Dietary influences on porcine postweaning diarrhoea

The occurrence of diarrhoea in piglets within three to 10 days of weaning (postweaning diarrhoea; PWD) has long been associated with the proliferation of certain serotypes of beta-haemolytic Escherichia coli in the proximal small intestine of affected animals (Richards and Fraser, 1961). Many of these E. coli isolates have been found to be enterotoxigenic [ETEC], producing either heat stable toxin 'a' [STa], heat stable toxin 'b' [STb], heat labile toxin [LT], or combinations of these; some strains also produce shiga-like vero cell cytotoxins [VT] (Smith et al., 1983; Morris and Sojka, 1985). Of these toxins, LT and STb cause fluid accumulation in intestinal loops of weaned pigs, but STa does not (Burgess et al., 1978). Forms of VT may be involved in the production of oedema disease in weaned pigs (Dobrescu, 1983; Smith et al., 1983), but their role in PWD is uncertain. Many ETEC from PWD lack the well-characterized adhesins found on E. coli from neonatal diarrhoea (i.e. K88, K99, 987P or F41), but may possess other uncharacterized adhesins (Nakazawa et al., 1987; Okerman, 1987). The haemolysin of E. coli isolates from PWD does not appear to play a part in the aetiology of the diarrhoea (Smith and Linggood, 1971), and non- haemolytic ETEC may on occasions also be recovered from natural cases of PWD (Hoblet et al., 1986). Initial infection with PWD-producing strains ofETEC can occur in the farrowing house, the organisms then being carried into the weaner house undetected in the intestinal tract (Miller et al., 1984a); alternatively the ETEC infecting weaned pigs can originate in the contaminated environment of the weaner house (Hampson et al., 1987). Despite the strong association between infection with haemolytic ETEC and occurrence of PWD, oral dosing of weaned pigs with these bacteria does not invariably reproduce the condition (Smith and Jones, 1963; Kenworthy and Allen, 1966; Armstrong and Cline, 1977). The organisms can also be found in the intestinal tract of healthy litter mates of pigs with PWD, although usually in lower numbers (Kenworthy and Crabb, 1963; Svendsen et al., 1974, 1978). These observations have led to a search for other factors influencing the occurrence of PWD, generally with the presumption that "predisposing factors" allow ETEC to establish themselves in susceptible portions of the intestinal tract in sufficient numbers to initiate diarrhoea. Although many such predisposing factors have been suggested, for example fluctuating environmental temperatures, chilling, crowding etc., the purpose of this paper is only to outline reported influences of diet and closely related factors on susceptibility to PWD, especially in relation to pigs weaned at three to four weeks of age ("early weaning")