Specific farming types and tasks have rarely been studied in relation to colorectal cancer (CRC). We evaluated associations between 5 types of livestock and 13 types of crops in relation to CRC and its subsites within the Agriculture and Cancer (AGRICAN) study.
AGRICAN cohort includes 181,842 agricultural workers living in 11 French geographical areas. Data on farming types and tasks was collected by self-administered questionnaires. We identified 2 609 CRC, 972 right colon, 689 left colon and 898 rectal incident cancer cases during follow-up from 2005 to 2015. Cox proportional hazards models were used to estimate hazard ratios (HR) and 95% confidence intervals (95% CI).
Significantly increased CRC risk was observed for farmers producing horses (HR=1.18, 95% CI 1.06–1.31), sunflower (HR=1.23, 95% CI 1.03–1.45) and field vegetables (HR=1.18, 95% CI 1.02–1.36). Positive associations were also observed for pig, poultry and wheat/barley. Some associations were observed only for specific subsites: left colon cancer was associated with fruit growing (HR=1.36, 95% CI 1.09–1.70) and potato (HR=1.28, 95% CI 1.05–1.57). Tasks related to livestock (animal care, insecticide treatment, disinfection of milking equipment and building) or to crop (haymaking, sowing, pesticide treatment, seed treatment, harvesting) were also associated with CRC. Duration and size of farming types/task increased the risk for some of the associations. Analysis stratified by sex suggested an interaction with several farming types/task.
The current study showed original and positive findings for several farming types and tasks and CRC risk, overall and by subsites.
- • Associations between farming, pesticides and colorectal cancer (CRC) have been poorly studied.
- • This is a comprehensive assessment of associations between farming types/tasks and CRC.
- • CRC risk among farmers is associated with the livestock and crop type, specific tasks, and varies by CRC subtype.
Colorectal cancer (CRC) was the third most common type of cancer with more than 1.9 million new cases and 0.9 million deaths in 2020 estimated worldwide . There has been a gradual shift in cancer incidences from the left to the right colon in the United States and Europe since 1980 s suggesting different risk factors for CRC subsites . Cancer of the right colon is more frequent among women whereas it is opposite for cancer of the left colon .
Associations with CRC have been reported for a number of factors including sex, genetic predisposition, inflammatory bowel disease, cigarette smoking, alcohol, being overweight/obese, high intake of red meat, low intake of whole grains, fruits, vegetables, dairy, low physical activity, hormone-replacement therapy, and aspirin use .
However, associations with occupational and environmental factors is not well-studied. Exposure to asbestos and night-shift work were associated with an increased CRC risk, while physical activity at work was associated with a decreased CRC risk . Exposure to diesel engine exhaust and metalworking fluids was associated with an increased risk of rectal cancer. Moderately increased risk of colon cancer was observed among workers exposed to benzene .
Farmers are generally at about 10–20% lower risk of CRC compared to general populations , and a deficit is commonly explained by lower rates of smoking, healthier diet and higher rates of physical activity among farmers. Excess mortality due to CRC was observed among Iowa and New-Zealand agricultural workers . There was no association between farming and CRC risk in an Australian cohort study . An increased CRC risk among farmers was observed in an Italian case-control study and poultry farming was positively associated with colon cancer in the Agricultural Health Study (AHS) .
Farmers are occupationally exposed to various agents (pesticides, dusts, diesel exhaust, disinfectants, endotoxins, zoonotic infections among others) while performing their routine tasks, and some of these agents have been positively associated with CRC risk. Ecological studies found elevated CRC incidence and mortality in regions with higher pesticide use in Brazil and Canada . Pesticide use was associated with an elevated CRC risk in a case-control study from Egypt . Positive associations were also observed for specific pesticide molecules in the AHS cohort studies . Various bacteria and viruses, which is a potential exposure in livestock farming, have also been linked to an increased CRC risk in previous studies .
There is an evidence that associations with CRC risk factors may vary by anatomical site . Association with smoking was stronger for rectal cancer than colon cancer in a case-control study of US veterans . Imazethapyr-exposed pesticide applicators in AHS study were at higher risk of colon cancer and excess risk was restricted to proximal colon . In addition, studies demonstrated that right and left colon cancer cases display different characteristics at the molecular, mutational and histological levels . Therefore, CRC subsites represent different disease entities and assessment of associations by CRC anatomical site may provide a better insight into CRC etiology and pathogenesis.
The Agriculture and Cancer (AGRICAN) is a large prospective cohort study conducted in France. It offers an opportunity for detailed evaluation of a large number of agricultural exposures. We aimed to evaluate associations between various farming types and tasks, including pesticide exposure through different tasks, and the risk of CRC, overall and by localization (colon overall, right colon, left colon and rectum) in the AGRICAN cohort.