Myopathy in American miniature horses.

The research investigates a case of myopathy observed in three American miniature horses that exhibited signs of organophosphates (OPs) toxicity due to a diet of pellets containing tetrachlorvinphos (TCVP), a type of OP.

Introduction

• The study revolves around organophosphates (OPs), a group of compounds that paralyzes insects by inhibiting cholinesterases, necessary enzymes for proper nerve and muscle function. OPs have multiple applications, from animal parasite treatments to pesticides. However, their toxicity varies among different species.
• In human beings, OP poisoning proceeds through three phases—an initial acute cholinergic phase, an intermediate syndrome displaying muscle weakness, and then a delayed nonlethal polyneuropathy. This intermediate syndrome, resulting from excessive calcium in the muscle cells causing necrosis, has not been reported in horses before.

Case Study

• The researchers examined three American miniature horses presenting an inability to eat, lethargy, muscle twitches, and weakness. These horses, along with two others that showed similar symptoms, shared a history of a diet containing TCVP. This organophosphate was being used as a ‘feed-through’ fly control mechanism. All affected horses were offsprings of the same sire. The one horse that did not exhibit these symptoms was from a different sire.
• Examination of muscle bioptates from two of the affected horses showed serious necrotising myopathy, especially severe in the masseter (chewing) muscles. The necrotic condition was found to a lesser extent in the tongue, neck and other muscles in the horses as well.

Investigation and Analysis

• Within these cases, whole blood selenium levels were normal while liver selenium level in one case was low. Whole blood cholinesterase activity was reduced in two cases, but rebounded after discontinuing the diet and initiating treatment.
• Despite many possibilities being offered such as genetics, developmental origin of the concerned muscle, the form of metabolism in muscle cells, nutritional myodegeneration, or a new syndrome associated with OP toxicity in miniature horses, the exact reason for the severe manifestation of muscle symptoms was uncertain.
• The researchers suggested that a combination of subpar selenium levels and chronic exposure to OP could potentially predispose these horses to the syndrome they presented.