Serum amyloid A increases following routine vaccination of healthy adult horses.

Overview

• This study investigates how routine vaccination affects serum amyloid A (SAA) levels in healthy adult horses.
• The researchers found that vaccination causes a temporary increase in SAA concentration, which peaks around day 2 and returns to normal by day 9.

Introduction

• Serum amyloid A (SAA) is an acute-phase protein, which means its concentration increases in response to inflammation or infection.
• SAA is often used as a biomarker to detect systemic inflammation or illness in horses and other animals.
• The goal of this research was to understand whether routine vaccinations cause an increase in SAA in healthy horses, which would be important for interpreting SAA levels clinically after vaccination.

Study Design and Methods

• Subjects: 36 healthy adult horses were studied—21 client-owned horses and 15 university-owned horses.
• Two experiments were conducted:
  • Experiment 1: A blinded, randomized, prospective crossover study with 8 horses.
    • Each horse received either a vaccine cocktail (against rabies, tetanus, West Nile virus, Eastern and Western equine encephalomyelitis, equine herpesvirus-1/-4, influenza) or saline.
    • SAA levels were measured at multiple time points: 6, 12, 24 hours, and daily up to 10 days post-treatment.
    • After a washout period, horses crossed over to the other treatment.
  • Experiment 2: A prospective observational study including 28 horses vaccinated with the same vaccine cocktail.
    • SAA was measured at 12 and 24 hours, then daily for up to 10 days after vaccination.
• Serum amyloid A levels were measured using a commercial lateral-flow immunoassay, a rapid diagnostic test.
• A linear mixed-effects model with repeated measures was used for statistical analysis, accounting for repeated SAA measurements on the same horses over time and comparing vaccinated versus unvaccinated groups.
• The correlation between body temperature and SAA was also evaluated to assess if fever was related to SAA changes.

Results

• Vaccination significantly increased SAA levels compared to saline treatment or baseline, even though horses showed no clinical signs of adverse reactions.
• In Experiment 1:
  • The median SAA concentration peaked on day 2 post-vaccination at approximately 1,872 µg/mL (with an interquartile range of 1,220.8 to 2,402.5 µg/mL).
  • SAA levels returned to normal levels (< 20 µg/mL) by day 9 post-vaccination (median 6 µg/mL).
• In Experiment 2:
  • SAA levels also increased over time following vaccination.
  • There was no significant correlation between body temperature and SAA (P = .78), indicating that SAA increase was not associated with fever.

Interpretation and Implications

• Routine vaccination in healthy adult horses causes a noticeable but temporary increase in SAA levels without causing clinical illness or fever.
• This transient increase reflects an expected immune response and inflammation as the horse’s immune system reacts to vaccine components.
• SAA returns to baseline within approximately 9 days, indicating a typical convalescence or recovery period after vaccination.
• Clinicians should be cautious when interpreting elevated SAA levels within 10 days following vaccination, as this rise may not indicate disease.
• Using SAA as a biomarker to diagnose illness immediately after vaccination may lead to false positives for systemic inflammation or infection.

Conclusions

• Routine vaccination in healthy horses increases serum amyloid A concentration significantly but transiently.
• The increase peaks around 2 days and returns to normal levels within 9 to 10 days post-vaccination.
• SAA elevation during this period should not be used alone to diagnose illness, as it is part of the normal immune response to vaccination.
• This insight helps veterinarians better interpret SAA levels and avoid unnecessary treatment or concern after routine vaccinations.