Microorganisms of Concern in Milk
This page describes milkborne pathogens and their associated illnesses, and several other important microorganisms in milk. A table summarizing the disease characteristics of the major milk pathogens is presented at the beginning of this section, followed by a more detailed discussion of the microorganisms in milk, listed in alphabetical order: Brucella spp., Campylobacter jejuni, Coliforms, Coxiella burnetii, Escherichia coli O157:H7, Listeria monocytogenes, Mycobacterium bovis and tuberculosis, Mycobacterium paratuberculosis, Psychrotrophic Bacteria, Salmonella spp., and Yersinia enterocolitica.
Cases of human illness associated with the consumption of dairy products are listed in the Disease Outbreaks Associated with Milk Products Section.
Table 1. Major milkborne pathogens and their associated diseases.
| Organism | Disease | Disease Symptoms | Source |
|---|---|---|---|
| Campylobacter jejuni | Gastroenteritis | Diarrhea, abdominal pain, fever | Intestinal tract and feces |
| Coxiella burnetii | Q fever | Chills, fever, weakness, headache, possible endocarditis | Infected cattle, sheep, and goats |
| Escherichia coli O157:H7 | Gastroenteritis Hemolytic uremic syndrome (HUS) |
Diarrhea, abdominal pain, bloody diarrhea Kidney failure, possible death |
Intestinal tract and feces |
| Listeria monocytogenes | Listeriosis | Flu-like symptoms, miscarriage, stillbirths, fetal death, and spontaneous abortion | Water, soil, environment |
| Mycobacterium bovis or tuberculosis | Tuberculosis | Lung disease | Infected animals |
| Mycobacterium paratuberculosis | Johne's (ruminants) |
Unconfirmed link to Crohn's disease in humans | Infected animals |
| Salmonella spp. | Gastroenteritis Typhoid fever |
Diarrhea, nausea, fever | Feces and environment |
| Yersinia enterocolitica | Gastroenteritis | Diarrhea, appendicitis | Environment, water, infected animals |
Brucella spp.
Brucella species (spp.) are found in many animal species including cattle, sheep, and goats. Brucella spp. are destroyed by pasteurization.
Brucella spp. cause illness with symptoms that are flu-like and include fever, sweats, headaches, back pain and physical weakness. In some cases long-lasting symptoms of fever, joint pain and fatigue may occur.
Campylobacter jejuni
Campylobacter jejuni is found in the intestinal tract, udder, and feces of cattle, in poultry and wild birds, and in contaminated water sources. Campylobacter jejuni is destroyed by pasteurization. Campylobacter jejuni is one of the most common bacterial causes of diarrheal illness in the US. Illness can often occur as sporadic events and in larger outbreaks.
Campylobacter jejuni generally causes illness 2 to 5 days after exposure, and illness typically lasts 5 to 10 days. Symptoms of campylobacteriosis include diarrhea, bloody diarrhea, abdominal pain, cramping, nausea, vomiting, and fever. Patients with Campylobacteriosis usually recover without specific treatment other than fluid and electrolyte replacement. In some persons with a compromised immune system, Campylobacter jejuni infection can lead to the more serious diseases Guillan-Barré syndrome and Reiter syndrome. Guillan-Barré syndrome is a disorder that results in temporary neuromuscular paralysis, although 20% of those infected may have long term disability and it may cause death. Reiter syndrome is a reactive arthritis that may affect multiple joints, particularly the knee joint.
The prevalence of Campylobacter jejuni is very widespread. It has been reported in bulk tank raw milk samples in Illinois, Michigan, Minnesota, Ohio, Pennsylvania, South Dakota, Tennessee, Virginia, and Wisconsin, suggesting that the organism is ubiquitous. In these studies, Campylobacter jejuni was found in 0.4 to 12.3% of the bulk tank milk samples (Doyle and Roman, 1982; Jayarao et al., 2001 and 2006; Lovett et al., 1983; McManus and Lanier, 1987; and Rohrbach et al., 1992).
Coxiella burnetii
Coxiella burnetii is found in many animals worldwide and is shed in the milk, urine and feces of cattle, goats, and sheep. Coxiella burnetii is considered to be the most heat resistant non-sporeforming pathogen commonly found in milk, and the established conditions for milk pasteurization are specifically designed to destroy this organism.
Coxiella burnetii causes Q fever, an illness characterized by a sudden onset of high fever, severe headache, nausea, vomiting, diarrhea, abdominal pain, chest pain, chills, sweats, sore throat, non-productive cough, and general malaise. Fever can last for 1 to 2 weeks. Most patients recover without any treatment, although Coxiella burnetii may result in death.
The prevalence of Coxiella burnetii was >94% in raw milk samples from the Northeastern, Midwestern, and Western regions of the US tested between 2001 and 2003 (Kim et al., 2005).
Escherichia coli O157:H7
Escherichia coli O157:H7 is one strain in a large family of bacteria. Strains of Escherichia coli (E. coli) are considered fecal coliforms. Most strains of Escherichia coli do not cause illness and live in the intestinal tracts of healthy humans and animals. E. coli O157:H7 is found in the intestinal tract and feces of cattle. E. coli O157:H7 is destroyed by pasteurization.
E. coli O157:H7 produces toxins that cause illness in humans. Symptoms of illness include bloody diarrhea and abdominal cramps. In some cases, particularly in young children, E. coli O157:H7 infection causes hemolytic uremic syndrome, which destroys red blood cells and causes kidney damage or failure, and in some cases death.
The prevalence of E. coli O157:H7 and Shiga-toxin producing E. coli has been reported for bulk tank raw milk samples in Minnesota, Pennsylvania, South Dakota, Wisconsin and Ontario. E. coli O157:H7 was found in 0.87 to 10% of the bulk tank milk samples tested (Jayarao et al., 2001 and 2006; Padhye and Doyle, 1991; and Steele et al., 1997).
Listeria monocytogenes
Listeria monocytogenes is found in soil and water and has been isolated from a large number of environmental sources. Listeria monocytogenes is destroyed by pasteurization, but if food products are contaminated after pasteurization, Listeria monocytogenes can grow at refrigerator temperatures. Illness can occur as sporadic events or larger outbreaks.
Listeria monocytogenes typically causes illness in pregnant adults, newborns, the elderly, and patients with compromised immune systems, but healthy adults and children may also become infected. Symptoms of Listeriosis include flu-like symptoms, fever, muscle aches, stiff neck, headache, septicemia, meningitis, miscarriage, stillbirth, premature delivery, abortion, or death.
The prevalence of Listeria monocytogenes has been reported for bulk tank raw milk samples in individual states (or grouped by region) for California, Colorado, Florida, Idaho, Illinois, Indiana, Iowa, Kentucky, Massachusetts, Michigan, Minnesota, Missouri, Nebraska, New Mexico, New York, Ohio, Pennsylvania, South Dakota, Tennessee, Texas, Washington, Wisconsin, Vermont, Virginia, and in Alberta and Ontario, Canada. Listeria monocytogenes was found in up to 12% of the bulk tank milk samples tested (Farber et al., 1987; Fedio and Jackson, 1990; Jayarao et al., 2001 and 2006; McManus and Lanier, 1987; Liewen and Plautz, 1988; Lovett et al., 1987; Rohrbach et al., 1992; Slade et al., 1988; Steele et al., 1997; and Van Kessel et al., 2004), illustrating the widespread presence of Listeria monocytogenes in unpasteurized milk.
Mycobacterium bovis and Mycobacterium tuberculosis
Mycobacterium bovis and Mycobacterium tuberculosis are found in infected cattle worldwide. Both of these organisms are destroyed by pasteurization.
Mycobacterium bovisand Mycobacterium tuberculosiscause tuberculosis, a lung disease. Tuberculosis in the US is not very common today, although historically milk was a common source of tuberculosis. Tuberculosis is a concern in many parts of the world.
Mycobacterium paratuberculosis
Mycobacterium paratuberculosis causes Johne's disease in cattle. It has been suggested that Mycobacterium paratuberculosis may be associated with Crohn's disease, an intestinal disorder, in humans, but this has not been confirmed.
Salmonella spp.
Salmonella species (spp.) contain several strains that cause illness in humans, the most common are the serotypes Enteriditis and Typhimurium. Salmonella has been found in the intestinal tracts of all warm-blooded animals including humans. Salmonella is destroyed by pasteurization.
Salmonella spp. causes illness that can develop 12 to 72 hours after exposure, and can last 4 to 7 days. Symptoms of Salmonellosis include diarrhea, abdominal cramps, and fever. Most people recover without treatment other than fluid and electrolyte replacement. Some cases may be severe and require hospitalization. A small number of people may develop Reiter syndrome, which is a reactive arthritis that may affect multiple joints, particularly the knee joint.
The prevalence of Salmonella spp. has been reported for bulk tank milk samples in individual states (or grouped by region) for California, Colorado, Florida, Idaho, Illinois, Indiana, Iowa, Kentucky, Michigan, Minnesota, Missouri, New Mexico, New York, Ohio, Pennsylvania, South Dakota, Tennessee, Texas, Washington, Wisconsin, Vermont, Virginia, and Ontario, Canada. Salmonella spp. were found in 0.17 to 8.9% of the bulk tank milk samples tested (Jayarao et al., 2001 and 2006; McManus and Lanier, 1987; Rohrbach et al., 1992; and Van Kessel et al., 2004), indicating the widespread presence of Salmonella in unpasteurized milk.
Yersinia enterocolitica
Yersinia enterocolitica is found in the intestinal tract of farm animals, especially pigs, and in the environment. Yersinia enterocolitica is destroyed by pasteurization, but if food products are contaminated after pasteurization, Yersinia enterocolitica can grow at refrigerator temperature.
Yersinia enterocolitica causes illness with symptoms of fever, abdominal pain and diarrhea.
The prevalence of Yersinia enterocolitica has been reported for bulk tank milk samples in Michigan, Minnesota, Pennsylvania, South Dakota, Tennessee, Wisconsin, Virginia, and Ontario, Canada. Yersinia enterocolitica was found in 1.2 to 18% of the bulk tank milk samples tested (Jayarao et al., 2001 and 2006; Moustafa et al., 1983; Rohrbach et al., 1992; and Schiemann, 1978). McManus and Lanier (1987) reported Yersinia enterocolitica in 48.1% of the samples tested, but they were all environmental, non-pathogenic strains.
OTHER BACTERIA OF SIGNIFICANCE
Coliforms
Coliforms are a large group of bacteria that are found in the intestines of warm-blooded animals. Most coliforms are not pathogenic, but their presence indicates contamination, usually from fecal sources. Coliforms are destroyed by pasteurization.
The prevalence of coliforms were detected in 62 to 95% of the raw bulk tank milk tested in regions that included California, Colorado, Florida, Idaho, Illinois, Indiana, Iowa, Kentucky, Michigan, Minnesota, Missouri, New Mexico, New York, Ohio, Pennsylvania, South Dakota, Tennessee, Texas, Washington, Wisconsin, Vermont, and Virginia (Jayarao and Wang, 1999; and Van Kessel et al., 2004), suggesting the ubiquitous presence of these organisms in unpasteurized milk.
Psychrotrophic Bacteria
Psychrotrophic bacteria are not a specific type or family of bacteria, but rather this is the name given to bacteria that are capable of growing at 44.6°F (7°C) or less. This group of microbes is a concern in dairy products because they grow at refrigerator temperature and cause spoilage, often resulting in off-flavors. The most common psychrotrophs are in the genus Pseudomonas. These organisms are killed by pasteurization, but may occur in milk from contamination after pasteurization. Some bacterial pathogens are psychrotrophic, including Listeria monocytogenes, Yersinia enterocolitica, some E. coli strains and some Bacillus strains.