Encephalopathy with idiopathic hyperammonaemia and Alzheimer type II astrocytes in equidae.

This study discusses a case of three mature female horses that exhibited episodes of colic and depression, followed by an encephalopathic disorder with severe behavioural changes. High levels of blood ammonia and the presence of a specific type of brain astrocytes, known as Alzheimer type II astrocytes, were found in these horses. This condition occurred without any identifiable liver or kidney lesions, suggesting a possible increase in ammonia absorption from the intestine or increased ammonia production by colonic bacteria.

Horses’ Clinical Symptoms

• The three mature female horses of different breeds presented with initial symptoms of colic and depression.
• This stage lasted for 14 days, after which the horses exhibited signs of an encephalopathic disorder.
• The horses’ behaviour became increasingly erratic, with maniacal behaviour, anxiety, and intense sweating. In one case, the horse developed terminal opisthotonus, a condition where the head, neck, and spinal cord are hyperextended backward in a severe muscle spasm.

Blood Ammonia Levels and Necropsy Findings

• Blood tests showed that the ammonia levels were approximately ten times higher than the normal range.
• Upon necropsy, the horses were found to have suffered from gastrointestinal serosal and mesenteric haemorrhages, i.e., bleeding in the lining of the digestive tract and the connective tissue that holds the small intestines in place.

Presence of Alzheimer Type II Astrocytes

• Alzheimer Type II astrocytes were found diffusely in the cerebral grey matter of all three horses.
• These astrocytes were mostly negative for Glial Fibrillary Acidic Protein (GFAP) or only very weakly positive, weakly S-100 positive, and negative for vimentin – proteins that are typically present in normal astrocytes.

Possible Causes

• The condition occurred in the absence of primary hepatic (liver) and renal (kidney) lesions, which often cause elevated blood ammonia levels, suggesting other potential causes.
• Increased intestinal ammonia absorption due to ileus (a disruption in the normal propulsive ability of the gastrointestinal tract) or increased ammonia production by colonic bacteria are hypothesized as potential causes. This could explain the abnormal behavior observed in the horses due to the impact of high ammonia levels on the brain.