Matrix Metalloproteinase-9 Concentration in Peritoneal Fluid of Horses with and Without Strongyle Infection

Strongyle infections are common in horses and can cause clinically relevant disease, yet their associated inflammatory responses remain incompletely understood. Matrix metalloproteinase-9 (MMP-9), a key mediator of extracellular matrix degradation and inflammation, has been proposed as a biomarker in various equine diseases. This study investigated whether strongyle infection is associated with increased MMP-9 concentrations in equine peritoneal fluid (PF). A total of 83 horses presenting with colic, weight loss, or faecal water were included, and strongyle infection was determined semi-quantitatively by combined sedimentation–flotation. MMP-9 concentrations in non-anticoagulated and citrate-anticoagulated PF were measured using ELISA, and non-parametric statistical tests were applied. MMP-9 measurements in non-anticoagulated samples were strongly correlated with the values from citrate-anticoagulated samples. No significant differences in MMP-9 concentrations, total nucleated cell count (TNCC), total protein (TP) or lactate were observed between strongyle-positive and -negative horses, nor across faecal egg count categories. In contrast, MMP-9 concentrations showed significant positive correlations with inflammatory parameters, including TNCC and TP, irrespective of strongyle infection. These findings indicate that peritoneal MMP-9 reflects non-specific inflammatory processes rather than strongyle infection. The lack of association may also reflect limitations of faecal egg counts and sampling site, suggesting that PF is not optimal for detecting parasite-specific inflammation.