The demand for raw milk is growing as more farmers and consumers learn about its nutritional benefits, delicious flavor, anti-inflammatory functional proteins, bioactive compounds, farm sustainability and beneficial animal and environmental stewardship.[i, ii] Raw milk farmers can thrive in this emerging market and help their customers achieve robust health with strong immune systems, less inflammation, and less asthma and allergies.[iii, iv, v]

Unfortunately, that service to consumers can be overshadowed if farmers aren’t diligent about managing the real food safety risks that can come along with raw milk and the farm environment. There is no such thing as perfectly safe food. Like other foods, if not properly produced and handled, raw milk may contain pathogens that may make customers sick. The resulting illnesses can be serious or, on rare occasions, even fatal.

People are depending on farmers to manage the food safety risks. Although the risks are small, they are real, and we encourage all raw milk farmers to take these risks seriously with ethical and moral commitment. The health of the individual cows/goats/sheep (or other mammals) and the overall raw milk dairy herd is of prime importance in ensuring that raw milk is low-risk and safe to drink. As more farmers become focused on producing low risk raw milk for direct human consumption, there is an increased interest in learning which specific human pathogens and herd health conditions may be of concern.

Pathogens and Zoonotic Diseases Versus Herd Health Concerns

The main microorganisms that may be of concern for raw milk dairy herds include different types of bacteria and viruses. Some of these microorganisms cause illness or disease that can be transmitted from animals to humans or vice versa (also known as zoonosis or zoonotic disease). We generally refer to these microorganisms as human pathogens.

Some other microorganisms of concern can cause health problems in the dairy herd, but are not generally considered to pose human health risks and/or are not transmissible through food. There is uncertainty about several microorganisms and possible connections to human health risks.

We recommend that every raw milk farmer build a good relationship with a veterinarian who can be trusted for advice in herd health management. As a helpful guide, this article provides an overview of the following microorganisms of concern.

Zoonotic microorganisms (known human pathogens): Verotoxin-producing Escherichia coli (VTEC, such as E. coli 0157:H7, Salmonella enterica spp., Campylobacter spp., Listeria monocytogenes, Shigella spp., Yersinia spp., Mycobacterium bovis (tuberculosis), Brucella bovis (brucellosis), Coxiella burnetii (Q Fever), Avian Influenza H5N1, Staphylococcus aureus (S. aureus).
Herd disease microorganisms (those not generally considered as human pathogens): Bovine Leukemia Virus (BLV), Mycobacterium avium subsp. paratuberculosis (MAP, Johne’s disease or paratuberculosis).

The guidance in this document is intended for educational purposes. Local disease conditions are important in determining the appropriate course of action, so it is recommended to consult with a local veterinarian in making decisions about herd health monitoring and testing.

Three Variables for Illness

Bacterial, fungal, and viral pathogens need a host to survive and thrive, yet pathogens do not cause illness in every host. For a pathogen to cause illness, three variables must align:

A pathogen must be present which is virulent and capable of producing harmful effects
The pathogen load must be high enough to trigger illness
The host must be susceptible to the pathogen

Farm families tend to have strong immune systems that are adapted to the microbial flora of the animals and the farm environment, so they are less likely to become ill from raw milk that could possibly contain some of these microbes. However, it is important for farmers to recognize that many consumers may have compromised immune systems or immune systems that have not been adapted to the farm microbial flora, such as those living in urban areas, those taking medications such as antibiotics, chemotherapy or other immune-suppressing drugs, and those exposed to toxins in the environment or in the food. Unlike the farmer and the farm family, these consumers may be at a higher risk of illness from exposure to pathogens that can be found routinely in the farm and animal environment.

Sampling Techniques for Microorganisms

Determining whether there are microorganisms of concern in the raw milk dairy herd generally requires observation, physical assessment, and specialized laboratory testing. It is important to work with the herd veterinarian, the diagnostic laboratory, and/or consultants to evaluate the presence of microorganisms of concern on the farm. The aim of testing is to determine the presence of specific microorganisms on the farm, where the hazards are present within the farm, and whether the microorganisms are present in the raw milk.

Depending on the goals of testing, the samples may come from numerous sources such as slurry, manure, water, milk from individual teats, milk from a single animal, blood, urine, animal tissue, milk filters, bulk tank milk, bottled milk, or vending machines. Some tests determine the presence of microorganisms directly in the milk. For some microorganisms, samples such as blood and milk are taken to detect antibodies or immune response in the animals against the microorganisms, rather than the microorganisms themselves.

Risk Reduction Strategies

Healthy goats and cows can produce healthy milk that will nourish farm families and customers, but diseased or unhealthy animals can pass illness through their milk or through milk contaminated with feces. With due diligence and care, raw milk can be a low-risk food for the nourishment of people.

Farmers can reduce the risks of human pathogens being present in the milk through various strategies including biosecurity measures, herd management strategies, milking practices, and milk handling measures. Some specific risk management strategies include:

maintaining a closed herd or implementing testing and quarantine of any new animals before they are introduced to the herd,
ensuring the animals are healthy and kept in comfortable conditions,
monitoring for signs of mastitis (udder inflammation),
monitoring for fever as a possible indicator of systemic disease,
thoroughly cleaning the teats prior to milking,
rigorously cleaning all equipment that will be in contact with the milk,
rapid chilling the milk to <40ºF (<4°C) within an hour (or less) or milking, and
performing bacterial testing of the milk on a regular basis for early detection of problems and to assure compliance with RAWMI Common Standards.[vi]

Regardless of the specific microorganisms that are present, conscientious raw milk producers should monitor their herds for illness and ensure that raw milk from unhealthy animals is not used for direct human consumption. Signs of illness can include poor appetite, lameness, poor body condition, rough fur or coat, runny nose, cough, watery eyes, diarrhea or discolored feces, fever, discolored urine, uterine discharges, swollen udder or quarter, reddened udder or teats, high somatic cell count (such as detected in the California mastitis test, even if not detected in every quarter), and/or abnormal milk (even if not detected in every quarter) such as milk that is thick, discolored, stringy, or salty.

Human Pathogens of Concern for Raw Milk in Western Countries

Currently in the western world, the four most common human pathogens that have been associated with raw milk-related illness in humans are verotoxin producing Escherichia coli (VTEC, such as E. coli O157:H7), Salmonella enterica spp., Campylobacter jejuni, and Listeria monocytogenes. Less commonly, Shigella spp. and Yersinia spp. have been associated with raw milk-related illness in humans. When any of these bacteria are present in the milk at levels that are sufficient to cause infection, susceptible people may experience intestinal illness that may include severe illness or death.

VTEC (such as E. coli 0157:H7) are of particular concern because these pathogens can produce severe illness even with a low pathogenic load (few bacteria are needed to cause disease). For instance, although “the total case numbers of E. coli O157:H7 infections are lower than those of other enteric pathogens such as Salmonella or Campylobacter spp., the diseases caused by E. coli O157:H7 showed much higher hospitalization and fatality rates... Human infection caused by E. coli O157:H7 can present a broad clinical spectrum ranging from asymptomatic cases to [in rare cases] death. Most cases initiate with non-bloody diarrhea and self-resolve without further complication. However, some patients progress to bloody diarrhea… In 5–10% of [these] patients, the disease can progress to the life-threatening sequelae, HUS [hemolytic uremic syndrome, leading to kidney failure] or thrombocytopenic purpura [blood clots that can restrict flow of oxygen to the organs].”[vii]

Thus, pathogens in raw milk need to be taken very seriously. Some of the most common sources of pathogens in milk are manure, mastitis, and improper cleaning of milking equipment resulting in biofilms of bacteria. Researchers from Canada and Europe have studied the safety of raw milk intended for direct human consumption and found that raw milk can be a low-risk food when farmers are trained in risk management practices, implement careful production practices, and test their milk regularly.[viii, ix]

Pathogen testing of raw milk can be used to determine whether pathogens are present, however this testing can be expensive and is only meaningful if it is performed frequently. For small-scale farms that cannot afford frequent pathogen testing, Raw Milk Institute recommends bacterial testing at least monthly for hygiene indicators by testing for coliforms and Standard Plate Count (SPC). The RAWMI Common Standards aim for a rolling three-month average of <5,000 cfu/mL for SPC and <10 cfu/mL for coliforms. [vi]

Although these two tests do not directly detect the presence of pathogens, these tests serve as general indicators that the milk is being produced hygienically and in such a way that pathogens are less likely to be present. Coliform and Standard Plate Count testing can be performed at an offsite lab or with an on-farm lab (with results in 24-48 hours). After an initial investment in equipment, on-farm labs can greatly reduce testing costs in the long-term because the on-farm test cost is only $1-3 per test.

Now, we will proceed to a description of some of the common herd health or zoonotic pathogens.

Mycobacterium bovis (Tuberculosis or TB)

Tuberculosis (TB) is a serious disease both in animals and humans. Cattle who are infected with Mycobacterium bovis (TB) may show no outward signs of infection or vague symptoms such as weight loss and low energy.[x] TB can infect humans through contact with infected cattle or through consuming raw products from infected animals. In humans, tuberculosis can cause prolonged cough, lung, kidney, brain, and spine damage, and ultimately death if not successfully treated.[xi]

In some continents and regions, bovine tuberculosis is one of the biggest challenges to low-risk raw milk production. For instance, many areas in Asia, Africa and South America have relatively high rates of tuberculosis in cattle and problems with TB contamination of raw milk.[xii, xiii, xiv, xv] For farmers in high-risk TB areas who want to produce raw drinking milk, a rigorous TB testing program of the entire herd and workers, combined with careful control to ensure that animals are kept out of contact with potential disease carriers, would be essential.

Nearly all states in the United States of America (USA) have successfully eradicated tuberculosis in dairy animals through national programs, involving extensive testing and culling of TB infected animals.[xvi] Canada is officially free of bovine tuberculosis.[xvii] In the European Union, 17 countries are officially TB-free.[xviii, xix]

There are wildlife reservoirs of TB in many countries that may have TB-free dairy herds. For instance, badgers in the United Kingdom and Ireland, white-tailed deer in the USA, elk and bison in Canada, and brushtail possums in New Zealand are common carriers of tuberculosis in the wild.[xx] Limiting interactions with wild animals is a very important strategy for protecting your herd from this disease.

In states and countries that have legal access to raw milk, annual TB testing for raw milk herds is commonly required by regulatory agencies. The Raw Milk Institute’s Common Standards recommend testing the dairy herd for TB at least annually.[vi] Farmers living in TB-free areas, where the herd has no contact with wildlife, may choose to rely on the tuberculosis-free status of their region in consultation with a local veterinarian. In some areas, it may be appropriate to test more often than annually, such as if TB is common in the region or known to commonly affect wildlife populations.

TB testing is generally performed by a trained veterinarian and involves giving a subdermal injection of TB antigens followed by measuring the swelling of skin at the injection site. Regulatory agencies take tuberculosis very seriously. Positive TB tests lead to immediate restrictions of movement of animals and suspension of raw milk dairy production.

If an animal is a reactor in the skin sensitivity test, then additional tests need to be performed on the animal and the whole herd. If the additional tests come back positive, then the animal is euthanized to investigate organs for disease involvement in a necropsy. Depending on local policies, if the autopsy finds no tissues affected by tuberculosis, the rest of the herd may be able to be spared.

Brucella bovis (Brucellosis)

Brucellosis is a serious zoonotic disease that also can cause disease in animals. In cattle, brucellosis can cause fertility problems including miscarriage, abortion, and retained placenta.[xxi] In people, brucellosis can cause undulant fever which can have serious short term and long term health effects. Brucellosis in people may include chronic fevers, liver inflammation, fatigue, bone and joint inflammations, and more serious inflammation of the heart and brain tissues.[xxii] There are brucellosis vaccines for cattle and some countries have official brucellosis vaccination programs. In the USA, the brucellosis vaccine may be required in some localities. Several years ago in the USA, this vaccine was improperly produced and caused Brucellosis to be contracted by those who drank raw milk from vaccinated animals.[xxiii]

In some countries, brucellosis is one of the biggest challenges to low-risk raw milk production. For instance, India and Central African countries have relatively high rates of Brucellosis in cattle.[xxiv, xxv] A rigorous brucellosis testing program, combined with careful control to ensure that animals are kept out of contact with potential disease carriers, are essential in areas such as these.

In the USA, nearly all states have successfully eradicated brucellosis in dairy animals.[xxvi] Canada is officially free of bovine brucellosis.[xxvii, xxviii] In the European Union, most countries are officially declared free of brucellosis.[xxix] However, there are still wildlife reservoirs of this disease in some parts of Europe.

In the USA, states have varying requirements for Brucellosis testing. For instance, Pennsylvania requires annual brucellosis testing whereas California requires quarterly brucellosis testing for raw milk herds. The Raw Milk Institute’s Common Standards generally recommends testing the dairy herd for brucellosis regardless of location.[vi] However, farmers living in brucellosis-free areas whose animals have no contact with wildlife may choose to rely on the brucellosis-free status of their region in consultation with a local veterinarian. In some areas, it may be appropriate to test more often than annually if brucellosis is common or known to commonly affect wildlife populations.

Generally, brucellosis testing is performed on bulk tank milk using the “ring test” which detects the presence of Brucella antibodies. Brucellosis is taken very seriously by regulatory agencies. If the milk tests positive, then further testing can be performed to determine which animals in the herd are affected. If it is verified that animals have Brucellosis, the affected animals must be culled from the herd. This decision is driven by local animal health regulations. Beware that the ring test is not generally considered to be effective for goats or sheep due to a high rate of false positive tests.[xxx]

Although brucellosis is mainly associated with domestic ruminants and pigs, there are wildlife reservoirs for the disease in bison and elk in the USA, buffalo in southeast Africa, and Alpine ibex in the French Alps.[xxxi] Limiting interactions with wild animals is an important strategy for protecting your herd from this disease.

Coxiella burnetii (Q Fever)

Q Fever is an illness caused by the bacteria Coxiella burnetii. In dairy herds, Q fever can lead to reproductive problems such as infertility and abortions. In humans, most Q fever cases are asymptomatic, but some people can develop mild flu-like symptoms. Rarely, some people can develop serious illness with chronic Q fever.[xxxii]

The primary way people are exposed to Coxiella burnetti is through breathing in dust or air that is contaminated by manure or the animals’ bodily fluids. Transmission can also occur through direct contact with mucus, aborted fetuses, amniotic fluid, placenta, milk, or ticks. In some areas, cases of Q Fever are required to be reported to public health agencies.

Coxiella burnetti testing of raw milk dairy herds is not typically required by regulatory agencies. Coxiella burnetti testing is typically performed with serum antibody testing to evaluate whether the animal has been exposed to Coxiella burnetti. Thus, a positive test does not necessarily indicate that an animal is contagious or virulent at the time of testing. Further testing may be needed to determine whether Q fever is currently present. When animals have positive antibody tests, it is recommended to work with a veterinarian to determine if there is an active infection ongoing in the herd.

For overall herd health, Raw Milk Institute generally recommends testing for Q Fever prior to purchasing animals. For existing herds with no clinical signs of illness, it is recommended to consult with a local veterinarian who is knowledgeable about local conditions in order to determine whether Q Fever testing is recommended.

Avian Influenza Virus H5N1

Avian influenza virus primarily affects birds and is spread across the globe with migratory birds. Avian influenza H5N1 is a type that has recently crossed over from wild birds into some bovine and porcine herds in the USA. Although avian influenza H5N1 can be highly pathogenic in birds, according to the US Department of Agriculture (USDA) Animal and Plant Health Inspection Service (APHIS), in cattle this illness primarily causes decreased lactation and reduced appetite.[xxxiii] The illness is generally mild, where many cattle in the herd show no signs of illness, and those who do generally recover within a couple weeks. Symptoms include weakness, dehydration, fever, lower milk production, and yellow-tinged, thick milk. The first symptom of Avian influenza H5N1 in cows is generally a fever which appears a few days prior to becoming clinically sick.

Knowledge about H5N1 avian influenza is limited. Like other flu viruses, H5N1 is a respiratory illness and not generally considered to be a foodborne illness. There have been reports of cats dying from exposure to H5N1 in milk, however it is not clear whether those cats had exposure to wild bird or poultry sources of the virus.

It is recommended to monitor the herd for illness and ensure that raw milk from unhealthy animals is never used for direct human (or pet) consumption. If there is an H5N1 outbreak in the herd, it is recommended to quarantine all symptomatic animals from the rest of the herd for 2-3 weeks, until all signs of illness have passed. It is recommended to consult with a local veterinarian to determine the best course treatment and any other precautions that should be taken. For existing herds with no clinical signs of illness, it is recommended to consult with a local veterinarian who is knowledgeable about local conditions in order to determine whether H5N1 testing is recommended.

Staphylococcus aureus (Staph A)

Staphylococcus aureus (S. aureus) is a common cause of mastitis in ruminants. It can be present in raw milk, but it is not generally considered to be a pathogen of human concern. However, it is a major and serious cause of chronic mastitis and degradation of the herd’s productivity.[xxxiv] S. aureus bacteria colonize the mammary glands and teats. Over time, scar tissue and abscesses develop in the udder and teats, thereby reducing milk volume and the infection may remain in the udder, even after numerous antibiotic treatments.

Milk culture testing can be used to determine if S. aureus is present in your herd. Most regulatory agencies do not require testing for S. aureus. However, there are exceptions such as New York, where the presence of S. aureus at low levels is allowed in raw milk, but high levels are not allowed.

Animals infected with S. aureus may initially show no clinical signs, whereas thereafter clinical mastitis (udder swelling, udder hardness, abnormal milk, and elevated somatic cell counts) may appear and result in chronic infection. S. aureus may be passed from mother to offspring through raw milk or nursing. S. aureus can also be spread among the lactating cows through milking equipment, teat dip cups, and the hands of farm workers. Flies may also transfer the bacteria.[xxxv]

S. aureus is generally resistant to many types of antibiotics and is difficult to eradicate from the herd. Not uncommonly, S. aureus can appear to be wiped out as determined via testing, but then it may appear again in later testing. Some small-scale farmers have reported success in treating S. aureus with the use of homeopathic medicines but it is not clear whether this could work in larger herds with widespread S. aureus. [xxxvi]

Because S. aureus can degrade the herd’s milk supply and cause recurrent mastitis, many farmers choose to perform herd milk culture testing. Strategies for managing S. aureus include: [xxxv]

Culling animals who test positive and maintaining a herd that is negative for S. aureus
Bottle feeding calves and kids to ensure that S. aureus is not passed from mother to offspring (and ensuring that milk from infected animals is not fed to offspring)
Segregating the milking herd to separate those who test positive for S. aureus
Milking S. aureus-infected animals last, to ensure that the bacteria are not passed to uninfected animals via the milk machine and hands of farm workers
Ensuring teat cups used on S. aureus-positive animals are not used on other animals (or rinsing and sanitizing such cups after milking infected animals)

Bovine Leukemia Virus (BLV)

Bovine Leukemia Virus (BLV) can affect the health and productivity of your herd. Although some animals with BLV may have signs of illness (such as labored breathing, loss of appetite, or tumors), nearly two-thirds of infected animals are sub-clinically infected (i.e. they show no signs of illness).[xxxvii] Some recent studies have theorized that BLV may be linked to breast cancer in humans.[xxxviii, xxxix] However, the research on this is inconclusive and was not able to make clear associations.

BLV is destroyed by pasteurization. Raw milk naturally contains bioactive anti-viral and anti-carcinogenic compounds (which are destroyed by heat).[xl] These bioactive compounds affect human immunity and it is not known how they would affect any potential interactions with BLV. According to one of the studies investigating the link between BLV and breast cancer, “Numerous prospective studies on dairy consumption in various defined populations, however, including one study that carefully evaluated unpasteurized milk consumption, found no significant relationship between cow’s milk consumption and breast cancer incidence.”[xxxix]

BLV testing of the raw milk dairy herd is not typically required by regulatory agencies. For overall herd health, Raw Milk Institute recommends testing for BLV prior to purchasing animals. In existing herds with no clinical signs of illness, it is recommended to consult with a local veterinarian who is knowledgeable about local conditions in order to determine whether BLV testing is recommended. Some types of BLV testing measure the relative levels of viral loads, with the results given as ‘Undetected, Low, Moderate, or High’. Animals who have no symptoms and test as Undetected or Low are considered to be low-risk animals for raw milk production.

Mycobacterium avium paratuberculosis (MAP, Johne’s disease, Paratuberculosis)

Johne’s disease is an infection in the small intestine of cows and other ruminants. It is caused by Mycobacterium avium subspecies paratuberculosis (MAP) and can easily spread in the herd. Johne’s disease is a chronic gut infection that leads to weight loss, diarrhea, loss of body condition, and reduced milk production. Animals may contract MAP or already be infected with MAP at birth, yet it is possible that no signs of illness may appear for years.[xli] Johne’s is generally not considered to affect humans, thus it is primarily a herd health issue.

Some researchers have theorized that MAP may be linked to Crohn’s disease in humans, however the research is inconclusive. For instance, a study looking for connection between MAP and Crohn’s found that “The results do not support the hypothesis that Map plays a causative role in the etiology of Crohn's disease.” [xlii] According to another study, “Despite numerous attempts to demonstrate causality by researchers, direct microbiological evidence of MAP involvement in [Crohn’s disease] remains elusive. Importantly, it has not been possible to reliably and reproducibly demonstrate mycobacteria in the tissue of [Crohn’s Disease] patients.” [xliii]

MAP testing of the raw milk dairy herd is generally not required by regulatory agencies in North America. For overall herd health, Raw Milk Institute generally recommends testing for MAP prior to purchasing animals. For existing herds with no clinical signs of illness, it is recommended to consult with a local veterinarian who is knowledgeable about local conditions in order to determine whether MAP testing is recommended.

When cows or goats have the clinical signs of diarrhea, weight loss, and poor body condition (skinny-looking) while still eating a normal ration, it should be considered that they may have MAP and testing is recommended. The milk test that is generally used for MAP indicates relative amounts of antibodies to MAP that are present in the milk, and does not strictly indicate that an animal has Johne’s disease. To protect the rest of the herd, culling should be considered for any animal that shows ongoing signs of Johne’s illness or tests positive on antibody screening tests.

My Herd Came Back Positive. Now What Do I Do?

We hope the above information clarifies some of the most common pathogens and herd health concerns. Be aware that testing results are not always clear. There are many kinds of testing protocols and different tests reveal different kinds of data. Depending on the type of test performed, a positive test result may be detecting the presence of antibodies against a disease (which would indicate a prior immune response to the illness rather than a current, active infection).

Additionally, some cows or goats may test positive for illness while appearing perfectly healthy with no signs of illness. In those cases, the farmer may choose to monitor the cows/goats closely and wait to cull until such time as there may be actual signs of illness. This is a decision to be made in consultation with a veterinarian.

When culling is necessary, it should be done at a USDA-approved slaughter plant. Slaughtered animals that are suspected to have been diseased will be inspected by a USDA inspector to determine if the animal is fit for human food consumption. At that time, an official USDA determination is made about the disease status of the animal that will either confirm or nullify the initial assumption or diagnosis of disease.

As an ethical raw milk producer, one of your most important and highest responsibilities is to assure that your raw milk herd is healthy and not spreading illness unto itself or any of your raw milk consumers. Whatever it takes to achieve and protect the optimal herd and consumer health will be the best decision. Be sure to have a good veterinarian to assist in these decisions. There are many kinds of tests, and being able to understand the specific test results will be critical to making the right decisions. The broader raw milk production community may also have answers to your challenge. Thus, stay connected to the larger community and its greater experience.

Thanks to the RAWMI Advisory Board for reviewing this document.

References

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[xvi] Animal and Plant Health Inspection Service. “Status of Current Eradication Programs.” United States Department of Agriculture. April 2024. https://www.aphis.usda.gov/livestock-poultry-disease/status-eradication-programs

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[xx] Miller, R. S., & Sweeney, S. J. (2013). Mycobacterium bovis (bovine tuberculosis) infection in North American wildlife: current status and opportunities for mitigation of risks of further infection in wildlife populations. Epidemiology and infection, 141(7), 1357–1370. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3684113/

[xxi] Larsen, J. (May 2023) “Overview of Brucellosis in Large Animals.” Merck Manual Veterinary Manual. https://www.merckvetmanual.com/reproductive-system/brucellosis-in-large-animals/overview-of-brucellosis-in-large-animals

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