Zoonotic infections infections in livestock and the risk to public health - Abstract

Open Farm and Other Farm Linked Cases

G C Pritchard

Veterinary Laboratories Agency, Rougham Hill, Bury St Edmunds, Suffolk IP33 2RX

Introduction

The Veterinary Laboratories Agency (VLA) may be requested by the Consultant in Communicable Disease Control of the Local Health Authority to investigate outbreaks of VTEC O157 infection in England and Wales thought to be linked to contact with animals. In 27 incidents investigated by the VLA between 1994 and 1998, dairy farms (48%) were implicated most frequently, with the probable route of infection being consumption of unpasteurised milk or cheese in most cases. About 30% of premises were mixed farms, beef units or stables and 22% were farms open to the public. Cattle were the species most commonly involved, followed by sheep, goats, birds and pigs.

Open farm investigations

Eight outbreaks (9% of all outbreaks in England and Wales 1994-99) were associated with open farms. Investigations by the VLA into such incidents include an assessment of risk factors, animal husbandry and hygienic practices. Animal faecal samples are collected on a statistical basis where possible, to include animals thought to have been in contact with human cases. VTEC O157 isolates identified from faecal culture are typed by the Laboratory of Enteric Pathogens, Colindale, using phenotypic and DNA-based methods. Human and animal isolates are subsequently compared to allow epidemiological links to be assessed.

Open farm case study (Pritchard and others, Veterinary Record In press).

An outbreak of VTEC O157 phage type 2 involved two separate family groups who visited a well operated open farm. Identical or very closely related strains of VTEC O157 PT2 were subsequently isolated from three of eight calves and two of 30 goats sampled. The same organism was not isolated again from the farm after removal of the infected animals and thorough cleansing and disinfection of the buildings. However, identical VTEC O157 PT21/28 strains were detected in two (0.4%) of 474 faecal samples collected from 15 resident animal species of various ages during a 12 month longitudinal bacteriological study on the farm. Both PT21/28 isolates came from calves originally obtained from different sources and sampled 146 days apart, suggesting the infection had persisted although not detected in other species. The same strain was subsequently isolated from another calf housed in the same pen as one of the infected calves. The longest excretion period detected was seven days. Infected animals were not allowed in contact with the public. VTEC O157 was not isolated from 204 added replacement animals (including 138 orphan lambs and 10 calves) from various local sources sampled whilst held in two weeks isolation prior to being introduced to the farm, or from environmental samples. During the study period a visitor became ill with VTEC O157 PT2. The isolate was clearly distinct from those recovered from the farm and there was no evidence to suggest the visit was the source of infection.

General conclusions

Open farms present a significant potential risk for human infection acquired directly from animals because of the large number of visitors - estimated at 13 million in England and Wales in 1998 alone. However, the number of cases relating to open farm outbreaks is very small - 51 in England and Wales 1994-99. Nevertheless, evidence to be presented by other speakers suggests that animal contact is a major risk factor in sporadic cases. The small chance of farm visitors, particularly young children, developing serious illness from VTEC O157 infection needs to be balanced against the undoubted educational and amenity value of farm visits. Adherence to the Health and Safety Executive guidelines, particularly separation of eating and animal handling areas, good washing facilities and supervision should reduce the risks to a minimum.

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