An acute-phase reaction is the body’s immediate reaction to tissue injury—inflammation, infection, trauma—ultimately resulting in hepatic and extra-hepatic synthesis of acute-phase proteins (APPs). The horse’s body produces many APPs, including positive ones that have increased de novo synthesis, such as fibrinogen, serum amyloid A (SAA), haptoglobin, C-reactive protein, and alpha 2-macroglobulin. Equine veterinarians commonly use fibrinogen and SAA in a clinical setting to assess the acute phase reaction.
“SAA is particularly useful because it is a ‘major’ APP in that its concentration increases manyfold in response to inflammation, commonly up to 1,000 times or more,” says Stine Jacobsen, DVM, PhD, Dipl. ECVS, professor of large animal surgery at the University of Copenhagen, Denmark.
Importantly, she notes, “SAA does not increase in response to noninflammatory stimuli.”
We’ll review why SAA is a “major” APP protein (double entendre intended) and provide examples of how practitioners can and cannot use SAA in a clinical setting based on the available literature.
Understanding SAA Levels
Resting SAA levels are very low in healthy horses, often measuring < 0.5 mg/L and sometimes immeasurable. In response to inflammatory stimuli mediated by pro-inflammatory cytokines, SAA levels increase rapidly, typically within eight to 12 hours, reaching 5,000 mg/L or more, depending on the severity of the inflammatory stimulus. Values peak rapidly, usually within 48-72 hours of stimulation, and remain high in the face of continued inflammation. Once inflammation resolves, SAA levels decrease quickly because of their short half-life (30-120 minutes). In contrast, fibrinogen takes longer to increase and breaks down more slowly, resulting in a longer half-life—days to weeks after inflammation has subsided.
“Because of the large and rapid changes that occur in response to inflammation as well as the rapid decline in plasma concentration following resolution of inflammation, SAA is therefore very useful for real-time monitoring of inflammation in a clinical setting, which means it can be used to support clinical decision-making, such as knowing when to stop antibiotics, for example,” explains Jacobsen.
She says SAA has a wide variety of clinical uses, including:
- Assessing inflammation during the diagnostic process to prioritize differential diagnoses or to detect subclinical inflammation.
- Serving as a prognostic indicator after diagnosis.
- Monitoring disease progression and response to therapy.
“This APP can assess response to therapy and direct cessation of treatment, detect relapse of disease, and identify complications,” she adds.
SAA in a Clinical Setting
Here we’ll review a few studies in which researchers evaluated SAA levels in horses with respiratory, neurologic, and synovial conditions.
Monitoring Clinical Response in Bacterial Pneumonia
In a 2022 study, Hepworth-Warren et al. evaluated SAA levels in horses with naturally occurring bacterial pneumonia and pleuropneumonia.
“These conditions often necessitate long-term antimicrobial treatment, and cessation of treatment is often based on clinical experience, normalization of hematologic variables (e.g., white blood cell [WBC] count), and improvement of thoracic imaging,” says lead author Kate Hepworth-Warren, DVM, DACVIM, assistant professor of equine internal medicine at North Carolina State University. “However, WBC count can be normal in horses with bacterial pneumonia, and fibrinogen is inconsistently elevated; thus, alternative monitoring strategies are needed.”
Using a hand-held monitor, the research team measured SAA levels in 18 hospitalized horses on Days 0 to 2 of admission, then once every three days until time of discharge. They also performed a complete blood cell count and measured fibrinogen on those days.
Results showed SAA at the time of admission was 530 ug/mL, and SAA levels peaked on Day 2 (1,037 mg/mL). After that, SAA decreased steadily until time of discharge. SAA at discharge was 0 mg/mL in 40% of the horses and lower than at the time of admission in 94% of the horses. These results were presented as geometric means, says Hepworth-Warren. (Note that ug/mL is the same as mg/L, which are frequently reported units for SAA.)
“There wasn’t a significant or consistent change in white blood cell count or fibrinogen, so regular monitoring of SAA in conjunction with imaging and the horse’s clinical picture (e.g., fever, cough, nasal discharge) may be more useful than CBC data,” she advises.
In 2017, Viner et al. evaluated SAA levels in horses with various respiratory conditions. That study involved 207 horses diagnosed with equine influenza virus, equine herpesvirus-4, Streptococcus equi subspecies equi, inflammatory airway disease, or serving as healthy controls. SAA levels were significantly greater for the horses with infectious respiratory conditions (influenza, herpesvirus-4, S. equi) compared to control horses. Further, SAA levels for the bacterial condition S. equi were higher than the viral conditions influenza and herpesvirus-4; however, there was substantial overlap between these two causes of respiratory infection.
Based on these findings, the study authors advised against using SAA levels alone to distinguish between bacterial and viral respiratory disease. However, SAA can “facilitate early detection of respiratory infections, help track disease progression, and aid practitioners in making recommendations about proper biosecurity and isolation of potentially contagious horses,” they said.
SAA Is Not Useful for Diagnosing EPM
Equine protozoal myeloencephalitis (EPM) is challenging to diagnose, often requiring a cerebrospinal fluid tap to demonstrate the presence of intrathecal antibodies against Sarcocystis neurona or Neospora hughesii. Spinal taps are more commonly performed in hospital settings than in barns, which means equine practitioners often rely on serum tests to diagnose EPM. Serum tests, however, have high false positive rates.
Because EPM could have a systemic inflammatory component, some researchers suggest measuring acute phase proteins such as SAA and C-reactive protein (CRP), combined with serology, might support a diagnosis in the field without a CSF tap. To determine if SAA could help diagnose EPM, Amy Johnson, DVM, Dipl. ACVIM (Large Animal Internal Medicine and Neurology), and colleagues from the University of Pennsylvania measured SAA in repository paired serum and CSF samples from horses with neurologic disease. Samples were collected during routine diagnostic evaluations and stored at -80 degrees C (-112 F). Ten of the cases were diagnosed with EPM and another 10 had cervical vertebral stenotic myelopathy (CVSM).
The researcher noted no elevation in serum SAA in any of the EPM or CVSM cases. Similarly, CSF SAA values were low in all EPM horses (mean < 0.1 mg/L), and those values did not differ significantly from CSF SAA levels in CVSM horses.
“These results show that SAA does not aid in diagnosing EPM,” says Johnson. “In my experience, horses with three of the most common neurologic conditions, which are CVSM, EPM, and equine neuroaxonal dystrophy/equine degenerative myeloencephalopathy (eNAD/EDM), do not have increases in SAA. However, we occasionally see horses with less common neurologic conditions, such as bacterial meningitis, that do have increased SAA.”
The lack of SAA increase in horses with EPM could mean S. neurona does not incite a systemic inflammatory response or clinical signs of disease do not develop until after systemic inflammation has waned or resolved.
“Therefore, EPM diagnosis is best achieved through careful neurologic examination, exclusion of other possible diseases, and SnSAG 2/4/3 serum:CSF titer ratio when possible,” says Johnson.
Monitoring Synovial Structure Infections
Acute injuries to horses’ distal limbs can result in synovial structure infection, hindering an animal’s future athleticism because of chronic lameness. Recognizing synovial infection early is “indispensable to prevent further damage of the affected synovial structure, such as cartilage destruction, osteoarthritis, or intrathecal adhesions and to significantly improve outcome,” reports Rocken et al. in their 2021 study.
That group conducted a prospective study designed to determine SAA’s clinical utility for monitoring disease progression and treatment response in horses with and without synovial structure infection secondary to acute limb injury. The study involved 55 horses that had suffered acute wounds less than 24 hours prior to presentation. SAA levels were determined at admission (prior to surgical debridement); 12, 24, and 48 hours after; and every 48 hours until SAA reached baseline. Each patient underwent a daily lameness examination, and long-term outcome was determined via telephone interview approximately two years after discharge.
Twenty-six of the horses had synovial structure involvement.
In all horses, SAA followed the “characteristic rise-and-fall pattern” typical of SAA in response to inflammation, the study authors reported. This included a significant increase to a peak SAA concentration within 48 hours followed by a steady decline. Three horses diagnosed with synovial involvement had a recurrence of synovial sepsis associated with an increase in SAA.
“Repeated measurements of SAA accurately reflected the course of synovial inflammation and thus provided a reliable and rapidly available tool to monitor the disease course and to adapt the treatment regimen,” said the authors. “SAA should be routinely added to the postoperative management of such cases.”
Other Applications for SAA
- The early diagnosis of Rhodococcus equi pneumonia in foals (Hassenpour and Moghaddam 2023). SAA values were significantly higher in 19 foals diagnosed with R. equi pneumonia than in healthy controls (2,715 mg/dL vs. 1,640 mg/dL, respectively, P < 0.001). “SAA has a short half-life, making it an ideal marker for the early stages of inflammation and monitoring the therapeutic response,” the authors wrote. “Therefore, a one-time analysis of SAA concentration identifies the possibility of inflammation or infection of the body. Serial analysis during the treatment of foals can be helpful for foals with R. equi.”
- Early recognition of surgical implant infection (SII) after internal fixation (Thurston et al. 2022). This study included 39 horses undergoing 40 orthopedic surgical procedures. SAA was measured serially in all horses. SAA and fibrinogen were both early predictors of SII, though SAA had greater diagnostic utility than fibrinogen. “Persistent elevations of SAA postoperatively are associated with the development of SII,” said the research team. “Serial monitoring of SAA can be used to help predict the development of SII in horses undergoing internal fixation. This may lead to earlier suspicion and, therefore, recognition and treatment of SII.”
- SAA can help predict sepsis and survival in neonatal foals < 14 days old (Hoeberg et al. 2022). Practitioners measured SAA levels in 599 hospitalized foals at the time of admission; foals were classified as septic or nonseptic. SAA was significantly higher in septic foals than in sick, nonseptic animals (1,079 mg/L vs. 312 mg/L, respectively). Admission SAA levels were lower in surviving foals. A cutoff for nonsurvival prediction was 1,250 mg/L, with a low sensitivity (22.1%) but a high specificity (90.8%). “SAA can therefore be used as a marker to rule out sepsis and nonsurvival,” the authors concluded.
- SAA does not predict survival in adult horses with colitis (Runge et al. 2022). SAA was measured in 176 horses with acute colitis at the time of admission. Those admission SAA levels were similar between survivors (548 mg/L) and nonsurvivors (396 mg/L, P < 0.43).
Using SAA in Healthy Horses
Interestingly, veterinarians can also use SAA levels to assess healthy horses.
“SAA can detect subclinical inflammation,” says Jacobsen. “This may be useful for horses undergoing elective surgery to ensure they are truly free from disease.”
In horses undergoing elective castration, for instance, postoperative infectious complications occurred more commonly in horses that had elevated preoperative SAA. Examples of conditions that might predispose horses to postoperative complications include distant site infections such as subclinical airway infections.
“Because distant site infection increases the risk of surgical site infection postoperatively, and because subclinical infections of the airways may hypothetically increase anesthetic risk, we advise against performing elective surgery in horses with preoperatively elevated SAA. We do test SAA in all patients undergoing surgery,” says Jacobsen.
Practitioners can also measure SAA in apparently healthy horses to ensure fitness or identify stress. For example, Jacobsen says SAA increases slightly during training programs lasting several months, and elevated SAA concentrations could indicate training exceeding the horse’s fitness level.
“We don’t know exactly why this happens (this is not my own research), but the thinking is that excessive training causes mild injury to musculoskeletal structures that result in mildly elevated SAA,” she says.
“Our unpublished data suggest that SAA could be used to detect infection before clinical signs occurred,” she adds, emphasizing that this is still a hypothetical use of SAA.
“In our clinic we use repeated SAA in high-risk patients post-surgically to identify infectious complications as early as possible,” Jacobsen explains. “This allows us to instigate treatment as early as possible in order to increase chances of a successful outcome. An example for this is after exploratory laparotomy where surgical site infections are fairly common, occurring in 20% of patients or more depending on the hospital.”
Other uses in healthy horses can include ensuring health prior to transportation, before moving the horse into a new facility (to ensure it does not have a subclinical infectious condition that would put the horses at the destination at risk), and to ensure fitness prior to a race or competition.
Concluding Thoughts and Recommendations
As beneficial as SAA measurements appear to be in both healthy and sick horses, the protein does have its limitations. Importantly, SAA cannot be used to make an etiologic diagnosis. Infection, aseptic tissue injury, and any form of inflammation either accidental or iatrogenic (e.g., vaccination) can all induce an SAA response. Further, some inflammatory conditions, such as equine gastric ulcer syndrome, do not reliably induce an SAA response.
Jacobsen says she likes to use SAA as part of an inflammatory “panel,” which includes APPs with both fast and slow responses.
“At our clinic we measure several inflammatory markers (all of which should be widely available), including white blood cell count, iron, SAA, and fibrinogen,” she says. “The first two react very rapidly (within a few hours) in response to inflammation, SAA has an intermediate reaction time, and fibrinogen is slower. Measuring all these markers enhances the chance of detecting inflammation, independent of stage/acuteness.”
Related Reading
- AAEP Health Coverage: PPG and FM Effect on SAA Response in Premedicated Healthy Adult Horses
- Serum Amyloid A as Survival Predictor for Colitis in Horses?
- ‘Mystery’ Cases and SAA Testing
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