A friendly reminder: serum amyloid A (SAA), a major acute-phase protein in horses, is routinely provided in all AHL comprehensive equine biochemistry profiles!
Acute-phase proteins are produced by the liver in response to inflammatory or infectious stimuli. Increased hepatic production begins within 4-5 h of a stimulatory event, leading to increased serum concentrations that are a reflection of both protein production and peripheral catabolism. Major acute phase proteins, such as serum amyloid A (SAA) in horses, are present in negligible amounts (AHL reference interval: 0-20 mg/L) in healthy animals but increase rapidly and dramatically (often 1,000-fold) following an inciting event. Moderate acute-phase proteins, such as fibrinogen, are found in low concentrations in health, and increase modestly (10-fold) and more slowly (within 24-72 h), in response to inflammatory stimuli. The acute phase protein profile differs from species to species; with some proteins exhibiting the characteristics of a major acute phase reactant in one species, yet only negligible increases to inflammatory stimuli within another.
SAA is a major acute phase protein in horses. SAA is thought to play a role in cholesterol transport, platelet and neutrophil function, and modulation of inflammatory processes. Increased serum concentrations are found in horses suffering from a variety of disorders including viral and bacterial infections, inflammatory disease, colic, neoplasia, trauma and surgery. Foals have higher physiologic concentrations than adults for ~7 d after foaling. Increases in SAA occur much earlier in the course of disease than changes in total leukocyte count or fibrinogen, thus facilitating timely clinical intervention. Although infectious stimuli generally elicit a greater SAA response than inflammatory stimuli, the magnitude of increase is non-specific in nature, and does not support a particular etiology. SAA is rapidly cleared from circulation, therefore the high concentrations accompanying active inflammation or infection will decrease quickly once the eliciting stimulus has subsided, or successful treatment has been initiated. Serial SAA determinations may be helpful in gauging response to treatment and possibly as a predictor of recovery from illness. Relapse or clinical deterioration will result in sustained increases in SAA, which are then often accompanied by concurrent increases in moderate acute phase proteins such as fibrinogen. Recent studies have included evaluation of SAA concentrations in synovial and peritoneal fluids, and its use as a marker of performance in endurance horses; the reader is referred to the literature for additional details.
Mean concentrations of SAA (µg/mL), haptoglobin (Hp; mg/mL) and fibrinogen (Fb; g/L) in 19 horses undergoing elective surgery and 8 horses undergoing nonelective surgery.
From: Pollack et al. Effects of surgery on the acute phase protein response in clinically normal and diseased horses. Vet Record 2005:156;538-542.
SAA has been included in the AHL comprehensive equine biochemistry profile since 2010, and it has been an invaluable addition to the traditional markers of equine inflammation. AHL
Belgrade RL, et al. Assessment of serum amyloid A testing of horses, and its clinical application in a specialized equine practice. J Am Vet Med Assoc 2013;243:113-119.
Westerman TL, et al. Evaluation of serum amyloid A and haptoglobin concentrations as prognostic indicators for horses with colic. J Am Vet Med Assoc 2016;248:935-940.