Epidemiological studies of Verocytotoxin-producing Escherichia coli infections in animals in Scotland

Abstract

This thesis is a summation of studies carried out between 1991 and 2004 and attempts to place the work in context with other knowledge to establish the role of animals as a source of human infection with verocytotoxin-producing Escherichia coli (VTEC).

In a preliminary prevalence study, VTEC 0157 was first isolated from cattle in Scotland in 1992. In this study, using basic techniques to examine faeces samples routinely submitted to the Scottish Agricultural College (SAC) Veterinary Centres, 0.25% samples from cattle were positive for VTEC 0157. The organism was more commonly isolated from calves less than two months of age.

A very large prevalence study was commissioned following the Central Scotland outbreak. Using what has now become the national reference method (immunomagnetic separation following enrichment of lg faeces in buffered peptone water with no antibiotics), prevalence levels were established with 95% confidence limits as follows. 7.9% (6.5%-9.6%) animals sampled (12-30 months of age) were shedding VTEC 0157. 22.8% (19.6%-26.3%) of farms had at least one animal shedding in the group sampled. There was a significant drop in the proportion of farms where shedding was detected between the three years of the study 1998, 1999 & 2000. When farms were repeatedly visited twelve times, the organism was detected on 87.5% farms. Because of the lack of sensitivity of the test and the uneven distribution of the organism in faeces, these are underestimates of the true prevalence.

A very large prevalence study was commissioned following the Central Scotland outbreak. Using what has now become the national reference method (immunomagnetic separation following enrichment of lg faeces in buffered peptone water with no antibiotics), prevalence levels were established with 95% confidence limits as follows. 7.9% (6.5%-9.6%) animals sampled (12-30 months of age) were shedding VTEC 0157. 22.8% (19.6%-26.3%) of farms had at least one animal shedding in the group sampled. There was a significant drop in the proportion of farms where shedding was detected between the three years of the study 1998, 1999 & 2000. When farms were repeatedly visited twelve times, the organism was detected on 87.5% farms. Because of the lack of sensitivity of the test and the uneven distribution of the organism in faeces, these are underestimates of the true prevalence.

A pilot prevalence study in sheep determined a Group Level Prevalence of 8% with 95% confidence of 2% to 19%) and an Animal Level Prevalence of 1%). Enumeration of VTEC 0157 organisms gave counts ranging from <5xl02g_1 to >104g"1. A similar study in deer in Scotland suggested that the prevalence was low.

Ninety-one farm investigations were carried out associated with 57 incidents of human E. coli 0157 infection, when animals were suspected as a possible source. In eighteen instances an indistinguishable isolate was obtained from an animal to a human case. The most likely modes of transmission were postulated in each incident. The routes included improperly pasteurised milk, raw milk, direct contact with cattle, contact with a contaminated environment, contaminated camp sites by cattle or sheep, untreated private water supplies contaminated by sheep, cattle or deer and contact with a cat on a farm.

Finally it is postulated that the regional variation in the rate of infection per unit population in Scotland and the difference between Scotland and England relate to the relative cattle and human populations in the areas being considered.