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Zoonotic infections infections in livestock and the risk to public health - Abstract |
B A Synge 1, G J Gunn 1, H E Ternent 1, G F Hopkins 2, F Thomson-Carter 3, G Foster 1 and I McKendrick 4
As cattle were suspected of being a reservoir of verocytotoxin producing Esherichia coli (VTEC) O157, work was initiated in SAC in 1992. Early studies confirmed that cattle were carriers and subsequent studies elucidated how some, but by no means all, human incidents may be associated with animal sources.
This paper describes the preliminary findings of two major studies in which the field work has just finished: the determination of the prevalence of E. coli O157 in Scottish beef cattle, funded by SERAD, and a study of the factors associated with shedding of the organism by beef suckler cows (cows that suckle their calves), funded by the Ministry of Agriculture Fisheries and Food (MAFF).
Determination of the Prevalence of E. coli O157 in Scottish Beef Cattle
The major purpose of this study was to determine the group level prevalence of verocytotoxin producing E. coli O157 for fattening cattle in Scotland using immunomagnetic separation (IMS) on 1 gram faeces samples. It is important to define the methods and the sampling frame before any comparisons in prevalence can be made. A validated questionnaire was used to collect information in order to determine statistical associations with shedding.
The study was conducted in beef cattle derived from either suckler beef, dairy farms or specialist finishing units. In all cases the animals were between 12 and 30 months and the nearest cohort to slaughter on any farm was selected. Nine hundred and fifty-two farms were sampled randomly throughout Scotland. The preliminary results given here are based upon 14564 samples collected from 924 farms, collected between April 1998 and May 2000. The following estimates and 95% confidence intervals were generated. (ie we are 95% certain that the prevalences quoted fall into the ranges shown in brackets).
Animal-level prevalence: 8.6% animals in sampling frame are shedding
E. coli O157 (7.3% - 10.0%)
Group-level prevalence: 23.7% groups of animals in the sampling frame have at least one shedding animal (21.0% - 26.5%)
Analysis of the shedding rates on individual farms shows that there are statistically significant differences between farms where the animals were housed when sampled, and those farms where the animals were at grass. Shedding rates were typically higher among housed animals (p=0.001). Shedding in housed animals is subject to seasonal effects, most noticeably a drop in shedding during the Winter (p=0.051), followed by an increase in the subsequent Spring (p=0.045). The pattern of shedding over time also appears to be different. Among animals at grass, no trend is apparent over the course of the study, while among housed animals the prevalence of shedding tended to fall throughout the study (p=0.004).
Analysis of the absence or presence of shedding animals on each farm also shows differences. Again there are statistically significant differences between farms where the animals were housed when sampled, and those farms where the animals were at grass. Farms with animals housed are more likely to be classed as a positive farm (p=0.03). Neither farms with animals at grass nor farms with animals housed showed any consistent trend or pattern in absence/presence of infection over the course of the study. It was found that farms which restocked their herds only from breeding within the herd had a statistically significantly lower probability of being classed as a positive farm than farms which bought in animals from external sources (p=0.02).
MAFF open contract 9616 was undertaken to try and identify factors associated with shedding of E. coli O157 in adult suckler beef cows. Between August 1997 and April 2000 thirty- two farms were visited and faecal samples screened repeatedly at approximately monthly intervals. Each farm was studied for at least ten months. Unlike the prevalence study, farms were not randomly selected. Selection was biased by the inclusion of twelve herds of known positive status. Six farms showed no evidence of shedding at any time. The majority (77%) of positive farms shed for less than five months of the year. There was a significant seasonal effect with shedding higher in the spring and the autumn (p=0.003). The researchers attempted to find out why by completing a farm management questionnaire at each visit. There are strong associations with the event of housing (p<0.001) and the feeding of home grown barley concentrate (p<0.05). More comprehensive analysis is ongoing. There seems to be no effect of using fertiliser / spreading manure, other species of animals on the farms or the feeding of forage crops.
The almost universal willingness of farmers to allow their livestock to be sampled and to answer the questionnaires is a great credit to the agricultural industry.
Colleagues in SAC Veterinary Science Division at Aberdeen, Inverness and Thurso contributed to these studies. Grampian University Hospitals Trust, Department of Medical Microbiology carried out the typing of isolates.
The financial support of the Scottish Executive Rural Affairs Department (SERAD) and the Ministry of Agriculture Fisheries and Food (MAFF) is gratefully acknowledged.
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