Bilateral laryngeal paralysis in the horse.

The study investigates a case where four Thoroughbred horses displayed laryngeal paralysis suspected to be due to possible exposure to a contaminated substance, specifically mineral oil thought to be tainted with an organophosphate compound.

About the horses and their condition

• The horses, all two-year-old Thoroughbreds, showed signs of an acute gastrointestinal illness after being treated with mineral oil, suspected to have been contaminated with an organophosphate compound.
• Three weeks following the ingestion of the oil, they all began to show signs of dyspnoea (difficult or labored breathing), prompting endoscopic examination. This examination revealed that they had developed bilateral laryngeal paralysis, a condition affecting both sides of the larynx and impeding breathing.

Progression of the condition and outcomes

• Two of these horses died six and eight months later due to severe dyspnoea episodes.
• The third horse was euthanized soon after.
• The fourth horse developed bilateral laryngeal paralysis two years later.

Findings from the Examination of the Nerves

• Upon examination of the left and right recurrent laryngeal nerves in these horses, significant loss of myelinated fibers, especially in the left nerve was detected.
• A less severe but similar lesion was observed in other long peripheral nerves including the phrenic and digital nerves of the third horse.
• Examination of the spinal cord in two horses showed signs of mild axonal degeneration; however, this wasn’t linked to any specific tract or location.

The Implication of Organophosphate

• The development of acute clinical signs and subsequent neurological sequelae in these horses is strongly indicative of accidental organophosphate toxicity.
• The correlation suggests that the mineral oil, potentially contaminated with an organophosphate compound, triggered the neurological dysfunction manifesting as bilateral laryngeal paralysis.

This research underscores the importance of careful handling and administration of treatment substances in horses, as contamination may lead to severe, sometimes fatal neurological conditions.