AETIOLOGY

Escherichia species share the common characteristics of the family Enterobacteriaceae. They are Gram-negative, medium sized (0.4—0.6 ? 2—3 μm) facultative anaerobic rods, catalase-positive, oxidase-negative, capable of fer­menting glucose and nitrate reductase-positive.

Serologic classification of E. coli, in particular of the O antigen of the lipopolysaccharide and the H antigen of the flagellum, allows discrimination between pathogenic and non-pathogenic strains. In addition, the capsular antigen (K) is also used for classification. Seven pathovars of intes­tinal E. coli currently exist, of which enterotoxigenic E. coli (ETEC) is the most common cause of diarrhoea in live­stock. Shiga toxin-producing E. coli (STEC), also known as verotoxin-producing E. coli (VTEC), are associated with diarrhoea and dysentery in neonates and young ruminants and oedema disease in swine, as well as cutaneous and renal glomerular vasculopathy in dogs. STEC strains causing distinct clinical signs in humans are known as enterohaemorrhagic E. coli (EHEC), of which the serotype O157:H7 is the most commonly implicated in human haemorrhagic enteritis. Enteropathogenic E. coli (EPEC) cause diarrhoea in several animal species, most importantly in rabbits, pigs and dogs. Pathogenic intestinal E. coli further include enteroinvasive (EIEC), enteroaggregative (EAEC), diffuse adherent (DAEC) and necrotoxic (NTEC) strains. The latter are also involved in septicaemia and urinary tract infections. The group of extra- intestinal E. coli (EXPEC) comprise bacteria, among them avian patho­genic (APEC) and uropathogenic (UPEC) E. coli, involved in a variety of infections including septicaemia, infections of the respiratory and urinary tract, metritis and mastitis⁽²⁾. Escherichia albertii has been associated with diarrhoea in humans but was also found in birds, causing subclinical infections and fatal enteritis⁽³⁾.