Bacterial Pneumonia in the Horse
Dr. Melissa R. Mazan
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.