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No major human disease outbreak has been accurately attributed to the contact human beings have had with horses. It is rare to find references in peer-reviewed literature on horses infecting people. Infectious equine diseases are usually solely of veterinary importance. The majority of zoonoses involve other domestic species or wildlife. Most of the diseases that can be passed from horses to humans are rare and don’t occur much in the US.
The diseases of greatest concern to humans listed by CDC/FoodNet differ substantially from those of concern to horses listed by the OEI/USDA. Vector-borne diseases are of concern to humans, but are especially of concern to horses that remain outdoors 24/7. Food- and water-borne diseases are of particular concern to humans, but only of veterinary concern to horses. Five pathogens associated with food- and water-borne illness are discussed in detail: bacteria E. coli 0157:H7, Salmonella species (spp); protozoa Cryptosporidium parvum and Giardia duodenalis; and a spirochete Leptospirosis interrogans. The horse in not significantly implicated in transmitting these diseases to humans. Based on information in the literature, reference texts, the internet, and interviews with veterinarians and medical doctors, human exposure to horse feces on trails near water and crops poses little significant health risk to humans.
The authors become concerned about information being provided to the public about the extent to which horses pose threats to human health from exposure to manure deposited on public trails. We decided to investigate the potential risk horses may pose to human health. Medical doctors, veterinarians, publications from agricultural universities, textbooks and the Internet were researched. This paper will show that the risk of horse fecal pathogens infecting humans (zoonosis) is extremely small.
The US Department of Agriculture (USDA) defines livestock as any animal raised for food or fiber. The National Agricultural Statistics Service (NASS), USDA Agricultural Statistics Board, gives United States (US) populations of the following livestock as:
The 9.2 million horses (USDA 2005, AHC 2005) in the US are a relatively minor livestock group compared to agro-business livestock production levels and stocking densities found on many high production farms. While some horses are still actively engaged as working ranch/farm livestock, many horses in the US are now considered to be "recreational animals." According to the American Horse Council and the USDA (2005), of 9.2 million horses, 3.9 million are actively involved in recreational activities. The rest are used in racing, showing, etc., and will not be found on trails.
Horses are classified recreational animals because they are not usually kept under the same circumstances as high intensity farming and feedlot animals. Because horses are kept in smaller numbers and in less intensive confinement, they are less likely to be subjected to the physical and psychological factors associated with proximity to other animals such as fecal contamination of food/water, fighting, stress, diseases, etc., as is found in animals held in feedlots. The lack of stressors associated with food production animals, more individualized care, vaccinations, access to clean water and food, and veterinary attention mean that horses are generally found in good health in the US.
According to the USDA Equine Report (2005), with the possible exception of "accidents," the horse is not instrumental in the chain of events that might precipitate significant human illness morbidity or mortality. Horses are not generally associated with transmitting diseases to man (Kester 1977, Magdesian 2008, Timoney 2002, DebRoy 2007, Mike Westendorf 2008). No major human disease outbreak has ever been accurately attributed to the intimate contact human beings have had with horses for thousands of years (Duell 1989).
It was difficult to find any references in peer-reviewed literature to horses infecting people. The majority of zoonoses involve other domestic species or wildlife. Infectious equine diseases are usually solely of veterinary importance (Walcott and Timoney 2002). That is, while equines may develop diseases, usually only the veterinary staff and care givers to the horses are the people with greatest potential to develop the disease itself. While episodic outbreaks of various pathogens are reported in horses, infection of humans by these disease outbreaks has not significantly occurred according to veterinarians Vic Spain and Karen Blount (2008) and our appendices on pages 26-31. Zoonoses transmitted by horses are rare. Adult horses kept in a clean environment pose minimal risk for transmitting disease. Most of the 200 or so diseases that can be passed to humans are fairly rare and don’t occur much in the US according to Leon Russell, DVM from Texas A&M (2008).
Humans and Exposure to Microbiology
According to researcher Bill Bryson (2003), microbes are always around us in inconceivable numbers. With average good health and hygiene, about 1 trillion microbes live off the 10 billion or so flakes of skin humans shed daily. There are trillions more microbes tucked away in our gut and nasal passages, hair and lashes, eyes, and teeth. Our digestive system is host to more than a hundred trillion microbes of over 400 types. Every human body consists of about 10 quadrillion native somatic cells, supporting 100 quadrillion bacterial cells. These cells process our wastes and make them usable again. They purify our water and keep soils productive. They synthesize vitamins in our intestines, convert food to useful sugars, and fight alien microbes we ingest (Bryson 2003). In the total biomass of the planet, microbes account for at least 80% (Woese 1996).
A handful of forest soil may contain ~10 billion bacteria, most of them unknown to science: 1 million yeasts, 200,000 fungi known as molds, 10,000 protozoa, assorted rotifers, flatworms, roundworms and microscopic creatures known collectively as cryptozoa (National Geographic 1993). Bergey’s Manual of Systematic Bacteriology lists about four thousand types of bacteria. However, in the 1980s Norwegian scientists Goksoyr and Torsvik collected one gram of random soil from a beech forest near their lab in Bergen and analyzed its bacterial content. It contained between 4-5,000 separate bacterial species, more than in the whole of Bergey’s Manual. They then sampled 1 gram from a coastal location nearby and found it contained 4-5000 other species. As Wilson (2003) observes: If over 9,000 microbial types exist in two pinches of soil from two locations in Norway, one wonders how many more await discovery in other radically different habitats. According to one estimate it could be as high as 400 million (Bryson 2003).
Not only are the sheer numbers of microbes that impact humans staggering, but also they can live for long periods of time in harsh environments. A live Streptococcus bacterium was recovered from the sealed lens of a camera that stood on the moon for two years (Davies 1999). A Salmonella bacteria purposely taken into space came back later more virulent than when it left earth. Mice fed the space germs were three times more likely to get sick and died more quickly than others fed identical germs that had remained behind on earth (Associated Press Sept. 25 2007).
Luckily, most microorganisms are neutral and/or beneficial to human health. Only about one species of microbe in a thousand is a pathogen for humans according to National Geographic (1993). Helicobacter pylori cause ulcers; other viruses, bacteria and protomes may cause heart disease, asthma, arthritis, MS, mental disorders, cancers, and obesity. Microbes also come and go, like the viruses that cause epi- and pandemics.
Likeliest Potential Pathogens of Interest in the Horse
Pathogenic microbes can cause illness in living organisms. Humans can get exposed to pathogenic organisms through contaminated food, water and fecal-oral routes and fomites (clothes, towels, etc.). Major human pathogen groups that may be associated with equines include fungi, helminths (worms), viruses, spirochetes, protozoa, and bacteria. Peer-reviewed literature and CDC statistics report that fungi and helminths have not been shown to readily transmit from horses to humans and will not be discussed in this paper. Generally speaking, a horse intestinal tract does not contain the 120 viruses and constituents of concern that are commonly found in human, dog and cat feces (Atwill 1998, Putnam 1983, Davis and Swinker 1996, Rugg 1998). Most pathogenic viruses cannot survive in the open environment away from a host. Viruses that affect horses are usually vector-borne from mosquitoes or ticks and therefore do not directly infect from horse to humans. Also, horses do not develop a high enough level of viremia (virus circulating in the blood) nor does the viremia last long enough for them to serve as a source of infection for mosquitoes. Because of this, the horse is called a dead-end host for most viruses (Walcott and Timoney 2002). Viruses will not be discussed in this paper either. This paper will discuss vector-, food-, and water-borne diseases.
The Office of International des Epizooties (www.oie.int), the animal health equivalent of the World Health Organization, lists very few zoonoses specific to the horse. Many of the diseases on this list are not found in the US. For others, vaccines exist and are given as needed. None of the diseases listed by OEI include the major pathogens associated with food production or water supplies for human consumption. Most of the diseases on the OIE list are transmitted directly by vectors, primarily mosquitoes or ticks. Horses are particularly susceptible to vector-borne diseases because they remain outdoors 24/7. Horses contract these diseases when the vector, such as a mosquito, feeds on an infected animal, such as a bird, and then bites a susceptible horse, which becomes the terminal host (Walcott and Timoney 2002, Timoney 2002). In the US, we control vector-borne disease fairly well. Therefore, the focus of this paper is on food- and water-borne pathogens.
In recent years, public concern regarding food and water safety has increased as a consequence of the outbreak of a number of diseases including Bovine spongiform encephalopathy (BSE or Mad Cow Disease) from cattle; Salmonella spp. from poultry meat and eggs; Listeria monocytogenes from dairy products, pate, and salads; Cryptosporidium spp. from water supplies, Escherichia coli 0157:H7 from beef, unpasteurized fruit juice, vegetables, and other diseases. Horses have not been implicated in these outbreaks.
Surveillance of food-borne illness is complicated by several factors. The first is underreporting: people often do not seek medical attention. Second, food-related pathogens can be transmitted directly from the food, from water sources, and from direct person-to-person contact, thus obscuring the role of food-borne transmission. Finally, some proportion of food-borne illness is caused by pathogens that have not yet been identified (Mead et al. 1999).
More than 200 known diseases are transmitted to humans through food. Fewer than 75% of food-borne diseases are caused by known pathogens. Unknown agents account for the rest. In the US, horsemeat is rarely consumed as a food, so the instances of food-borne disease from horses that may be a factor elsewhere in the world are not a concern here.
Three pathogens, Salmonella, Listeria, and Toxoplasma, are responsible for ~1,500 deaths each year in the US. Overall, food-borne diseases appear to cause more illnesses but fewer deaths than previously estimated (Mead et al. 1999). Five pathogens account for over 90% of estimated food-related deaths: Salmonella (31%), Listeria (28%), Toxoplasma (21%) a disease primarily from felines, swine, and rodents, Norwalk-like viruses (7%) which is not an equine disease, Campylobacter (5%) which is also not a prevalent equine disease, and E. coli O157:H7 (3%) (Mead et al. 1999 pg. 612).
FoodNet was established in 1996 as a collaborative effort by the Centers for Disease Control and Prevention, the U.S.D.A the Food and Drug Administration, and selected state health departments. FoodNet conducts active surveillance for seven bacterial and two parasitic food-borne diseases within a defined population of 45.5 million (15%) of Americans who are monitored periodically. Information for these nine surveillance pathogens is updated based on 2007 data.
Livestock or pet fecal matter should never knowingly be deposited in streams or other water bodies. The USDA RCDs have many Best Management Practices (BMPs) written to help keep clean water clean as it moves through horse farms. Horses used in the backcountry should be kept at least 100 feet from water bodies, except when they drink.
Water-borne pathogens of greatest concern to humans that may be transmitted by livestock are listed by Atwill (1997). The bacteria Aeromonas spp. has also been suggested to be of concern to humans in a paper by Derlett and Carlson (2004). Aeromonas is ubiquitous bacteria found in both contaminated food and water. It is frequently found in fish, shellfish, meat and poultry. It is capable of causing illness in fish and amphibians, as well as in humans. Little is known about its virulence and presumably not all strains are pathogenic, given its ubiquity. Exposure is by ingestion or open wounds. Illness is individual, sporadic and not associated with outbreaks (USEPA 2008). California is the first state in which this disease has become reportable. The CDC does not list them as pathogens of interest and there is virtually no data on Aeromonas associated with horses in the literature (USDA Bad Bug Book 2008), we have not included it in this paper. The Canadian Material Safety Data Sheets (2008) note that there are neither known zoonoses nor known vectors for Aeromonas.
Many infectious diseases have zoonotic potential and can, under certain circumstances, be transmissible from animals to man. Notwithstanding the diversity of infections involved, relatively few are derived from horses or other members of the equid family. Most of the more frequently encountered zoonotic diseases are contracted through direct or indirect human contact with other domestic species or with different species of wildlife (Timoney 2002, Atwill 2008, Kester 1977, Magdesian 2008, DebRoy 2007, Mike Westendorf 2008).
People vastly outnumber horses on trails in the US. Because horses are encountered infrequently by most people, it is likely that their perceived risk of exposure to horse feces is actually much higher than their true risk. There are 9.2 million horses (USDA, Amer. Horse Council 2005) and 301.2 (July 2007 est.) million people in the US. The ratio of horses to people is 0.01%. Of the 9.2 million horses, only 3.9 million are used in recreation. Probably less than half of the recreational horses potentially use trails (~2 m), as many recreational horses are kept at home simply as pets or old friends. The chance that the horse on the trail carries a zoonotic pathogen is too small to be meaningful and is a negligible health risk for contaminating food crops, water or people encountered on trails.
Horses are not raised for food or fiber in the United States, thus eliminating food-borne concerns about horses. Horses that are used as recreational animals are not usually kept under the same circumstances as high intensity farming and feedlot animals. Most horse feces are deposited in home paddocks/pastures. Because horses are kept in smaller numbers, and they are not kept in concentrations that are found in feed lots, they are less likely to be subjected to the physical and psychological factors of proximity to other animals such as fecal contamination of food and water, fighting, stress, diseases, etc. The lack of stressors associated with food production animals, vaccinations, access to clean water and food, and available veterinary care mean that horses are generally found in good health in the US. As a species, horses are unlikely to transmit pathogenic microbes to humans, compared to other humans, wildlife or other domesticated species.
No major human disease outbreak has ever been accurately attributed to the intimate contact humans have had with horses for six thousand years. (Duell 1989). Horses do not play a significant role in spreading diseases to humans (Kester 1977, Magdesian 2008, Timoney 2002, DebRoy 2007, Mike Westendorf http://esc.rutgers.edu/ask_expert/ate_fpmmm.htm (2008), and the USDA www.animalid.aphis.usda.gov/nais/index). Horse feces desiccate and decompose rapidly in the environment. It is unlikely that the average hiker practicing conventional hygiene will experience adverse effects from exposure to horse feces on a trail. Nor is it likely, as reported by Dr. DebRoy of the National E. Coli Reference Center, Dr. Madgesian of UC Davis and others professionals who testify in our appendices, that horse feces deposited near growing crop fields will be a problem.
Vector-borne diseases are of concern to humans, but are especially of concern to horses that remain outdoors 24/7. Food and water-borne diseases are of particular concern to humans, but only of veterinary concern to horses. Livestock or pet fecal matter should never knowingly be deposited in streams or other water bodies. Five pathogens associated with food and water-borne illnesses were discussed in detail: bacteria E. coli 0157:H7, Salmonella spp.; protozoa Cryptosporidium parvum and Giardia duodenalis; and a spirochete Leptospirosis interrogans. The horse is not significantly implicated in transmitting these diseases to humans.
People are exposed to a variety of risks every day and must make decisions about which risks we can ignore and which to actively manage. Pathogens affecting humans as reported by the active surveillance of ~45.5 million Americans through FoodNet are not commonly found in horses, and if found in horses are primarily of veterinary importance. Dr. Aaron Wildavsky, Professor at UC Berkeley has written about Americans: “The richest, longest-lived, best protected, most resourceful civilization is on its way to becoming the most frightened. Government has contributed to this process by taking responsibility for risk management away from individuals.” The authors of this paper believe that sufficient resources are available that can help dispel the fear of diseases transmitted from horses to humans. Based on information in the literature, the Internet, interviews with veterinarians and doctors, and reference texts, there is little significant health risk to humans, water or crops from horse feces on trails.
Download the complete text of Diseases of Humans and Horses (Zoonosis) in MS Word format with tables and references (65 pages; 12.5 mb)
These materials have been prepared by the authors and EnviroHorse for information purposes only and are not legal advice. Subscribers and online readers should not act upon this information without seeking professional counsel. All cited web information was obtained between dates stated in the paper, and is subject to change. Every attempt has been made to assure that the information contained in this publication is accurate. EnviroHorse and the authors assume no responsibility and disclaim any liability for any injury or damage resulting from the use or effect of any product or information specified in this publication.