Meet the pathogens
this is an excerpt from the Briefing Pack v2:
The chance of finding any pathogens in carefully produced certified raw milk is remote. But it can happen, even in pasteurised milk—no food can be made totally safe.
So let’s now take a brief look at the four pathogens responsible for most food-borne illness: Campylobacter jejuni, Shiga Toxin producing E. coli (E. coli O157:H7), Listeria monocytogenes and Salmonella spp. These pathogens may be present in any food, including dairy.
Campylobacter, Salmonella, E. Coli and Shigella are water borne. Therefore water quality at the dairy is vitally important, as well as the use of organic acid-based sanitisers, which are highly toxic to these bacteria—they do not acquire resistance. Organic acids are traditionally used by organic dairy farmers.
Listeria is associated with putrefying vegetable and animal tissues, and is favoured by refrigeration temperatures (long cold chain) where there is no competition from lactic acid bacteria, which have a minimum growth/activity temperature of 7°C. It is for this reason that fermented foods are safe above 7°C, but unpackaged, may become contaminated at refrigeration temperatures used in supermarkets and their supply chains.
The most common pathogen currently associated with raw milk outbreaks is Campylobacter. The virulent forms of Campylobacter can cause serious diarrhea. Campylobacter grows only inside living animal cells. The most common source is the intestinal tract of poultry. Infected chickens do not get sick, but they carry the organism in their faeces. Chicken meat can be contaminated. The most common reservoir is water contaminated by poultry manure. People with diarrhea caused by Campylobacter shed extremely high concentrations of the virulent bacteria in their stools.
Unexpectedly, the potential risk, though extremely low, is increased with raw milk that is ‘too fresh.’ Over time, the antimicrobial components of raw milk kill Campylobacter, so any potential risk diminishes as the milk ages under refrigeration. Longer storage time and exposure of the milk to air decrease the risk to raw milk drinkers. Keeping infected poultry and people that carry Campylobacter away from milk-handling areas reduces the risk.
Campylobacter is the second most common cause of all human food-borne illness. The illness usually goes away without treatment after a bout of unpleasant diarrhea, but there can be severe complications in rare cases.
Data from the United States indicate that Campylobacter in raw milk is responsible for a minute proportion of the cases of domestically acquired diarrhea caused by food contamination.
Shiga toxin-producing E. coli (E. coli O157:H7)
This pathogen grows in the intestinal tracts of warm-blooded animals. The most common source is faecal matter of infected humans. The most common reservoir is cows that are shedding virulent subtypes. Factors that increase the risk to raw milk drinkers include dairy animals contaminated with faeces from high-shedding animals, and milk handlers shedding during and after infection. Factors that decrease the risk to humans include closed herds,
managing dairy herds to minimise the spread of bacteria from colonised animals, and keeping people who are shedding away from milk processing and herds.
In the United States, E. coli O157:H7 in raw milk is responsible for a minute proportion of the cases of domestically acquired diarrhea caused by food contamination. The overall human public health impact of E. coli O157:H7 is small, but highly publicised because of a rare illness called haemolytic uremic syndrome.
The strains of E. coli implicated with haemolytic uremic syndrome are believed to have adapted to the changed environment in the rumens of cattle fed diets with high levels of grain or by- products of food manufacture. If certified raw milk producers are required to feed their animals a diet of predominantly pasture and/or hay, with only minimal grain and no by-products, the risk of contamination with these strains can be eliminated.
Listeria monocytogenes is the most serious and deadly of the contemporary food-borne pathogens. Yet it is also widespread in our environment. Scientists understand a lot about the factors that are necessary before certain subtypes of Listeria are able to cause disease.
Listeria can alternate between two growing modes: it grows in animal cells, or it can switch to growing in decomposing plant materials. Listeriosis is a significant health problem in domestic animals. The most common sources are poorly managed silage; amniotic fluid, placenta and foetal tissues from abortions resulting from infection in cows; and meat processing plants and their equipment.
The most common reservoir is the environment, particularly if cool, wet and undisturbed. Listeria is present as well in our homes and on our bodies.
People regularly ingest Listeria without becoming ill. You must ingest huge numbers of a virulent strain to cause gastroenteritis. Listeria in raw milk has never been a significant public health risk. The most serious public health risk from Listeria comes from contaminated ready- to-eat processed foods, particularly meats.
According to Dr Hull, there have been rare outbreaks associated with cheese, because of the long processing times and storage conditions. This is a risk with cheese made from both raw and pasteurised milk. In Australia, meat pâté is the major source of listeriosis, leading to revised dietary recommendations for pregnant women—but meat pâté remains a legal food.
Salmonella grows inside animal cells as well as in food and feed with high protein content, especially when stored in warm conditions. The most common source is infected humans and animals, as well as contaminated animal feeds and re-warmed foods. The most common reservoir is contaminated water. Inadequate refrigeration of raw milk increases the risk to consumers; eliminating sources of Salmonella decreases the risk.
Salmonella is the most common cause of food-borne illness, but raw milk is implicated extremely rarely.
For more information on how to identify and manage risks on the farm see the 11 categories of the Risk Identification and Risk Reduction program: