Acute phase protein response in an experimental model of ovine caseous lymphadenitis.

Background: Caseous lymphadenitis (CLA) is a disease of small ruminants caused by Corynebacterium pseudotuberculosis. The pathogenesis of CLA is a slow process, and produces a chronic rather than an acute disease state. Acute phase proteins (APP) such as haptoglobin (Hp) serum amyloid A (SAA) and alpha1 acid glycoprotein (AGP) are produced by the liver and released into the circulation in response to pro-inflammatory cytokines. The concentration of Hp in serum increases in experimental CLA but it is not known if SAA and AGP respond in parallel or have differing response profiles.

Results: The concentration in serum of Hp, SAA and AGP in 6 sheep challenged with 2 x 105 cells of C. pseudotuberculosis showed significant increases (P < 0.05) compared to 3 unchallenged control sheep. By day 7 post infection. (p.i.) the Hp and SAA concentrations reached mean (+/- SEM) values of 1.65 +/- 0.21 g/L and 18.1 +/- 5.2 mg/L respectively. Thereafter, their concentrations fell with no significant difference to those of the control sheep by day 18 p.i.. In contrast, the serum AGP concentration in infected sheep continued to rise to a peak of 0.38 +/- 0.05 g/L on day 13 p.i., after which a slow decline occurred, although the mean concentration remained significantly higher (P < 0.05) than the control group up to 29 days p.i.. Specific IgG to phospholidase D of C. pseudotuberculosis became detectable at 11 days p.i. and continued to rise throughout the experiment.

Conclusion: The serum concentrations of Hp, SAA and AGP were raised in sheep in an experimental model of CLA. An extended response was found for AGP which occurred at a point when the infection was likely to have been transforming from an acute to a chronic phase. The results suggest that AGP could have a role as a marker for chronic conditions in sheep.