Evaluation of an Indirect Fluorescent IgM Antibody Test for the Detection of Pigs with Recent Infection of Porcine Reproductive and Respiratory Syndrome Virus

Porcine reproductive and respiratory syndrome virus (PRRSV) has caused major economic losses to swine producers in recent years due to severe reproductive failure in pregnant sows and respiratory disease in young pigs. Antibodies to PRRSV in swine sera can be tested by various serologic methods including immunoperoxidase monolayer assay, 12 indirect fluorescent antibody (IFA) test, 14 enzymelinked immunosorbent assay, and serum neutralization (SN) test 3,15 However, none of the tests can adequately determine the time of infection. Development of a serologic test that could estimate the infection time or identify a recent infection would be valuable to swine farmers when introducing pigs from a PRRSV-infected farm. Demonstration of specific IgM antibodies has been a useful marker for recent infection to different viral infections. Virusspecific IgM antibody has been detected regularly in primary infection with herpes simplex virus, rubella virus, cytomegalovirus, hepatitis A and B viruses and pseudorabies viTUS. For example, IgM antibody to pseudorabies virus in pigs was first detected in serum 6 days postinoculation (PI), reached the highest titers 10 days PI, and was detectable up to 35 days PI. The objectives of this study were to evaluate an IFA test for the detection of IgM antibody specific to PRRSV and to compare the detection of IgM antibody and the presence of virus in sera from pigs with or without known days PI. PRRSV MN-1b was used for the IFA test, and the MN-H and MN-W isolates were used for animal inoculation. A permissive clone (MARC-145) of the African green monkey kidney (MA-104) cell line was used for virus propagation, virus isolation, and serology. The MARC-145 cells were maintained in Eagle’s minimum essential medium supplemented with 3% fetal bovine serum, 0.15% sodium bicarbonate, and antibiotics. Sera used were collected at known intervals from 3-weekold pigs following experimental infection with PRRSV MNH. Sera from sows infected experimentally with PRRSV MN-W or MN-H were also used. Field serum samples were collected from pigs of known age in 2 different farms with endemic PRRSV infection, and the samples were transported on the same day to the laboratory for virus isolation and serology. Virus isolation from sera was performed using 24-well microplates by inoculating 0.05 ml of the undiluted sera to 1 ml of MARC-145 cell suspensions (2 x 10 cell/ml). Infected monolayers were observed daily for cytopathic changes