Mannheimia haemolytica A1-induced Fibrinosuppurative Meningoencephalitis in a Naturally-infected Holstein–Friesian Calf

Mannheimia haemolytica is an opportunistic bacterium that is widely recognized among the bovine respiratory disease complex as the predominant pathogen causing broncho- and pleuropneumonia in cattle. Among the characterized M. haemolytica serotypes, A1 is the major cause of severe pulmonary lesions in cattle. This report describes post-mortem findings in a Holstein–Friesian calf with fibrinosuppurative meningoencephalitis and fibrinonecrotizing, haemorrhagic broncho- and pleuropneumonia, from which M. haemolytica and bovine viral diarrhoea virus (BVDV) were isolated. Microscopical evaluation showed expansion of the brainstem and cerebellar leptomeninges by neutrophils and fibrin, associated with gram-negative coccobacilli. Occasional blood vessels within the midbrain and cerebellum contained fibrin thrombi. Bacterial culture of cerebellum and lung yielded M. haemolytica with unusually high haemolytic activity. The isolates were confirmed as serotype A1 by rapid plate agglutination. Lung tissue was positive for BVDV by polymerase chain reaction. The broncho- and pleuropneumonia in this calf were consistent with typical mannheimiosis due to serotype A1; however, extrapulmonary infections due to M. haemolytica, as seen in this case, are rarely reported. To our knowledge, this is the first documentation of a natural BVDV and M. haemolytica co-infection associated with fibrinosuppurative meningoencephalitis in a calf.