Of all the virulence factors associated with Mannheimia species, leukotoxin is the most characterised. A panel of Mannheimia sp. isolates obtained from cases of ovine mastitis in south eastern Australia were initially assessed for the presence of the gene encoding the toxin, lktA. The sequences of the lktA gene generated from these isolates were compared. Cross neutralising capacities of rat antisera raised against the leukotoxin of one M. glucosida, one haemolytic M. ruminalis and two M. haemolytica isolates were investigated.
A higher overall nucleotide distance was found between the lktA sequences of the M. haemolytica isolates than between those of the M. glucosida isolates. Based on the sequence of the lktA gene, the M. haemolytica isolates could be categorised in 2 groups (A and B).
The neutralising capacities of polyclonal sera were tested against homologous and heterologous leukotoxins. A neutralisation titre of 32 was observed for each antiserum against its homologous leukotoxin, whereas the titres 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. The leukotoxin of M. glucosida had the highest similarity to the toxins produced by M. haemolytica (Group A) and the haemolytic M. ruminalis.
Our findings suggest that a vaccine based on M. glucosida may offer the broadest protection against ovine mastitis cause by Mannheimia species.