Persistence of the carrier status in haemorrhagic septicaemia (Pasteurella multocida serotype 6:B infection) in buffaloes.

Fifty-seven young buffaloes were experimentally infected or naturally exposed to haemorrhagic septicaemia (HS). Of these animals 32 became immune carriers. They were observed in groups for periods up to 360 days to monitor the appearance of pasteurellae in the nasopharynx and antibody status. In most animals pasteurellae appeared in the nasopharynx for a short period initially and disappeared. The organism reappeared intermittently and the longest observed period of reappearance was 215 days after exposure. All animals showed rising antibody titres with a peak lasting for 150 to 180 days and declining thereafter. Pasteurellae persisted in the tonsils and were isolated from 20 out of 27 carriers after slaughter. The longest period when isolation was made after slaughter was 229 days. The organism lodged in the tonsils was unaffected by antibacterial therapy using drugs to which the organism displayed in vitro sensitivity. It is concluded that in HS, carrier animals exist in an active as well as a latent state, the former appearing for short intermittent periods between long latent periods, when pasteurellae continue to remain in the tonsils which constitute a long-term reservoir.