Shared Science: Human and Equine Health Similarities

There are many similarities between veterinary medicine and human medicine, particularly when it comes to horses.

Photo: iStock

Studying diseases across species can benefit horses as well as people

Similarities between veterinary medicine and human medicine abound, particularly when it comes to horses. Horses, after all, suffer from many of the same conditions people do: cardiovascular disease, salmonellosis, Lyme disease, joint disease, the eye disease uveitis, tendon issues, and cancers such as melanoma. The two species also exhibit similar clinical signs, even though the root conditions can be different. For example, equine grass sickness and Alzheimer’s; self-mutilation in horses and cutting disorders in humans; foal rejection in mares and postpartum depression in women; equine metabolic syndrome in horses and diabetes in people; and asthma in humans versus what was classically called heaves in horses.

Enter the “One Health” movement, which has evolved over the years to mean “one medicine for all” and involves the collaboration of more than 850 physicians, osteopaths, veterinarians, nurses, dentists, health officials, behaviorists, and environmental and other scientists.

Craig Carter, DVM, PhD, Dipl. ACVPM, DSNAP, is the director of the University of Kentucky’s Veterinary Diagnostic Laboratory, as well as a visiting professor in the school’s College of Public Health, in Lexington. As such, he researches and teaches One Health principles.

“It is a multispecies thing that just has so much promise for the future of solving a lot of problems, from cancer to infectious disease, heart disease, and joint problems,” says Carter. “A lot of folks don’t understand that diseases in all mammals are pretty much a mirror of the physiology and pharmacology of people, with some key differences. But the use of antibiotics, anti-cancer drugs, and treatment protocols are all very similar.”

Carter says human and equine organ systems are quite comparable, having similar functions, but disease processes can affect those systems differently, creating distinct clinical signs. 

One of Carter’s roles as the new president of the American Veterinary Epidemiology Society (AVES) is to help people recognize One Health as an emerging field in the allied health professions—currently he estimates only 20-25% of veterinarians have bought into the One Health movement. The mission of AVES is to advance the field of veterinary epidemiology and public health to help improve the quality of life for all people and animals through science-based One Health principles. 

Stomach Similarities

Horses and humans have remarkably similar digestive tracts, which has given researchers of both species valuable insight into gastric ulcers and other disease processes, says Frank Andrews, DVM, MS, Dipl. ACVIM, LRMA, equine committee professor and director of the Equine Health Studies Program at Louisiana State University, in Baton Rouge.

Andrews, who has a special interest in equine gastroenterology, says the digestive disease that humans and horses most commonly battle is abdominal pain, known as colic in horses. Also, cecal impactions, infections, or ruptures in horses produce clinical signs similar to those of appendicitis in people. “We also have inflammatory bowel disease in horses as well as people,” he says. “It’s probably underdiagnosed in horses.”

Research on the equine digestive tract has centered around one important organ, the stomach, for the benefit of human research and vice versa (human stomach research benefits horses, too), especially in regard to ulcers. Both horses and humans are monogastric animals, meaning they have a single-chambered stomach—as opposed to ruminants, which have four. The horse’s stomach has two regions: the glandular, which covers the bottom two-thirds, and the nonglandular (squamous), which covers the top third of the stomach and is lined with the same tissue type that lines the esophagus. The nonglandular tissue of both species is susceptible to damage by stomach acid. In horses, acid splashes up onto the sensitive tissue, causing ulceration, whereas in humans acid refluxes into the terminal esophagus—which it can do because the valve leading to the stomach is not as tight as it is in horses—leading to damage, heartburn, and ulcers. 

“In gastric ulcer disease in horses, the esophageal portion (nonglandular mucosa) is most susceptible, so those ulcers we liken to gastroesophageal reflux disease (GERD),” says Andrews. “And that’s probably the most common cause of heartburn in people.” 

Treatment for equine gastric ulcer syndrome (EGUS) in horses and GERD in people are the same—administration of acid suppressants such as omeprazole (GastroGard for horses, Prilosec for people), among others.

Exercise intensity and diet (e.g., high-grain) are known risk factors for horses developing gastric ulcers, so it’s not surprising that this painful condition is prevalent in racehorses and sport horses. But are human athletes predisposed to get ulcers, as well? 

The answer is yes, says Andrews. In a 2012 study published by the American College of Sports Medicine, Waterman and Kapur found that gastrointestinal (GI) complaints—which include GERD, nausea, vomiting, gastritis, peptic ulcers, GI bleeding, or exercise-related transient abdominal pain—occurred in 30-70% of athletes studied. The type and intensity of the sport were contributing factors. 

Horses and people don’t share as many similarities in ulcers affecting the glandular mucosa, which differ in appearance and physiology from those in the nonglandular mucosa. Instead, in horses they more closely resemble inflammatory bowel-like lesions in humans, says Andrews. In people, 80% of the ulcers in the glandular mucosa are related to the bacterium Helicobacter pylori and treated with antibiotics and bismuth subsalicylate (Pepto-Bismol). Researchers have never isolated Helicobacter spp bacteria in the equine stomach, so treatment for glandular mucosal ulcers involves similar coating agents and acid suppressants, along with diet modification.

The Breathing Connection 

Human and equine respiratory systems are also very similar in anatomy, function, and fragility, says Laurent Couetil, DVM, PhD, Dipl. ACVIM, professor of large animal internal medicine at Purdue University, in Lafayette, Indiana, and president of the American College of Veterinary Internal Medicine, large animal medicine specialty. 

Over the years, research into equine respiratory conditions has confirmed that both horses and humans suffer from asthma. In horses, this condition was originally called chronic obstructive pulmonary disease (COPD), renamed recurrent airway obstruction (RAO), and is now simply called equine asthma due to its similarity to human asthma in clinical signs, causes, diagnosis, and treatment. Like people, horses with asthma develop a chronic cough, mucus in the airway, and problems breathing. 

Both horses and people with asthma have adverse reactions to dust, mold, and other irritants in their environment. Couetil feels there’s a lot to be gained for asthmatic people by studying asthma in horses. “You can look at the effect of environment, since we know people working in horse barns for a long period of time are more likely to suffer from asthma or allergic disease,” he says. “You certainly can use that to see if improvement of the horse environment will also improve the people that work in that environment.” 

Practitioners of both species can also share diagnostic methods, such as lung function tests and bronchoalveolar lavage (a “lung wash”), which involves collecting mucus and cells from the lung and viewing them under a microscope to look for inflammatory cells and respiratory pathogens and to determine the white blood cell profile. Unlike in humans, however, blood tests are not helpful for diagnosing equine asthma and skin allergy tests are not accurate for pinpointing associated allergies, say researchers.

Treatment across species includes corticosteroids and bronchodilators, with horses wearing mask systems for medication treatments rather than using inhalers, because horses can’t be trained to inhale on command. Humans and horses can each benefit from nebulizer machines, which vaporize medications for inhalation over a time period; however, they can be cost-prohibitive for horse owners. 

To prevent this respiratory condition, removing the allergens is your best bet. Couetil says even good-quality—and certainly moldy—hay can contain mold particles. For asthmatic horses, switching from feeding hay to feeding a pelleted or a complete feed and turning the horse out on 24/7 pasture is more beneficial and cheaper in the long run, says Couetil. 

Comparisons in Consciousness

John Madigan, DVM, MS, Dipl. ACVIM, ACAW, professor of medicine and epidemiology at the University of California, Davis (UC Davis), focuses his research on equine and comparative neurology. He and his team have made several discoveries during their years of studying equine neonates and, now, human newborns, including a comparison between maladjusted foals and autistic children.

One of Madigan’s most prominent research findings involved the discovery that many cases of neonatal ­maladjustment syndrome (aka dummy foals) do not result from hypoxia, or lack of oxygen during birth, as traditionally thought. Instead, the condition can be caused by persistently elevated in utero hormones called neurosteroids, whose job during pregnancy is to keep the fetus “asleep.” 

While trying to determine how the foal’s body knows to “switch off” these neurosteroids so it can “wake up” after birth, Madigan’s team found that the birth canal’s squeezing pressure is a major signal. He hypothesized that the final stage of labor was too quick or otherwise affected (e.g., due to birth complications), thereby failing to prompt the neurosteroid switch. These neurosteroids then stay elevated and keep the foal in a more sedated state, as is seen in maladjusted foals who wander, are unaware of their environment, and show no desire to bond with and nurse from the mare. Madigan calls this “a failure to transition to ­consciousness.”

Dr. John Madigan developed a novel "foal squeeze" system to help maladjusted foals transition to consciousness.

Photo: Joe Proudman/UC Davis

Madigan’s group has discovered that squeezing a maladjusted foal’s body with a rope harness for 20 minutes can simulate the pressure from the birth canal during labor, thus flipping the switch to lower the neurosteroid levels. In observational studies, the dummy foals squeezed with the rope system “wake up” and seem to have no lingering effects from the previously elevated neurosteroids.

Madigan’s team also performed a brain-wave study on eight newborn foals to explore what is called the “flopping reaction” to tight restraint for a procedure. “Using a novel foal squeeze system, now called the Madigan foal squeeze, we determined that foals go into slow-wave sleep (and essentially flop, or go limp), and at the end of the 20 minutes, there were actually some hormone changes that occurred,” he says. This could be the birth canal signal to wake up.

In the wake of this discovery, owners and veterinarians have tried this method to help their maladjusted foals. Madigan says it has saved many from expensive round-the-clock care, which 80% of dummy foals typically need to survive. “We’re doing a study now comparing about 100 foals that have had the squeeze treatment in the field to about 100 that were treated conventionally,” he says.

Madigan’s group has also experimented with infusing normal foals with neurosteroids, which resulted in altered behavior. When neurosteroid levels returned to normal, the foals’ behavior returned to normal. “So we’ve determined that there’s not just an association, but that these neurosteroids do alter behavior,” he says.

Now, back to the human tie-in. The Comparative Neurology Research Group, which Madigan is a part of, is currently studying neurosteroid levels in banked blood samples from autistic children of various ages to see if they are elevated.

But this isn’t the only group looking at these values in autistic children. Madigan cites peer-reviewed research results from scientists in Europe who found elevated neurosteroid levels in the saliva of some autistic children. Four of the neurosteroids mentioned are the same ones found in maladjusted foals.

Madigan is also working with researchers from Stanford University to compare neurosteroid levels in children shortly after birth to those of children who receive kangaroo mother care (when a mother keeps continuous skin-to-skin contact with her newborn baby). 

Madigan describes one instance he relays in all of his speeches—a newborn boy was pronounced dead in a major hospital, and the parents used kangaroo mother care and he “came alive.” “We can explain that by the idea that after the baby is born, evolutionary biology is waiting for the mother to hold the infant,” he says. “That child may have woken up because (he) had such a high level of these neurosteroids that it slowed down so much in the cellular machinery that they thought the infant was dead. And then when the mother holds it, perhaps it gets the signal to lower them just like when we squeeze the foals and they wake up.”

Results of a yet-to-be published study of 40 infants show some differences in neurosteroid levels between those who received kangaroo mother care and those who didn’t. Madigan says there is also an ongoing study at UC Davis Medical Center evaluating neurosteroids in neonatal infants receiving critical care. “We know that you have to have some signal to transition consciousness from the in utero sedated state to the extrauterine life, which is something all mammals have in common,” he says. “We are focusing our research on failure to transition to consciousness, which is a syndrome that’s never been reported, and it likely occurs in foals, cattle, zoo animals, and humans.” 

Madigan says all this research into neurosteroids could have a global impact on infant health and mortality. And his research results have already revolutionized how people treat maladjusted foals.

Connecting the Dots

Comparing what we know from equine medicine with human medicine (and vice versa) can potentially save lives and prevent disease in both species. To quote Barbara Natterson-Horowitz, MD, a professor of medicine at the University of Califiornia, Los Angeles, who serves as a cardiovascular consultant to the Los Angeles Zoo: “After all, we humans are animals, too, and it’s time for us physicians to embrace our patients’ and our own animal natures and join veterinarians in a species-spanning approach to health. Because it turns out, some of the best and most humanistic medicine is being practiced by doctors whose patients aren’t human. And one of the best ways we can take care of the human patient is by paying close attention to how all the other patients on the planet live, grow, get sick, and heal.”

About the Author

Sarah Evers Conrad

Sarah Evers Conrad has a bachelor’s of arts in journalism and equine science from Western Kentucky University. As a lifelong horse lover and equestrian, Conrad started her career at The Horse: Your Guide to Equine Health Care magazine. She has also worked for the United States Equestrian Federation as the managing editor of Equestrian magazine and director of e-communications and served as content manager/travel writer for a Caribbean travel agency. When she isn’t freelancing, Conrad spends her free time enjoying her family, reading, practicing photography, traveling, crocheting, and being around animals in her Lexington, Kentucky, home.

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