A reproductive syndrome of unknown etiology described in the United States in 1989 was the first documented clinical PRRS virus infection of swine.1 The causative arterivirus is now endemic in most swine-producing countries, and with an estimated cost of $560 million annually, PRRS is the most economically important disease affecting the US swine industry.2 In inflation-adjusted terms, this estimate greatly exceeds the estimated annual economic burdens of both hog cholera ($364 million in 2004 dollars) and pseudorabies ($36 million in 2004 dollars) viruses before national programs were undertaken to control those diseases.2 Like other RNA viruses, PRRS virus has marked genetic heterogeneity that is reflected in variability in virulence, clinical manifestations, and epidemiological attributes. Variants of the PRRS virus in Chile appear to have relatively low virulence, whereas highly virulent variants have been implicated in widespread and severe disease outbreaks in China.3,4 The propensity of PRRS to cause reproductive disease is strain dependent, and variants of the virus also differ with respect to their potential for aerosol transmission.5,6
The foremost obstacle to PRRS control is the ability of the virus to spread among neighboring farms despite strict biosecurity measures. Elimination of PRRS from individual herds has been regularly achieved by use of a variety of approaches,7–9 but these efforts are frequently undermined by reintroduction of virus via routes which, in most cases, are either unknown or speculative. In addition to obvious routes of introduction in pigs or semen, experimental studies have confirmed a variety of potential routes for PRRS virus transmission among herds, including people, fomites, transport vehicles, insects, and aerosols.10–13 Coordinated efforts to control PRRS in France had qualified success,14 and a government-led eradication program in Chile appears to have been successful.15 Likewise, an emergency government program immediately following the initial detection of PRRS in Sweden resulted in elimination of the virus.16 However, in each of these countries, a low proportion of herds was infected when these programs were initiated. The American Association of Swine Veterinarians has stated a long-term goal of PRRS elimination in the United States.17 However, in regions with a high herd prevalence of PRRS infection, considerable obstacles need to be overcome before any coordinated control program will be practical.2
Effective regional control of animal disease requires an understanding of all reservoirs of infection and of mechanisms and risks of pathogen transmission. The PRRS virus, like other arteriviruses,18 is highly host specific, and it is unlikely that nonporcine species could be reservoirs of the virus. Consequently, when developing regional control strategies for PRRS, attention may be focused entirely on populations of Suidae. Although commercial swine herds (ie, herds kept with the primary objective of income generation from pork production) constitute most of the US swine population, many other swine populations exist. These include feral or wild pigs, pet pigs, hobby farm pigs, and pigs reared in youth education programs (eg, FFA and 4-H). Feral or wild pig populations are important reservoirs of infectious diseases, including brucellosis and pseudorabies,19,20 and are also potentially important reservoirs for PRRS virus.21 Although widespread in much of the United States, feral pigs as yet are not known to have become established in Minnesota. Youth education programs that include rearing and showing of pigs attract broad participation in many states, including Minnesota. According to the 4-H National Headquarters, the numbers of youth involved in the swine educational curricula ranged from approximately 140,000 to 212,000/y from 1996 to 2003. Currently, there is little documentation of the relationships these youth have with the commercial swine industry. The purpose of the study reported here was to characterize the swine population shown by 4-H participants and to evaluate its potential importance to the epidemiology and control of PRRS.
Future Farmers of America
Local indicator of spatial autocorrelation
Minnesota Board of Animal Health
Minnesota Pollution Control Agency
National Agricultural Statistics Service
Porcine reproductive and respiratory syndrome
ArcGIS 9.0, Environmental Systems Research Institute, Redlands, Calif.
GeoDa, version 0.9.5-i, Spatial Analysis Laboratory, University of Illinois, Urbana-Champaign, Ill.
Trapp P, Minnesota Pollution Control Agency, Saint Paul, Minn: Personal communication, 2004.
Wiklund D, Minnesota Board of Animal Health, Saint Paul, Minn: Personal communication, 2004.
Questionnaire available upon request from corresponding author.
Excel, version 2003, Microsoft Corp, Redmond, Wash.
HerdChek PRRS Antibody 2XR ELISA test kit, IDEXX Laboratories Inc, Westbrook, Me.
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Daniels S, Seaboard Foods, Dalhart, Tex: Personal communication, 2004.
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Carlsson U, Wallgren P, Renström LH, et al. Emergence of porcine reproductive and respiratory syndrome in Sweden: detection, response and eradication. Transbound Emerg Dis 2009; 56:121–131.
Gresham CS, Gresham CA, Duffy MJ, et al. Increased prevalence of Brucella suis and pseudorabies virus antibodies in adults of an isolated feral swine population in coastal South Carolina. J Wildl Dis 2002; 38:653–656.
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