This is our Friday rubric: every week a new Science Page from the Bob Morrison’s Swine Health Monitoring Project. The previous editions of the science page are available on our website.
This week Arnaud Lebret, Valérie Normand, Pauline Berton, Théo Nicolazo, Charlotte Teixeira Costa, Céline Chevance, Mathieu Brissonnier and Gwenaël Boulbria share a descriptive study comparing alternative sample types for PRRS type 1 surveillance.
Background:
Knowing porcine reproductive and respiratory syndrome (PRRS) status is essential for designing herd management protocols. For this, weaning-age pigs are a key subpopulation. Recently, different alternatives to blood sampling have been introduced because they are easier, welfare-friendly and cost-saving tools. The first objective of our study was to compare the rate of detection of PRRSV-1 by RT-qPCR in individual serum samples, family oral fluid samples (FOF) and udder wipes (UW) collected the day before weaning. The second objective was to evaluate the suitability of pooling.
Materials and Methods:
This descriptive study was conducted on a commercial 210-sow farrow-to-finish pig herd located in Brittany, France. This herd was confirmed PRRSV-1-positive unstable category I-A (according to AASV classification) before the beginning of the study. Four consecutive batches were included. In each of them, 30 litters were sampled the day prior to weaning. In the morning, litters were sampled using the following methods: blood sample from one piglet per litter, one Family Oral fluid (FOF) collected by presenting an untreated cotton rope to the sow and its piglets and one Udder Wipe (UW) collected by wiping all the underline skin of the sow’s udder.
All samples were analyzed individually and after pooling (1:3 and 1:5) by RT-qPCR. A sample was considered positive if the cycle threshold (Ct) value was ≤ 40 and the curve had a specific exponential look.
The agreement between sera and FOF was assessed at the litter level using a concordance test (kappa statistics). At the batch level, the ability of pools (1:3 and 1:5) to detect the virus was assessed. The relation between the individual’s Ct value and pool’s Ct value was assessed using linear models. Then, the Spearman coefficient between the individual Ct value and pooled Ct value was determined. For each statistical analysis, the different levels of pooling (1:3 and 1:5) were taken into account.
Results
In total, 120 litters were sampled in four batches. In each batch, at least one blood or one FOF sample was positive (Table 1). The value of the Cohen’s kappa between sera and FOF was 0.84, indicating an almost perfect concordance but Ct values were significantly lower in sera than in FOF (p = 0.0006). UW were unable to detect PRRSV in three out of four batches.
| No. of Litters | RT-qPCR + | |||
| Serum | FOF | UW | ||
| Batch 1 | 30 | 3 | 1 | 0 |
| Batch 2 | 29 | 7 | 5 | 0 |
| Batch 3 | 30 | 2 | 2 | 1 |
| Batch 4 | 30 | 1 | 1 | 0 |
| Total | 119 | 13 | 9 | 1 |
Pooling was evaluated only for sera and FOF due to the lack of positivity with UW. For sera, there was a strong correlation between individual Ct and pool Ct values (r = 0.96, p < 0.001). Two out of 12 serum samples returned negative after pooling by 3 or 5. This result did not impact the qualification of batches. Regarding FOF, after pooling by 3, seven out of nine samples returned negative, misclassifying two batches out of four. After pooling by 5, eight out of nine samples returned negative, misclassifying three batches.
Conclusions
In the conditions of our study, conducted in one specific farm PRRSV-1 infected, FOF seem to be a good alternative to blood samples, but only when analyzed individually and not after pooling. UW seems to be not suitable for PRRSV-1 surveillance.
Find the full article at: https://www.mdpi.com/2306-7381/10/9/558