Zoonoses: What Horse Owners Need to Know

Horses can become infected with leptospirosis by drinking affected standing water.

Photo: Photos.com

Reprinted from The Horse Report with permission from the Center for Equine Health, School of Veterinary Medicine, University of California, Davis.

Zoonoses—diseases that can be transmitted to humans from animals—are a concern for all who live or work with animals. Our common ancestry with animals means that we share much of the same biochemistry and therefore much of the same susceptibilities. Humans and animals are at risk for cancer because our bodies are made up of large numbers of cells. If a mutation makes a single cell deaf to the needs of its body, it can develop a tumor.

Our common ancestry with animals is also the source of many dangerous infectious diseases. You might not think you have all that much in common with a chicken, but to an influenza virus humans and birds are promising hosts alike. In fact, 70% of infectious diseases in humans got their start in animals. Conversely, animals can acquire diseases such as tuberculosis from humans.

The connections between the health of humans, animals, and the environments in which they live have been known for ages. Hippocrates recognized that environmental factors can impact human health and that public health depends on a clean environment. In the nineteenth century, the physician Rudolf Virchow recognized the link between diseases of humans and animals and coined the term zoonosis. He said, “Between animal and human medicine there are no dividing lines—nor should there be.”

Emerging infectious diseases represent substantial threats to global health, and AIDS (or acquired immune deficiency syndrome) ranks as one of the most important infectious diseases facing humans in the 21st century. It is believed that HIV (human immunodeficiency virus) could have originated through contact with the infected blood of a primate during hunting and butchering of meat, and that this occurred in the early 1900s. It took many years for the social, economic, and behavioral changes to develop and provide the perfect storm in which the virus could expand and reach epidemic proportions. Since it was first identified around 1981, HIV has infected at least 60 million people worldwide and caused more than 25 million deaths.

Tuberculosis is another significant disease that impacts both humans and animals. Susceptibility to Mycobacterium tuberculosis is relatively high in humans, primates, and guinea pigs. Cattle, rabbits, and cats are susceptible to the M. bovis strain, and swine and dogs are susceptible to both M. bovis and M. tuberculosis.

Horse owners who are familiar with the zoonoses that affect horses are better equipped to manage their animals and property to minimize disease transmission. Awareness is the necessary element for prevention. Among the more serious diseases that can affect horses and are potentially infectious to humans are rabies, leptospirosis, Salmonellosis, and MRSA (methicillin-resistant Staphylococcus aureus). We discuss these further in a moment.

But first, did you know that veterinarians play an important role in safeguarding public health and in preventing the spread of disease from animals to people? Animal owners mostly see their veterinarians in a clinic or field setting where they provide medical care. But veterinarians also play a crucial role in ensuring public health by improving agriculture and food systems, advancing biomedical research, addressing zoonotic diseases, enhancing environmental health, and helping manage new public health challenges.

The Role of Veterinarians in Public Health

James Steel, an American veterinarian, is known as the “father of veterinary public health” because he pioneered efforts to prevent the spread of disease from animals to people. In 1942, a year after Steele received his DVM from Michigan State University, he became one of the first veterinarians to receive a master’s degree in public health from Harvard.

In 1945, he started the veterinary public health program at the United States Public Health Service in Washington. In 1947, he and the unit moved to Atlanta to what is now called the Centers for Disease Control and Prevention.

Since 1892, more than a dozen diseases have been eliminated from the U.S. equine, poultry, and livestock populations. The elimination of these livestock diseases, along with research in animal health, is key to the remarkable gains in the efficiency of U.S. animal production and in overall public health.

Today, veterinarians and physicians are working together increasingly to solve problems that affect both animals and humans. Knowledge gained from one species often benefits another, since animals suffer from many of the same chronic diseases as humans: heart disease, cancer, diabetes, asthma, and arthritis, to name a few.

Sometimes a disease is recognized in animals long before it is recognized in humans. Rous sarcoma virus, the first virus known to cause tumors, was discovered in chickens in 1911. The researcher who made the discovery, a physician named Peyton Rous, was awarded the Nobel Prize for identifying this tumor-inducing virus and for the impact it would have on future research. The virus has been used to study the development of cancer and subsequently helped the discovery of other tumor-causing viruses.

Humans, animals, and animal products now move rapidly around the world and pathogens are adapting, finding new niches, and jumping across species into new hosts. Lessons learned from SARS (severe acute respiratory syndrome), West Nile virus, and avian influenza are reminders of the need to view diseases globally; integrate animal and public health surveillance; and create new strategic partnerships among animal public health professions.

The future will most likely bring collaborations of veterinarians with scientists from multiple professions such as human medicine, bioengineering, animal science, environmental science, and wildlife. Together, they will be stronger to fight disease, and they will be wiser.

Illustration of a Zoonosis

What do horses have in common with sea otters? Here is an illustration of how these two very different species can share the same disease, facilitated by the ecosystem.

Equine protozoal myeloencephalitis (EPM) is a neurologic disease that occurs when protozoal parasites invade and infect a horse’s central nervous system. Infection with this parasite results in characteristic lesions in the brain and spinal cord and causes incoordination and muscle atrophy.

The two protozoal parasites responsible for EPM have been identified as Sarcocystis neurona and, less commonly, Neospora hughesi. Opossums are considered the definitive host for S. neurona, shedding the infective sporocysts (egg-like stage of development) in their feces. (A “definitive host” is a host in which reproduction of pathogens takes place.) Horses become infected by ingesting food or water that has been contaminated with opossum feces containing the infective sporocysts. The definitive host for N. hughesi has not been identified but is assumed to be a domestic or wild carnivore, possibly a dog or coyote.

Opossums are considered the definitive host for S. neurona.

Photo: Thinkstock

In approximately 2 to 4% of cases, the sporocysts ingested by a horse migrate from the intestinal tract into the bloodstream and cross the blood/brain barrier. There, they attack the horse’s central nervous system. The onset of the disease may be slow or sudden, and the signs vary depending on the type of damage to the central nervous system. If left undiagnosed and untreated, EPM can cause devastating and lasting neurologic deficits.

In April 2004, an unusually high number of dead or stranded sea otters from the Morro Bay, California, area were found to be infected with the same parasite that causes EPM in horses: S. neurona. Such deaths have been reported previously, but the number of otter deaths during this particular time period greatly exceeded that in previous years, and localized clustering of S. neurona infections had not been documented before.

Many otters stranded alive had clinical signs suggesting brain damage. Affected otters tested negative for several pathogenic viruses, including West Nile virus. The harmful algal bloom toxin—domoic acid—could have contributed to the deaths of a few sea otters, as well as a second protozoal parasite—Toxoplasma gondii—which is excreted by cats in their feces. Scientists believe that the Toxoplasma parasite ended up in the ocean through freshwater runoff. It is feasible that the parasite was then concentrated by ocean filter feeders such as shellfish, which were then eaten by the otters. Something similar could have occurred with S. neurona through infected opossum feces.

T. gondii causes toxoplasmosis in humans and is the reason pregnant women are told to avoid cats’ litter boxes. Since the 1920s, doctors have recognized that a woman who becomes infected during pregnancy can transmit the disease to the fetus, in some cases resulting in severe brain damage or death. Toxoplasma is also a major threat to people with weakened immunity. In the early days of the AIDS epidemic, Toxoplasma was to blame for some of the dementia that afflicted many patients at the disease’s end stage. Healthy children and adults can often fight off the protozoan, although some believe that it remains dormant inside brain cells. About one-third of the world’s population is infected by Toxoplasma.

This example of protozoal parasites appearing in two quite different species underscores the importance of understanding the basic mechanisms by which all disease spreads and how humans, animals, and ecosystems are profoundly interconnected.

Zoonoses and Horses

Horse owners who are familiar with the zoonoses that affect horses are in a much better position to prevent them. Here are four of the more serious ones.

Rabies—Rabies is a zoonotic disease caused by viruses in the genus Lyssavirus and has the highest case fatality ratio of any infectious disease; the disease is invariably fatal in horses. Wildlife have accounted for more than 90% of rabid animals reported in the United States since 1980. The primary reservoir species responsible for maintaining rabies are raccoons, bats, skunks, and foxes. It is a significant threat to horses in South and Central America, but is not a major cause of death in North America due to successful vaccination efforts.

With the increasing urbanization of areas in which the disease is regularly found in wildlife populations, the risk of exposure continues to be a concern for veterinarians and horse owners. The Centers for Disease Control reported rabies infection confirmed in 47 horses and mules during 2012, an increase over the 44 cases reported in 2011.

Rabies infection usually occurs by the bite of a rabid animal but can also be transmitted when fresh saliva from an infected animal comes into contact with a wound or mucous membranes on another animal or human. Encounters between a horse and a rabid animal rarely are witnessed, and bite wounds, which can be puncture-like in nature, can be difficult to find.

The delayed development of clinical signs has been attributed to the undeveloped state of the virus, which must first replicate in the muscle at the wound site before it migrates into the nervous system. The clinical signs of rabies in horses include inability to stand, ataxia and muscle weakness, excessive sensitivity to touch, muscle tremor, lameness, anorexia, loss of tail and anal sphincter tone, loss of sensory perception in the hind limbs, fever (greater than 101°F), colic, depression, convulsions, and aggressiveness. However, because rabies can “look like anything,” it is difficult to make generalizations. Most horses do exhibit some degree of sensitivity to touch, fever, ataxia, and muscle weakness. Survival time after the onset of signs can range from two to five days and can be as long as two weeks, although that is unusual.

While the incidence of rabies in horses is relatively low, the disease is invariably fatal and has considerable public health significance. The American Association of Equine Practitioners recommends that rabies vaccine be a core vaccine for all equids. University of California, Davis (UC Davis) veterinarians recommend that adult horses be vaccinated against rabies annually after receiving the appropriate vaccination protocol early in life.

Leptospirosis—Leptospirosis is a potentially fatal disease that humans and animals can contract through contaminated urine or water. Although leptospirosis is considered the most common zoonotic disease of people worldwide, our understanding of its epidemiology is minimal and further research is needed.

The causative agents are a small group of spirochete bacterial species in the genus Leptospira. These spiral shaped bacteria are contracted across mucous membranes and then may invade kidney and liver tissue, initially causing nonspecific, flulike signs. If left untreated, it can cause organ failure and death.

Older literature attributed each Leptospira serovar to a specific animal host (e.g., canicola to dogs, hardjo to cattle) and claimed that cases occurred in tropical areas with abundant rainfall. However, we now recognize a much more complicated, very poorly understood epidemiological web of serovars infecting multiple hosts, including rats, skunks, opossums, raccoons, and foxes. (Serovar refers to distinct variations within a species of bacteria or viruses.)

Horses can become infected by eating hay or grain that has been contaminated by infected urine, or they can contract it by drinking from standing water that has been similarly affected. In some cases, horses can be affected by the direct splashing of infected animals’ urine into the eyes or mouth.

Leptospirosis is now classified as a re-emerging infectious disease by the Centers for Disease Control and the World Health Organization. Increased attention has revealed numerous human and animal cases worldwide, including in the continental United States. Surveillance in children in inner cities found exposure and cases in Baltimore and Detroit, while clinical cases occurred in adult participants of water-related endurance sports and recreation, including in California. A whopping 65% of dairy workers in one study had exposure to Leptospira.

Canine, cattle, and wildlife cases are common in some areas of the United States, including California. Vaccines with limited efficacy are available for dogs and cattle, but not for horses or humans.

An active ERU episode (top) is accompanied by redness to the cornea, excessive tearing, and a hazy appearance. End-stage ERU (bottom) often has extensive corneal scarring.

Photo: Courtesy Dr. Steven Hollingsworth, University of California, Davis, School of Veterinary Medicine

The incubation period for leptospirosis in horses is one to three weeks. Horses might experience a variety of clinical signs, including fever, loss of appetite, swelling of the eyes, light sensitivity, tearing, ocular discharge, eye cloudiness, and redness around the eye, as well as lethargy and mid- to late-term abortion. Adult horses have been known to develop jaundice and even die from kidney and/or liver failure. Diagnosis of leptospirosis can often be overlooked because the clinical signs of the disease are common to other diseases. Only laboratory tests of blood or urine can confirm if leptospirosis is present.

A complication of leptospirosis in horses is equine recurrent uveitis (ERU, or moon blindness), the leading cause of blindness in horses. The blindness is caused not by the leptospirosis infection itself, but by the immune response to the initial infection, including severe inflammation inside the eye. Thus, horses with chronic ERU are not necessarily contagious for leptospirosis.

As the name of the disease implies, ERU typically occurs multiple times, which increases the chances for damage to the eye and eventual vision impairment. The clinical signs mimic trauma or injury to the eye and include watering of one or both eyes, squinting, redness, and a cloudy and/or opaque cornea. The condition is very painful and often the horse is very sensitive to light. Currently, treatment of recurrent uveitis is to aggressively treat acute attacks and also minimize the frequency and severity of recurrences.

Any horse with a suddenly painful, teary eye should receive immediate attention from a veterinarian who can then make a definitive diagnosis and initiate appropriate treatment. New methods used by veterinary ophthalmologists to treat ERU involve implantation of medicated “wafers” or injections of medication into the back of the eye. These anti-inflammatory drugs are then absorbed slowly over the course of years.

Good farm management techniques can help reduce the risk of infection. Keep wildlife away from feed sources and do not allow standing water to accumulate. Do not allow horses (or any of your animals) to drink from stagnant water sources as standing water might be contaminated with leptospirosis-tainted urine from wildlife or cattle. Good disinfection programs will help reduce the risk of exposure to leptospirosis and many other diseases.

Salmonellosis—Salmonellosis is one of the most commonly diagnosed infectious causes of diarrhea in adult horses. Clinical manifestations range from no abnormal clinical signs (subclinical shedder) to acute, severe diarrhea and even death. The disease is seen sporadically but can become an epidemic, depending on the virulence of the organism, level of exposure, and host factors.

Caring for Salmonella-Positive Animals

Animals are diagnosed for Salmonella by fecal culture. Animals that are sick or have been sick have altered intestinal flora and fauna, which makes them more likely to be colonized by Salmonella. In these animals, exposure to as few as 1/100th of a drop of Salmonella culture could result in temporary colonization and fecal shedding.

Animals that have recovered after a clinical course of salmonellosis are evaluated for diarrhea or other illness that could be attributed to Salmonella and are discharged when these signs are not present. However, they could be shedding small numbers of Salmonella. Since the number of bacteria required to cause illness in healthy normal animals is very high, it is unlikely that the discharged animal would pose a risk to others on returning home, provided that good farm management techniques are in place and that precautions are taken to protect vulnerable animals and humans.

  • Any humans with immunosuppressive diseases, taking immunosuppressive or antimicrobial drugs, or having cancer or sickle cell anemia, along with elderly people, pregnant women, and children should avoid contact with the animal until it is culture negative.
  • Keep the recovering animal separate from others, especially from the most susceptible animals (those that are stressed, sick or on antibiotics), especially young animals, for a period of six to eight weeks.
  • Wear latex gloves and wash your hands with soap thoroughly after handling this animal. Alcohol-based hand sanitizers are effective as well.
  • Clean the infected animal’s stall last and disinfect rakes, forks, and any other implements used with 1 part bleach in 10 parts water or other disinfectant as recommended by your vet (Accel, Trifectant, and Stall Safe are others).
  • Wear eye and hand protection when handling bleach, and use only in a well-ventilated area. Please review disinfectant use and protocols with your veterinarian.
  • Foot baths with a similar concentration of bleach or other disinfectant can be placed outside the stall or pen. Note that different disinfectants must not be used simultaneously or allowed to contact one another because toxic byproducts can form.
  • Do not use manure on fields or in the garden. Do not use the same carts for feed and manure.
  • Signs of salmonellosis in humans include diarrhea and fever. Most animals stop shedding Salmonella by eight weeks after regaining normal health and bowel function. Your veterinarian should obtain fecal cultures (or polymerase chain reaction, or PCR) once weekly, and when one of the samples is negative, then four additional daily samples should be obtained. When the horse has five negative cultures, it would be considered negative.

Gary Magdesian, DVM, Dipl. ACVIM, ACVECC, ACVCP

Salmonella is spread feco-orally between animals by manure that contaminates feed or water or by contact with animals actively shedding the bacteria. A foal might pick it up when nursing a mare or nuzzling her flank if she has lain on dirty bedding or her tail has flicked feces onto her body. A foal will also eat manure and can pick it up that way. Mice droppings in hay or grain can also be a source of Salmonella. Barnyard chickens, pigeons, and wildlife can be a source of Salmonella, especially if they defecate on hay or other feed.

Stress appears to play an important role in the progression of the disease—a history of surgery, transportation, or change in feed; concurrent disease, particularly gastrointestinal disorders (colic); or treatment with broad-spectrum antimicrobial drugs often precedes the diarrhea.

Three forms of salmonellosis have been recognized in adult horses. One is the subclinical shedder, which has the potential to transmit the bacteria to susceptible animals either by direct contact or by contamination of the environment, water, or feed sources. Multiple fecal cultures could be necessary to identify low-level shedders because the microorganism is shed in the feces intermittently and in small numbers. If stressed, the carriers can develop clinical disease. The national prevalence of fecal shedding of Salmonella enteric by normal horses in the United States is estimated to be less than 2%. However, the proportion of hospitalized horses shedding is much higher (approximately 8%), likely due to stress and concurrent illness.

The second form of the disease is characterized by a mild clinical course, with signs of depression, fever, low-grade colic, anorexia, and soft but not watery feces. Affected horses could have increased susceptibility to infection, and clinical disease can last 4 to 5 days. Because recovered horses can continue to excrete the organism in their feces for days to months, isolation of the shedding horse and thorough cleaning and subsequent disinfection of the contaminated area are recommended until fecal cultures are negative.

The third form of salmonellosis is characterized by the sudden onset of severe depression, anorexia, profound neutropenia (decreased neutrophils that result in increased susceptibility to infection), and frequently abdominal pain. Diarrhea develops within 24 hours after the onset of fever. Affected horses dehydrate rapidly, and metabolic acidosis and electrolyte losses occur as the horse deteriorates. Clinical signs of sepsis (a potentially fatal whole-body inflammation caused by severe infection) and hypovolemic shock (heart failure) can progress rapidly.

Treatment in animals and humans is primarily supportive, consisting of fluid and electrolyte administration, as well as the use of intestinal protectants. Systemic antimicrobials are used only when the risk of septicemia is high, such as when treating young, elderly, and immunocompromised patients, or when treating particularly invasive strains such as Salmonella typhimurium. The use of antibiotics has little benefit in treating primary gastrointestinal infections, does not eliminate subclinical states, and could contribute to the development of antibiotic resistance.

Tips from Gary Magdesian, DVM, Dipl. ACVIM, ACVECC, ACVCP, equine critical care specialist at the UC Davis William R. Pritchard Veterinary Medical Teaching Hospital, on caring for Salmonella-positive animals can be found in the sidebar above.

MRSA—MRSA is a type of bacteria that is resistant to certain antibiotics. These antibiotics include methicillin, cloxacillin, dicloxacillin, oxacillin, and nafcillin, as well as a closely related class of drugs known as cephalosporins (e.g., cephalexin). Overuse of antibiotics and the use of more powerful drugs than necessary for less serious infections could be some of the causes of the development of MRSA.

Approximately 1 to 2% of people carry MRSA on their skin or in their nose. Infections caused by MRSA, for the most part, are not different from any other Staph infection, although some strains of MRSA can be more aggressive than regular Staph isolates.

Caring for MRSA-Positive Horses

  • Any horse testing positive for MRSA should be quarantined on the farm or stable to prevent colonization and/or infection of other animals or people.
  • The quarantined horse should be kept in a stall where it has no nose-to-nose contact with other horses. It should not be turned out into areas where there are other animals.
  • Only a small number of people should handle the horse. No children, elderly, immunocompromised (e.g., HIV), or people with wounds or incisions should come in contact with the horse.
  • Horse handlers should wear gloves at all times when handling the horse and should wash hands thoroughly and/or use an alcohol-based hand sanitizer to disinfect hands afterward.
  • If the horse has an incisional infection or wound that requires cleaning, we recommend that you follow this procedure: Once the infected site is cleaned, the bandages and disposables used for cleaning should be discarded in a plastic bag with a small amount of disinfectant such as dilute bleach (one tablespoon bleach:one quart water), betadine, or nolvasan added.
  • If there is a risk of body fluid spillage from the cleaning process, contact your veterinarian for assistance with the process and for recommendations on how to do this safely.
  • To know if your horse has cleared a carrier state or infection, it should have two negative cultures of the nasal passages and the wound or incision taken by your veterinarian.

Gary Magdesian, DVM, Dipl. ACVIM, ACVECC, ACVCP

The diagnosis of an MRSA infection requires laboratory testing. Testing can also be important since MRSA’s antibiotic resistance may make it more difficult to manage and information is needed to guide treatment. First, there is a distinction between MRSA colonization and MRSA infection. Colonization is the presence of the bacteria with no signs of illness or infection. MRSA infection involves clinical signs of illness or inflammation due to tissue damage caused by invasion by the bacteria.

Infection requires treatment, while colonization does not. Most people do not know they are colonized because the bacteria are present but are not causing any signs of disease. Good hand hygiene and proper wound care help ensure that bacteria do not enter the body through breaks in the skin and potentially cause infections.

MRSA is present in a wide range of animal species, including horses. Food animals and pets likely play a role in human MRSA infections. Some groups of individuals who work closely with animals, such as veterinarians, have high MRSA colonization rates. It is certain that animals are a source of human MRSA infection in some circumstances, but humans can also serve as sources of infection in animals.

There is clear evidence of interspecies transmission of MRSA between horses and humans, resulting in both colonization and infection. Higher rates of MRSA colonization than would be expected have been identified in horse owners and equine veterinarians. One study reported colonization of 13% of horse owners and the presence of a colonized person on every farm that had one or more colonized horses, with indistinguishable human and equine strains.

Most colonized horses do not develop clinical infections, but colonization at the time of hospital admission is a risk factor for the development of such infections at catheter sites, wounds or incisions. MRSA-colonized horses are also of concern because of the potential for transmission to other horses, humans, and potentially other animal species.

The UC Davis William R. Pritchard Veterinary Medical Teaching Hospital has an aggressive infection control program in place for prevention of infections. It also allows rapid recognition and isolation of cases if clinical infection does occur. Every horse is treated as a potential carrier of MRSA. The program emphasizes the use of gloves to handle all patients as well as hand washing after handling every patient.

According to the Centers for Disease Control and Prevention, it is well-documented that the most important measure for preventing the spread of pathogens is hand-washing. This is certainly true for preventing the transmission of MRSA. In addition, the use of disposable gloves reduces skin-to-skin contact and therefore further reduces the risk of transmission.

Owners of horses who have tested positive for MRSA, either as a carrier or with an active infection, should follow Magdesian's care recommendations in the sidebar above.

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