The 'shock organ'
Every species has a "shock organ"—an organ whose response to a given medical condition determines the nature and, to a large
extent, the outcome for a given disease, says Orsini. "We always knew the horse's foot was at risk, especially during any
systemic illness, such as enterocolitis, pleural pneumonia and septicemia. And in cases of contralateral limb laminitis we
often see failure of the foot as well, but for different reasons. Contralateral laminitis differs because it is not the septicemia
or system disease that results in failure of a supporting foot but the prolonged loading of the foot and lamellar tissues
associated with a severe musculoskeletal injury in the opposite limb.
"The foot is bathed in a complex network of veins and arteries, allowing it to readily compensate for changes in environmental
temperature and body conditions. The temperature of the foot can fluctuate almost moment-to-moment. This is one of several
reasons that the foot is so susceptible to systemic illnesses and behaves like a shock organ, as other organs do in other
species," continues Orsini.
Another important consideration is the lamellar tissue's need for glucose—a need that is higher for the foot than it is for
the brain. "Add the mechanical loading of the foot during laminitis, and we have the 'perfect storm' for failure of the foot's
supporting structures," says Orsini.
During an inflammatory condition, such as laminitis, the needs for glucose are even greater. "This is one of the big differences
that we see with the horse's foot compared with other animal species. The inflammatory condition overwhelms the foot, and
the dermal lamellar tissue cannot get enough life support (glucose and oxygen) to sustain itself during these severe conditions,"
Orsini says. "This is one reason that the foot is considered a shock organ during multiple organ dysfunction syndrome.
"We know the kidneys, intestinal tract and coagulation system can fail," Orsini continues. "In the horse, it's interesting
we don't see as many cases where the lung is the primary organ of failure. There are other systems that fail first. Comparatively,
the horse's lungs may get pulmonary edema but not the pneumonia and severe respiratory problems that determine life and death
as in people."
In the horse's foot, however, there are elusive circulating factors, termed laminitis trigger factors, that cause an acute inflammatory condition that could lead to laminitis, says Orsini. The systemic inflammatory response,
termed systemic inflammatory response syndrome, results in such a marked inflammatory response that the outcome is loss of the suspensory apparatus supporting the coffin
bone (P3) with resulting sinking or rotation.
"The ultimate question is, How do we protect the foot as one of the horse's shock organs?" Orsini says. "There are proven
preventive measures like cryotherapy for the very sick horse with diarrhea. With endocrinopathic diseases, such as Cushing's
disease and equine metabolic syndrome, we have done a good job recognizing the disease much sooner and prescribing drugs (such
as pergolide) specifically formulated for the horse to manage the disease and prevent laminitis as a sequela. With these endocrine
diseases, the foot does not behave as the shock organ as described earlier. But using the 'all roads lead to Rome' theory,
the supporting tissues of the foot—the lamellar tissue—fail and founder the outcome."