Distinct Mannheimia haemolytica serotypes isolated from fatal infections in veal calves and dairy cows

An emergence of fatal Mannheimia haemolytica (M. haemolytica) infections in cattle in the Netherlands between 2004 and 2018 showed two distinct disease presentations, both associated with M. haemolytica and significantly increasing over time. These presentations were acute polyserositis in veal calves and acute fibrinous pleuro-pneumonia in adult dairy cattle.The occurrence of these two M. haemolytica disease presentations with presumed different pathogeneses, appear to be caused by genetically distinct strains with different antibiotic resistance gene patterns. Furthermore this study shows that M. haemolytica serotype A2, generally considered to be a non-virulent commensal bacterium in cattle, appears to be a cause of fatal polyserositis in veal calves in the Netherlands.