How the equine immune system uses inflammation to defend against pathogens

In our anything-but-sterile world, horses are both filled with and surrounded by microscopic organisms—bacteria, viruses, and protozoa—as well as more complex parasites. Some, especially those that make up the normal microbiome, are beneficial and support horse health. However, the environment is also filled with pathogenic microbes that put horses at risk of disease. If those microbes enter the body successfully, the horse becomes a host to invaders that set up shop, churning out replicates of themselves in massive numbers and eventually spreading to new hosts.  

To fight that scenario the body comes equipped with a multipart defense plan: Recognize invaders and sound the alert; send out first-line defense mechanisms; crush the enemy with tailor-made attacks for that specific invader; and get the body back to “normal”—a state known as homeostasis, says Caroline Chauché, DVM, MSc, PhD, a research fellow at the University of Edinburgh, in Scotland.

It’s a system that works well—keeping horses living for decades rather than months or years, she says. But it’s not perfect. In particular, inflammation resulting from immune responses, while it represents “an essential step in the battle against a pathogen’s progression and its related disease processes,” can also undermine the general health and welfare of the host itself.

Inflammation: At the Center of the Defense Plan

When a pathogen slips into the body—through the lungs, the gastrointestinal tract, or the skin, for example—the immune system sends out alerts to all its components. “It’s saying, ‘There’s something foreign, and you should do something about it,’ ” says Christiane Schnabel, DVM, a researcher in equine immunology at the University of Leipzig, in Germany.

It next initiates an immediate general kind of attack by producing cytokine (special messaging protein) and other types of immune mediators, which attracts and activates local and peripheral (blood) innate immune cells, says Chauché. That attack isn’t very precise, but it gives the body a fighting chance while it configures a more specific strategy for that pathogen.

“After 12 hours or so, you have (even) more leukocytes and inflammatory cells that get attracted there, to take care of the foreign material,” Schnabel explains. This next step of the defense is a more customized battle called the adaptive immune response, which uses pathogen-specific antigens that, ideally, destroy the invaders, she says. “Usually, this results in the body eventually clearing the pathogen through a cascade of first-defense cells and then some other (adaptive) cells.”

The Inflammatory Response: Swelling, Secretions, Redness, Pain, Heat

The second part of the immune system response—the general attack, also known as the innate response—spurs inflammation, Schnabel says. Cytokines have just signaled alerts in the brain and throughout the body, calling for extensive shipments of combat supplies to the affected region.

In the lungs and other organs, much of that process happens at a microscopic level or in other ways that are invisible to the observer, says Alicia Long, DVM, an emergency and critical care fellow at the University of Pennsylvania School of Veterinary Medicine’s New Bolton Center, in Kennett Square. It’s a process that’s very similar to what happens visibly on skin (e.g., with a wound), where you might see redness, swelling, pain, and sometimes oozing, she says