IMPROVING PATIENT CARE WITH EQUINE SERUM AMYLOID A TESTING SIDDRA HINES, DVM, PHD, DACVIM Veterinary Scientist, VMRD, Inc.
KEY HIGHLIGHTS
1 SAA INCREASES RAPIDLY AND DRAMATICALLY WITH ACUTE SYSTEMIC INFLAMMATION Early identification of infection in the horse is both critical and challenging, with initial clinical signs often being subtle. To this end, the acute phase protein serum amyloid A (SAA) is a sensitive indicator of inflammatory status, even more so than elevated body temperature. SAA is virtually undetectable in normal animals but increases within 6-12 hours in acute inflammation, reaching up to 1000-fold baseline values. This is far more rapid and dramatic than alterations in fibrinogen or WBC count. SAA is rapidly responsive to clinical changes and begins dropping within 12-24 hours as inflammation starts to resolve. 2 ELEVATION IN SAA IS COMMONLY CAUSED BY BACTERIAL OR VIRAL INFECTION SAA increases with any acute systemic inflammation; however viral or bacterial infections are common culprits. Bacterial infections in particular cause high levels of SAA. Non-infectious etiologies may cause mild elevation, but minimal increase is seen with chronic or localized disease. SAA can be used to differentiate between disease processes with similar clinical presentations and help identify those requiring antimicrobials. By this same token, SAA can help assess potential etiology and disease severity in horses with non-specific signs such as ADR or fever of unknown origin.
Equine veterinarians often use SAA to differentiate pneumonia from uncomplicated equine asthma. Significant elevation strongly supports a primary or secondary infection, as allergic etiologies alone stimulate minimal SAA production. If a bacterial infection is suspected, antimicrobial therapy can be instituted, and efficacy monitored through repeat SAA measurement. 3 TRACKING SAA OVER TIME IS PARTICULARLY VALUABLE Veterinarians can monitor the trend of SAA values over time to track clinical progress, including response to treatment. The dynamic nature of SAA makes it an ideal marker for this purpose. SAA measured at a single time point may not reflect overall peak value, as it will increase for 2-4 days following an acute inflammatory insult and may not yet have peaked when the horse is first evaluated. Appropriate application of follow-up testing can avoid confusion by characterizing the peak value to help with interpretation of subsequent results. (continued)
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