pulling blood

Hearing that surgery was successful and your horse is back on his feet can be an immense relief to anxious owners following orthopedic procedures, but this doesn’t mean your horse is out of the woods just yet. Closely monitoring horses’ serum amyloid A (SAA) levels, however, can help veterinarians rapidly identify and address postoperative infections.

Many orthopedic surgical procedures in horses involve surgical implants such as bone screws, plates, and wires. Those implant sites are at risk of infection, and an alarming one in four horses with surgical implants develops postoperative infections (Thurston et al., 2022). These infections are quite serious; even with aggressive therapy, horses that develop implant infections are more than 12 times more likely to not survive to hospital discharge.

“Diagnosing surgical site infection (SSI) in these cases has been based on clinical signs including drainage at the incision, increased lameness, increased rectal temperature, culture of the surgical site, and bloodwork including fibrinogen,” said Catherine Thurston, VMD, a third-year resident in large animal surgery at the University of Pennsylvania’s New Bolton Center, in Kennett Square.

Over the past several years SAA has gained popularity in equine medicine as an invaluable tool for measuring inflammation and infection. This acute phase protein is almost undetectable in a healthy horses’ circulation but increases rapidly and markedly in the face of infection.

“Previous studies show that SAA blood values in horses undergoing surgical procedures have a predictable response,” said Thurston. “They peak within 36 to 72 hours postoperatively then return to low baseline levels by Days 7 to 8 postoperatively in uncomplicated cases.”

But what are the trends when a surgical complication, such as an implant infection, develops?

Looking to answer this question, Thurston and colleagues performed serial SAA measurements in horses that underwent a variety of orthopedic surgeries with implants. Following surgery, 12 of 40 procedures (30%) developed SSI. Signs of infection included discharge from the surgical site, lameness, fever, and radiographic evidence of infection.

In uncomplicated cases without evidence of infection, SAA levels peaked on Day 3 postoperatively and decreased on Days 5 and 7, returning to near normal levels.

“SAA values were also high on Day 3 in horses that developed infection, but those levels either remained elevated or continued to rise until the end of the study period at 14 days,” Thurston said.

Using SAA levels, veterinarians could diagnose an SSI as early as five days postoperatively, but 7.5 days was average.

Persistently elevated SAA values on Days 5-7 postoperatively might therefore raise the index of suspicion of implant infections, said Thurston. With early recognition, veterinarians can initiate aggressive therapies such as systemic antimicrobials and regional limb perfusions to improve outcomes. The cost of running serial SAAs is insignificant compared to the total surgical costs, she added, especially if complications develop. 

It is important to appreciate that veterinarians should only be evaluating SAA trends at this time, said Thurston. Absolute values vary markedly and can be affected by other factors, such as degree of trauma at the time of fracture, postoperative soft tissue trauma unrelated to infection, and even general anesthesia.

The study, “Serum amyloid A and fibrinogen as markers for early detection of surgical site infection associated with internal fixation in the horse” was published in Frontiers in Veterinary Science in October 2022. The full-length article is available online.