This project investigates mortality in a cohort of workers in the meat industry identified from a meatcutters union in Baltimore, because they were exposed at work to transmissible agents that are known to infect or cause cancer and other diseases in cattle, pigs and sheep, and were also occupationally exposed to known chemical carcinogens. The study is of importance as it adds to the growing evidence that meat workers are at increased risks of developing cancer and other diseases. Workers in the meat industry who handle cattle, pigs and sheep or their raw products are heavily exposed to a plethora of transmissible agents such as prions, viruses, bacteria, protozoa, etc., that are known to cause disease in these animals, including cancer and neurologic diseases. For example, bovine leukemia virus (BLV) commonly infects and causes lymphosarcoma in cattle and sheep, and the prevalence of infection in herds can be as high as over 40% (Burny and Mammerickx, 1987). Lung cancer in sheep is caused by the jaagsiekte sheep retrovirus (Palmarini & Fan, 2001). Prion agents cause the subacute severe neurologic disease known as bovine spongiform encephalopathy (BSE) or 'mad cow' disease in cattle, and scrapie in sheep. In an epidemic of BSE in the United Kingdom more than one-third of dairy herds were affected and over 200,000 animals died (Belkin, 2003). BSE prion is believed to be the cause of a variant of Creutzfeldt-Jakob disease (vCJD) in humans (Weihl & Roos, 1999). Losses in sheep from scrapie can vary from 15 to over 20% annually (Gravenor et al, 2004). There is historical and current evidence that some of these agents are transmissible to workers and subjects in the general population, and cause zoonotic acute infection and disease in them such as leptospirosis, Q-fever, brucellosis, vesicular stomatitis, etc., as reviewed in Johnson (1984). However, although the acute effects of exposure to some of these agents are well established, the long-term effects of the majority have received little attention. It is not known for example whether those microbial agents which cause cancer and other diseases in cattle, pigs and sheep also cause cancer in humans. The study thus has implications not only for the workers but also for the general population who are also exposed to these agents through contact with live animals and their raw products, and ingestion of raw or inadequately cooked meat products. Cohort mortality studies have been the mainstay for evaluating cancer and other chronic disease risks in subjects resulting from exposures in the workplace. They have proven effective and reliable in detecting the carcinogenic effects of agents such as asbestos, benzene, aniline dyes, etc. in workers. They have also been the main sources of the epidemiologic evidence that these agents are carcinogenic in humans. They have led to regulatory actions that have resulted in the dramatic reduction of such harmful exposures in the workplace, hence succeeded in protecting workers' health. However, although a large number of these studies have been conducted in various occupational and industrial groups, much attention has not been paid in the United States to workers in the meat industry who are highly exposed to transmissible agents known to be potent causes of cancer, neurologic diseases such as 'mad cow' disease, and other severe infectious diseases in the animals they handle at work. To investigate if exposure to transmissible agents present in food animals at work is associated with excess occurrence of cancer and other diseases, we initially studied mortality (cancer and non-cancer) for the period 1949 to 1989 in an exposed cohort of 19,849 subjects who worked in abattoirs/slaughterhouses, beef and pork processing plants, and supermarkets, where exposure to these transmissible agents occurred, from handling cattle, pigs and sheep or their products, and an unexposed control group of 6,149 subjects without such exposures ( Johnson 1994; Johnson et al, 1995; Johnson et al, 2007a; Johnson et al, 2007b). Exposed and unexposed subjects were identified from the rosters of a local meatcutters' union in Baltimore, Maryland. Compared to the United States general population, and the internal control group, excess risks of certain causes of death including cancers were observed in the exposed meat workers cohort. Because only 25% of the study population had died at the time, and there were not enough deaths to adequately investigate cancer and other less common chronic diseases in the initial study, for this current grant award, we updated mortality in the cohorts through the end of 2006, during which time a total of 12,931 deaths (50%) had occurred, and we examined mortality risk for a total of 185 individual causes of death as compared to 60 causes of death investigated in previous studies. "Infectious Diseases" is a NIOSH priority area. This new study confirmed that several cancers and other chronic diseases, including neurological diseases, were occurring in excess in the exposed cohort, as well as identifying new causes occurring in excess. The excesses included lung cancer, cancer of the buccal cavity & pharynx, colon cancer, cancer of the esophagus, as well as diseases such as bacterial diseases, infections, diabetes and kidney diseases, including neurological diseases such as senile and presenile psychotic conditions (Alzheimer's disease). These results are very interesting, but it is not known from this type of study the specific exposure(s) responsible in each case, and confounding factors were not controlled for. Now that this large study has demonstrated clearly that these workers are dying at excessive rates from particular causes of death, it is now urgently required for proper, large-scale studies that are adequately designed to find out the specific exposure(s) responsible such as nested case-control studies to be carried out in future.
Eric S. Johnson, M.D., Ph.D., M.P.H., D.T.P.H., University of North Texas Health Science Center, Department of Epidemiology, School of Public Health, UNT Health Science Center, 3500 Camp Bowie Blvd., Fort Worth, TX 76107