Acute systemic anaphylaxis in the horse

1. Histamine in small doses caused systemic depressor responses in horses, whereas greater doses caused biphasic effects. All doses of 5-hydroxytrypt-amine (5-HT) were pressor and all doses of bradykinin depressor. All three active substances raised pulmonary artery pressure and lowered central venous pressure. 5-HT reduced ventilation volume. Histamine caused brief apnoea followed by hyperpnoea only.2. Acute anaphylaxis in the horse was accompanied by a severe systemic arterial depressor response, a pressor response in the pulmonary artery and vena cava, and alternating phases of apnoea and dyspnoea.3. During anaphylaxis, profound haemoconcentration, leucopoenia, thrombocytopoenia and hyperkalaemia were in evidence. Early during anaphylactic shock (2 to 4 min) there were profound increases in plasma histamine (five to six-fold) and plasma kinin activity (four to five-fold). Plasma 5-HT concentrations were reduced initially but recovered. Later in anaphylaxis (10 to 20 min) whole blood histamine concentration fell significantly. This coincided with the most profound period of leucopoenia.4. No significant differences were observed in histamine concentration in any of five tissues between six ponies subjected to anaphylaxis and six controls. Mast cell numbers were not reduced but mast cells were more metachromatic (pink) and there was spilling of mast cell granules.5. Gross pathological changes were noted mainly in the lungs which were extensively oedematous and congested. Inflamed, congested and oedematous areas in the large colon and caecum were seen, and the kidneys, spleen and liver were engorged. Alveolar emphysema, peribroncheolar oedema (containing mononuclear cells and neutrophils) were recorded. Alveoli contained erythrocytes.