MRLS was initially characterized by four syndromes: (1) EFLs, (2) LFLs and the nonreproductive syndromes, (3) unilateral uveitis, (4) pericarditis, and later (5) Actinobacillus encephalitis.
Clinical signs for LFLs included explosive parturition, dystocia, foaling while standing, premature placenta separation, and foals either stillborn or born weak.