Metabolic Causes of Encephalopathy in Horses

Metabolic Causes of Encephalopathy in Horses: Hyperammonemia secondary to intestinal disease or dysfunction appears to be more common in horses than in other species. The reason for the relatively high incidence of this disease in horses is unknown. The exact cause of the disorder is also unknown but is presumed to be a result of excessive ammonia production within the intestinal tract. The hypothesis is that this excessive production and absorption of ammonia would overwhelm normal liver metabolism causing ammonia toxicity and the encephalopathic signs. There is some possibility that intestinal overgrowth of hyperammonia-producing bacteria may be responsible for the disorder although no specific bacteria have been documented to be associated with the condition. Overgrowth of specific hyperammonia-producing bacteria and increases in intestinal pH are known to increase intestinal ammonia production in cattle, although neurologic signs were not reported in the cattle. Most horses with the hyperammonemia encephalopathy exhibit either colic and/or diarrhea during or preceding the neurologic signs. This provides circumstantial evidence of intestinal dysfunction or disease being associated with the presumed overgrowth of the hyperammonia-producing bacteria and/or increased absorption of ammonia. A recent epidemiologic investigation of 13 documented cases did not find any breed or gender predisposition to the disease; the average age was 12 years (range 4–24 years). Most cases presented between March and October, and most affected horses had access to pasture but no specific type of pasture, hay, or grain could be found in common to all cases. In cattle, there are more hyperammonia-producing bacteria in forage-fed cattle than in grain-fed cattle, which might be consistent with the evidence that most horses with hyperammonemia are not on a high grain diet. In most reports, only a single horse on the farm is affected, although in one report, 2 horses on a single small farm were affected on the same day. One recent report has also identified 10 foals with intestinal hyperammonemia.