Seroprevalence of Indirect Fluorescent Antibody to Porcine Reproductive and Respiratory Syndrome Virus in Selected Swine Herds (1993)

Porcine reproductive and respiratory syndrome (PRRS) virus has recently been identified in the USA as a cause for reproductive failure in pregnant sows and respiratory disease in young pigs. 1-4,6 The syndrome has been commonly referred to as swine infertility and respiratory syndrome in this country but there was a general agreement to adopt the name PRRS at an international PRRS symposium (St. Paul, MN, May 1992). A virus designated as Lelystad virus has also been reported to cause a similar syndrome in several European countries. 1,5 Both viruses can cause significant economic problems for swine producers because of acute reproductive losses and piglet mortality. Chronic production losses involving increased mortality and reduced growth rate in pigs at postweaning age have also been reported on some infected farms. 1 In our recent study, an indirect fluorescent antibody (IFA) test was developed to detect and quantitate PRRS virus antibody in swine sera. 7 This IFA test for swine sera collected between 1981 and 1991 revealed evidence of PRRS virus infection in this country as early as April 1986. The purpose of the present study was to report the recent Seroprevalence status of IFA to PRRS virus by testing sera collected from selected swine herds during the first 6 months of 1992. Swine sera submitted January-June 1992 for serodiagnosis to the Oxford Veterinary Laboratory, Worthington, Minnesota, were used. Since October 1988, the laboratory has provided a swine antibody profile service to practicing veterinarians for different pathogens. The service has included serology for Actinonobacillus pleuropneumonia serotypes 1, 5, and 7, encephalomyocarditis virus, porcine parvovirus, Leptospira spp., and group D Streptococcus. The IFA test for PRRS virus was included starting in January 1992. Between January and June 1992, a total of 2,787 sera from 263 swine farms (an average of 10.6 serum samples per farm), were submitted from 13 different states. Veterinarians provided a brief farm history and requested serology for specific agents. Of the 263 farms, 225 requested PRRS virus serology. For the PRRS virus IFA test, aliquots of each sample were sent to the swine virology laboratory, University of Minnesota. ethanol was used to fix the cell monolayers in each well. Test sera, diluted 4-fold serially from 1:4 to 1:1,024, were added to test plates. The plates were incubated, washed, and incubated again with an optimal dilution of rabbit anti-swine IgG conjugated with fluorescein isothiocyanate. The plates were then washed and examined under a fluorescent microscope. Negative and positive reference sera and an uninfected SAM control for each serum were included in each test. The IFA titers were recorded as the highest serum dilutions with specific cytoplasmic fluorescence, and titers of ≤ 1:4 and ≥ 1: 16 were considered negative and positive, respectively. Of 2,787 sera from 263 swine herds tested, 979 sera (35.1%) had PRRS IFA titers of 21: 16 and 148 farms (56.3%) had 1 or more pigs that were PRRS virus IFA test positive (Table 1). Of 1,726 sera from 148 farms with PRRS virus seropositive pigs, 747 sera (43.3%) had IFA titers of ≤ 1:4 and 979 sera (56.7%) had the titers of ≥ 1:16. Of 979 seropositive samples, 222 (22.7%) 314 (32.1%), 283 (28.9%), and 160 (16.3%) samples had IFA titers of 1:16, 1:64, 1:256, and ≥ 1: 1,024, respectively. Of 225 farms requesting PRRS serology, 129 (57.3%) had seropositive pigs, whereas 19 of 38 farms (50.0%) that did not request PRRS virus serology were positive. Results of the farms with seropositive pigs were 1 1/16 (68.8%), 16/41 (39.0%) 35/65 (53.8%) 21/35 (60.0%), 37/61 (60.7%), and 28/45 (62.2%) farms for January, February, March, April, May, and June, respectively. No significant difference for seropositive farms was found between the months by chisquare analysis (P = 0.190). The positive farms were located in 11 different states.