Review of literature for epidemiology and control of Porcine Reproductive and Respiratory Syndrome virus (PRRSV) in North America: lessons learned and knowledge gaps

In 1989, a syndrome characterized by reproductive disorders in sow herds and respiratory disease in growing pigs was first reported in the southeastern United States (US) and disease reports rapidly followed throughout North America [1, 2]. A number of known pathogens were initially implicated as the cause of this disease and the major breakthrough came in 1991 when a virus, initially referred to as Lelystad virus, was identified in The Netherlands [3, 4]. Subsequently, Koch’s postulates were fulfilled when experimental aerosol exposure to sows with cell cultured Lelystad virus reproduced the clinical manifestation of the disease [4, 5]. In the United States, a prototype virus referred to as VR2332, which was isolated from continuous cell lines, was first identified in 1992 [3, 6, 7]. Although several names were originally applied to this syndrome, the designation of Porcine Reproductive and Respiratory Syndrome (PRRS) was chosen at the 1st International Symposium on Emerging and Re-emerging Pig Diseases in St Paul, Minnesota in 1991 [7]. A retrospective analysis of samples in the United States suggested no evidence of the PRRSV prior to 1980, whereas 1/26 herds were positive in 1985, and nearly 63% of samples were positive in 1988 [8]. In Canada, it has been suggested that PRRS antibodies were present as early as 1979, although clinical signs were not recorded until the mid to late 1980’s [9]. The European strain is referred to as type I PRRS virus and the North American strain as type II. Most PRRSV infections in the US are due to type II viruses [10] and type I viruses remain a relatively unimportant component of PRRS outbreaks in North America [11].

Cost of PRRS virus was estimated for the United States to be approximately $560 million in 2005 and $664 million in 2011 [12, 13]. PRRS incidence has been difficult to estimate because differentiating new PRRSV introductions from resident or vaccine strains is technically challenging and expensive [14-16]. A coordinated action item from an American Association of Swine Veterinarians PRRS task force, referred to as National PRRS Incidence project, was initiated in 2011. In this project, voluntary participants agreed to share PRRS status of their sow herds using standardized terminology for status and new infection [17, 18]. Also agreeing to share retrospective data to 2009, this project now has four years of incidence data being reported across 14 production companies, 374 sow farms, and 1.2 million sows [19]. This project has revealed striking repeatability of the annual PRRS epidemics in the participating herds. For each assessed year (2011-2013), the incidence has exceeded the annual average in mid-October, signaling the onset of an epidemic of PRRS virus. The highest and lowest incidence rates occur in late winter and mid-summer, respectively.