chronic pain in horses

When a horse limps, bobs his head, and moves gingerly, you know he’s lame. But sometimes we don’t have the luxury of such obvious signs. Subtle lameness can affect our horses’ mood, health, performance, and welfare. While researchers are working on ways to pick up on subtle lameness—including saddle slip and facial expressions of pain—recognizing those signs remains difficult and subjective.

That’s why Chilean researchers have been looking into lameness markers—in spinal fluid. Their work is revealing physiopathological processes that occur specifically when lameness goes from acute (short-term) to chronic (long-term).

“Sometimes we forget that most lameness originates from a painful condition that can affect more than just a single limb,” said Hedie Bustamante, PhD, of the Universidad Austral de Chile Faculty of Veterinary Sciences, in Valdivia. “This might originate in an acute condition, but it’s very common that horses with chronic conditions like osteoarthritis, navicular disease, and chronic laminitis end up developing chronic pain.”

In their study, Bustamante and her fellow researchers evaluated the shapes and qualities of certain cells in the five lame horses’ spinal fluid. They found significant differences between those that were acutely and chronically lame—indicating a sort of metamorphosis of specific cells that deal with pain signals.

“Science has to move forward into finding potential biomarkers of chronic pain,” Bustamante said. “While other plasmatic markers have been identified and shown to increase during acute pain, none has previously really proven reliable to confirm the presence of chronic pain.”

The sensation of pain is a chemical, mechanical, and even electrical process within the body. Scientists recently discovered, in mice and rats, that a special kind of nervous system cell called a “glial” cell evolves in a way to signal the brain to feel pain. Specifically, they’ve noted that a subcategory of glial cells, known as astrocytes and microglia, send out electrical information to neurons (nerve cells) to make them extra-sensitive. As a result, those neurons start to spontaneously signal the brain to feel pain, without any kind of injury provoking them to do it. That’s a basis for chronic pain.

Bustamante and colleagues decided to investigate astrocyte and microglia changes in lame horses’ spinal cords. To do so, they checked for the presence of a certain molecule (ionized calcium binding adaptor molecule 1 [Iba-1], which microglia express when they signal pain) and a certain protein (glial fibrillary acidic protein [GFAP], which causes astrocytes to change shape and overreact to nerve damage).

The team evaluated lameness in the five horses before euthanizing them. Then, they dissected the affected limbs to confirm the lameness. They identified the lameness, based on their findings and the horse’s history, as being either acute (less than six months’ duration) or chronic (more than six months’ duration).

They also took spinal cord samples from cord branches on the same and opposite sides of the affected limbs. Previous research has shown that the astrocyte and microglia change locally, on the same side as the painful limb, and not throughout the entire spinal cord.

Three horses had chronic lameness (two with osteoarthritis and one with laminitis). The other two had bruising of the foot or leg, causing acute lameness.

They found that microglia expressed more Iba-1 on the same side of the affected limb in all the lameness cases. In acute cases, the microglia had very similar shapes. But in the chronic cases, their shapes varied considerably—meaning some had moved into a sort of chronic and spontaneous activation phase.

As for the GFAP, it didn’t show more astrocyte activity in lameness cases. However, light tests showed they were more fluorescent in two of the chronic cases. What this means with regard to pain signaling is yet to be determined.

Once the biomarkers are confirmed through more tests and larger samples, scientists might be able to develop treatments to prevent chronic pain.

“Hopefully in the near future we will be able to potentially use blood biomarkers to avoid this transition between acute and chronic pain,” Bustamante said. “This will eventually increase the use of new treatment strategies, including the use of newer drugs, and advance the recognition of potential therapeutic targets.”

The study, “Microglia and astrocyte activation in the spinal cord of lame horses,” was published in Veterinary Anesthesia and Analgesia.