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January 24, 2003

Delmarva Community Health Study: Health Risks of Agricultural Antibiotic Use in the Poultry Industry

Researchers from the Johns Hopkins Bloomberg School of Public Health are conducting a study of the health concerns and increased infections reported by workers, growers, and others associated with the Delmarva poultry industry. Ellen Silbergeld, PhD, professor of environmental health sciences, and her colleagues have focused on the use of antibiotics, at subtherapeutic doses, which are used to promote growth in broiler chickens. Preliminary results from the study were presented during a community forum held January 23, 2003, in Salisbury, Md.

As Dr. Silbergeld explained, antibiotic resistant infections are an increasingly serious clinical problem in the United States. All uses of antibiotics inevitably select for resistance. The same classes of drugs used in food animal production are also used to treat people. However, Dr. Silbergeld noted that her decision to study agricultural uses of antibiotics is not to imply that this use is the sole cause of antibiotic resistance in pathogenic bacteria, but to investigate a part of the problem that has not been studied. Understanding all aspects of this increasingly serious public health problem is critical to developing interventions.

The agricultural aspect of antibiotic resistance has been primarily considered a food safety problem, because of the potential for food consumers to come into contact with drug resistant bacteria. Considerable investments have been made by government and industry to ensure food safety through control of hazards in food processing, storage, and preparation. However, food is not the only potential route of exposure to drug-resistant pathogens from poultry production. Poultry workers – including growers – are in contact with live chickens and may come into contact with their waste. The waste problem (2.5 lbs per chicken) can also result in contamination of drinking water and dispersal in air and soils.

Phase 1 of the Hopkins study is to determine if exposure to antibiotic-resistant bacteria is occurring in Delmarva community residents, including poultry workers and growers. The Hopkins team studied the potential for exposure of the families of workers and growers, who may have come into contact with bacteria, brought home from poultry houses on clothing. For the study, Dr. Silbergeld and her colleagues collected information from chicken catchers, live hangers, growers, household contacts, and unrelated community residents, which included socioeconomic data, work history, health status, and employment in the poultry industry. Microbiological analyses were done on stool samples from each individual in the study. Funding for Phase 1 was provided by the U.S. Food and Drug Administration.

Preliminary results of the first phase of the study show that workers handling live chickens are at increased risk of exposure to bacteria from poultry, specifically Enterococcus faecium and Campylobacter jejuni. According to Dr. Silbergeld, the prevalence of C. jejuni carriage was almost four times higher in poultry workers as compared to studies of persons from regions without poultry industry. Moreover, these bacteria were frequently drug resistant. In addition, the household members of chicken workers were also at increased risk of exposure to these bacteria. The results of this study indicate that occupation and person-to-person contact can result in exposure to bacteria from poultry houses. Dr. Silbergeld stressed that the study does not provide information that can link or explain illnesses in any individual. However, it is important to note that carrying pathogens is a public health issue, even if the carrier is not sick. “Typhoid Mary” was a public health menace, and the early spread of HIV/AIDS occurred precisely because the carriers were not sick. 

During the community forum, the Hopkins team announced the beginning of Phase 2 Delmarva Health and Exposure Study, which is funded by several foundations, including the Winslow Foundation of Maryland. During Phase 2, the team will continue to investigate exposures by increasing the number of subjects in the Phase 1 study, especially inviting the participation of growers and their families. The impacts of poultry farming and clinical antibiotic use will also be directly studied in a new collaboration with health care institutions in Delmarva. In addition, the Phase 2 study will include a six-month prospective study of workers, growers, household members, and community residents to determine the longer-term risks of exposure and health effects that occur over this period that may be related to exposures. The Hopkins team will also examine gastrointestinal and respiratory exposures and health outcomes.

Public Affairs Media Contacts for the Johns Hopkins Bloomberg School of Public Health: Kenna Brigham or Tim Parsons @ 410-955-6878 or

Photographs of Ellen Silbergeld are available upon request.