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Raw Milk & Listeriosis, Where's The Evidence? Part 2

Written by Daniel Roytas (MHSc Human Nutrition), BHSc (Naturopathy), Dip. RM

When we think about raw milk, getting food poisoning from Listeria monocytogenes (L. monocytogenes) is probably the first thing that comes to mind. Now I’m not saying people don’t get sick from drinking contaminated milk, but as I highlighted in part 1 of this blog, the evidence proving that L. monocytogenes is the cause of disease, is less than convincing to say the least. In part 2 of this blog, I’m going to provide alternative hypotheses as to the reasons why people might get sick from contaminated milk. It’s important to note, that in no way am I making definitive claims, I am merely highlighting that there are other plausible explanations. It's also important to note that contaminated milk refers to any source of contamination, not just bacterial contamination. 

It’s probably important to note that when I say “contaminated milk”, I am referring to both Pasteurised and raw dairy, because illness can arise after consuming either of these products. This is evidenced by a 2015 study by the Centres for Disease Control and Prevention (CDC) comparing the number of outbreaks occurring from Pasteurised and raw dairy products. This study found that out of 121 listeria outbreaks occurring between 1193 – 2006, 73 were from raw dairy products and 48 were from Pasteurised products.

Whilst there does seem to be a correlation between the consumption of dairy products containing L. monocytogenes and disease1,2, it’s important to remember that scientists found it all but impossible to infect animals via the natural route of infection (oral consumption) by exposing animals to just the bacteria. Not surprisingly, when they gave the animals “contaminated milk” which also contained L. monocytogenes, they became sick. This suggests that it may not actually be the bacteria causing the problem, but rather, some other toxic substance in the milk.

So this begs the question, what else could be in milk that is making people sick? During my search of the scientific literature, I found a very recent paper published in January 2022 stating;

“Although pathogenic microorganisms are considered the main hazard that threatens the safety of milk, they do not represent the highest percentage of reported cases. The contaminants that have been more reported in the literature are of chemical origin. Among chemical contaminants, the most reported are heavy metals (22.18%), pesticides (22.05%), and antibiotics (22.18%); due to bad practices in agriculture and cattle”3.


This got me thinking, could listeria colonise milk in an attempt to break down poisonous contaminants such as pesticides, heavy metals and antibiotics? In 2012, a study was published that failed to detect any listeria in milk samples taken from 210 different dairy farms, however they did find organophosphates and carbamates (pesticides and insecticides) in approximately 75% of the samples. The authors concluded that these toxic chemical residues were a more important risk factor to human health than “bacterial pathogens”4. To date, there has not been any specific investigation as to whether a correlation between pesticides and the presence of listeria in milk exists, however there is evidence to show that fruits and vegetables containing pesticide residues are 2.2 times more likely to be contaminated with bacteria such as listeria5.

Interestingly, Pasteurisation has been shown to degrade pesticides found in dairy milk6,7. Could the reduced risk of illness from Pasteurised dairy products be a result of toxin degradation, rather than from the destruction of bacteria? Is it possible that listeria present in milk might be there to break down pesticides or other toxic contaminants, or for some other reason all together?

Heavy metals

Whilst milk contains many essential minerals such as iron, zinc and calcium, it may also be contaminated with toxic heavy metals such as cadmium, lead, arsenic and mercury8. There is evidence to suggest that L. monocytogenes is resistant to heavy metals including arsenic and cadmium9. Could it be possible that listeria is present in milk contaminated with toxic heavy metals to try and break these heavy metals down? This notion isn't as outlandish as it may seem because in in 2017, it was discovered that specific strains of Escherichia coli (E. coli) found in the human gut are capable of breaking down heavy metals such as mercury and lead found in contaminated dairy products10.

Could listeria play a similar role to E. coli in cow’s milk and the human gut? Is it possible that instead of causing disease, listeria might be trying to break down heavy metal contaminants in milk? Could this mean that the symptoms we see, are from the metabolic waste products being produced from the process of breaking down heavy metals and not necessarily from the bacteria itself?


Bioremediation is a process that uses mainly microorganisms, plants, or microbial or plant enzymes to detoxify contaminants in the soil and other environments. Interestingly, L. monocytogenes has been proposed as a potentially useful bacteria for bioremediation purposes, not only due to its resistance against a range of toxins including tellurite, acridine dyes, lithium chloride, nalidixic acid, cycloheximide11 and heavy metals9, but also for its ability to break down a range of environmental pollutants and toxins12,13. If science is only just beginning to understand the potential role of L. monocytogenes in bioremediation of the environment, why is it so outlandish to think that this is precisely the role listeria has in milk, instead of being a pathogenic, disease-causing bacteria?

Animal Welfare

It is well known that stress, environmental conditions and dietary quality are risk factors for listeriosis in cows14,15. There are serious concerns about the living conditions of dairy cows, which could affect milk quality. For example, some dairy cows may spend their entire lives inside an indoor, concrete-floored enclosure16, given anti-biotics, hormones and other drugs17, and fed diets which may contain plastic18, cardboard, coffee grounds19, and xenobiotics20.

It might seem like common sense, but many people don’t realise, that what you feed a cow comes out in the milk. A 2019 study found that pesticides and antibiotics were commonly detected in milk samples from conventional farms, but not in organic farms21. A 2012 study found no listeria present in milk samples taken from organic farms, but did find listeria in samples taken from conventional farms. This same study found listeria in cow feed, but not in grass samples on these farms22. Why didn't the milk from cows on organic farms contain listeria? Was it due to their improved living conditions and the fact they were being fed pasture rather than a heavily processed diet?

Cows are commonly fed silage. Silage is produced from forage, crop residues or agricultural and industrial by-products. The silage is preserved by acids, either artificially added or produced by natural preservation, and stored in bails. It is well established that when silage starts to spoil and decompose, listeria feed on the decomposing plant material23. When cows eat this decaying food, they are said to become sick with “listeriosis” due to a bacterial infection of listeria24.  How can we say with any certainty that listeria caused the illness, when it is commonly found in both clinically healthy25,26 and sick cows?

Final Thoughts

Has anyone stopped to think, that when a cow is fed a diet of processed feed containing decaying plant material, cardboard and plastic, kept inside for its entire life, pumped full of antibiotics and growth hormones, artificially inseminated and stripped of its calf soon after birth, that the animal might be stressed, toxic, nutrient deficient, grieving and therefore not producing the most healthy milk? Could it be that the listeria then comes along to break down all of the toxins in the milk? We drink that milk, become sick and then blame the whole thing on an innocent little microbe trying to clean up the mess? What an interesting thing to ponder...

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  1. McLauchlin J, Greenwood MH, Pini PN. The occurrence of Listeria monocytogenes in cheese from a manufacturer associated with a case of listeriosis. International Journal of Food Microbiology. 1990;10(3-4):255-262. doi:10.1016/0168-1605(90)90073-E
  2. Lyytikäinen O, Autio T, Maijala R, et al. An Outbreak of Listeria Monocytogenes Serotype 3a Infections from Butter in Finland. The Journal of Infectious Diseases. 2000;181(5):1838-1841. doi:10.1086/315453
  3. Calahorrano-Moreno MB, Ordoñez-Bailon JJ, Baquerizo-Crespo RJ, Dueñas-Rivadeneira AA, B. S. M. Montenegro MC, Rodríguez-Díaz JM. Contaminants in the cow’s milk we consume? Pasteurization and other technologies in the elimination of contaminants. F1000Research. 2022;11:91. doi:10.12688/f1000research.108779.1
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  6. Pardío Sedas VT, López Hernández KM, Flores Primo A, Uscanga Serrano R. Efecto de la pasteurización en la concentración de diclorodifeniltricloroetano (DDT) y hexaclorociclohexano (HCH) en leche de bovino. Revista Mexicana de Ciencias Pecuarias. 2021;12(2):318-336. doi:10.22319/rmcp.v12i2.5483
  7. Singh S, Nelapati K. Effect of food processing on degradation of hexachlorocyclohexane and its isomers in milk. Veterinary World. 2017;10(3):270-275. doi:10.14202/vetworld.2017.270-275
  8. Ismail A, Riaz M, Akhtar S, Goodwill JE, Sun J. Heavy metals in milk: global prevalence and health risk assessment. Toxin Reviews. 2019;38(1):1-12. doi:10.1080/15569543.2017.1399276
  9. Parsons C, Lee S, Kathariou S. Heavy Metal Resistance Determinants of the Foodborne Pathogen Listeria monocytogenes. Genes. 2018;10(1):11. doi:10.3390/genes10010011
  10. Soltani N, Shaheli M. Cow Milk Contamination with Heavy Metals (Mercury and Lead) and the Possibility of Heavy Metals Disintegration by the Human Intestinal Bacteria. Journal of Medical Microbiology & Diagnosis. 2017;06(04). doi:10.4172/2161-0703.1000267
  11. Khan JA, Rathore RS, Shaheen K, Hussain FM, Iqbal A. Role of iListeria monocytogenes/i in human health: disadvantages and advantages. In: The Handbook of Microbial Bioresources. CABI; :193-203. doi:10.1079/9781780645216.0193
  12. Dutta V, Elhanafi D, Osborne J, Martinez MR, Kathariou S. Genetic Characterization of Plasmid-Associated Triphenylmethane Reductase in Listeria monocytogenes. Applied and Environmental Microbiology. 2014;80(17):5379-5385. doi:10.1128/AEM.01398-14
  13. Ghasemi M, Kazemi S. Evaluation Of Growth And Cell Morphology Of Listeria Monocytogenes PTCC 1297 As Affected By Various Concentrations Of Toxic Heavy Metals. Iranian Journal of Public Health. 2014;43(2):123-123.
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  15. Vilar MJ, Yus E, Sanjuán ML, Diéguez FJ, Rodríguez-Otero JL. Prevalence of and Risk Factors for Listeria Species on Dairy Farms. Journal of Dairy Science. 2007;90(11):5083-5088. doi:10.3168/jds.2007-0213
  16. Jacobs A. Is Dairy Farming Cruel To Cows. New York Times.
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  18. Grant H. Legal plastic content in animal feed could harm human health, experts warn. The Guardian.
  19. van Wyk L, Clark G. Potential for use of Spent Pleurotus Mushroom Substrate as Feed for Dairy Cattle. McGill University. Published online 2021.
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  21. Welsh JA, Braun H, Brown N, et al. Production-related contaminants (pesticides, antibiotics and hormones) in organic and conventionally produced milk samples sold in the USA. Public Health Nutrition. 2019;22(16):2972-2980. doi:10.1017/S136898001900106X
  22. Konosonoka IH, Jemeljanovs A, Osmane B, Ikauniece D, Gulbe G. Incidence of Listeria spp. in Dairy Cows Feed and Raw Milk in Latvia. ISRN Veterinary Science. 2012;2012:1-5. doi:10.5402/2012/435187
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  24. Nucera DM, Grassi MA, Morra P, Piano S, Tabacco E, Borreani G. Detection, identification, and typing of Listeria species from baled silages fed to dairy cows. Journal of Dairy Science. 2016;99(8):6121-6133. doi:10.3168/jds.2016-10928
  25. Unnerstad H, Romell A, Ericsson H, Danielsson-Tham ML, Tham W. Listeria monocytogenes in Faeces from Clinically Healthy Dairy Cows in Sweden. Acta Veterinaria Scandinavica. 2000;41(2):167-171. doi:10.1186/BF03549648
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