Prompt detection and effective control of porcine reproductive and respiratory syndrome virus (PRRSV) during outbreaks is important given its immense adverse impact on the swine industry. However, the diagnostic process can be challenging due to the high genetic diversity and high mutation rate of PRRSV. A diagnostic method that can provide more detailed genetic information about pathogens is urgently needed. In this study, we evaluated the ability of Oxford Nanopore MinION direct RNA sequencing to generate a PRRSV whole genome sequence and detect and discriminate virus at the strain-level. A nearly full length PRRSV genome was successfully generated from raw sequence reads, achieving an accuracy of 96% after consensus genome generation. Direct RNA sequencing reliably detected the PRRSV strain present with an accuracy of 99.9% using as few as 5 raw sequencing reads and successfully differentiated multiple co-infecting strains present in a sample. In addition, PRRSV strain information was obtained from clinical samples containing 10 4 to 10 6 viral copies or more within 6 hours of sequencing. Overall, direct viral RNA sequencing followed by bioinformatic analysis proves to be a promising approach for identification of the viral strain or strains involved in clinical infections, allowing for more precise prevention and control strategies during PRRSV outbreaks.2 of 18 against a new introduction from outside the farm indicates a need for enhanced biosecurity, while a re-break of a resident strain suggests better strategies for an elimination or vaccination program are needed. Another big challenge for PRRS control is that PRRSV vaccine is not completely effective at preventing and controlling infection due to the high genetic diversity of the virus, thus outbreaks still occur in vaccinated herds [13][14][15][16]. A diagnostic method which can provide genetic information about the strain causing infection would allow for identification of potential reasons for vaccination failure, such as limited cross-protection due to high genetic divergence from vaccine [17], or in the case of genetic similarity to vaccine, perhaps a reversion to virulence (escape mutation) of the vaccine itself [18]. In addition to the clinical challenges mentioned above, PRRSV has widely divergent genetic lineages and is a rapidly evolving pathogen with novel variants which seem to be more divergent and virulent than those in the past [10,[19][20][21]. The continuous emergence of new virulent strains causes unexpected devastating outbreaks, such as the severe outbreaks of HP-PRRSV in China and MN184 and NADC30 outbreaks in the United States [22][23][24][25]. The increasing incidence of co-infections of multiple strains further complicates PRRS diagnosis and control [26]. Hence, diagnostic tools that can provide more genetic information are extremely important for investigation, prevention, and control strategies for PRRSV outbreaks.Prompt detection of pathogens during an outbreak is essential for efficient disease control. Real-time quantitative...