A longitudinal study into the interaction between PRRSV and Mycoplasma hyopneumoniae in finishing pigs

Abstract - A synergistic interaction between the Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) and other porcine respiratory infectious agents for the Porcine Respiratory Disease Complex (PRDC) in growing and finishing pigs has been suggested mostly based on empirical observations in the field. So far, most scientific investigations have been conducted under experimental conditions, as infection with several viral or bacterial agents during the acute stage of PRRSV infection, i.e. the interval between the PRRS and the second infectious agent, has always been relatively short. Little is known, however, about the real time span between the infections in case of dual infections with PRRSV and Mycoplasma hyopneumoniae under field conditions. The presented investigation was a longitudinal study over two years in a fattening herd with an all-in/all-out system. Altogether 3574 pigs (8 fattening groups) with 4-11 different origins per group were clinically investigated every second day during the whole fattening period. Ten percent of the pigs from each fattening group were individually marked and bled at 7 points of time. The sera were tested for antibodies against PRRSV by means of an IPMA (carried out at the Federal Research Centre for Virus Diseases of Animals, Tübingen, Germany) and for antibodies against M. hyopneumoniae by means of an ELISA (carried out with the ELISA test kit by Bommeli AG, Bern, Switzerland). Growth performance was calculated for all serologically investigated pigs, and slaughter checks were done as well. Taking published results into account, a time interval of 14 days after the last possible point of infection with the PRRSV was defined for estimating the frequency of infections with M. hyopneumoniae during the acute stage of PRRSV infection. For determining the time of infection with the PRRSV, a time interval of 7 days, and for M. hyopneumoniae a time interval of 21 days between infection and seroconversion was assumed. At the beginning of the grow-finish period, when the pigs were on average 75 days old, 19% had antibodies against PRRSV and only 0.6% antibodies against M. hyopneumoniae. After two weeks, 53% and after four weeks 95% of the pigs had antibodies against PRRSV. By the sixth week, all pigs, which had been PRRSV antibody negative at the time of arrival on the farm, seroconverted. During the first weeks of the grow-finish period, only very few pigs seroconverted against M. hyopneumoniae, and no remarkable increase of seroconversion against M. hyopneumoniae took place before the time between the sixth and ninth week. Altogether 48% of the pigs had antibodies against M. hyopneumoniae at the end of the fattening period. The seroconversion against M. hyopneumoniae correlated significantly with the prevalence of coughing. Altogether only 11% of the pigs had a seroconversion against M. hyopneumoniae and PRRSV within a short period of time. Further seroconversions against M. hyopneumoniae took place in a distance of several weeks when no or low risk for interactions is suggested. In total, pigs that seroconverted against M. hyopneumonia showed a significant growth retardation and pneumonic lesions were significantly increased, but there was no influence of the infection with M. hyopneumonia during the acute stage of PRRSV. Under the field conditions described, it appears that fattening pigs have only a low risk for infection with M. hyopneumoniae during the acute stage of PRRSV infection, since the seroconversion against M. hyopneumoniae took place only several weeks after the PRRS infection. This time interval is considerably longer than the time interval between infection and seroconversion in M. hyopneumoniae determined by most authors, which means that the majority of the infections with M. hyopneumoniae clearly took place long after the acute phase of PRRS. Based on the obtained results, there is no evidence for a significant involvement of PRRSV in the porcine respiratory disease complex (PRDC), as supposed by some researchers. No difference in the average daily weight gain and the frequency of pneumonic lesions at slaughter has been observed between pigs seroconverted against both agents within a short period of time, compared to pigs seroconverted within a period of several weeks/month.