How many clinically distinct forms of AHS exist, and what are they?

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AHS in horses presents with syndromes defined by which organ system bears the major burden, because the virus damages vascular endothelium and leads to organ-specific edema and dysfunction. The four clinically distinct forms most commonly recognized are the pulmonary form, which shows rapid, severe respiratory distress due to pulmonary edema; the cardiac form, marked by edema of the head and neck and often fluid in body cavities (hydrothorax) from cardiac failure; the fever form, where the horse runs a fever with little or no edema or major respiratory signs; and the mixed form, where features of both pulmonary and cardiac involvement are present. A neurologic form is described in some discussions but is not consistently treated as a separate, routinely recognized clinical form, so it’s not counted here. Thus, the standard four clinically distinct forms are pulmonary, cardiac, fever, and mixed.