‘Mystery’ Cases and SAA Testing

swollen hind leg horse
Is it trauma or cellulitis? SAA testing can help veterinarians determine infectious versus non-infectious causes of limb swelling.

Phoebe Smith, DVM, DACVIM-LA, is a consultant and internal medicine specialist at Riviera Equine Internal Medicine & Consulting based in Los Olivos, California. The vast majority of her equine patients come to her through referral. Because of that, she said she is often referred “mystery” cases where primary care veterinarians request her help in figuring out what is going on with a horse.

Editor’s note: This article shares information about Smith’s use of Zoetis Equine’s Stablelab® stall-side test that measures and quantifies Serum Amyloid A (SAA) levels—a biomarker of possible infection in horses—in less than 10 minutes.

“I work with a wide variety of patients, from foals to elite sport horses,” said Smith. “To unravel the mystery, I start by obtaining a thorough history, reviewing medical records and lab work. Serum Amyloid A is a very useful test in many cases, helping to distinguish infectious from non-infectious causes of a problem.”

Stablelab’s SAA test is part of an “inflammatory profile” that she often requests the primary veterinarian run before she sees the horse, or she runs the initial evaluation herself. She uses SAA in addition to other tests, including white blood cell counts, fibrinogen, chemistry panel, and specialized tests for specific diseases when indicated.

Smith said many of her patients are “super mystery” cases. “That’s about 50% of the cases I see,” she said with a laugh. “I often use SAA to help me know which way to proceed in the initial evaluation. Using a decision-tree approach, SAA test results can help guide which next steps are most appropriate.”

When SAA Became a Stall-Side Test

Internal medicine specialists tend to be detectives at heart, and they love a mystery to solve. “I was among the early adopters of the [Stablelab] stall-side SAA test,” Smith noted.

She had followed the early research and wanted to incorporate SAA testing in her practice to learn which cases SAA testing added value to.

Smith said she has now run Stablelab tests on thousands of equine cases. “I’ve learned how [SAA testing] fits into the diagnostic process, what information it provides and when and how often to use it for different disease processes,” she stated.

SAA in Practice

Since Stablelab is a quick, stall-side test that takes 10 minutes, Smith said she often collects blood for SAA and other testing, then her assistant runs the test immediately while she is talking to the horse owner.

“It gives me some help quickly to know if I’m dealing with allergy, an infection or non-infectious inflammation,” said Smith. “I use it in a wide variety of cases.”

Smith said that trends in SAA values are more important than a single number. “One [Stablelab test result] is just one point in time,” she noted. “If the SAA test result is equivocal, either I or the primary veterinarian will check SAA on the horse again in a couple of days.”

Likewise, elevated Stablelab SAA test results are rechecked during the treatment period to assess efficacy of the selected treatment.

The frequency of rechecks with Stablelab SAA testing will depend on the disease and the animal. For example, “with foals, I recheck frequently, especially with neonatal foals because they are so immunologically vulnerable,” she said.

As an internal medicine specialist, Smith doesn’t see lameness cases. “The types of cases I see are horses that come in with problems such as weight loss, endocrine and neurological dysfunction, fever of unknown origin, poor performance not related to lameness, and foals,” she said. “The sport horse cases have often been thoroughly evaluated for soundness by the primary care veterinarian and are referred to me after musculoskeletal causes of poor performance are ruled out.”

The Stablelab SAA value in those cases helps distinguish a potential low-level infection from other causes of poor performance as an important starting point.

Smith said it is important for veterinarians to be familiar with SAA testing and how it relates to different medical conditions. She said if they are not familiar with SAA testing, they should reach out to a colleague who is.

“In some cases, you expect a certain result, and that’s not what you get,” she said. “It doesn’t mean the test is inaccurate because the result is a real value, but that the condition must be reconsidered.”

For example, she said, Corynebacterium pseudotuberculosis (aka pigeon fever or dryland distemper) can produce [Stablelab] results with a wide range—from zero to 2000.

“You may see an enormous abscess and have a low SAA value, or no visible abscess and a very high SAA,” said Smith. “Stage of disease and the presence of a thick capsule or walled-off infection affect SAA results.”

‘Fat-Legged’ Acute Horses

Smith said that while primary care veterinarians often see and diagnose cellulitis cases, occasionally she sees an acute case of limb swelling. “Initially, I may not know if it is trauma or cellulitis,” she said.

While Smith noted that some cellulitis cases have fever when she examines them, limb swelling can develop before fever appears, or an owner might have given the horse NSAIDs that reduced its fever.

“I like to start a [Stablelab] test running while I examine the patient,” she said. “This test helps distinguish between infectious and non-infectious causes of limb swelling.”

While some horses with cellulitis might have apparent abscesses, sometimes there is nothing to culture. And even in the cases with abscesses where cultures are taken, “while awaiting results, we have to select an appropriate antimicrobial,” said Smith. “Tracking the trend of Stablelab SAA results in this type of case is quite useful for monitoring response to therapy.”

Response to antibiotics is not “linear,” Smith said, “so the trend of riding or declining SAA value is far more useful than a single value at any time point.”

Smith added that with some infectious disease cases, she also will use Stablelab SAA testing about five days after completing antimicrobial therapy to confirm there is no regrowth of microbes.

About Dr. Smith

Phoebe Smith, DVM, DACVIM, leads the Riviera Equine Internal Medicine & Consulting veterinary practice near Los Olivos, California. Smith provides specialized internal medicine services for horses in the central coast and southern California regions.

“We work with veterinarians seeing complex internal medicine cases, trainers and riders working to maximize athletic performance, and owners requesting a second opinion or specialist evaluation,” she said. “Equine medicine is a rapidly advancing field and one simply cannot know it all. Specialization allows for in-depth knowledge and expertise in a specific area of medicine.

“We believe the best medicine is delivered by a team approach,” continued Smith. “Professionalism, collegiality and collaboration are foundations of any successful team, and we strive to build this with every consultation.”

Smith received her DVM from Auburn University and her DACVIM, Large Animal, from the University of California, Davis. She started Riviera Equine in July 2021 after tenures at The Ohio State University and as the staff internist at Alamo Pintado Equine Medical Center.

Stablelab is a registered trademark of Epona Biotech Limited, used under license. STB-00174

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