Avian pathogenic Escherichia coli (APEC)

Avian pathogenic Escherichia coli (APEC) cause aerosacculitis, polyserositi s, septicemia and other mainly extraintestinal diseases in chickens, turkey s and other avian species. APEC are found in the intestinal microflora of h ealthy birds and most of the diseases associated with them are secondary to environmental and host predisposing factors. APEC isolates commonly belong to certain serogroups, O1, O2 and O78, and to a restricted number of clone s. Several experimental models have been developed, permitting a more relia ble evaluation of the pathogenicity of E. coli for chickens and turkeys. He nce, virulence factors identified on APEC are adhesins such as the F1 and P fimbriae, and curli, the aerobactin iron sequestering system, K1 capsule, temperature sensitive hemagglutinin (Tsh), resistance to the bactericidal e ffects of serum and cytotoxic effects. Experimental infection studies have shown that the air-exchange regions of the lung and the airsacs are importa nt sites of entry of E. coli into the bloodstream of birds during the initi al stages of infection and that resistance to phagocytosis may be an import ant mechanism in the development of the disease. They have also demonstrate d that F1 fimbriae are expressed in the respiratory tract, whereas P fimbri ae are expressed in the internal organs of infected chickens. The role of t hese fimbrial adhesins in the development of disease is not yet, however, f ully understood. The more recent use of genetic approaches for the identifi cation of new virulence factors will greatly enhance our knowledge of APEC pathogenic mechanisms. Diagnosis of APEC infections is based on the clinica l picture, lesions and isolation of E. coli. This may be strengthened by se rotyping and identification of virulence factors using immunological or mol ecular methods such as DNA probes and PCR. Approaches for the prevention an d control of APEC infections include the control of environmental contamina tion and environmental parameters such as humidity and ventilation. Antibio therapy is widely used, although APEC are frequently resistant to a wide ra nge of antibiotics. Vaccines containing killed or attenuated virulent bacte ria protect against infection with the homologous strain but are less effic ient against heterologous strains. Hence, vaccination for colibacillosis is not widely practised because of the large variety of serogroups involved i n field outbreaks. (C) Inra/Elsevier, Paris.