Leukotoxin is the best characterised virulence factor of Mannheimia species, and can be produced by isolates of Mannheimia species from ovine mastitis.  The sequence diversity of the lktA gene in a panel of isolates of Mannheimia species obtained from cases of ovine mastitis was compared and the cross-neutralising capacities of rat antisera raised against the leukotoxin of one M. glucosida, one haemolytic M. ruminalis, and two M. haemolytica isolates (isolates A and B) were investigated. 

The results indicated a higher overall nucleotide distance between the lktA gene sequences of M. haemolytica isolates compared to that between M. glucosida isolates.  The M. haemolytica isolates could be divided into 2 groups based on their lktA gene sequences.

The neutralising capacities of polyclonal sera were tested against homologous and heterologous leukotoxins.  Each antiserum had a neutralisation titre of 32 against its homologous leukotoxin, while the titre differed when tested against heterologous toxins.  The antigenic similarity coefficient calculated from the cross neutralisation data revealed that the leukotoxins from the two M. haemolytica isolates had the least similarity among the strains tested, while leukotoxin from M. glucosida had highest similarity to those from the M. haemolytica isolate A and the haemolytic M. ruminalis.