Anecdotal evidence has suggested that enteric disease during the pre-weaning period is a major cause of economic loss to both large-scale and smallholder pig production in Vietnam. However, the contribution of enterotoxigenic E. coli (ETEC) to the problem is currently unknown. The aims of this study were to determine the significance of ETEC as a causative agent of pre-weaning diarrhoea in North Vietnam, identify and compare the major ETEC pathotypes, serotypes and antimicrobial resistance profiles present in commercial piggeries and village-based industries in Vietnam and commercial piggeries in Australia and identify representative Vietnamese ETEC strains for future vaccine development.
A six-month epidemiological investigation in five commercial piggeries in North Vietnam confirmed that diarrhoea in piglets from birth to weaning is a serious problem, as demonstrated by the high
mean incidence rate (71.5% of 568 litters) and associated mortality (12.6%). Incidence rates were significantly higher during the summer period (April to June) and were shown to decrease with increasing sow parity, with a peak incidence during the first farrowing. There was a slight increase in the incidence rate of diarrhoea during the first week of life compared to other stages of the suckling period, but the rate was unaffected by either the breed of pig or the size of the litters.
Several bacterial enteropathogens were identified in the study, of which ETEC were the most predominant, being involved in the aetiology of 31% of all cases of diarrhoea. Serotyping and pathotyping of ETEC isolates obtained from commercial (n=200) and village pigs (n=62), showed that isolates belonging to serotype 0149:K91 and possessing the F4/STa/STb/LT pathotype were the most prevalent in both production systems (33.3% of commercial pig isolates and 45.5% of
village pig isolates). In addition, a large proportion of ETEC isolates (22.2%) from commercial pigs belonged to serogroup 08; these strains carried all three enterotoxin genes (STa, STb and LT), but did not possess any of the recognised porcine-associated fmibriae (F4, F5, F6, F41 and FI8) and were thus designated 5F- ETEC. Isolates of serotype 0149:K91 possessing the F4/STa/STb/LT pathotype were also predominant in neonatal and post-weaning diarrhoea in Queensland piggeries, and in post-weaning diarrhoea in New South Wales. However, isolates obtained from neonatal pigs in New South Wales showed a much greater diversity of pathotypes and serotypes compared to Queensland isolates.
Antimicrobial resistance profiles for 16 different antimicrobials and their respective MIC values were obtained for 244 isolates from Vietnam (126 ETEC and 74 non-ETEC from commercial pigs and 44 ETEC from village pigs), and 32 ETEC isolates from
Australia. High rates of resistance were recorded for some of the most frequently used antimicrobials, including tetracycline, streptomycin (>70% of isolates from both countries), amoxicillin, trimethoprim/sulfamethoxazole (>65% for Vietnamese isolates), and lincospectinomycin (>80% for Australian isolates). Prevalence of resistance to individual antimicrobials reflected the availability and regulation of antimicrobial use in each country. Multiple-resistance to three or more antimicrobials was widely distributed in isolates from both Vietnam (79.4-94.6%) and Australia (87.5%). Vietnamese ETEC isolates were more susceptible to antimicrobials than non-ETEC isolates obtained from the same source.
All five 08 strains possessing the 5F7STa/STb/LT pathotype that were tested caused agglutination of guinea pig, sheep, human and chicken red blood cells that was not inhibited by the presence of 1.5% D-mannose. In addition, all five
strains were shown to adhere to isolated porcine brush borders in a similar ratio to the F4 positive control strain, whereas a confirmed fimbriae negative strain failed to attach. One of the 5F- strains, together with four strains that were representative of the predominant ETEC serotypes and pathotypes in North Vietnam and one F4 strain of Queensland origin, were demonstrated to colonise the small intestine and cause diarrhoea in one-day-old colostrum-deprived piglets. Clinical signs, necropsy and histological examinations of infected piglets resembled natural field cases of the disease, and interestingly, the 5F- strain was shown to only attach to the ileum. These results confirm that the 5F- serotype 08 strains may possess novel, previously undescribed fimbriae.
A multivalent killed whole-cell vaccine prepared with aluminium hydroxide adjuvant
from the five selected Vietnamese ETEC strains was shown to be safe and effective in protecting newborn piglets from experimental challenge in a small-scale trail conducted in Vietnam.