Horses can have a range of severity and duration of clinical signs. Early stage, acute clinical signs include fever, inappetence and lethargy. A subacute form exists in which horses have these acute signs with an intermittent fever, weight loss and pale mucus membranes. They may also appear jaundiced with yellowed mucus membranes, such as the white of their eyes (sclera). Bleeding and/or bruising (petechiation and/or ecchymoses) may also be visible on mucus membranes.
Horses can be chronically infected, sometimes referred to as a ‘carrier state’. Clinical signs can be absent or non-specific and include weight loss and poor performance. Horses can also become carriers of the infection with no overt clinical signs although flare ups of disease are possible and may be associated with factors causing immunosuppression, such as stress following transportation. Severe disease can be fatal. If a pregnant mare is infected, the infection can be transmitted to the unborn foal, causing abortion, or neonatal infection which can be fatal.