First case report of Corynebacterium pseudotuberculosis biovar equi in semiarid region of Brazil.

Corynebacterium pseudotuberculosis is a bacterium that causes significant economic damage to farmers, mainly in the semiarid region of Northeastern Brazil. In this context, only biovar ovis has been reported, and it is widespread in small ruminants. However, this report describes the first equine case in Brazil attributable to a biovar equi. Clinically, it is the causative agent of ulcerative lymphangitis, mastitis, necrotic and ulcerative dermatitis. A Quarter Horse from the semiarid Northeast presented ulcerative lymphangitis. Culture, nitrate reduction test, MALDI-TOF, and whole-genome sequencing identified the Cp_Eq_BR01 strain as biovar equi. The 2.37 Mb genome shared > 98% nucleotide identity with other equi strains and clustered most closely with the Chilean isolate E19. Although no unique determinants of virulence or resistance were found, the observed genomic clonality suggests potential for rapid transboundary spread. This first Brazilian detection broadens the recognized host range, alerts the farmer's national equine to a new threat, and highlights the need for routine nitrate testing, and updated movement control policies for equines.