A virulence-associated plasmid of some 85 kbp is absolutely required for infection of foals (Takai et al., 1991 ▸; Tkachuk-Saad & Prescott, 1991 ▸) and encodes a key factor for infection and intracellular multiplication: virulence-associated protein A (VapA; Jain et al., 2003 ▸). The gene discussed is VAPA; the disease is infection.