The objective of the present study was to gain more insights into the relationship between sample-to-positive (S/P) ratios in OF and sera on one hand and into infection profiles on farms on the other hand.

Material and Methods

In 20 Belgian farrow-to-finish operations, 3 OF samples were collected in each farm in pens of animals of three age categories as follows: (1) 4 to 7 weeks, (2) 8 to 12 weeks, (3) 13 to 21 weeks. Each pen housed 5 to 13 pigs. Corresponding serum samples were collected from all pigs. All serum and OF samples were assayed for PRRSV antibodies using commercial indirect ELISA kits (PRRS X3 AB and PRRS OF respectively, IDEXX). A positive result was defined as a S/P ratio ≥ 0.4. In addition, presence of viral RNA was detected on the OF samples by RT-PCR (Virotype PRRS).

Results

The correlation coefficient between the average serum S/P ratio and the OF S/P ratio was 0.68; 0.87 and 0.78 respectively in the 3 different age groups. In the two youngest groups, PRRS S/P ratios of both OF and serum showed large variation within farms. In the oldest group however, PRRS serum and OF S/P values were both either very low or very high. The former were farms in which PRRSV circulation, indicated by positive RT-PCR results in the OF samples, was low/absent. The number of PRRS RT-PCR positive OF samples out of 20 was 6, 14 and 8 in the 3 different age groups respectively. All isolates belonged to the European type.

Discussion

The lower correlation coefficient in the pigs from the early nursery likely resulted from the high variation in S/P results between piglets from different farms, as this diverse group included animals with high and low levels of maternal antibodies as well as recently infected animals. Furthermore, the results indicated that most PRRSV infections was occurring between the end of the nursery and the beginning of the fattening period. Therefore, understanding the degree of farm infection using RT-PCR may be the most successful at the late nursery or early fattening phase, and should include antibody monitoring at multiple times during the fattening phase as well.

Conclusion

In conclusion, the results indicated that OF can be a useful tool for PRRSV monitoring, even under endemic conditions. Sampling in multiple age groups on a regular basis provides timely information for improved PRRSV management.

Presented at the 2015 European Symposium of Porcine Health Management

October 2015