Haptoglobin in oral fluid samples from pens of pigs can potentially be used to estimate herd inflammatory status

Until recently, plasma haptoglobin (Hp) content has been used as a biomarker for immune system activation in pigs (Parra et al. 2006). The major limitation of this is the need to collect representative blood samples, which can be problematic and create errors in terms of collecting representative samples of a herd. Alternatively, measuring Hp in oral fluid samples to discriminate an unhealthy herd has been proposed (Soler et al. 2013). Therefore, the hypotheses of the present study were that (1) immune system activation of grower/finisher pigs can be discriminated by measuring Hp content in oral fluid (OF) samples, and (2) the Hp contents in the OF samples collected from pigs in a pen (the group) are significantly correlated with that in the OF samples collected from individual pigs in that group.

To test the hypotheses, seven commercial farms of varying health status (healthy v. unhealthy, as assessed by veterinarians) were selected. To test the first hypothesis, five pens per farm were randomly selected for OF and plasma sample collection from individual pigs. A total of 340 grower pigs were sampled for plasma and OF. To test the second hypothesis, two additional pens were randomly selected from each farm and an individual OF sample and a single-point OF sample were collected by hanging a cotton rope for 40 min in a pen. A total of 110 individual OF samples and 12 group pens OF samples were collected in this process. One group pen OF sample was removed from the dataset due to dehydration, as the water supply was interrupted on the sampling day. The samples were analysed for Hp content using a commercially available ELISA kit (Aviva Systems Biology, San Diego, CA, USA). Correlation and a simple linear regression analyses were conducted using Genstat 15 (VSN International, Hemel Hempstead, UK).

The mean (± s.e.) plasma and OF Hp contents determined from 340 pigs were 0.73 (0.03) mg/mL and 0.51 (0.04) µg/mL, respectively. The Hp contents in the OF samples were significantly correlated with that in the plasma samples (r = 0.44, P < 0.001). The cut-off point of the OF Hp content for discrimination between healthy and unhealthy pigs was 1.5 µg/mL (Fig. 1a). The single-point OF sample collected by hanging a cotton rope in a pen linearly correlated with the mean Hp content determined by collection of OF samples from individual pigs in the same group (r = 0.967, P < 0.001, Fig. 1b). This suggests that OF is a potentially useful sample for measuring the degree of immune system activation. However, and as the slope indicates in the regression equation, the Hp content in the group OF sample was higher by 2.56 µg/mL per unit of Hp than the average of individual OF samples. This would indicate an adjustment for the cut-off point may be required when single-point OF sample is used to evaluate inflammatory status of a herd.

Fig. 1.  Oral fluid haptoglobin concentration in individual pigs (a), and the relationship between mean haptoglobin content in individual v. group oral fluid samples (b).