Disease pathogens associated with livestock may be present in manure, and these pathogens may affect humans coming in contact with the animals or by the manure contaminating the human water supply. Some of the chemicals and drugs used for promoting animal health and performance may become mixed with animal manure. It is important to understand the potential areas of risk and then to minimize the potential for disease transmission or contamination of the water supply.
Controlling the presence of pathogens on a livestock farm is very important for the health and performance of the animals, profitability, and in minimizing the risk to human health. Four approaches to controlling pathogens on animal operations are:
Many of the aspects relating to waste treatment and pathogen control have been discussed in detail in other chapters, especially as they relate to collection, storage, and land application of manure. Pathogenic bacteria require an adequate food source, appropriate temperature, and adequate moisture to survive. The survival of pathogenic bacteria that are found in manure can be lessened by providing a clean, dry environment in animal facilities.
Potential risk has been identified for the transfer of pathogens in the airborne dust particles from livestock farms, with most of the focus being on swine and poultry operations. Limited data are available to date on the concentration of bacteria in particulate matter from livestock farms and its relative risk to human health. See Chapter 8, Odor and Dust Emission Control, for more information on dust-control measures.
Although a number of diseases are shared by both man and animals, many of them require specific conditions, such as a mosquito or tick bite, for their transmission. Examples of these include Lacrosse encephalitis, West Nile fever, and Lyme disease. Fortunately, the number of diseases that are transmitted from animals to man, either directly or by contact with animal manure, is rather small. These diseases are all infections caused by organisms classified as bacteria, viruses, fungi, or protozoa. The more important of these diseases are discussed here.
Brucellosis is also known as “undulant fever” in humans and is spread mainly by contact with aborted fetuses and milk. Although it was once a serious problem, brucellosis has been essentially eradicated in the United States through an active testing and monitoring program (www.aphis.usda.gov/vs/ceah/ncahs/nsu/surveillance/bru.htm). If brucellosis should occur in a herd, the infected herd is quarantined until all known infected animals are removed.
Bovine Tuberculosis is spread by aerosols and by contamination of food and the environment. Tuberculosis is also nearly eradicated from the United States. Human and bovine tuberculosis are caused by different bacterial strains. Whereas humans are susceptible (mainly through drinking water) to bovine tuberculosis, cattle are relatively resistant to the human form. Pasteurization of milk and meat inspection procedures have virtually eliminated the exposure of people to both brucellosis and bovine tuberculosis.
Salmonella, Campylobacter, and Escherichia coli are bacteria that inhabit the intestines of many animals and human beings. People usually get exposed to these bacteria by contamination of their food through unsanitary food processing or handling practices, contaminated water supplies, or by drinking unpasteurized milk. These bacteria are commonly involved in cases of food poisoning, and the source of the bacteria may be both infected animals and other humans. Pasteurization of milk, thorough cooking of foods, and routine water chlorination usually kills these bacteria, and it is often post-treatment, or post-cooking, contamination that leads to illness in people.
Direct transmission of these pathogens to humans following contact with farm animals or farm environments can and does occur. This is of primary concern for high-risk groups, such as young children (less than five years of age), the elderly, and immuno-compromised individuals. Much of this risk can be mitigated with proper hygienic practices, such as hand washing.
Listeria is a bacterium that lives in the intestines of many types of animals and occasionally humans. It may be found in the milk of infected cows. Often it causes no harm, but the environment may become contaminated, thus leading to exposure of other animals and people. The bacteria can live in the environment for a long time, including the environment inside food-processing plants. Some outbreaks in people have been associated with contamination of food products in the processing plant where Listeria survived on inadequately sanitized equipment or in air-handling systems. Other outbreaks have been traced to improperly pasteurized milk or uncooked, or improperly cooked, foods.
Leptospirosis is caused by several species of the Leptospira genus of bacteria. These bacteria live in the urinary tracts of many species of animals, including rats, mice, dogs, raccoons, deer, and muskrats as well as cattle. Most people get exposed by direct contact with the urine of infected animals or by swimming or wading in ponds or other contaminated water sources.
Rabies is the only virus of importance to mention here. It is transmitted by a bite or other direct contact with saliva from an infected animal, and all warm-blooded animals are susceptible. The reservoir for the virus in the United States is predominantly wild animals. Raccoons, skunks, foxes, and bats are the principal species involved. In the past decade, all human cases developing from exposures in the United States have been traced to bats. Livestock species are infected primarily through bites of wild animals, and humans are only at risk when they have close, direct contact with them. (See OSU Extension fact sheet VME-1-97, Rabies Prevention in Livestock, at ohioline.osu.edu/vme-fact/0001.html.)
Ringworm is the common name for skin infections caused by a number of fungi. Athlete’s foot is an example of a fungal infection unique to humans. Ringworm is most common in cattle and occasionally seen in horses and sheep. Transmission to humans occurs, but it is not common. The fungi that cause ringworm are spread by direct contact with the infected animal and by contamination of clothing and other objects, such as grooming equipment, with fungal spores. Although people occasionally become infected by contact with livestock, the general public is more at risk by contact with pet animals that may also carry various types of fungi.
Cryptosporidia and Giardia are found in the intestinal tracts of many species of livestock and other domestic animals, as well as human beings and some wild animals. People usually become exposed through contaminated food or water. It was once thought that these organisms were rather freely transmitted between animals and people. Newer scientific techniques have shown us that this may not be the case.
Although people can become infected by the strains of cryptosporidia that infect cattle, we now know that some people may harbor a strain transmissible only to other people. Since some of these newer scientific tools have become available, it has been suggested that most cases of cryptosporidiosis in people, where there are outbreaks, have come from other human sources.
Giardia can be found in some surface water supplies. Although chlorination of drinking water does not destroy these organisms, other water treatment processes, such as sand filtration required for municipal water systems, make the water safe. Properly sealed and maintained water wells should not be contaminated with these organisms.
Most rural residents have private wells, but many rural and most urban residents obtain water from public wells, lakes, or streams. Organisms of primary concern for contamination of water supplies include Salmonella, Campylobacter, E. coli, Leptospirosis, Cryptosporidia, and Giardia (see previous section for discussion about each organism).
Well contamination is usually caused by surface water running into the top of the well or problems in the well casing that permit entry of contaminated water. If the well is properly constructed and is of adequate depth (i.e., to bedrock), it should be safe. Local health departments can check wells for bacterial contamination.
Pathogens in manure tend to be retained at or near the soil surface. As a result, surface runoff is the primary cause of pathogen transport. Soil may retard and filter bacteria by their absorption to organic matter; thus soil may entrap bacteria during leaching events. Higher soil organic matter would promote more entrapment, and therefore, the organic matter content of the manure applied may impact movement of the bacteria. Finer textured soils are more likely to entrap bacteria. Also, grass buffer strips are quite effective at filtering out pathogens.
The Environmental Protection Agency (EPA) requires municipal water systems to have filtration systems in-place, which, when functioning properly, will remove cryptosporidia oöcysts. While chlorination will not kill cryptosporidia, it is very effective in killing bacteria, such as E. coli and Salmonella spp.
Antibiotics are primarily administered to animals either by addition to feed or by injection. Antibiotics in the feed that are not absorbed will be excreted in the manure. Absorbed antibiotics or injected antibiotics are generally metabolized and the resulting metabolites excreted in urine (these metabolites do not function as antibiotics). Therefore, antibiotics can enter the manure storage facility from discarded feed with antibiotics, manure from animals receiving antibiotics, dumped milk from treated dairy cows, and footbaths (see the section on footbaths). Antibiotics in the manure are in very low concentration and likely will be denatured in vegetative buffer strips and in soil. The highest risk for antibiotics getting in the water supply is caused by runoff directly into a water source. Judicial use of all medications for animals is expected, and antibiotic usage will continue to be under close scrutiny.
Milk from dairy cows treated with antibiotics, most commonly for an udder infection known as mastitis, must be discarded for the number of milkings described on the product’s label. Although only a single quarter may be infected and treated on a cow, the milk from all four quarters must be discarded for the specified withholding time period. Antibiotic testing kits are available to farmers to test questionable milk. Generally speaking, antibiotics are not used extensively or indiscriminately on dairy farms; thus, dairy farms are unlikely to be a source of water contamination by antibiotics.
Footbaths are commonly used for cattle, especially dairy, and sheep to control foot rot and other infectious foot diseases. The most common product used in the footbath is copper sulfate (usually mixed to provide a 5% solution), but antibiotics and formalin may sometimes be included in the solution. The frequency of usage is variable, but the animals may be required to pass through the shallow bath several times per week during selective times of the year. The bath becomes contaminated with manure, and a new solution is mixed periodically. Another footbath with just water or mild detergent solution prior to the medicated one will help to extend the life of the solution and increase the effectiveness of the medicated footbath.
The contents of the footbath are usually emptied into the manure-storage facility. A footbath 6 ft x 34 in x 6 in (3.4 cu ft) at 75% capacity (2.5 cu ft) will contain about 18.5 gallons of solution. Although this amount of solution mixed into a manure-storage facility will usually result in negligible increases in the concentration of copper and the other compounds in the manure, awareness of this risk is important.
Detergents and acidic chemicals are used in milking parlors to wash and sanitize equipment. Also, chemical solutions for disinfecting teats to prevent the spread of mastitis are used at each milking, with most farms dipping teats before and after milking. The primary active ingredients used in the products include chlorhexidine, iodine, linear dodecyl benzene sulfonic acid, and sodium hypochlorite, among others (www.nmconline.org/docs/Teatbibl.pdf). For more information about the disposal and handling of milking parlor wastes, see Chapter 3, Manure-Management Systems.
Eastridge, M. L., K. Hoblet, W. Shulaw, and T. Wittum. 2001. Questions Pertaining to Large Dairy Enterprises in Ohio: Animal and Human Health. AS-13-01, Ohio State University Extension, Columbus. ohioline.osu.edu/as-fact/0013.html
Geohring, L. D. 2001. Keeping pathogens out of water. Pages 353-365 in Proceedings, Dairy Manure Systems: Equipment and Technology. NRAES-143, Ithaca, New York.
Shulaw, W. P., and G. L. Bowman. On-Farm Biosecurity: Traffic Control and Sanitation. VME-6-01, Ohio State University Extension, Columbus. ohioline.osu.edu/vme-fact/0006.html