Pattern of Infection with the Porcine Reproductive and Respiratory Syndrome Virus on Swine Farms in Belgium

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Abstract
On four closed breeding-fattening farms, 17 sows and their litters were examined for porcine reproductive and respiratory syndrome virus (PRRSV) antibodies in the blocking enzyme-linked immunosorbent assay (ELISA). On each farm, the pigs were born within the same week and remained together for 12 weeks. The pigs were followed serologically to determine the antibody profile with the purpose of establishing the infection pattern of PRRSV. A total of 13 sows had antibodies and a positive correlation existed between their titre and the maternal antibody titre of their litters at 2 weeks of age. The maternal antibodies were detectable until 4–10 weeks of age. On two farms, no infection occurred during the time of the experiment as no seroconversion was observed in the eight litters. On the two other farms, infection was observed in eight of the nine litters between 4 and 12 weeks of age while one litter became infected during the fattening period. In some litters, a few pigs seroconverted between 6 and 8 weeks, others between 8 and 10 weeks and still others between 10 and 12 weeks. This observation indicated a rather slow spreading pattern of PRRSV. At the time of entry on four commercial fattening farms, which is 10–11 weeks of age, 15%, 50%, 75% and 60%, respectively, of the pigs had antibodies to PRRSV. Based on the rate of decline of maternal antibodies as determined by the studies on the closed breeding-fattening farms, it was indicated that these positive pigs had already been infected on the farm of origin. Occasionally, pigs at that age with a low antibody titre may still have waning maternal antibodies. The latter pigs and the pigs negative at entry had seroconverted 4 weeks after arrival, indicating that a PRRSV infection had occurred on all four fattening farms.