A Closer Look at Comorbidities in Horses

Horses suffering from multiple conditions simultaneously can be challenging to diagnose and treat

It can be incredibly satisfying when things fit neatly in a box—including diagnoses for equine ailments. But not all health problems are so simple. Horses can experience more than one disease process at the same time, an occurrence known as comorbidity. (Morbidity means being affected with disease.) The presence of more than two diseases at a time is called multimorbidity. These scenarios can make for confusing clinical signs and long lists of diagnostic tests. They’re important to recognize because if you’re treating only one of multiple reasons for a horse’s clinical signs, you’re not going to solve the ­problem.

In this article we’ll look at the equine diseases veterinarians most commonly see in conjunction with other conditions.

Pituitary Pars Intermedia Dysfunction

One disease that frequently coexists with other health issues in horses is pituitary pars intermedia dysfunction (PPID). It’s caused by an enlargement of the pituitary gland’s middle lobe (the pars intermedia), which results in an overproduction of hormones that regulate bodily functions. The disease generally occurs in horses older than 18, although it can also manifest in individuals in their mid-teens.

In a recent study British researchers looked at nine common diseases in more than 100,000 horses over a 26-year period and found that 12% of the horses experienced one of those diseases: cancer, PPID, equine metabolic syndrome (EMS), grass sickness, laminitis, navicular syndrome (now known as podotrochlosis), osteoarthritis, recurrent airway obstruction, and sarcoids (Welsh et al. 2016). They identified multiple chronic disease processes in 1.2% of the study population.

In that study 49% of horses with comorbidities were affected by PPID. Because it’s a systemic problem, veterinarians know it is not uncommon to see PPID in concert with other conditions.

Michelle Coleman, DVM, PhD, Dipl. ACVIM, assistant professor of large animal internal medicine at Texas A&M University, in College Station, describes a variety of conditions and characteristics reported in conjunction with PPID:

  • Laminitis in 24-82% of PPID horses;
  • Abnormal sweating patterns such as anhidrosis (not sweating enough or at all) or hyperhidrosis (sweating too much) in 14-67%;
  • Abnormal fat deposition in 9-67%;
  • Increased susceptibility to secondary infections in 33-55%; and
  • An association with suspensory ligament degeneration.

The increased infection susceptibility is due to reduced immune responses, says Michelle Linton, BVMS, Dipl. ACVIM, staff veterinarian at the University of Pennsylvania School of Veterinary Medicine’s New Bolton Center, in Kennett Square. “Studies have shown that some cells responsible for a healthy immune system don’t function adequately in horses with PPID,” she says. “Therefore, these horses are at higher risk for any type of infection, including skin infections, foot abscesses, dental disease, sinusitis, conjunctivitis, and pneumonia.”

If horses with PPID do develop infections or wounds, these altered immune responses can make healing challenging.

A Closer Look at Comorbidities in Horses

Obesity and Endocrine Disease

Obesity leads to many significant health problems, including insulin resistance (IR, a reduced sensitivity to insulin that results in high blood insulin levels) or insulin dysregulation, EMS, low-grade systemic inflammation, laminitis, and fertility issues. Horses with EMS are typically overweight with a body condition score above 6 or 7 (on a scale of 1-9) and obvious fat deposits along the neck, loins, shoulders, sheath, or udder, for example. They are usually middle-aged.

“We refer to EMS horses as having insulin dysregulation rather than just resistance,” says Linton. “The term dysregulation better describes these individuals’ abnormally high resting insulin blood levels, abnormal insulin responses to feeding and glucose administration, and tissue insulin resistance.” Coleman emphasizes, however, that not all obese horses are insulin-dysregulated and not all insulin-dysregulated horses are obese.

“PPID and EMS are both endocrine diseases, so one disease/syndrome could make the horse at higher risk for developing the other,” says Linton. Only occasionally does she see a PPID horse develop EMS. “More often,” she says, “we see that some horses with EMS subsequently develop PPID.”

Coleman concurs, adding that horses with EMS might be at greater risk of developing PPID as they age and should be monitored and tested for it.

“In people, there’s a link between ­obesity and inflammation, which is involved in the (development) of several chronic diseases,” says Linton. “The same can be said for horses, with research now identifying that obesity-induced chronic inflammation identified in the bloodstream may lead to development of inflammatory conditions such as osteoarthritis.”

Studies in humans and rats have shown that low-grade inflammation initiated by obesity can transfer from mother to offspring. Similar issues have emerged in equine studies: Maternal obesity in mares alters metabolism, increases the likelihood of insulin resistance, and increases low-grade systemic inflammation in both dams and foals. Inflammatory effects in the brain can lead to anxiety, behavioral abnormalities, and learning difficulties. In one study more foals born to obese mares developed osteochondrosis in joints than did foals born to nonobese mares.


In the aforementioned U.K. study of more than 100,000 horses, the authors noted that 74% of multimorbidity cases involved laminitis, which is often a systemic disease coupled with a number of physiologic derangements. In affected horses the tissues that attach the hoof to the coffin bone within fail, possibly resulting in rotation or sinking of the bone.

“Laminitis development in the context of underlying endocrinopathic disease (i.e., EMS or PPID) has been referred to as endocrinopathic laminitis and is the most commonly reported form of laminitis,” says Coleman. “Recent studies have highlighted the role of excess circulating insulin in the bloodstream (hyperinsulinemia) as a risk factor for (its) ­development.

“In a recent study … risk factors for development of pasture- and ­endocrinopathic-associated laminitis include obesity, prior diagnosis of metabolic disease, and recent corticosteroid administration,” she adds.

Linton sums up the likelihood of a comorbidity that includes laminitis: “Laminitis is a complex disease process, and its relationship to other disease complexes may have different causes.” She lists the possible relationships between obesity, insulin dysregulation, and laminitis:

  • Insulin dysregulation impacts nutrition, health, and hoof cell function, leading to laminitis.
  • There is a relationship between obesity-induced chronic systemic inflammation and inflammatory molecules in the blood and the hoof.
  • Insulin dysregulation changes blood vessels and blood flow in the hoof.

Coleman also describes how laminitis is associated with sepsis (a systemic inflammatory response to infection) in adult horses. This is often a complication of diseases such as gastrointestinal (GI) tract twisting (strangulation), colon inflammation (colitis), complicated bacterial pneumonia (pleuropneumonia), and uterine infection.

“Carbohydrate overload, such as might occur from excess grain ingestion, is associated with disruption of the bowel lining, potentially leading to leakage of toxins and inflammatory components into the systemic circulation—this can predispose to laminitis,” she says.

“Additional studies have noted alterations in cecal and colonic microflora and pH due to carbohydrate-rich diets,” she continues. “This may increase systemic absorption of triggers to laminitis.

“Alterations in the GI tract that increase intestinal permeability are known as ‘leaky gut,’  ” Coleman adds. This compromises the intestinal barrier’s protection against harmful toxins. She says leaky gut results from a variety of conditions, including physical stressors, decreased blood flow to the intestine, inflammatory GI disease, and infections. It can result in systemic inflammation affecting multiple organ systems.

“Increased risk of infection can be life-threatening, while laminitis can be a debilitating disease in the advanced stages or when it becomes uncontrolled,” says Linton.

Similarly, a horse that’s undergone successful surgical treatment for strangulation colic could still ultimately succumb to the effects of laminitis, says Coleman.

Lung Disease

Typically, equine asthma is a result of allergens that trigger lung inflammation, which can lead to systemic inflammation and resulting multimorbidity.

Coleman says that in human asthma systemic involvement is associated with increased inflammatory markers in the blood. These include inflammatory precursors, called cytokines, and acute phase proteins, which the liver produces and are known to rise very rapidly in response to infection and inflammation.

“Comparison of acute phase proteins and cytokines in healthy horses and horses with equine asthma holds some promise for markers of acute and chronic systemic inflammation,” she says. 

Lyme Disease and EPM

Clinical signs of Lyme disease caused by tick bite inoculation with Borrelia burgdorferi bacteria could be associated with co- or multimorbidities because it creates complex signs in various organ systems. Lyme disease can affect the heart, joints, eyes, muscles, respiratory tissues, and nervous system, along with behavior. Despite multiple physical signs, all aberrations can be assigned to one disease. The same applies for equine protozoal myeloencephalitis (EPM)—one or several lesions from Sarcosystis neurona or Neospora hughesi protozoa in the spinal cord can make the horse appear to be affected by more than one disease when, in fact, the source is EPM caused by those pathogens. It’s possible for horses affected by either Lyme disease or EPM to also be affected by other infectious diseases.

Take-Home Message

Horses—particularly aging ones—can be plagued by two or more health problems simultaneously. Systemic diseases such as PPID and laminitis are two conditions commonly accompanied by a variety of others, and they might contribute to myriad disease problems.

It is important that you and your veterinarian try to track down all abnormal issues in your horse to be sure you treat the primary cause. Owners can help their vets sleuth the problem(s) by providing a complete history. Keep a log detailing any subtle or overt behavioral or physical changes in your horse, and share it with your veterinarian to ensure a thorough clinical workup. The best resolution comes with early diagnosis.