Bacterial Pneumonia in the Horse

Bacterial Pneumonia in the Horse

Pneumonia refers to inflammation of the lungs. In horses, this is usually due to a bacterial infection, most commonly the streptococcal species.

Most horses get pneumonia by inhaling bacteria, which are often normal inhabitants of the upper airways. The lungs are usually able to clear the bacteria rapidly. However, if your horse has a depressed immune system, or has taken in an overwhelming dose of bacteria, then he may not be able to get rid of the bacteria, and pneumonia (lung infection) will develop.

There are many risk factors for bacterial pneumonia: those most important include a recent viral upper respiratory infection, moderate to severe exercise or overtraining, long-distance transport, overcrowded barns, and esophageal obstruction (choke).

What to Watch For

  • Rapid breathing (tachypnea)
  • Difficulty breathing (dyspnea)
  • Fever
  • Nasal discharge that may be clear or may look like pus
  • Poor appetite
  • Weight loss
  • Decreased exercise tolerance
  • Dullness
  • Fetid nasal breath
  • Enlarged submandibular lymph nodes (they lie under your horse's jaw)
  • Cough, especially if productive – horses often look as though they are chewing or swallowing after a productive cough


  • Your veterinarian will first perform a thorough medical history and physical examination, including careful auscultation (listening with a stethoscope) and percussion (tapping on the chest wall).
  • Your veterinarian will usually take blood for a complete blood count to look for infection and chemistry profile to check that all the other body systems are working properly.
  • Your veterinarian may choose to do an ultrasound of the lungs. This can show whether there is consolidation in the lungs, or a build-up of fluid.
  • Frequently, your veterinarian will perform a transtracheal aspirate, to obtain a culture of the fluid in the lungs.
  • Your veterinarian may choose to look at the trachea (windpipe) and beginning of the bronchi (breathing tubes) with an endoscope.


  • Antibiotic therapy is necessary in order to treat pneumonia. Your veterinarian will probably start out with a broad-spectrum antibiotic that is likely to kill streptococcal species, and then will change antibiotics if the culture results warrant a change.
  • The length of treatment will vary with the severity of the pneumonia. It may range from 7 days to 4 weeks or longer.

    Home Care

    Monitor your horse's rectal temperature once or twice a day. The horse's normal temperature ranges between around 99° F to 101° F. Also monitor your horse for any increase in coughing or nasal discharge. Note whether his appetite is good, and whether he looks bright and alert when you enter the barn.

    Keep your horse in a temperate, well ventilated but draft-free environment. Avoid damp, poorly ventilated barns. Try to avoid dusty bedding, or moldy, dusty hay.

    It is extremely important that you follow all instructions for antibiotic treatment. Your horse is likely to look and feel better long before the infection is completely cleared. If you stop antibiotic treatment too early, the infection will recur, and may be more difficult to cure the second time.

    Give your horse plenty of time to recover if he has just undergone strenuous exercise – his immune system will be down. Whenever possible, don't tie your horse's head up when shipping, and avoid hay bags.

    Have any horse with a cough or fever evaluated by your veterinarian; most horses will have a viral infection, but a few will go on to get pneumonia.

    Pneumonia refers to inflammation and consolidation of the lungs. When areas of the lung that are normally filled with air become filled with inflammatory secretions like mucous and pus, this is called consolidation.

    In horses, pneumonia is usually characterized by colonization and infection of the bronchioles, air sacs, and parenchyma by bacteria. Rarely, fungi and parasites may also cause pneumonia, as well.

    The most commonly involved bacteria are streptococcus species that are gram positive, which refers to the type of cell wall. We often find a variety of gram negative bacteria, which are bacteria that have a thin cell wall and carry endotoxins as well.

    The most common route of infection is inhalation (meaning that the horse breathes in the bacteria). Horses can also, less commonly, develop pneumonia through a primary infection in the blood.

    Pneumonia in horses is generally not contagious; that is, there is no rapid spread from individual to individual. However, the same risk factors that apply to one horse in the barn may well apply to others, so occasionally you may have multiple horses with pneumonia.

    Horses have a tremendous respiratory reserve, so pneumonia may be well established by the time you see signs such as an increased respiratory rate, cough, or fever. Severe heaves, a non-infectious cause of respiratory difficulty, may have many of the same signs as pneumonia, but the treatments are very different. It is important to pursue a diagnosis and treatment promptly so that your horse has the best chance of recovery.

    Risk Factors

  • A common misconception for many years is that pneumonia in horses arises from a head cold. This led to the use of vaporized oils for the treatment of the nasal passages and throat. However, viral upper respiratory infection – Equine Influenza Virus and Equine Herpes Virus, in particular – can impair the respiratory system's ability to clear bacteria, thus making it easier for pneumonia to become established.
  • Physical stress – especially endurance rides and racing. This type of exercise has been shown to suppress the ability of some of the white blood cells to fight infection.
  • General anesthesia. Horses are so heavy that even with the best of padding, their lungs tend to experience partial collapse (atelectasis) due to the weight of their own bodies pressing on their lungs when they are down for long periods of time. Lungs with atelectasis are more susceptible to bacterial infection. When horses are anesthetized, they often have an orotracheal tube placed. This tube goes from the mouth to the trachea, and allows the anesthetist to regulate the horse's breathing. Bacteria from the mouth can contaminate the trachea during intubation. The pressure of the tube can cause inflammation of the trachea, and consequent difficulty in clearing the respiratory system. Finally, after anesthesia, horses sometimes have difficulties swallowing properly, which increases the risk of aspiration pneumonia.
  • Exercise induced pulmonary hemorrhage (EIPH, or bleeding) may also contribute to pneumonia. The blood that ends up in the lungs is a good growth medium for bacteria, so some veterinarians will routinely treat horses that have bled with antibiotics. There is no proof that this prevents pneumonia.
  • Other systemic illnesses that may contribute to immune system suppression, for example, colic, blood parasites, and gastrointestinal parasites.
  • Long-distance transport. The physical stress of transport can suppress the immune system. Although there is not a definition of what constitutes long distance, most veterinarians consider trips greater than 6 to 8 hours to be potentially stressful. Studies have also shown that when horses' heads are tied up for even a few hours, the bacterial load in the trachea and bronchi increases. This is because the horse is not physically able to drain his airways. Also, the common practice of hanging hay bags in trailers or vans actually serves as a good vehicle for respirable particles of dust and hay – which bring bacteria with them.
  • Overcrowded barns and poor barn hygiene increase the contact between horses and subsequently the spread of disease. Poor barn hygiene causes increased levels of ammonia, which can impair the lung's ability to clear itself of bacteria.
  • Esophageal obstruction (choke) can cause horses to inhale particles of food and saliva, along with their accompanying bacteria, as they try to swallow.
  • Indwelling intravenous catheters may become infected, and the infection can spread to the lungs.
  • In some horses, pneumonia can develop into pulmonary abscesses, or pleuropneumonia. With pleuropneumonia, the infection spreads to the pleural space, which surrounds the lung within the chest cavity. Lack of response to treatment may make your veterinarian suspect that one of these complications has arisen.
  • Syringing medications through the mouth may cause the horse to inhale some of the medication along with his own saliva, thus contributing to an aspiration pneumonia.
  • Smoke inhalation
  • Prolonged cold stress, such as exposure to cold rain, snow, and wind.

    Veterinary care should include diagnostic tests and subsequent treatment recommendations


  • Your veterinarian will perform a thorough medical history and physical examination and will want to know what kind of work your horse does, what level of contact he has with other horses, how well he is vaccinated against viral diseases, if other horses in the barn have had a cough or fever, among many questions. Your veterinarian will spend time carefully listening to your horse's chest with a stethoscope (auscultation) for abnormal breathing sounds. She may also use a rebreathing bag on your horse. This is simply a small trash bag, or rectal sleeve, that is held over your horse's nose. This causes your horse to breathe in more deeply, which in turn makes it easier to hear abnormal noises.
  • In the field, your veterinarian will also be able to perform ultrasound of your horse's chest. This can help to determine if there are areas where there is so much infection that the lung has become consolidated, or if there is a visible abscess in the lung.
  • X-rays. In some cases, at least part of the lung can be X-rayed in the field.
  • Complete blood count (CBC) to look for signs of infection. An increase in the cells called neutrophils or an increase in a protein called fibrinogen can both indicate the presence of inflammation and infection.
  • A chemistry profile to assess whether other organs, such as the kidneys, are functioning properly, and whether globulins are elevated. Globulins include antibodies and other proteins that are stimulated by inflammation.
  • Transtracheal aspirate (TTA). When your veterinarian performs a TTA, he inserts a large needle into the trachea, through which he passes a very thin, sterile catheter. He then obtains a sample of the mucous and bacteria that are deep within your horse's lungs. Your vet can then look at a portion of this aspirate under the microscope, and can have a good idea about what kind of cells and bacteria are present. There will usually be inflammatory cells called neutrophils present, there may or may not be visible bacteria. Your vet will then have a laboratory culture the bacteria so that he will know exactly what bacteria are present, and to which antibiotics those bacteria are sensitive.
  • Endoscopy or bronchoscopy. In this procedure, an endoscope, which is a long, slender, fiberoptic instrument through which your veterinarian can observe your horse's trachea and the beginning of the bronchi.
  • If your horse is admitted to a hospital, he will have radiographs (X-rays) of the lungs taken. This is usually the most effective way of evaluating the extent of the pneumonia. Normal lungs are primarily made up of air, thus the X-ray beam is able to penetrate a good portion of the lung, and then go on to expose the X-ray film. The end result is that you see a very dark, or black radiograph. If your horse has pneumonia, many of the normally air-filled spaces will now be filled with inflammatory exudate and consolidation, which makes it difficult for the X-ray beam to penetrate and thus fewer of the X-rays are able to expose the X-ray film. This results in a lighter, or whiter radiograph.
  • An arterial blood gas may be done to determine if your horse is able to exchange oxygen and carbon dioxide adequately – this is, after all, the ultimate job of the lungs.


  • Antibiotic therapy is the mainstay of treatment for pneumonia. Treatment will usually be more successful if your veterinarian can base her treatment on the results of the culture and sensitivity.
  • Because the results of the culture and sensitivity will not be available for 2 to 4 days, your veterinarian will want to start preliminary treatment based on what the most common bacteria are. This is referred to as empirical treatment, or a knowledgeable guess, based on the typical bacteria found in the horse, and the local area.
  • Bacterial pneumonia in horses usually involves a streptococcal species, so empirical treatment usually targets this bacterium. Most streptococcal species will respond to penicillin, which must be administered intramuscularly or, with some formulations, intravenously.
  • Gram negative bacterial species are also often involved, your veterinarian may want to combine the penicillin with aminoglycoside drugs, such as gentomycin. Other potentiated penicillins, such as ampicillin or penicillin-type drugs, such as ceftiofur (Naxcel®) will have action against both types of bacteria. Trimethoprim-sulfa (often referred to as SMZ) can also be a successful first line of defense, although this drug has been used so extensively in the past years that many streptococcal species have developed resistance to it.
  • In cases that do not respond to treatment, or when the culture and sensitivity indicates the need, your veterinarian may need to broaden the spectrum, or coverage, of antibiotic treatment to include anaerobic species. These are bacteria that do not need oxygen in order to survive. It may seem surprising that anaerobic bacteria can survive in the lung, where oxygen levels should be high! However, when the lung is severely damaged, parts of it may die (become necrotic), in which case those areas will contain no oxygen, and will become a perfect environment for oxygen-hating bacteria to set up housekeeping.
  • The length of treatment depends on the severity of pneumonia. The minimum treatment time is usually 7 days, but antibiotics may be required for several weeks.

    Your veterinarian will usually want to re-check your horse within the next few days, depending on how sick your horse is. At this time, he will listen to your horse's lung sounds, to determine if things are improving. He will probably also take another complete blood count, to see if the body's inflammatory response system is responding to the antibiotics.

    The results of culture and sensitivity are usually available within 3 to 5 days. There is really no good way to make the results come back any faster – the laboratory is completely dependent upon how fast the bacteria grow in culture. If your horse is doing well, it frequently turns out that the empirical choice of antibiotics was correct. If your horse is not improving, or is not doing as well as you and your veterinarian might like, this is usually the time to change antibiotics.

    Your veterinarian will usually continue with a given antibiotic for at least 48 hours before concluding that it is not working.

    If your horse's pneumonia was severe enough that it required a visit or a stay at the hospital, you will probably be advised to bring your horse back for re-check x-rays at the time when you will be discontinuing antibiotics. This will enable your veterinarian to determine if there are any small vestiges of disease left.

    After your veterinarian has pronounced your horse cured, remember that he has gone through a long and physically exhausting illness. It is important to give him plenty of rest (usually at least one month of pasture rest) before bringing him back into training.

    When you do bring your horse back into training, do so gradually and slowly. It will take time to get back to the same level of fitness that he had prior to the pneumonia. Although most horses maintain their level of fitness for 4 to 6 weeks, recovery from pneumonia takes longer, so expect to spend 8 to 12 weeks getting your horse back into top condition.

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