Identification and phylogenetic analysis of Mannheimia haemolytica isolated from nasal samples of horses.

The Mannheimia genus, which is part of the upper respiratory tract microbiota, harbours species that have the potential to cause progressive infections, such as haemorrhagic septicaemia and enzootic pneumonia, when the host immune system is suppressed. Among these species, Mannheimia haemolytica is considered the most pathogenic, and horses can be a host for this agent. This study aimed to investigate the occurrence of Mannheimia haemolytica in horses with and without respiratory clinical signs and to assess the potential pathogenic significance of isolates recovered from animals through serotyping, detection of virulence-associated genes, and phylogenetic analysis. In this study, a total of 134 nasal swab samples were collected from horses reared in 15 farms in five provinces of Türkiye. Of these animals, 20 exhibited clinical signs of respiratory disease, while 114 were clinically healthy. All samples were analysed using cultural, phenotypic, and molecular methods for the detection of M. haemolytica. Ten (7.46%) isolates were identified as M. haemolytica by culture and subsequently confirmed by 16 S rDNA-PCR analysis. All M. haemolytica isolates were obtained exclusively from horses with clinical signs such as high fever, lethargy, loss of appetite, rhinorrhea, and coughing. Serotyping revealed that nine isolates belonged to Serotype 2 and one to Serotype 1. The leukotoxin (lkt) gene was detected in only one (11.11%) Serotype 2 isolate. Similarly, the Ssa-1 protein gene was detected in one Serotype 1 strain. One Serotype 2 isolate carrying the lktCABD operon clustered closely with leukotoxin-associated sequences of both M. haemolytica and M. glucosida, while remaining consistent with its independently established species identity. These findings indicate that M. haemolytica, including strains carrying virulence-associated markers, can be recovered from the upper respiratory tract of horses exhibiting respiratory disease. The exclusive recovery of M. haemolytica from clinically affected horses suggests a potential association with equine respiratory disease, although its causal role requires further investigation.