Respiration may become labored, rapid, and shallow. Mucous membranes can become dark and congested. Headpressing may occur. The legs, sheath, and angle of the jaw may also swell. The last sign to appear is usually a substantial, frothy nasal discharge.
Death occurs approximately two days after clinical signs appear.
After death, humans and horses show similar pathology. During autopsy or necropsy, fluid and blood will be found in the lung area, and small red hemorrhages will show on the lung surface. Fluids will ooze from lung surfaces when cut, and whit, blood-tinged foam often fills the airways. Under a microscope, lung lesions look similar to those caused by pneumonia. The only known outbreak of EMV occurred in Thoroughbred horses on a limited number of premises in the Brisbane area of Queensland, Australia, during September 1994.