Estimation of the transmission parameters for swine influenza and porcine reproductive and respiratory syndrome viruses in pigs from weaning to slaughter under natural conditions.

In the present study, the transmission parameters of swine influenza virus (SIV) and porcine reproductive and respiratory virus (PRRSV) have been calculated using the basic reproductive rate (R) parameter in two commercial pig farms (F1 and F2). In order to do this, a serological (PRRSV genotype 1 and SIV) and virological (SIV) follow-up of a batch of animals was carried out weekly from 3 weeks of age until the age of slaughter on each farm. Results of the analysis for SIV and PRRSV showed different transmission profiles depending on the farm, the pathogen, and time of transmission. In F1, transmission of both viruses was detected throughout the sampling. The R (R for a given period of time) value for SIV ranged from 1.5 [0.9-2.3] to 3.6 [2.3-4.9] from farrowing to the beginning of the fattening period, and the R value for PRRSV was 3.3 [2.9-4.3] to 3.5 [2.8-4.1] from farrowing until the slaughter age. These results indicated that both viruses were transmitted enzootically in that farm for these periods of time. A different transmission pattern with a higher incidence was also observed during the fattening period in F1 (after 15 weeks of age) for SIV, coinciding with the entrance of a new subtype. In this case, R value for SIV reached 3.3 [1.65-4.9]. On the other hand, in F2, SIV and PRRSV seemed to be restricted to the fattening period. R reached a value of 6.4 [4.1-8.8] for SIV and 7.1 [3.5-10.6] for PRRSV. These findings suggest a different origin of the virus, as well as a more epidemic circulation, especially for SIV, where most of the new cases were observed in a one week period. In conclusion, the present study offers a reliable estimation of the range of R values for SIV and genotype 1 PRRSV transmission under field conditions, suggesting that enzootic circulations of both viruses are similar in terms of transmission, probably higher for PRRSV, but also that transmission of SIV is more efficient (or epidemic) than transmission of a genotype 1 PRRSV isolate in naïve animals given the new cases observed in only in F2.