Serum amyloid A increases following routine vaccination of healthy adult horses

Abstract OBJECTIVES To measure the effect of routine vaccination on serum amyloid A (SAA) concentration in apparently healthy horses. We hypothesized that routine vaccination would increase SAA in healthy horses. ANIMALS 21 apparently healthy client-owned horses and 15 Kansas State University College of Veterinary Medicine–owned horses. METHODS In experiment 1 (n = 8 horses), a blinded, randomized, prospective, crossover study was performed. Horses were either vaccinated (rabies, tetanus, West Nile, Eastern and Western equine encephalomyelitis, equine herpesvirus-1/-4, influenza) or administered saline, and SAA was measured at 6, 12, and 24 hours and daily until day 10 with a commercial lateral-flow immunoassay. In experiment 2 (n = 28 horses), a prospective, observational study measured SAA after vaccination at 12 and 24 hours and daily until day 10. A linear mixed-effect model with repeated measures over time blocked by horse tested the effect of treatment on SAA. A repeated-measures correlation tested the correlation between SAA and temperature. RESULTS Over time, vaccinated horses had increased model-adjusted SAA compared to unvaccinated horses without clinical evidence of adverse reaction ( P < .01). In experiment 1, the model-adjusted SAA after vaccination peaked on day 2 (median, 1,872 µg/mL; IQR, 1,220.8 to 2,402.5 µg/mL) and returned to normal (< 20 µg/mL) by day 9 (median, 6 µg/mL; IQR, 0.8 to 23.5 µg/mL) after vaccination. In experiment 2, vaccinated horses had increased SAA over time; temperature and SAA were not correlated ( P = .78). CLINICAL RELEVANCE Results of this study indicated that routine vaccination results in increased SAA concentration and provided evidence for a period of convalescence following vaccination. Measuring SAA for 10 days following vaccination cannot be used as an indicator of illness.