Safety Risks in the Milking Parlor - Biological Exposures

The design of the parlor, along with the throughput and proximity of cows, can facilitate exposure to various bioaerosols ranging from dust to biological hazards to gases. The risk in the parlor may be greater when compared to performing other tasks on the farm. Further exposure to pathogens in the parlor can occur from direct contact with feces, urine, milk, or water droplets.

When monitoring bioaerosols in the parlor, sampling indicated that parlor workers were at an increased risk of inhaling compounds of gram-negative bacteria, endotoxin and 3-OHFA, muramic acid, which also is a component of some bacteria, ammonia, and dust (Davidson et al., 2018). Adverse respiratory outcomes have been linked to these exposures. Some of the respiratory conditions previously linked to organic dust exposure include mucous membrane irritation, bronchitis, and reduction in pulmonary function.

Interestingly, improvement in parlor design was suggested as a reason for lower inhalational exposure than previously reported in other studies (Nonnenmann et al., 2017). In this study, exposure to inhalable dust, endotoxin, and muramic acid was monitored for 62 dairy workers in three mid-western states. The respiratory health outcomes included in the study were pulmonary function tests, pulmonary symptom questionnaires, and exhaled nitric oxide (eNO) which is a biomarker for inflammation, all of which were recorded pre-shift and post-shift. A significantly lower number of participants reported any of the listed respiratory symptoms post-shift when compared to before working, and the eNO level was lower, while no significant difference was observed with another metric of pulmonary function.

Another study of worker exposure to bioaerosols on dairies in Colorado also used pre-shift and post-shift questionnaires, pulmonary function tests, and gauged exposure to dust, bioaerosols, and ozone (Martenies, et al., 2020). The exposure to dust, bioaerosols, and ozone was not significantly associated with changes in pulmonary function; however, a notable association was reported between dust and endotoxin exposure and an increase in one of the function metrics (peak expiratory flow rate). The results of these studies demonstrate a need for additional research to fully understand the relationship between respiratory health and exposures experienced by workers on dairy farms.

Increasing attention to emerging pathogens, including viruses, begs us not to overlook biological hazards associated with milking parlors. Antimicrobial resistant genes (ARG), viral pathogens, and the microbial communities hosted within the human body were included in a review of bioaerosol exposure on dairies (Seidel et al.,  2023). These authors observed that bioaerosol concentration on farms was independent of herd size. Another study from the same research group reported 68% of the participants had at least one positive test for influenza D virus from nasal lavages during the study period (Leibler et al., 2022). Note that the public health significance of this virus is unclear, and they were not testing for highly pathogenic avian influenza A, which was confirmed in dairy cattle in March 2024 (USDA APHIS, 2024).  

Exposure via non-aerosol routes to pathogens including E. coli O157:H7, Salmonella spp., and Cryptosporidium spp. for dairy farm workers in Arizona was assessed by swabbing the clothing and skin of the workers at the end of their shift (Wagoner et al., 2024). All but one of the 16 individuals who completed the questionnaire reported milking as a task they performed. Most workers (94%) wore disposable gloves, 81% wore aprons, and 100% donned rubber boots, while 62.5% indicated they wore eye protection. Handwashing at the end of the shift was reported by 75%, while one-half wore their boots home, and all replied that they wore their work clothes home. These behaviors are noteworthy as culture-positive samples were reported for E. coli O157:H7 (8/60). Droplet Digital PCR (ddPCR) assays detected Salmonella spp. in 23 of 60 samples and Cryptosporidium spp. in 1 of 60 samples. Note that the group of questionnaire respondents all worked on the same farm, so the farm may have policies or the workers may have received similar training on using personal protective equipment (PPE).  

The milking parlor, calf rearing areas, hospital, and maternity areas workers were included In another study on two Colorado dairies (Palomares Velosa et al., 2020). E. coli (6%), Salmonella spp. (27%), and Campylobacteria spp. (21%) were recovered from a portion of the sample from the workers' boots, gloves, and layer that was placed over their clothing. Some of the environmental samples (n=11) collected in the milking parlor also were positive for Salmonella and Campylobacter. An accompanying questionnaire on the prevention of zoonotic disease indicated that 30% of the respondents did not think PPE could reduce the risk of pathogen transmission and 25% did not think handwashing reduced the risk. These findings reinforce the need for education on the prevention of zoonotic disease.    

Another recent study explored the efficacy of PPE and handwashing against Cryptosporidium parvum (Mraz et al., 2023). C. parvum was detected and quantified in 12.9% of 70 fecal samples from seven dairy farms in New Jersey. A way of assessing risk, or quantitative microbial risk assessment (QMRA), was employed to determine the risks associated with certain behaviors, including handwashing and the use of PPE such as gloves and masks. Farmers who do not use PPE or wash their hands on a regular basis had a 29% greater risk, but the risk declined to 11% when wearing a mask, 4.82% when wearing gloves, and handwashing demonstrated a 14.72% reduction. Combinations of these practices resulted in greater decreases in risk. The researchers made some interesting observations including that farmers seem to be more concerned with using PPE to protect milk quality compared to human health. They also noted handwashing was consistent before milking.

While researchers have identified the presence of biological hazards in milking parlors, more research is needed to determine the associated health outcomes and interventions for reducing or preventing exposure and to understand the complexities and interactions of the work environment in milking parlors. As research continues, handwashing and using PPE are relatively inexpensive interventions that require very little extra time when compared to the potential benefits. Yet some dairy producers are reluctant to make these practices part of their routine. Handwashing and glove usage can work both ways to reduce potential contamination of milk in the parlor while also protecting the milker from pathogens transferred from contact with cows. Handwashing and PPE should not be overlooked or taken for granted as a frontline defense for protecting farm workers from pathogens. Additionally, good parlor hygiene, ventilation systems, and education to increase the awareness of exposure to hazards can be implemented.

References:

Davidson, M. E., Schaeffer, J., Clark, M. L., Magzamen, S., Brooks, E. J., Keefe, T. J., Bradford, M., Roman-Muniz, N., Mehaffy, J., Dooley, G., Poole, J. A., Mitloehner, F. M., Reed, S., Schenker, M. B., and Reynolds, S. J. (2018). Personal exposure of dairy workers to dust, endotoxin, muramic acid, ergosterol, and ammonia on large-scale dairies in the high plains Western United States. J Occup and Environ Hygiene 15(3):182-193. doi.org/10.1080/15459624.2017.1403610. 

Leibler, J. H., Abdelgadir, A., Seidel, J., White, R. F., Johnson, W. E., Reynolds, S. J., Gray, G. C., and Schaeffer, J. W. (2022). Influenza D virus exposure among US cattle workers: a call for surveillance. Zoonoses and Public Health. 70:166-170. doi.org/10.1111/zph.13008. 

Martenies, S. E., Schaeffer, J. W., Erlandson, G., Bradford, M., Poole, J., Wilson, A., Weller, Z., Reynolds, S. J., Magzamen, S. (2020). Associations between bioaerosol exposures and lung function changes among dairy workers in Colorado. J Occup and Environ Med 62(6):424-430. doi.org/10.1097/JOM.0000000000001856. 

Mraz, A. L., Mutyala, N., Cleary, S., and Seals, B. F. (2023).  Is personal protective equipment worth the hassle? Annual risk of cryptosporidiosis to dairy farmers and how personal protective equipment and handwashing can mitigate it. Microorganisms 11:2341. doi.org/10.3390/microorganisms11102413. 

Nonnenmann, M. W., Gimeno Ruiz de Porras, D., Levin, J., Douphrate, D., Boggararam, V., Schaeffer, J., Gallagher, M., Hornick, M., and Reynolds, S. (2017). Pulmonary function and airway inflammation among dairy parlor workers after exposure to inhalable aerosols. Am J Industrial Med 60:255-63. doi.org/10.1002/ajim.22680.

Palomares Velosa, J. E. P., Salman, M. D., Roman-Muniz, I. N., Reynolds, S., Linke, L., Magnuson, R., McConnel, C. S., and Sangeeta, R. (2020). Socio-ecological factors of zoonotic diseases exposure in Colorado dairy workers. J Agromed.  doi.org/10.1080/1059924X.2020.1725700. 

Seidel, J., Magzamen, S., Wang, Y. H., Neujahr, V., and Schaeffer, J. W. (2023). Lessons from dairy farmers for occupational allergy and respiratory disease. Current Allergy and Asthma Reports 23:325-339. doi.org/10.1080/15459624.2020.1717502.

United States Department of Agriculture Animal and Plant Health Inspection Service. (2024). Highly pathogenic avian influenza H5N1 genotype B3.13 in dairy cattle: national epidemiologic brief. (PDF)

Wagoner, R., Benally, K. A., Cabrera, D., Lopez, G., Lopez-Galvez, N. I., and Diaz, D. (2024). Prevalence of E. coli O157:H7, Salmonella, and Cryptosporidium among Arizona dairy workers using post-work swabbing. J Ag Safety Health 30(1):17-34. doi.org/10.13031/jash.15680.