Fowl cholera.

Pasteurella multocida subspecies multocida is the most common cause of fowl cholera, although P. multocida subspecies septica and gallicida may also cause fowl cholera-like disease to some extent. However, the virulence properties of the different subspecies for various hosts have not been elucidated. The severity and incidence of P. multocida infections may vary considerably depending on several factors associated with the host (including species and age of infected birds), the environment and the bacterial strain. No single virulence factor has been associated with the observed variation in virulence among strains. Possible virulence factors include the following: the capsule, endotoxin, outer membrane proteins, iron binding systems, heat shock proteins, neuraminidase production and antibody cleaving enzymes. No RTX toxins (repeats in toxin) appear to be produced by P. multocida, but P. multocida exotoxin (PMT) could contribute to virulence in some avian infections. The epidemiology of fowl cholera appears complex. Traditional serotyping systems are only of limited use in epidemiological studies. In recent years, molecular typing methods have been applied to avian strains of P. multocida of different origin. The results obtained using these newer methods indicate that wild birds may be a source of infection to commercial poultry. Documentation suggesting that mammals play a similar role is not as comprehensive, but the possibility cannot be excluded. Carrier birds seem to play a major role in the transmission of cholera. Surviving birds from diseased flocks appear to represent a risk, but more recent investigations indicate that carriers of P. multocida may exist within poultry flocks with no history of previous outbreaks of fowl cholera. The significance of this awaits further investigation. The site of infection for P. multocida is generally believed to be the respiratory tract. The outcome of infections may range from peracute/acute infections to chronic infections. In the former type of infections, few clinical signs are observed before death and the lesions will be dominated by general septicaemic lesions. In chronic forms of P. multocida infections, suppurative lesions may be widely distributed, often involving the respiratory tract, the conjunctiva and adjacent tissues of the head. Diagnosis is always dependent upon isolation of the organism. For the detection of subclinical infections, mouse passage of relevant samples is recommended, but polymerase chain reaction and isolation attempts on selective media may represent alternatives. Confinement is probably the most effective way to prevent introduction of P. multocida. However, extensive management systems dominate in many parts of the world, and under such circumstances vaccination is recommended as a preventive measure. Unfortunately, the development of safe and efficient live vaccines still poses problems. As a result, control remains dependent on bacterins which exhibit significant disadvantages compared to live vaccines.