E coli outbreak linked with visits to dairy farm

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Aug 23 (CIDRAP News) – An outbreak of Escherichia coli O157:H7 infection has been linked with visits to a Pennsylvania dairy and petting farm that allowed public access to animals. Findings of the extensive scientific investigation of the outbreak are reported in the Aug 22 issue of the New England Journal of Medicine. The article highlights the need to consider zoonotic transmission during searches for the source of such outbreaks.

Diarrheal illness suggestive of E coli infection was first reported during Sep 2000 in Montgomery County, Pennsylvania, and surrounding counties. Health officials quickly recognized that the outbreak seemed related to visits to a traditional dairy facility that had hosted visits by the public, especially groups of children, for several decades. Visitors were allowed to touch the animals, and food and beverages were available for snacks or lunch that often was eaten in areas housing the animals. Handwashing facilities were limited, not configured for use by children, and unsupervised.

Officials at the Montgomery County Health Department and the Pennsylvania Department of Health promptly instituted active case finding and eventually located 15 confirmed and 36 probable cases of E coli O157:H7 in persons ranging in age from 1 through 52 years. More than 90% of the infected patients were 10 years old or younger. Sixteen patients were hospitalized, and hemolytic-uremic syndrome developed in eight, all of whom were age 10 or younger. End-stage renal failure occurred in 1 of the 8 patients and required renal transplantation. No patient died, and no further cases were reported once the farm was closed to the public.

Investigators from the Centers for Disease Control and Prevention (CDC) joined the search for specific causes of the disease outbreak. The farm's entire population of domestic animals was tested for E coli O157:H7 and subtyped with the use of pulsed-field gel electrophoresis (PFGE). In addition, swabs from railings around animal enclosures were obtained, and water samples were collected at sites around the farm. All samples were cultured for E coli and subtyped with PFGE.

Of the 216 cattle on the farm, 33 (15%) were colonized with E coli O157:H7, and 28 of the infected samples had the same PFGE pattern as did isolates from visitors who became ill. Infections were more often found in calves and heifers than in older cattle. The proportion of cattle colonized with E coli at the farm was higher than the typical range of less than 2%. This high rate may indicate that the E coli strain had recently been introduced to the farm, leading to the increased incidence often seen when a new strain moves into a previously unexposed herd. The high prevalence probably contributed to the contamination of animal hides and the environment in general. However, farm workers reported no illness, which might reflect the fact that previous infection and frequent reexposure may confer some protection.

Statistical analysis of infected patients and noninfected control groups showed that risks were highest for patients who had viewed and presumably touched calves less than 6 weeks of age (odds ratio [OR], 3.9; 95% confidence interval [CI], 1.1 to 17.3; P=0.027) and calves 6 to 35 weeks of age (OR, 3.3; 95% CI, 1.3 to 8.8; P=0.007). Handwashing seemed to offer protection from infection, although not to a strictly statistically significant level (OR, 0.5; 95% CI, 0.2 to 1.1; P=0.081).

The team of investigators, headed by John A. Crump, MB, ChB, DTM&H, of the Foodborne and Diarrheal Diseases Branch of the National Center for Infectious Diseases, CDC, notes that E coli can survive in the environment for months and may provide an ongoing source of infection. They suggest that all cattle be handled as if they are colonized and that all cattle environments be approached as if they are contaminated.

In addition, farm environments can be made safer for visitors by encouraging careful handwashing, controlling and supervising contact with animals, and separating food-related activities from areas housing animals.

In an editorial accompanying this article, Sarah J. O'Brien, FFPHM, and Goutam K. Adak, Ph.D., of the Public Health Laboratory Service Communicable Disease Surveillance Centre, London, state that "Investigating an emerging pathogen is like trying to complete an intricate jigsaw puzzle without the picture on the lid of the box as a guide and without knowing whether or not all the pieces are there."

The editorial writers go on to note that E coli infection does not respect administrative, geographic, or professional boundaries and that synergy among clinical, laboratory, veterinary, epidemiologic, and public health disciplines have greatly expanded knowledge and unlocked opportunities for prevention. They conclude: "There is no doubt that this particular jigsaw puzzle is much more complicated than it first appeared to be, and it is unlikely that the latest pieces that have been added are going to be the last."

Crump JA, Sulka AC, Langer AJ, et al. An outbreak of Escherichia coli O157:H7 infections among visitors to a dairy farm. N Engl J Med 2002;347(8):555-60
Abstract

O'Brien SJ, Adak GK. Escherichia coli O157:H7—Piecing together the jigsaw puzzle. (Editorial) N Engl J Med 2002;347(8):608-9

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