## Overview Pneumonia is infection and inflammation of the lung parenchyma (the functional tissue of the lungs). In horses it is not a single disease. Adult horses most often develop bacterial pneumonia when organisms that normally live in the upper airway reach the lower lungs and overwhelm local defenses. Viral respiratory infections (especially equine influenza and equine herpesvirus) rarely cause pneumonia by themselves but damage the airway lining and predispose to secondary bacterial infection. Aspiration of feed, saliva, or medication can also produce pneumonia. Long-distance transport with the head elevated (shipping fever) is a well-recognized risk factor that allows bacteria to colonize the lower airways during prolonged transport and can progress to the more serious complication of pleuropneumonia. Foals, particularly those one to five months of age, face a different and often more serious form of pneumonia caused by the intracellular bacterium Rhodococcus equi. This organism produces abscesses in the lungs and can affect other organs. Because the causes, clinical course, diagnostic approach, and treatments differ substantially between adult bacterial pneumonia, viral-associated disease, aspiration pneumonia, shipping-related disease, and Rhodococcus equi pneumonia in foals, management must be tailored to the specific situation. ## Causes In adult horses the most common bacterial isolate from pneumonia is Streptococcus equi subspecies zooepidemicus, a normal inhabitant of the upper airway. Mixed infections that include other aerobic bacteria (such as Escherichia coli, Klebsiella species, Actinobacillus species, and Pasteurella species) and anaerobic bacteria are common, especially in more severe or chronic cases. Viral infections such as equine influenza and equine herpesvirus type 1 or 4 damage the mucociliary clearance mechanism and open the door to these secondary bacterial invaders. Aspiration pneumonia occurs when foreign material enters the lungs. The most frequent triggers are esophageal obstruction (choke), difficulty swallowing due to neurologic disease, improper administration of oral medications or mineral oil, and complications of general anesthesia. Transport-associated pneumonia (shipping fever) develops when horses are hauled long distances, particularly with the head tied in an elevated position. Inability to lower the head impairs normal airway clearance, allowing bacteria to colonize the lower respiratory tract during prolonged transport. This process can progress to pleuropneumonia if infection spreads to the pleural space. In foals one to five months old, Rhodococcus equi is the predominant and most serious cause of pneumonia. The organism is widespread in soil and feces. Foals become infected by inhalation or ingestion in the first weeks of life, but clinical signs usually appear later. Other bacteria can also cause pneumonia in foals, but R. equi is distinctive for its ability to survive inside cells and form large abscesses. ## Diagnosis Your veterinarian begins with a thorough history and physical examination, including careful auscultation of the lungs for abnormal sounds such as crackles or wheezes. Bloodwork often shows elevated white blood cell counts and inflammatory proteins. Thoracic ultrasound is useful for detecting peripheral lung consolidation, abscesses, or pleural fluid. Radiographs can reveal deeper lung patterns, including the nodular abscesses typical of Rhodococcus equi in foals. Lower-airway sampling, especially sterile tracheal or transtracheal aspirates for cytology, culture, and susceptibility testing, is most useful for confirming bacterial infection and guiding therapy in severe or nonresponsive cases. Bronchoalveolar lavage is not an equivalent primary diagnostic for pneumonia. In foals suspected of Rhodococcus equi, polymerase chain reaction testing for the virulence-associated protein A (vapA) gene on airway samples can support etiologic confirmation. Thoracic ultrasound is widely used for screening and monitoring of foals on endemic farms, but it is not a standalone gold standard for confirming R. equi pneumonia. Viral testing (nasopharyngeal swabs for polymerase chain reaction) is appropriate when a viral outbreak is suspected. ## Treatment Treatment depends entirely on the cause and the individual patient. For bacterial pneumonia in adult horses, antimicrobial therapy is directed at the most likely organisms (especially Streptococcus zooepidemicus) while awaiting culture results. Broad-spectrum coverage is often needed when mixed or anaerobic infections are possible. Supportive care includes anti-inflammatory medication, rest, and a clean, dust-free environment. Viral respiratory infections are managed primarily with rest and supportive care. Antibiotics are not effective against viruses and are reserved for confirmed or strongly suspected secondary bacterial pneumonia. Aspiration pneumonia requires prompt, aggressive broad-spectrum antimicrobial therapy that includes anaerobic coverage, plus supportive care and measures to prevent further aspiration (for example, treating the underlying choke or dysphagia). Shipping-fever pneumonia is treated with antimicrobials and supportive care. If the infection has already progressed to pleuropneumonia, additional interventions such as pleural drainage may be required. (Pleuropneumonia is covered in a separate article on this site.) For foals with clinical Rhodococcus equi pneumonia, a macrolide antibiotic combined with rifampin remains the traditional recommended treatment. Evidence is evolving, resistance is an increasing concern, and treatment of small subclinical ultrasound lesions is often unnecessary. Supportive care and monitoring for complications are essential. Antibiotics appropriate for adult bacterial pneumonia are generally not effective against R. equi. ## Recovery and Prognosis Survival is variable and depends on the case mix, severity, and presence of complications. Adult hospitalized cohorts report survival to discharge around 73% to 78%, with worse outcomes when pleural disease, anaerobic bacteria, or markers of systemic illness are present. Horses that develop large lung abscesses or require prolonged drainage have a more guarded outlook for return to full athletic performance. Foals with clinical Rhodococcus equi pneumonia have variable outcomes that depend on the severity of lung involvement, the presence of extrapulmonary abscesses, and whether antimicrobial-resistant strains are present. Some foals on endemic farms develop subclinical lesions that resolve without treatment. Aspiration pneumonia carries a more variable and often poorer prognosis, especially if large volumes of material were inhaled or if treatment is delayed. ## Prevention Vaccination against equine influenza and equine herpesvirus reduces the risk of primary viral infection and subsequent secondary bacterial pneumonia. Biosecurity practices (isolation of new or returning horses, good ventilation, and limiting nose-to-nose contact) further lower the chance of respiratory outbreaks. The strongest support for reducing transport-associated risk is allowing horses to lower their heads periodically, limiting prolonged head restraint, and improving monitoring after long-distance travel. Avoid shipping horses that are already showing signs of respiratory disease. For Rhodococcus equi, no routine vaccine is available. Evidence for hyperimmune plasma is mixed, and farm hygiene measures are promising but not definitive. Serial ultrasound screening is used on some endemic farms, but treatment of small subclinical lesions is often unnecessary. Prophylactic antibiotic use in foals is discouraged because it promotes resistance. Good dental care, gradual feed changes, and careful administration of oral medications reduce the risk of choke and subsequent aspiration.
Fever; cough; nasal discharge (clear, mucoid, or purulent); increased respiratory rate or effort; lethargy or depression; reduced appetite; in foals, poor weight gain or failure to thrive; in severe cases, labored breathing or respiratory distress
## Overview Pneumonia is infection and inflammation of the lung parenchyma (the functional tissue of the lungs). In horses it is not a single disease. Adult horses most often develop bacterial pneumonia when organisms that normally live in the upper airway reach the lower lungs and overwhelm local defenses. Viral respiratory infections (especially equine influenza and equine herpesvirus) rarely cause pneumonia by themselves but damage the airway lining and predispose to secondary bacterial infection. Aspiration of feed, saliva, or medication can also produce pneumonia. Long-distance transport with the head elevated (shipping fever) is a well-recognized risk factor that allows bacteria to colonize the lower airways during prolonged transport and can progress to the more serious complication of pleuropneumonia. Foals, particularly those one to five months of age, face a different and often more serious form of pneumonia caused by the intracellular bacterium Rhodococcus equi. This organism produces abscesses in the lungs and can affect other organs. Because the causes, clinical course, diagnostic approach, and treatments differ substantially between adult bacterial pneumonia, viral-associated disease, aspiration pneumonia, shipping-related disease, and Rhodococcus equi pneumonia in foals, management must be tailored to the specific situation. ## Causes In adult horses the most common bacterial isolate from pneumonia is Streptococcus equi subspecies zooepidemicus, a normal inhabitant of the upper airway. Mixed infections that include other aerobic bacteria (such as Escherichia coli, Klebsiella species, Actinobacillus species, and Pasteurella species) and anaerobic bacteria are common, especially in more severe or chronic cases. Viral infections such as equine influenza and equine herpesvirus type 1 or 4 damage the mucociliary clearance mechanism and open the door to these secondary bacterial invaders. Aspiration pneumonia occurs when foreign material enters the lungs. The most frequent triggers are esophageal obstruction (choke), difficulty swallowing due to neurologic disease, improper administration of oral medications or mineral oil, and complications of general anesthesia. Transport-associated pneumonia (shipping fever) develops when horses are hauled long distances, particularly with the head tied in an elevated position. Inability to lower the head impairs normal airway clearance, allowing bacteria to colonize the lower respiratory tract during prolonged transport. This process can progress to pleuropneumonia if infection spreads to the pleural space. In foals one to five months old, Rhodococcus equi is the predominant and most serious cause of pneumonia. The organism is widespread in soil and feces. Foals become infected by inhalation or ingestion in the first weeks of life, but clinical signs usually appear later. Other bacteria can also cause pneumonia in foals, but R. equi is distinctive for its ability to survive inside cells and form large abscesses. ## Diagnosis Your veterinarian begins with a thorough history and physical examination, including careful auscultation of the lungs for abnormal sounds such as crackles or wheezes. Bloodwork often shows elevated white blood cell counts and inflammatory proteins. Thoracic ultrasound is useful for detecting peripheral lung consolidation, abscesses, or pleural fluid. Radiographs can reveal deeper lung patterns, including the nodular abscesses typical of Rhodococcus equi in foals. Lower-airway sampling, especially sterile tracheal or transtracheal aspirates for cytology, culture, and susceptibility testing, is most useful for confirming bacterial infection and guiding therapy in severe or nonresponsive cases. Bronchoalveolar lavage is not an equivalent primary diagnostic for pneumonia. In foals suspected of Rhodococcus equi, polymerase chain reaction testing for the virulence-associated protein A (vapA) gene on airway samples can support etiologic confirmation. Thoracic ultrasound is widely used for screening and monitoring of foals on endemic farms, but it is not a standalone gold standard for confirming R. equi pneumonia. Viral testing (nasopharyngeal swabs for polymerase chain reaction) is appropriate when a viral outbreak is suspected. ## Treatment Treatment depends entirely on the cause and the individual patient. For bacterial pneumonia in adult horses, antimicrobial therapy is directed at the most likely organisms (especially Streptococcus zooepidemicus) while awaiting culture results. Broad-spectrum coverage is often needed when mixed or anaerobic infections are possible. Supportive care includes anti-inflammatory medication, rest, and a clean, dust-free environment. Viral respiratory infections are managed primarily with rest and supportive care. Antibiotics are not effective against viruses and are reserved for confirmed or strongly suspected secondary bacterial pneumonia. Aspiration pneumonia requires prompt, aggressive broad-spectrum antimicrobial therapy that includes anaerobic coverage, plus supportive care and measures to prevent further aspiration (for example, treating the underlying choke or dysphagia). Shipping-fever pneumonia is treated with antimicrobials and supportive care. If the infection has already progressed to pleuropneumonia, additional interventions such as pleural drainage may be required. (Pleuropneumonia is covered in a separate article on this site.) For foals with clinical Rhodococcus equi pneumonia, a macrolide antibiotic combined with rifampin remains the traditional recommended treatment. Evidence is evolving, resistance is an increasing concern, and treatment of small subclinical ultrasound lesions is often unnecessary. Supportive care and monitoring for complications are essential. Antibiotics appropriate for adult bacterial pneumonia are generally not effective against R. equi. ## Recovery and Prognosis Survival is variable and depends on the case mix, severity, and presence of complications. Adult hospitalized cohorts report survival to discharge around 73% to 78%, with worse outcomes when pleural disease, anaerobic bacteria, or markers of systemic illness are present. Horses that develop large lung abscesses or require prolonged drainage have a more guarded outlook for return to full athletic performance. Foals with clinical Rhodococcus equi pneumonia have variable outcomes that depend on the severity of lung involvement, the presence of extrapulmonary abscesses, and whether antimicrobial-resistant strains are present. Some foals on endemic farms develop subclinical lesions that resolve without treatment. Aspiration pneumonia carries a more variable and often poorer prognosis, especially if large volumes of material were inhaled or if treatment is delayed. ## Prevention Vaccination against equine influenza and equine herpesvirus reduces the risk of primary viral infection and subsequent secondary bacterial pneumonia. Biosecurity practices (isolation of new or returning horses, good ventilation, and limiting nose-to-nose contact) further lower the chance of respiratory outbreaks. The strongest support for reducing transport-associated risk is allowing horses to lower their heads periodically, limiting prolonged head restraint, and improving monitoring after long-distance travel. Avoid shipping horses that are already showing signs of respiratory disease. For Rhodococcus equi, no routine vaccine is available. Evidence for hyperimmune plasma is mixed, and farm hygiene measures are promising but not definitive. Serial ultrasound screening is used on some endemic farms, but treatment of small subclinical lesions is often unnecessary. Prophylactic antibiotic use in foals is discouraged because it promotes resistance. Good dental care, gradual feed changes, and careful administration of oral medications reduce the risk of choke and subsequent aspiration.
Fever; cough; nasal discharge (clear, mucoid, or purulent); increased respiratory rate or effort; lethargy or depression; reduced appetite; in foals, poor weight gain or failure to thrive; in severe cases, labored breathing or respiratory distress