms, evenly distributed across four areas (two with high pig density and two with low pig density, as shown in Figure 1), along with 11 gilt- and piglet-producing farms that supplied animals to at least one of the participating farms. A biosecurity questionnaire was conducted on each of the farms. The collected data encompassed, for example, location, management of animal flow, and animal contacts between herds.

Blood samples from 30 piglets at the end of the nursery phase in pools of five, or, in case only fatteners were present, six oral fluid samples were analysed via PRRSv RT-qPCR and ORF5 sequencing from one sample with the highest viral load. Phylogenetic analysis of ORF5 aimed to identify close matches (≥98% nucleotide identity) and trace them back to potential direct or indirect contacts between farms. Chi-squared tests were applied to detect differences between areas using contingency tables.