Towards a better understanding of salivary and meat juice acute phase proteins determination in pigs: an expression study

Acute phase proteins (APPs) determination in different fluids like serum, saliva and meat juice measured with ultrasensitive assays can be used to evaluate the disease status of porcine populations under field conditions. Liver is the main production site of serum APPs, but the origin of APPs that can be determined in body fluids different from blood remains unknown. The objective of this study was to clarify the origin of three APPs: C-reactive protein (CRP), serum amyloid A (SAA) and haptoglobin (Hp) in saliva and meat juice. The mRNA expression of these proteins was measured in liver, salivary gland and diaphragmatic muscle by quantitative PCR and compared with the protein levels in serum, saliva and meat juice, respectively in healthy and naturally diseased animals. As expected, concentrations of all APP were significantly higher in all body fluids from diseased animals. Levels of all APPs mRNA were very low in diaphragmatic muscle tissue, and the expression was independent of the disease status. In contrast, we found higher expression levels of SAA and Hp mRNA in the salivary gland of diseased animals, while CRP mRNA was not detected. Our data indicate that the APP present in meat juice derived predominantly from serum. This assumption is also supported by the good correlation of the levels of both proteins in meat juice with those in serum. Further, the lower variability of the APP levels within the two groups of animals, suggests meat juice as an alternate sampling material. The APP levels that are determined in saliva, however, appear to result from an increased local production except for CRP, indicating that the salivary gland responds to disease. These findings are relevant for the establishment of saliva as the preferred diagnostic sample for health monitoring programmes, due to the technical and ethical advantages of the collection.