In Summary
- Experiments have found that pathogenic bacteria from beef cattle are able to contaminate nearby crops, with the risk increasing the closer the crops are to the cattle.
- Current evidence suggests that the route of contamination may be from insects and direct airborne transmission.
- Livestock kept uphill of crops may also pose contamination risks, due to surface run-off.
Livestock in close proximity to crops
Berry et al. (2015) have investigated the impact of proximity to a beef cattle feedlot on the contamination of leafy greens by E. coli O157:H7. Over two years, leafy greens were planted to plots located 60, 120, and 180 metres from an adjacent cattle feedlot. Leafy greens and manure samples were collected from June to September in each year. Both E. coli O157:H7 and total E. coli numbers were recovered from leafy greens at all plot distances with the pathogen isolated from 3.5% of leafy green samples per plot at 60 meters. Overall, there was a decrease in contamination as distance from the feedlot was increased. Although E. coli O157:H7 was not recovered from air samples at any distance, the study authors recovered generic E. coli from air samples at the feedlot edge and all plot distances. The study authors believed those recoveries indicated airborne transport of the pathogen could occur. If that is true, airborne transmission directly from livestock is a relatively-recently identified hazard and auditors and assessors might not yet be aware of the potential issues of proximal livestock.
More recently, Berry et al. (2019) have expanded on their original observations of spread from cattle in close proximity to produce using the same field site. The premise for the investigation was that pest flies could harbour E. coli and may disseminate it to fresh produce. It is well established that flies can harbour human pathogens (Khamesipour et al., 2018). The researchers determined the occurrence of E. coli O157:H7–positive flies in leafy greens planted up to 180 m from a cattle feedlot and assessed their relative risk to transmit this pathogen to leafy greens. The primary fly groups captured on sticky traps at the feedlot and leafy greens plots included house, face, flesh and blow flies. E. coli O157:H7 carriage rates of house, face, flesh, and blow flies were similar, ranging from 22.3 to 29.0 flies per 1,000 flies. In contrast, the carriage rate of stable flies was lower at 1.1 flies per 1,000 flies. The researchers believed that the differences in carriage rates were due to different behaviours of the insects around fresh bovine faeces. E. coli O157:H7 carriage rates of total flies did not differ over the 0 to 180 m range from the feedlot. Most fly isolates were the same predominant strain types found in feedlot surface manure and leafy greens, providing some evidence for flies in transmitting E. coli O157:H7 from manure to leafy greens.
A second hazard from livestock comes in the form of overland transmission, especially if the animals are uphill of cropped areas. Blaustein et al. (2015) assessed livestock manure as a source of faecal indicator bacteria and the role of (simulated) rainfall in spreading contamination. Two key factors were identified as important by Blaustein and colleagues: the gradient of soiled land and the volume of rainfall. Steeper gradients and higher rainfall volumes increase the overland transmission of faecal indicator organisms. Bacteria were released from manure into overland surface runoff and also into captured manure leachate at similar concentrations depending on slope. The concentration of E. coli in the runoff was around 200,000 cells per ml of liquid.
The USDA advises leafy green field distance guidelines of 120 meters from livestock, which should be considered the minimum proximity to livestock. 120 meters will limit, but might not entirely eliminate, the transmission of zoonotic agents such as E. coli O157:H7, Cryptosporidium parvum and Cyclospora cayanensis to produce crops.
References
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