Equine Emerging Diseases Reviewed

Tracking emerging and re-emerging equine diseases helps the horse world attempt to stay a step ahead of economically devastating and deadly outbreaks. For that reason, equine veterinarians and industry members gathered on June 14, for Merck Animal Health’s "Equine Emerging and Re-emerging Disease Luncheon" at the 2013 American College of Veterinary Internal Medicine Forum in Seattle, Wash., to discuss which infectious diseases most concern leading veterinarians and researchers.

Wendy Vaala, DVM, Dipl. ACVIM, senior equine technical veterinarian with Merck, moderated the presentation and subsequent discussions, which covered equine coronavirus (ECoV), equine protozoal myeloencephalitis (EPM), Lyme disease, equine influenza virus (EIV), and Corynebacterium pseudotuberculosis (the bacterium that causes pigeon fever, also known as dryland distemper).

In addition to Vaala, presenters included Nicola Pusterla, DVM, PhD, Dipl. ACVIM, of the University of California, Davis (UC Davis), School of Veterinary Medicine, and Amy Johnson, DVM, Dipl. ACVIM, of University of Pennsylvania’s New Bolton Center.

Equine Coronavirus in Adult Horses: High Morbidity, Low Mortality

Pusterla started the luncheon by asking how many attendees had treated or seen cases of ECoV in adult horses, a RNA virus that causes respiratory and enteric (digestive tract) diseases. Only a few in the crowded room raised their hands.

Considering the equine form of coronavirus first gained recognition as a cause of disease in 2010, the sparse response didn’t seem to surprise Pusterla.

“But that doesn’t mean this pathogen hasn’t been around earlier on,” Pusterla responded, adding that in many cases its possible researchers and clinicians had missed ECoV as a cause of disease.

Species-specific coronavirus affects a variety of avian and mammalian species, including humans (enteric and respiratory coronavirus), cattle (bovine coronavirus), and dogs (canine respiratory coronavirus) just to name a few. Clinical signs of ECoV in adult horses include:

  • Anorexia (loss of appetite);
  • Lethargy;
  • Fever;
  • Diarrhea; and
  • Colic.

A study of all reported U.S. outbreaks from November 2011 and April 2013 included six stables in five states: California, Texas, Wisconsin, Massachusetts, and New Jersey. Interestingly, Pusterla noted, the outbreaks happened in a variety of, what he considers, small to midsized equine boarding facilities and riding schools ranging in size from 28 to 70 horses each (for a total of 248 animals). The disease mostly affected adult horses (ages 1-32, “It’s more common in older horses,” Pusterla said), and between 20-57% of those infected with ECoV showed clinical signs of disease.

The study also found that ECoV is a self-limited disease in adult horses, and clinical signs in individuals resolved in one to four days. “Generally these outbreaks will resolve within three to four weeks,” Pusterla said.

Bovine coronavirus is anecdotally considered a cold-weather disease, with people in the industry referring to it as “winter dysentery.” Whether that’s really the case or applies to ECoV is debatable, Pusterla said.

Secondary complications of the disease in horses included gastrointestinal tract translocation and possible metabolic derangements, he added. “My theory: This is a disease of high morbidity, low mortality,” Pusterla concluded.

EPM: More than One Organism

EPM is a neurologic disease caused by two distinct protozoa: Sarcocystis neurona and Neospora hughesi. Most of what’s documented about EPM surrounds S. neurona, which is passed to horses through opossum scat.

Less is understood about N. hughesi, Pusterla said, adding that horses are believed to serve as the organism’s intermediate host, but no definitive host known. Evidence of transplacental transmission of N. hughesi from mare to foal also exists, Pusterla said. Horses in 27 states have tested positive for N. hughesi.

No vaccine for EPM exists. So for disease prevention, Pusterla recommended owners:

  • Minimize horses' stress;
  • Reduce exposure to opossum feces;
  • Avoid feeding horses on the ground;
  • Offer fresh water; and
  • Prevent contact to wildlife, especially opossums.

Prophylactic treatments include:

  • Ponazuril;
  • Sulfa/Pyrimethamine; and
  • Diclazuril.

Equine Lyme Disease: Many Clinical Signs

yme disease in horses is notoriously baffling to diagnose, frustrating to treat, and difficult to study, said Johnson. “I could’ve named this presentation, ‘Why I hate Lyme disease,’” she said good naturedly, eliciting agreement from the audience.

One reason Johnson and others in the room find Lyme disease so maddening: “Most confirmed cases of neurologic equine Lyme disease are dead,” Johnson said.

Lyme disease is caused by the bacterium Borrelia burgdorferi and is transmitted to horses (and other species) by a bite from an infected blacklegged (deer) tick.

Clinical signs of Lyme disease in horses include:

  • Lethargy, depression, and altered mental status;
  • Anorexia and weight loss;
  • Pyrexia (fever);
  • Neck and back pain;
  • Ataxia (incoordination), paresis (impaired movement), and recumbency (an inability to rise after lying down);
  • Hyperesthesia (hypersensitivity to touch and sound);
  • Tremors;
  • Uveitis;
  • Polysynovitis (a condition in which multiple joint surfaces are inflamed);
  • Vestibular disease (balance issues);
  • Facial paralysis; and
  • Dangerous behavior.

Existing research shows that some horses deemed free of the disease based on clinical findings or serology were actually positive and some horses that showed compatible clinical signs and positive serology did not have the disease. This makes establishing a “gold standard” for clinical diagnosis nearly impossible, Johnson said.

When horses have or are suspected of having Lyme disease, it's often difficult for clinicians to know how to treat them, Johnson said. And, with horse owners accessing sometimes unsubstantiated information about vague clinical signs online, gathering case history and making an accurate diagnosis becomes even more difficult. “Lyme gets blamed for just about everything,” she said.

Johnson’s current approach to Lyme disease: “Consider it in cases with certain signs, such as fever, meningitis, polyneuropathies, test for every reasonable alternative, and then treat the horse with appropriate antimicrobial therapy.”

Equine Influenza: Back on the Radar

“Equine influenza needs to be back on your radar,” Vaala told the audience as she opened the EIV presentation.

Her reasons: Frequency of this serious and highly contagious respiratory disease in U.S. horses has increased significantly in the past two years, and the affected horse population isn’t as straight forward (or limited) as once thought.

“This is no longer the disease of just 2- and 3-year-old racehorses,” explained Vaala, referring to the long-held horse industry belief that EIV is primarily a young-horse disease associated with related competitions, sales, transport stress, and/or lack of vaccination.

While it’s true that immune-compromised populations are more susceptible to flu (in human terms, consider how the elderly, infants, and school children receive vaccination priority over healthy adults), experts are recognizing the risk of EIV to mature, previously vaccinated horses, as well.

“Anywhere horses might congregate creates an opportunity for the virus to spread,” added Pusterla, who studies disease epidemiology (the movement of a disease within a population). He noted three important factors related to these recent EIV outbreaks:

  1. EIV is one of the leading respiratory pathogens in the United States, creating a significant economic loss to the industry;
  2. Veterinarians have seen an increased frequency of EIV detection in the past two years; and
  3. Horses vaccinated with killed vaccines are developing the disease despite inoculation.

Since 2013, the respiratory surveillance program, sponsored by Merck Animal Health in collaboration with the diagnostic lab at UC Davis veterinary school, has identified 16 EIV outbreaks in nine U.S. states, including (from west to east): Oregon, California, Idaho, Nevada, Montana, Utah, Arizona, Texas, Oklahoma, and Florida.

One California case originated in a horse imported from Germany who’d gone through a three-day quarantine. The horse started showing clinical signs during transport and ended up at UC Davis’ School of Veterinary Medicine, where he went into isolation and veterinarians confirmed EIV.

Pusterla described EIV as a “cold weather disease due to horse husbandry during winter.” That includes keeping horses in close quarters, where the virus is easily spread.

Horses under transport or competition stress are also more susceptible to EIV, and horses are now more mobile nationally, as well as internationally. For that reason, Pusterla said, “it’s time to push the issue of vaccinating for influenza.”

Pigeon Fever: Moving East

Practitioners in the Western United States have gained experience with Corynebacterium pseudotuberculosis over the past 20 years. However, the intracellular bacterial disease (more commonly called pigeon fever) is on the move. “For those of you in the Midwest and East, if you haven’t seen it, you will,” warned Pusterla.

C. pseudotuberculosis is a soil-borne organism that is transmitted to horses via flies. The flies land on horses’ open abrasions or wounds, infecting the horse. “The disease will persist in the soil for up to eight months,” Pusterla said.

Pigeon fever isn’t related to birds but is named for the abscesses sometimes seen on infected horses’ chests. These abscesses give the horses’ a pigeon-like appearance, because their chest looks puffed up like a pigeon breast.

The abscesses aren’t always on the chest, Pusterla said, adding that they can also form on the sheath or abdomen. “We’ve seen more and more complications,” he added about his experience with the disease.

“Pigeon fever diagnosis is pretty straight forward,” Pusterla said. “Ultrasound is probably the best tool we have to hunt down the origin of the abscesses.”

Additional tools for diagnosis include culture, clinicopathologic data, and serology.

Pusterla recommended individualized treatment for each patient based on disease severity, the amount of soft-tissue inflammation, and maturity of abscess. The outcome relies on establishing pus drainage and antimicrobial treatment, as well as the presence or absence of ulcerative lymphangitis (a limb infection) and internal infection.

Currently no vaccine is available to prevent pigeon fever. To prevent the disease Pusterla recommends good sanitation and fly control measures, regular manure disposal, proper wound care of horses with actively draining abscesses, and prevention of ventral midline dermatitis (which opens a horse’s skin for infection exposure).

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.