OBJECTIVES Aberrant wound healing with formation of exuberant granulation tissue (EGT) occurs frequently in horses and may affect their athletic career and quality of life. The objective of the study was to determine mRNA expression levels of serum amyloid A (SAA) in normal and aberrant wound healing in horses.

METHODS Experimental wounds were made in six horses on both metatarsi and on regio brachii. One limb was bandaged to provoke formation of EGT. Biopsies were collected on day 21 and were divided in three groups: body wounds (regio brachii), unbandaged limb wounds (normal healing), and bandaged limb wounds (aberrant healing with formation of EGT). All biopsies were examined for the relative mRNA expression level of SAA using qRT-PCR. Differences in SAA expression levels between the three groups were analyzed by Kruskal-Wallis test and Dunns test.

RESULTS SAA mRNA level was significantly higher (P < 0.01) in limb wounds healing with EGT formation than in body and limb wounds with normal healing. In body wounds and limb wounds with normal healing SAA expression was very low, in EGT SAA expression levels varied from low to very high.

CONCLUSIONS SAA is a major equine acute phase protein, which is produced in the liver and several extrahepatic tissues during inflammatory conditions. This study shows that cells in EGT derived from horses produce SAA. This may be related to the length of the inflammatory phase of the wound healing, which is short (approximately 3 days) in wounds with normal healing, but protracted in wounds developing EGT.
Chronic inflammation might facilitate binding of SAA-containing high density lipoproteins (HDL) to extracellular vascular proteoglycans, which would favour retention and modification of HDL by the vascular matrix. Such changes could be the pathophysiological reason underlying endothelial cell dysfunction and subsequently hypoxia, which has been implicated in EGT formation.