Don’t gamble with your horse's respiratory system.

Experts advise: When it comes to respiratory illness, save the gambling for Vegas. Be aware of the major respiratory diseases that horses are susceptible to at each point of their lives and use preventive measures, both with management and vaccination, to head them off.

Foals: Rhodococcus equi

Rhodococcus equi is an intracellular bacterium found in soil and air. Overall, it is the most important cause of pneumonia in foals 3 to 20 months old: R. equi is estimated to be the third-leading cause of illness and the second most common cause of death in foals, as Steeve Giguère, DVM, PhD, Dipl. ACVIM, professor of veterinary medicine and the Marguerite Thomas Hodgson chair in equine studies at the University of Georgia's Department of Large Animal Medicine, reported at the 2009 World Equine Veterinary Association (WEVA) Congress. Pneumonia due to R. equi is expensive to treat and requires long-term therapy.

Foals with R. equi pneumonia develop a fever, decreased appetite, lethargy, an increased respiratory rate, labored breathing, cough, and bilateral nasal discharge.

R. equi can be cultured from the environment of virtually every horse farm, yet not all farms have problems related to R. equi, and not all foals are susceptible to the bacterium. So how do we know which foals will succumb to this hardy bug?

"Based on our current understanding of the equine immune system, foals appear to be deficient in their ability to produce an important anti-inflammatory mediator called interferon-γ (gamma)," relays Giguère.

Giguère's research shows that foals infected with only a low number of bacteria can develop mild lesions in the lungs but no clinical signs of Rhodococcus pneumonia. In these foals levels of interferon-γ are at least as high and potentially even higher than those measured in healthy adult horses. However, if foals are overwhelmed by exposure to very high numbers of -bacteria, their bodies mount a different type of immune response, and these foals develop pneumonia.

For the past 20 years, the only R. equi pneumonia treatment has been a combination of the antibiotics erythromycin and rifampin. However, other treatment options are sought-after because the erythromycin-rifampin combination needs to be administered frequently (three to four times a day for erythromycin and twice daily for rifampin) and can be accompanied by serious side effects (e.g., diarrhea; hyperthermia, also known as heatstroke).

A different combination, clarithromycin-rifampin, appears more effective than the erythromycin-rifampin combination, at least for the most severely affected foals, according to Giguère.

R. equi lives inside the foal's macrophages (a type of white blood cell). Since it is more challenging to create an effective vaccine against microorganisms that live inside cells instead of outside, there is currently no vaccine available for R. equi. In addition to the lack of a vaccine, veterinarians are often not able to identify which foals are susceptible to infection. Together, these factors make prevention a real clinical challenge. Some farms with continual R. equi problems elect to treat all foals with hyperimmune serum and/or administer azithromycin during the neonatal period. At the 2009 American Association of Equine Practitioners' (AAEP) Annual Convention, Jeanette McCracken, DVM, and Nathan Slovis, DVM, Dipl. ACVIM, CHT, both veteriarians at Hagyard Equine Medical Institute in Lexington, Ky., described an "ultrasound screening program." This program involves performing ultrasound scans of foals' chests every two weeks from four to eight weeks of age.

McCracken and Slovis suggest this ultrasound screening program enables veterinarians to detect R. equi pneumonia early, and that treating apparently healthy foals that have signs of lung lesions on the ultrasound reduces the number of foals that develop full-blown pneumonia requiring hospitalization.

Young Horses: Herpesvirus

Equine herpesviruses-1 and -4 (EHV-1 and EHV-4) are common causes of upper respiratory tract infections in young horses. Approximately 80-90% of horses are infected by two years of age. Equine herpesviruses are a major economic drain on the equine industry, spread quickly in naive (unexposed) horse populations, can lie dormant in healthy horses, and are surreptitiously shed from these asymptomatic horses to naive horses (i.e., the young stock) without warning.

EHV-4 is primarily limited to the respiratory tract, causing fever and upper respiratory tract inflammation (in the pharynx, trachea, and bronchioles). While EHV-4 causes respiratory disease more often than EHV-1, the latter virus is the more dangerous. EHV-1 is ill-behaved and does not stay in the respiratory tract like EHV-4. Instead, EHV-1 also causes abortion, neonate death, myeloencephalopathy (if the neurologic strain of EHV-1 is involved), and ocular (pertaining to the eyes) disease.

Management, not vaccination, is the most effective method of controlling herpesvirus and decreasing the chances of major disease outbreaks. This is because immunity to all of the currently available EHV vaccines is incomplete (meaning there is poor evidence it decreases clinical disease, let alone prevents disease) and is short-lived. This is not to say vaccination isn't important; it simply means horse owners and barn managers are encouraged to consider such factors as hygiene and appropriate isolation protocols on a day-to-day basis to best minimize spread of disease. Consult your veterinarian to establish if your horses would benefit from administration of an EHV vaccine.

Mature Horses: Heaves

Equine recurrent airway obstruction (RAO, heaves) is a common, debilitating asthma-like condition in horses typically eight years of age or older. RAO-affected horses suffer from inflammation and constriction of the airways (bronchoconstriction) and mucus accumulation leading to obstruction. Mold spores, dust, and a variety of inhaled particles all can cause inflammation, bronchoconstriction, and mucus accumulation.

Heavey horses cough with increased frequency and begin to show signs of exercise intolerance. Gradually, over the course of the next year or two, the cough worsens and the ability to perform athletically declines to the point that horses with heaves have an increased respiratory rate even while at rest. In long-standing, untreated cases, affected horses develop a heave line--a groove in the external abdominal oblique muscle that runs diagonally from the point of the hip forward to the lower edge of the ribs. This line is caused by the persistently increased respiratory effort. If the condition is severe enough, a heavey horse also will lose weight because of how difficult it is to eat while trying to breathe.

Diagnosis is fairly straightforward in most cases. Certain medications are effective and usually include a corticosteroid (e.g., dexamethasone, prednisolone) to decrease inflammation and a bronchodilator (e.g., clenbuterol, albuterol) to increase the diameter of the airways, allowing more air to flow to and from the lungs. Non-steroidal anti-inflammatory drugs (e.g., phenylbutazone or "Bute") and antihistamines are not recommended for the treatment of heaves in horses

Prevention and Control

Vaccines are an essential part of disease control, but they are not foolproof. H. Steve Conboy, DVM, a private practitioner from Lexington, Ky., suggests the following steps to prevent and control infectious and contagious equine disease effectively:

  1. Maintain a carefully designed vaccination program;
  2. Implement a pre- and post-arrival policy;
  3. Isolate new arrivals and horses returning to the premises after an event;
  4. Isolate any horse that is sick or suspected of being sick or shedding bacteria or viruses;
  5. Avoid overpopulating;
  6. Avoid mixing foals and yearlings with older horses, particularly pregnant mares; and
  7. Use preventive hygiene measures.

In the case of RAO the most important means of preventing disease-onset or exacerbation of the condition is to improve the horse's environment. Ed Robinson, BVetMed, MRCVS, PhD, the Matilda R. Wilson chair in large animal clinical sciences at Michigan State University and the director of the Equine Pulmonary Research Laboratory, says, "It is critical for horse owners to focus on the horse's breathing zone--a two-foot sphere around the horse's nose from where he draws his breath.

"I always advise owners to scatter the hay on the ground instead of providing a round bale. This reduces the number of particles and mold spores that the horse inhales and also encourages the drainage of mucus from the airways during feeding," Robinson adds.

The horse also should spend the majority of his time outside the barn, where there are less particles in the air, as long as he is adequately protected from the elements.

Take-Home Message

According to Robinson, a small investment in prevention is better than a large expenditure in treatment that is less likely to be effective.

Pay attention to those minor coughs and runny noses because they can be signs of problems that could become serious, he adds. "Saving a little money by not vaccinating or by buying cheap, poor-quality hay may seem wise at the time, but it could lead to much bigger expenditures."

About the Author

Stacey Oke, DVM, MSc

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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