There is overwhelming evidence that animals are reservoirs for several of the most important food-borne and waterborne agents. Manure and wastewater from animal feeding operations have the potential to contribute pathogens and pollutants, such as antibiotics, nutrients (nitrogen and phosphorus), hormones, sediments, heavy metals, organic matter, and ammonia, to the environment and eventually could impact human health. Some of the illnesses resulting from these agents cause only temporary health problems, while others cause severe crises and even death.
Years ago, diseases such as tuberculosis, brucellosis, diphtheria, and scarlet fever were commonly spread through milk (Table 1). TB testing, brucellosis eradication, and pasteurization resulted in a dramatic decline in milk-borne disease outbreaks. From 1950 through the 1970s, several disease outbreaks were caused by pathogens in processed milk and manufactured dairy products (e.g.,Salmonellae in dried milk products, toxicogenic Escherichia coli and Staphylococcus aureus in certain cheeses). In the 1980s several previously undetected bacteria were related to illnesses (Listeria, Campylobacter, and Yersinia).
Even now, pathogen contamination of water and food supplies has received national attention. Salmonellae contamination of pasteurized milk affected hundreds of thousands. Listeriosis in cheese has caused dozens of deaths. Cryptosporidium parvum from farm runoff in the Milwaukee water supply sickened more than 400,000. E. coli O157:H7 has been found in hamburger (often from spent dairy cows), and recently several people were sickened and some children died because of a contaminated water supply at a fairground which may have been polluted by livestock waste.
|Years||Notable Diseases or Pathogens Associated with Dairy Products|
|1900-1940s||Diptheria, tuberculosis, brucellosis, Q-fever, septic sore throat|
|1950-1975||Salmonellosis, Staphylococcus enterotoxins, brucellosis, Bacillus cereus, Enterotoxigenic E. coli|
|1975-1986||Hemorrhagic E. coli, Yersinia enterocolitica, Campylobacter jejuni, Salmonella typhimurium, Listeria monocytogens|
Most dairy food-borne illness outbreaks are due to microorganisms in food. Bacteria in dairy food can cause intoxications or infections. Infections occur when the microorganism is eaten and established in the body, usually multiplying inside the intestinal tract and irritating the lining of the intestines. Intoxications occur when bacteria grow in foods and release preformed toxins. Bacteria have recently been linked to illnesses caused by milk that previously have never been or were rarely known to have been associated with milk or dairy foods.
The Center for Disease Control and Prevention has recently estimated that 76 million cases of gastrointestinal illnesses are caused annually by food, resulting in 325,000 serious illnesses requiring hospitalization and 5,000 deaths. The CDC reports that more than 95% of estimated food-related deaths are due to six pathogens: Salmonella, 31%, Listeria, 28%, Toxoplasma, 21%, Norwalk-like viruses, 7%, Campylobacter, 5%, and hemorraghic E. coli, 3%. Of these six, Salmonella, Campylobacter, hemorraghic E. coli and Listeria have been associated with dairy foods.
It must remembered that unknown agents account for approximately 81% of food-borne illnesses and hospitalizations and 64% of deaths. Ninety percent of Listeria cases require hospitalization because it causes serious illness, whereas, much of the E. coli and Salmonella illnesses are not reported. Food-borne transmission is attributed to 36% of acute gastroenteritis, and of hospitalizations caused by bacterial pathogens, 60% were due to food-borne transmission.
Many of the pathogens of greatest concern today were not recognized as causes of food-borne illness 20 years ago (hemorraghic E. coli, Listeria, Campylobacter, Cryptospordium). CDC has targeted four bacterial pathogens – E. coli O157:H7, Salmonella enteritidis, Listeria monocytogenes, and Campylobacter jejuni – as those of greatest concern
Most cases of food-borne and waterborne illness in healthy adults are self-limiting and of short duration. Diarrhea, cramps, nausea, and vomiting are the most common acute symptoms of many food-borne and waterborne illnesses, which can range from mild to severe. These symptoms are commonly found regardless of pathogen.
Most of the pathogens can be found as contaminants of raw milk, eggs and poultry products, meats, shellfish, vegetables, and, last but not least, water. Many of these illnesses can be transmitted by other vectors, such as contaminated or infected food handlers or equipment. Pregnant women, newborns or young children, elderly people, and the infirmed are more prone to these illnesses, although many also attack healthy people.
While the likelihood of serious complications is unknown, the U.S. Food and Drug Administration estimates that about 2 to 3% of all food-borne illness cases lead to secondary long-term illnesses. For example, E. coli O157:H7 can cause kidney failure in young children and infants; Salmonella can lead to reactive arthritis and serious infections; Listeria can cause meningitis and stillbirths; and Campylobacter may be the most common precipitating factor for Guillain-Barre syndrome.
According to the CDC, Campylobacter is the most common bacterial cause of diarrhea in the United States, resulting in 1 to 6 million illnesses each year. Also each year, an estimated 1,850 people become seriously ill from Listeria monocytogenes. The disease primarily affects pregnant women, newborns, and adults with weakened immune systems.
Many emerging zoonotic pathogens (disease-causing organisms that transfer from animals to humans under natural conditions) are becoming increasingly resistant to antimicrobial agents, largely because of the widespread use of antibiotics in the animal reservoir. Therefore, public health concerns include the patterns of antimicrobial use in agriculture, as well as in human medicine.
Briefly described below are microbes that can be transmitted from animals to humans directly or through dairy foods and water supplies and thus are a potential threat to human health. Listed are only those that may be transmitted through milk or meat from spent dairy cows. Pathogens such as human enteric viruses and Giardia have not been included.
|Hemorraghic E. coli||Listeria||Campylobacter||Staph aureus||Streptococci|
The U.S. Food Safety Inspection Service has proposed a concept of pathogen reduction that traces back to the dairy, farm, or ranch. However, eradication on the farm seems very unlikely for major food-borne pathogens. For one thing, the concept of food safety through on-farm testing will be expensive to implement. It is important to recognize common sources for herd or farm exposure to many of these pathogens. According to Jim Cullor, Ph.D., (University of California, Davis), probable on-farm critical control points for many of these human pathogens will be: (1) housing and bedding, (2) water and waste management areas, (3) hospital pens, calving pens, treatment areas, (4) bulk tank milk, and (5) young stock and cull animals.
Steps to Take
- Identify and reduce stresses, such as overcrowding, among healthy animals.
- Clean and chemically sanitize reasonable contact areas, such as sick cow stalls, maternity pens, and contaminated feed materials and water.
- Isolate animals with diarrhea.
- Provide proper clothing and show covers for workers and visitors to the farm.
- Have foot baths available for people and clean them on a regular basis.
The most common method for destroying pathogenic bacteria and other organisms and also for eliminating organisms that spoil milk or cause off-flavors is pasteurization. According to the CDC, raw milk drinkers are 158 times more likely to contract illness caused by Salmonella than those who consume properly pasteurized milk.
However, some microorganisms, not the pathogens, survive pasteurization. Milk producers must continue with farm practices that protect the public from food-associated hazards even though few illnesses can be traced to the farm. Producers must be aware of public perceptions of possible hazards and risks so they can strengthen current quality practices and prevent adverse circumstances.
The primary entry points of a pathogen into the milk supply are cows, people, and equipment. Cows are normal carriers of many pathogens, either on the hair or skin from exposure to soil, manure, surface water (ponds and creeks), and other environmental sources.
People can also carry pathogenic bacteria. Because many of the organisms have their origin in human feces or infections, people can directly cross-contaminate the milk, the cow, or the equipment through poor personal hygiene. Employees can also become contaminated from soil and manure on the cow and, in turn, cross-contaminate the milk or equipment.
Unsanitary equipment primarily results from improper cleaning and sanitizing. When milk contacts unsanitary surfaces, such as pipelines, milker parts, the bulk tank, or other sources, it can be contaminated with pathogens. Even if contact surfaces are properly cleaned and sanitized, they can contaminate milk if they come in contact with other disease carriers, such as rodents, roaches, flies or other insects, manure, or contaminated water. We must remember that pasteurization of any sort is no match for bad sanitation and substandard practices.
Many of the pathogens listed (except Listeria monocytogenes) will not grow and multiply below 40 F. Refrigerating milk will not kill the organisms, but it will limit their ability to reproduce. Milk producers must ensure temperature control according to regulations. Cool milk rapidly (less than two-hour chill time) to less than 40 F and maintain the temperature at that level. However, Listeria is a cold-loving bacteria that can grow at 37 F or higher. While cool temperatures are the best way to control most pathogens, stop Listeria before it enters the system by cleaning and sanitizing everything properly.
Young animals are most susceptible to many of these organisms. They are more frequently found with the stress of inadequate housing. Cattle, especially young stock, should be kept clean and well nourished. Manure should be stored and spread when risk of disease transmission, such as times of high risk of runoff, is low. Narrow filter strips reduce the risk of stream contamination.
Management, Housing, and Feeding Precautions
- Clean and dry maternity and sick stalls. Animals with diarrhea should be segregated. Calves should be born in a clean, manure-free environment.
- Remove newborn calves from dam at birth and before nursing. Give newborn colostrum from dam or disease-free cows. Avoid feeding pooled waste milk.
- Keep young cattle separated from milking or dry cows.
- House in clean and dry loafing areas or environments. Cleanliness is important for cows’ teats and udders, housing and milking equipment, and milkers’ hands. Remove hair from udders, and milk clean, dry teats.
- Thoroughly clean and sanitize milking equipment and replace gaskets and other parts before they show wear.
- Water troughs should be cleaned weekly.
- Old feed should be removed daily from feed bunks. Feed storage areas and mixing equipment should be cleaned regularly. Cattle should receive fresh feed through a balanced ration.
- Caution should be exercised in selection of any outside animals. Animals introduced from outside should be isolated from the rest of the herd.
- Milk from cows with clinical mastitis or elevated somatic cell counts should not be added to the bulk tank for shipment.
- Implement an effective fly control program.
- Test water for bacterial quality and determine if it needs to be chlorinated.
- Monitor milk temperatures.
Food will always represent some biological risk. Food-borne illnesses are preventable. Adherence to good hygienic practices can result in food safety and ensure food quality. But you are not alone because food safety is the shared responsibly of governments, academia, the food industry, and the consumer.
Cullor, J.S. 1997. Mastitis and dairy environment pathogens of public health concern. Pages 20-32 in Proc. 36th Annual Meeting, Nat’l Mastitis Council, Madison, WI.
Partnership for Food Safety Education. Food-borne illness: A constant challenge. http://www.fightbac.org/content/view/154/86/
Pell, A.N. 1997. Manure and microbes: Public and animal health problem. J. Dairy Sci. 80:2673-2681.
Tauxe, R.V. 1997. Emerging Food-borne Diseases: An Evolving Public Health Challenge. National Conference on Emerging Food-borne Pathogens: Implications and Control. Emerging Infectious Diseases, vol 3., http://www.cdc.gov/ncidod/eid/vol3no4/tauxe.htm
Tybor, P.T. and W. Gilson. 1992. Dairy Producer’s Guide to Food Safety in Milk Production. Univ. of Georgia Bull. 1084, Athens. http://www.foodsafety.org/ga/ga005.htm
USDA, USEPA Unified national strategy for animal feeding operations. March 9, 1999.
U.S. Food & Drug Administration, Center for Food Safety & Applied Nutrition (FDA-CFSAN). Food-borne Pathogenic Microorganisms and Natural Toxins Handbook
Vasavada, P.C. 1988. Pathogenic bacteria in milk – A review. J. Dairy Sci. 71:2809-2816.
Gerald M. Jones
Virginia Polytechnic Institute and State University (Virginia Tech)