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Raw Milk & Listeriosis, Where's The Evidence? Part 1

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

When I mention to people that I am doing a raw milk fast, their immediate reaction is, “That’s so dangerous, you could get sick from listeria and die”. Naturally, I wanted to dig deeper in to the scientific evidence supporting the theory that listeria is the cause of disease.

Listeria (L. monocytogenes) is said to be the bacteria responsible for causing food poisoning (listeriosis) from the consumption of contaminated raw dairy products. The symptoms of listeriosis range from no symptoms at all (asymptomatic), to a severe infection resulting in diarrhoea, fever and in extreme causes, septicaemia and even death. To prevent listeriosis, raw dairy products are required to be Pasteurised, which is the process of heating milk to a temperature of approximately 65°C for 30 minutes, or temperatures above 90°C for just a few seconds1. It’s important to note that just because milk has been Pasteurised, doesn’t automatically make it “safe”.

Several studies have found the presence of listeria to be higher in Pasteurised dairy milk compared to raw milk. A 2015 study assessed 443 milk products for the presence of L. monocytogenes, finding 40% of Pasteurised products contained listeria, compared to just 18.9% of raw milk products2. A more recent study in 2019, assessed the prevalence of L. monocytogenes in milk products from many different countries around the world. The prevalence of raw milk products containing L. monocytogenes was significantly less, compared to Pasteurised milk products3. Despite the widespread availability (and likely consumption) of Pastuerised milk and dairy products containing L. monocytogenes, listeriosis is somehow still considered a “rare” disease4. If Pasteurised dairy products contain listeria, why aren't outbreaks more common?   

There is no doubt that people become sick after consuming spoiled dairy products, but what is the scientific evidence to prove listeria is actually the cause? Could there be another explanation for this? You may be forgiven for thinking that there is an abundance of irrefutable, scientific proof showing L. monocytogenes causes food poisoning, however many readers will be surprised to learn, that there is in fact, very little (if any) evidence supporting this assumption.

First of all, we need to think about how a scientist would go about proving L. monocytogenes is the cause of food poisoning. The most logical way of proving such a thing, would be to take some contaminated milk, isolate just the L. monocytogenes bacteria and then get a healthy person or animal to consume said bacteria. If the person gets sick, this would be a fairly good indication that L. monocytogenes is the causative agent of disease. How many experiments do you think have been done using this methodology? Well, after searching through the scientific literature, I found none. That’s right, no experiments have ever been done (in humans at least) proving L. monocytogenes causes listeriosis. Rather than taking my word for it, let’s have a look at what the literature says.

According to Hoelzer et al. the experimental feeding of L. monocytogenes to humans is not a viable option and therefore the current understanding of listeriosis is mainly based on epidemiological (aka observational) data5. It’s important to remember, that from a purely scientific perspective, correlation, does not prove causation. Other scientists conclude that the lack of direct experimental evidence in humans is due to ethical and safety concerns preventing such studies from being undertaken. It has already been established that a significant portion of Pasteurised dairy products contain listeria and people are likely consuming these products without getting ill, so by the same token, surely the sale of these products should also considered unethical? Scientists argue that there are a number of animal studies proving L. monocytogenes is the cause of food poisoning and that the evidence is sufficient to extrapolate this to humans6. But as will be discussed later, even the animal trials leave a lot to be desired.

Furthermore, given the lack of human experiments, there is currently no known infective dose of L. monocytogenes7,8. It is suggested that oral doses of 2.5 x 109 to 4.3 x 109/L cause listeriosis, information which comes from case studies. Take for example, the case study where a single person became sick after eating soft cheese. Interestingly, many other individuals (both normal and high-risk individuals), consumed the same contaminated soft cheese containing high numbers of L. monocytogenes, none of whom developed listeriosis9. If L. monocytogenes is indeed the cause of listeriosis, it is difficult to comprehend how it has a peculiar ability to infect (supposedly) some people, but not others.

The results of animal trials are even more surprising. Many scientists acknowledge the difficulty of infecting animals by feeding them listeria. For example, and I quote “Oral transmission of L. monocytogenes has not been widely used over the past few decades due to a high degree of innate resistance in many animals”10 and “It is extremely difficult to experimentally produce listeriosis in non-pregnant animals that resembles naturally occurring disease, (it) proves extremely difficult” and “Even at high doses, (L. monocytogenes) rarely leads to disease in non-pregnant animals”5. It seems that it’s not just humans that are difficult to infect with listeria, apparently animals are too.

One experiment conducted by Farber et al. found that monkeys fed sterile whole milk contaminated with high levels of L. monocytogenes, became unwell11. Many would use this as proof that L. monocytogenes in fact does cause illness, however in order to prove a bacterium causes disease, only the isolated bacterium should be given, rather than mixing it with other substances (like milk). Who’s to say that it was not the milk itself, or an unknown substance in the milk which induced illness?

Given the difficulty of inducing illness through the oral administration of L. monocytogenes to animals, it seems more drastic measures were undertaken by scientists to “prove” their hypothesis. Quote Early feeding trials suggested that many animals are highly resistant to oral infection, and the more reproducible intravenous or intraperitoneal routes of inoculation soon came to be favoured”10 and “Intravenous, intraperitoneal and intracerebral routes of exposure reliably lead to disease in non-pregnant animals, but their relevance for naturally occurring disease appears questionable” 5.

Needless to say, I was quite surprised to learn that the way scientists were “infecting” animals with listeria was by injecting the bacteria directly in to their blood stream, abdominal cavities or brains, and not by feeding them the actual bacteria. Such experiments in no way resemble the purported natural route of infection. Could it be that the very act of injecting any foreign substance (bacteria or otherwise) in to the brain or blood or abdominal cavity of a living creature is the reason why the animals in these studies became sick?

I also found it interesting that many of the papers I was reading, made reference to the fact that it was extremely difficult to infect animals with L. monocytogenes, unless they were pregnant. In several of these papers, they made reference to a study where 33 pregnant Rhesus monkeys were fed L. monocytogenes. Ten (30%) of the monkeys gave birth to stillborn young, one (3%) gave birth to a premature young (with no signs of listeriosis), whilst the other twenty-two (66%) gave birth to healthy young12. This study seems to be used as evidence that L. monocytogenes causes adverse effects in pregnant animals. I was intrigued to learn that it is not uncommon for the prevalence of stillbirths in caged monkeys to be anywhere up to 39%, resulting from physical trauma to the unborn foetus from the mother, or its cage-mates13. Once again, how can we be sure the stillbirths were caused by the bacteria and not from other factors like physical trauma or the fact the animal is locked in a cage and being experimented on?

As I mentioned earlier in this blog, I am not denying that people get sick from drinking “bad milk”, I’m just not convinced that a bacterium is to blame. It seems that the mere presence of L. monocytogenes in milk and then finding it in the stool samples of sick animals or humans, is enough to draw the conclusion that this is the causal agent. In my opinion, this is guilt by association, a case of mistaken identity. When you drive past a house fire and see the fire brigade there, do you automatically assume that the firefighters started the fire? Or course not!

o if not a bacterium, what might be the actual cause of listeriosis? Make sure to check out part 2 of this blog, where I will discuss why people might get sick from drinking “bad milk”, the pro's and cons of raw milk and Pasteurised milk, and much more.

Note - I am not suggesting that there is no conclusive evidence proving listeria is the cause of listeriosis, I just haven't been able to find any after an exhaustive search. I would be more than happy to adjust my position on this after being presented with a peer reviewed scientific journal article proving this causal relationship.

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  1. Deak T. Food Safety Management. Elsevier; 2014. doi:10.1016/C2009-0-01959-X
  2. Seyoum ET, Woldetsadik DA, Mekonen TK, Gezahegn HA, Gebreyes WA. Prevalence of Listeria monocytogenes in raw bovine milk and milk products from central highlands of Ethiopia. The Journal of Infection in Developing Countries. 2015;9(11):1204-1209. doi:10.3855/jidc.6211
  3. Ulusoy BH, Chirkena K. Two perspectives of Listeria monocytogenes hazards in dairy products: the prevalence and the antibiotic resistance. Food Quality and Safety. Published online November 5, 2019. doi:10.1093/fqsafe/fyz035
  4. Donovan S. Listeriosis: a Rare but Deadly Disease. Clinical Microbiology Newsletter. 2015;37(17):135-140. doi:10.1016/j.clinmicnews.2015.08.001
  5. Hoelzer K, Pouillot R, Dennis S. Animal models of listeriosis: a comparative review of the current state of the art and lessons learned. Veterinary Research. 2012;43(1):18. doi:10.1186/1297-9716-43-18
  6. Rahman A, Asgary A, Munther D, Fazil A, Smith BA, Wu J. An agent-based simulator for the gastrointestinal pathway of Listeria monocytogenes. International Journal of Food Microbiology. 2020;333:108776. doi:10.1016/j.ijfoodmicro.2020.108776
  7. Lamont RF, Sobel J, Mazaki-Tovi S, et al. Listeriosis in human pregnancy: a systematic review. Journal of Perinatal Medicine. 2011;39(3). doi:10.1515/jpm.2011.035
  8. Farber JM, Ross WH, Harwig J. Health risk assessment of Listeria monocytogenes in Canada. International Journal of Food Microbiology. 1996;30(1-2):145-156. doi:10.1016/0168-1605(96)01107-5
  9. 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
  10. D’Orazio SEF. Animal models for oral transmission of Listeria monocytogenes. Frontiers in Cellular and Infection Microbiology. 2014;4. doi:10.3389/fcimb.2014.00015
  11. Farber JM, Daley E, Coates F, Beausoleil N, Fournier J. Feeding trials of Listeria monocytogenes with a nonhuman primate model. Journal of Clinical Microbiology. 1991;29(11):2606-2608. doi:10.1128/jcm.29.11.2606-2608.1991
  12. Smith MA, Takeuchi K, Anderson G, et al. Dose-Response Model for Listeria monocytogenes -Induced Stillbirths in Nonhuman Primates. Infection and Immunity. 2008;76(2):726-731. doi:10.1128/IAI.01366-06
  13. Sesbuppha W, Chantip S, Dick EJ, et al. Stillbirths in  Macaca fascicularis. Journal of Medical Primatology. 2008;37(4):169-172. doi:10.1111/j.1600-0684.2007.00275.x



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