SAA: A Magic 8 Ball for Detecting Infection in Horses?
The acute phase protein serum amyloid A (SAA) is a powerful tool for equine veterinarians. SAA is a more reliable indicator of both inflammation and/or infection than white blood cell count and fibrinogen and can also help veterinarians recognize when a horse is returning to health.
A serum protein is considered an acute phase protein (APP, a protein produced by the liver and circulating in the bloodstream) if it increases or decreases more than 25% in response to inflammation. If it increases, it is considered a positive APP.
Most of the APPs used in veterinary medicine are positive APPs, including SAA, C-reactive protein, and fibrinogen. SAA stands out among those positive APPs, as it is the only “major” APP used in equine medicine.
“SAA has a favorable ‘signal to noise ratio,’ meaning that there are normally low circulating levels of SAA in the bloodstream normally,” explained Rodney L. Belgrave, DVM, MS, Dipl. ACVIM, staff internist at the Mid-Atlantic Equine Medical Center, in Ringoes, New Jersey, during the Northeast Association of Equine Practitioners Convention, held Sept. 21-24, 2022, in Saratoga Springs, New York. “In response to inflammation, there is a rapid increase in SAA, in some cases upward of 10,000-fold. SAA also has a short half-life, so there is a rapid decline as the inflammation resolves.”
In contrast, mild and moderate APPs fibrinogen and C-reactive protein, respectively, are already present at low levels in the blood and gradually increase only one- to tenfold in response to inflammation. The increase in fibrinogen, for example, can take 24-72 hours in contrast to the immediate “spike” in SAA blood levels.
Monitoring APPs can help veterinarians differentiate between diseases that do and do not have inflammatory components. For example, they can use SAA to determine if the horse has a bacterial infection.
“APPs like SAA can also provide information regarding prognosis and survival and can also be used in monitoring the efficacy of our treatment protocols,” Belgrave said.
Other Uses for SAA
Belgrave noted these additional uses for SAA:
- Foals with umbilical cord infections, which can have SAA levels about 195 mg/L.
- Septic foals, which can have SAA levels about 280 mg/L.
- Postsurgical SAA elevations. “Even minor surgical procedures cause inflammation,” he said. “SAA can remain elevated at 400-600 mg/L for up to three days post-surgically.” If those SAA elevations persist, however, veterinarians can use them to identify postoperative complications.
- The tick-borne disease anaplasmosis in the case of horses with fevers, particularly in the Northeastern United States.
- Septic joints, identified by measuring SAA in synovial fluid. The SAA concentration in synovial fluid should be 0. So even if SAA levels are low, around 300-400 mg/L, you should be concerned, said Belgrave. “Even if the SAA in synovial fluid is normal and the horses is non-weight-bearing lame, you’re likely going to treat it the same as if it were infected. Joints are always septic until proven otherwise,” he said.
SAA Limitation: Asthma
SAA increases in response to a variety of conditions, including Streptococcus equi infections, bacterial pneumonia, cellulitis (skin infections), colitis/diarrhea, and peritonitis (abdominal infection).
“SAA is not, however, increased in horses with equine asthma,” said Belgrave. “It is more reliably elevated with infections of the respiratory tract, both viral and bacterial, whereas asthma typically has normal SAA except if the horse has a secondary bacterial component.”
Nonetheless, Belgrave said it might be worthwhile to measure baseline SAA in horses with suspected equine asthma.
“You want to make sure the horse does not have a concurrent secondary bacterial component, as many asthma cases will be getting a steroid,” he said.
SAA Limitation: After Vaccination
In some situations measuring SAA levels is not valuable, such as when evaluating post-vaccination fevers.
“SAA may be increased following vaccination with equine herpesvirus-1 or -4,” said Belgrave. “Don’t be surprised to see SAA levels of about 1,300 mg/L in horses with post-vaccination fevers. Increases post-vaccination are common and expected to be higher, especially in those that experience a reaction. If the horse’s condition post-vaccination prompts testing of SAA and it is increased, the test should be repeated to confirm that it is trending back down toward normal.”
SAA Limitation: Rhodococcus Equi
Belgrave also discouraged veterinarians from using SAA to screen foals for Rhodococcus equi. He said SAA has a poor predictive value in this capacity, likely because R. equi is a chronic disease, not acute.
“Ultrasound is still a better choice for identifying subclinically infected foals,” Belgrave said.
Tips for Using SAA
“Generally speaking, we are looking for trends in SAA levels, not single values,” said Belgrave.
He showed a graph illustrating the sharp peak of SAA occurring on Days 3-5 in an inflammatory situation.
“We want to take serial measurements of SAA to identify this peak,” he said. “If a horse is tested on Day 2 on the upside of the curve, then not again until Day 5 or 6 on the downside of the curve, the SAA value could still be higher than the original measurement, but it is actually on its way down. A practitioner might think the horse is not responding and change the treatment when in reality SAA is decreasing, and the treatment is working,” he explained.
In other words, SAA could initially measure 600 mg/L, and on Day 5 it might measure 1000 mg/L, but the peak has already happened, and levels are going back down. Instead, measuring on Days 1, 3, 5, and 7 would more clearly show the horse is recovering, said Belgrave.
He also advised sticking with the same type of sample for all subsequent evaluations, either blood or plasma. In addition, dilute the sample to get an actual number rather than being satisfied with a value that reads >3000 mg/L.
Assessing SAA levels can help veterinarians diagnose many equine conditions. However, the tests do have their limits, and SAA is not useful for diagnosing and monitoring certain infections or conditions. Additionally, veterinarians must consider SAA levels over time and use them as a piece of the puzzle versus the only factor in a potential diagnosis.
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