Adult Horses Get Pneumonia, Too

This inflammation of the lungs most commonly affects foals, but that doesn’t mean your mature horse isn’t at risk.
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Adult Horses Get Pneumonia, Too!
Racehorses in training are one of the populations at highest risk. | Photo: Anne M. Eberhardt/The Horse

This inflammation of the lungs most commonly affects foals, but that doesn’t mean your mature horse isn’t at risk

The pneumonia-causing pathogens Rhodococcus equi and Streptococcus zooepidemicus are notorious for setting up shop in foals’ respiratory tracts, thanks to the youngsters’ relatively undeveloped immune systems. That doesn’t mean, however, that pneumonia of a different breed doesn’t pack a punch when it infects mature horses. Those at particular risk are young athletic horses, horses that travel and compete, and geriatric horses. Understanding why pneumonia—at its most basic, lung inflammation caused by bacterial or viral infection—occurs in horses beyond weanling age can go a long way toward prevention. That’s what we’ll be discussing in this article.

Lower Respiratory Tract Contamination

Bacteria, viruses, fungi, and debris assault the equine respiratory tract constantly. A competent immune system keeps these organisms at bay and curtails foreign matter’s impact on the lower airways. Mature horses usually have well-developed immune systems that function effectively—their endogenous (naturally occurring) antibodies and white blood cells scavenge and attack potentially harmful foreign proteins.

Foreign material can enter the lower respiratory tract a few ways:

  • Through inhalation, in which external material (debris, dust, fungal spores, etc.) enters as the horse breathes.
  • Through aspiration, a process through which respiratory secretions move from the upper respiratory tract into the lower tract.  

“It is normal for some bacteria to get into the trachea from the upper respiratory tract through aspiration, but these bacteria are rapidly cleared (from the lower respiratory tract) by the immune system,” explains Harold McKenzie, DVM, MS, Dipl. ACVIM, FHEA, professor of large animal medicine at the Virginia-Maryland Regional College of Veterinary Medicine. “The upper airway area of the trachea is always a bit inflamed because it serves as the area of immunologic activity and filtration of debris and bacteria between the larynx and the mainstem bronchii of the lungs.”

He adds that the vast majority of adult pneumonia cases results from aspiration or inhaled sources. Rarely, horses develop pneumonia through a bloodstream (hematogenous) infection.  

Setting the Stage

Sometimes the upper respiratory tract’s function as a filter for the lower airways gets derailed: Viral infections can compromise its ability to clear foreign matter, while also causing cellular damage within the airway lining. Such an infection can set the stage for bacteria to enter and colonize.

“Most respiratory disease goes undiagnosed,” McKenzie says. “Many respiratory viruses, such as equine herpesvirus-4 (EHV-4), usually create a mild infection that is treated symptomatically with no further investigation, especially in the field. Rarely is a practitioner looking for a bacterial component.”  

But as we just learned, veterinarians need to be vigilant for opportunistic bacteria.

Usually a viral infection takes a short toll on the horse, with rapid recovery within five to seven days. “But,” McKenzie says, “once the horse demonstrates a persistent cough, nasal discharge, and fever, and it is clear that the horse simply isn’t doing right, it is then assumed that there is a secondary bacterial infection,” and your veterinarian should pursue further diagnostic measures.

The best way to prevent this scenario is immunizing against respiratory viruses. “Vaccines are beneficial to decrease the incidence and severity of viral infections,” McKenzie stresses. “The level and duration of protection may not be 100%, but a young horse or one living a lifestyle with a high risk of exposure benefits from respiratory immunizations that help to decrease his susceptibility to viral infections.”

Parasitic pneumonia from lungworm or roundworm is a rare event, due to the wide availability of deworming medications, but veterinarians should still consider it a possible source of lung inflammation.

The High-Risk Horse

Pneumonia in adult horses typically results from bacterial infection of the lower airways. “For young equine athletes, pneumonia is often transport- and training-related,” McKenzie says. “For the geriatric horse pneumonia tends to follow an episode of choke.”

At one end of the spectrum is the not-quite-mature, young adult horse. “One of the highest risk populations occurs in the 2-year-old racehorse or long yearling that is relatively immunologically naive,” McKenzie reports. “These individuals experience a stressful lifestyle with hard training coupled with transport.”

He says strenuous work at full speed also affects the lower respiratory tract because it overwhelms the upper respiratory tract’s ability to warm, humidify, and filter air. Cold, dry air then travels down to the lower respiratory tract and causes inflammation. “Added to this, (aspirated) bacteria, and in particular streptococcal organisms from nasopharyngeal secretions, enter the bronchial tree and increase contamination,” McKenzie explains.  

If the horse is also being shipped, the negative immunological effects of transport stress can also facilitate bacterial colonization in the lower respiratory tract. Like a falling set of dominoes, this sequence of lifestyle events can cascade toward pneumonia.  

Another risk factor for bacterial colonization in adult horses—especially racehorses—is exercise-induced pulmonary hemorrhage (EIPH, or “bleeding”). “In addition to blood being a great culture medium for bacteria, inflammatory exudates (pus) in the lungs also provide a growth medium and reservoir for bacteria,” McKenzie says.

Transport Stress-Related Pneumonia

Researchers have placed a lot of focus on the phenomenon of transport stress as it affects the lower respiratory tract and the development of “shipping fever.” McKenzie describes one study in which horses had their heads tied in an elevated position during a long haul. The researchers determined that this combination of factors (transport stress plus head elevation) impairs horses’ ability to clear secretions from the lower respiratory tract and also increases the burden of inhaled and aspirated bacteria in the respiratory tract.

He cited another study in which horses were simply cross-tied with heads elevated in a stationary horse trailer. Within 12 hours they all displayed increased respiratory tract secretions and inflammation.  

When horses travel, their restrained head position and debris-ridden trailer environment, coupled with the general systemic stress of transport, can suppress local airway protective mechanisms and reduce systemic immunity, McKenzie says. This sets up perfect conditions for bacterial infection.

To minimize transportation’s adverse impacts on the respiratory tract, many veterinarians suggest unloading horses at intervals along a journey to allow them to lower their heads and drain their airways. However, not everyone has the opportunity to unload and reload horses safely.  

At the very least, “it is important to stop every 4 to 6 hours and loosen the head ties to give horses some freedom to lower their heads to clear respiratory secretions and debris,” McKenzie says. “They should have access to water and forage. In fact, so much debris blows around in most trailers that the presence of hay is not all that significant. Although hay feeding in the trailer may not be ideal for the respiratory tract, the benefits of food accessibility to gastrointestinal function are important to decrease the incidence of colic.”

Choke-Related Pneumonia

The other adult at high-risk for developing pneumonia is the choked horse. Choke is more prevalent in older horses with poor dentition, but it can occur at any age. McKenzie says 10-20% of pneumonia cases he sees are in geriatric horses following a choking episode. He suspects the rising incidence of post-choke pneumonia is due to a growing population of senior horses (thanks to caring owners).

“Not all choking horses develop aspiration pneumonia, despite the presence of food and mucus in the trachea,” he says. “The biggest problem is the geriatric population and their decreasing immune function with age. The greatest battle to wage is against the volume of exposure—the longer the choke goes on, the greater the contamination. The key to pneumonia prevention is to recognize immediately that a horse is choking and to quickly call for veterinary attention.”  

Therefore, always call your veterinarian if a horse is coughing profusely. You might think he has a respiratory problem when, in fact, it’s a choke. (Learn more about clinical signs of choke at

Treatment Strategies

Veterinarians identify pneumonia based on a horse’s clinical presentation—cough, fever, lethargy, difficulty breathing, loss of appetite—and a thorough physical exam that includes listening to the lungs with a stethoscope. Additional testing, such as blood work, samples from the trachea or bronchi, cytology (cellular evaluation under a microscope) and bacterial culture of these samples, chest radiographs, and ultrasound, helps pin down a diagnosis. Once the veterinarian makes a definitive diagnosis, he or she can implement effective treatment.

Based on bacterial culture and sensitivity results, McKenzie notes that one of the most common causes of bacterial pneumonia is Streptococcus equi subspecies zooepidemicus, which is part of the upper respiratory tract’s normal bacterial population and the same organism that infects foals. In early or mild cases it might be appropriate to treat with oral antibiotics, but seriously ill horses in respiratory distress call for more broad-spectrum treatment with intravenous or intramuscular antibiotics.

McKenzie likes to combine systemic treatment with aerosolized antibiotics. Using a nebulizer (a machine that aerosolizes a medication), veterinarians can administer the drugs directly into the horse’s lungs. Prior to nebulizing, he recommends using a metered dose inhaler (MDI) with a bronchodilator such as albuterol to prevent reflex airway constriction. This decreases reactive airway issues and improves the drug’s penetration into the lungs. In the 1990s McKenzie demonstrated that nebulized gentamicin (an antibiotic) does not cause inflammation or other adverse lung or systemic health effects. Researchers have also shown the antibiotic ceftiofur to be effective, and it also does not induce untoward effects when administered with a nebulizer.

For bronchitis cases—where the infection is confined within the large airways and has not reached deeper into the lungs—McKenzie notes it might only be necessary to use aerosol antibiotic therapy. However, in pneumonia cases where there is lung consolidation (the lung tissue fills with liquid) and/or breathing is causing the horse pain, veterinarians must administer systemic antimicrobials. McKenzie considers the nebulizer an adjunct treatment strategy for bronchitis cases.  

Rest is Essential

For any respiratory infection, and pneumonia in particular, rest is paramount to healing. In many cases rest can help prevent pneumonia following a viral respiratory infection. Once the acute phase of a respiratory virus is over (usually within three to five days) and the horse looks and feels better, an owner might be tempted to put the horse back to work. But McKenzie stresses that the trachea and lower airway mucociliary clearance mechanisms take a month or more to normalize following insults such as influenza infection. “Premature return to work perpetuates the inflammatory process and creates an increased risk of developing bacterial pneumonia,” he says.

He recommends allowing a horse to have three to four weeks of rest following a viral respiratory infection. If he goes back to work too soon, he could end up on layup for many months with a secondary bacterial infection or pneumonia.

“In addition to rest and treatment, the horse’s environment should be managed to minimize inhaled material that could add fuel to the fire,” McKenzie says. “Soaking hay (to reduce the amount of particles the horse inhales while eating) is beneficial.”  

Other “good air” strategies include using low-dust bedding, ensuring the barn is well-ventilated, and/or moving a horse outside into fresh air.

Take-Home Message

Pneumonia can be a very serious and debilitating disease that causes significant performance and financial losses. The best cure is prevention: Owners should implement biannual vaccinations against respiratory viruses, take care to minimize stress and environmental contaminants during long-distance transport, and provide a clean-air environment on the farm. For choke-prone horses, it’s important that you recognize clinical signs and call your veterinarian out quickly to minimize the risk of aspiration pneumonia. Remember that your veterinarian is best-equipped to handle any sign of respiratory disease to determine the severity of the problem and the best strategy for healing.


Written by:

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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