With estimated costs exceeding 140 billion dollars annually, the burden of gastrointestinal (GI) disorders is considerable in the United States.¹ Affecting~60% of adults in any given week, GI illness symptoms are common.² White females, as well as younger and more educated individuals, are more likely to report GI symptoms.²

Farmers are also considered to be at higher risk of some GI disorders. Prior studies have shown farm workers and residents of watersheds with high concentrations of farm animals are more susceptible to severe GI infections.^3–7 For dairy farm workers, this is probably related to contact with animal excreta secondary to livestock management, manure handling, and related exposures to antimicrobial-resistant microorganisms.⁸ These observations are not uniform, however, as a survey in Quebec found residents of the most intense farming regions had 30% lower odds of acute GI illness.⁹ Few studies have examined associations between dairy farm work and general GI illness symptoms, which are precursors to clinical GI disorders.

The sample included 70 individuals in the dairy farm group and 74 in the matched, non-farm group (4 replacements in this group were retained in analyses). The two groups were similar on most baseline characteristics, but the non-farm group had a significantly greater percentage of individuals with a college degree, obesity, and were current smokers (Table 1). Overall, 77% of participants reported at least one GI symptom during their enrollment period, with diarrhea (46%) the most common, followed by constipation (40%), abdominal pain (40%), reflux (40%), dyspepsia (32%), and nausea (20%).

In univariate models, dairy farm workers had significantly lower GISSI scores for abdominal pain (mean±SE = 3.9 ± 0.8 dairy vs. 7.9 ± 1.2 non-farm, p = .007), diarrhea (3.1 ± 0.6 dairy vs. 7.1 ± 1.1 non-farm, p = .002), constipation (2.1 ± 0.4 dairy vs. 6.5 ± 1.0 non-farm, p < .001), and dyspepsia (1.8 ± 0.4 dairy vs. 4.0 ± 0.6 non-farm, p = .004). Nausea (2.1 ± 0.4 dairy vs. 3.6 ± 0.8 non-farm, p = .091) and reflux (3.3 ± 0.6 dairy vs. 4.6 ± 0.6 non-farm, p = .144), while trending lower in dairy farm workers, were not significantly different. In the covariate adjusted models summarized in Table 2 and Figure 1, the magnitude and significance of these associations were slightly attenuated, but remained similar. With the exceptions of smoking and gender in the dyspepsia and nausea models, no covariates were significant predictors of GISSI component scores, nor did they appreciably shift any univariate estimates of differences between groups (indicating no confounding).

As expected,² GI illness symptoms impacted most participants at some point during the 3-year DOME study. The overall burden of GI symptoms, however, was consistently lower in dairy farm workers. In particular, constipation and diarrhea scores were 2–3 times greater in the non-farm group. This was intriguing given that prior research suggests farm workers, as well as those who live in watersheds with high concentrations of livestock, are more apt to develop severe GI infections.^3–7

Dairy farm workers are in regular, close contact with cows (e.g., milking, cleaning animals and facilities) and handling of antimicrobial saturated manure (shedding up to 10⁷ Escherichia coli bacteria per gram¹⁵ is an occupational hazard that can, for example, double the risk of Cryptosporidium infection, which can trigger life threatening diarrhea.¹⁶ Our study findings, however, were actually more consistent with a survey by Febriani and colleagues,⁹ who found residents of the most intense farming regions of Quebec reported persistent vomiting and diarrhea less often than residents of other watersheds. Discrepant findings in this body of literature are likely a function of methodological differences in GI distress measures and target samples/regions, including animal mixes and watersheds with varying diversity of bacterial and protozoal pathogens. It could be that farm workers are both more susceptible to acute GI infection resulting in hospitalization, yet experience sub-clinical GI symptoms less often.

Causal mechanisms for the GI illness symptom protection observed in dairy workers in DOME are unknown. The associations were robust in multivariable models that accounted for some typical GI risk factors such as age, gender, smoking, and obesity, but other potential confounding variables like antibiotic use were not measured. Superior diet quality in farm workers seems an unlikely explanation,¹⁷ but greater abundance of beneficial microbes in dairy farmer workers’ GI tract could be a contributor.¹⁸ As observed in farm children, regular exposure to animals primes immune-protective pathways that result in fewer cases of asthma and atopic diseases,^19,20 and per-haps prevention of other pro-inflammatory conditions.²¹ Additionally, farm workers have greater microbial diversity in their nasal cavity, which is hypothesized to protect against staphylococcal infection.²² As microbiomes and resistomes can readily shift for people exposed to farm environments (including non-farmers),²³ future analyses should consider bacterial enterotype comparisons between individuals with and without GI disorders to examine how protective gut microbiota may differ in dairy farm workers, analogous to studies by Costea and colleagues.²⁴

Strengths of this study included the prospective cohort design, matched analyses, and defined source population. The chief limitations were the small sample size and self-reported GI outcomes. This precluded examination of important subgroups and increased risks of recall and self presentation biases (e.g., farmer stoicism) of symptom severity. The degree to which increased GISSI scores translate into clinical GI disorders remains unknown, as less than 20% of adults seek medical care following GI symptom onset,²⁵ and some potential confounders (e.g., antibiotic use, dietary habits) were unavailable. In addition, the source population was racially homogenous and focused on dairy farming, thereby limiting generalizability to more diverse groups or other agricultural specialties. Future research should target broader populations, along with more robust control matching, assessments of nutrition and health attitudes, and detailed information on animal exposures.

Working on a dairy farm was associated with significantly less frequent/severe GI illness symptoms in adults. Causal factors underlying this observation remain unknown, but highlight the need for more research into potentially beneficial environmental exposures where interactions with farm animals (and associated microbiota) are routine. This could eventually guide therapeutic remedies to help prevent clinical GI disorders.