A View From Within

Trans-tracheal "wash," or aspiration, is a valuable diagnostic procedure that is often performed in order to better guide the treatment of difficult or chronic lung disorders. It can be used to aid in the diagnosis of pneumonia, and potentially allow for identification of the causative organism, thus allowing for testing that can help determine the most effective therapy.

Pneumonia, defined as an inflammation of the lungs, can be of bacterial or viral origin. When a bacterial cause is suspected, antibiotics are usually warranted. But realistically, once the presumptive diagnosis of bacterial pneumonia is made, the first choice antibiotic is an educated guess. At the time of diagnosis, there is generally no specific information available that will tell us what bacteria is involved or the individual characteristics of the causative bacteria. First-choice antibiotics are based on what bacteria are the most common pathogens in the lung, and what is known about those bacteria's antibiotic resistance factors. As bacteria continue to evolve, the development of resistance to antibiotics to which they are exposed can occur at an alarming frequency and is a serious problem with which physicians and veterinarians have to contend.

Typically if an infection fails to respond to the chosen therapy within three days, resistance factors concerning the current antibiotics should be considered. Trans-tracheal aspiration can help determine if such resistance factors are responsible for treatment failure. In cases such as this, antibiotics are often discontinued (based on the condition of the horse) for 12-24 hours prior to obtaining a trans-tracheal aspiration, as concurrent antibiotic therapy can reduce the chance of culturing the causative organism.

Pneumonia also can be caused by the migration of certain parasites (worms) through the lungs. A common intestinal "round worm" (they are almost all round, so that term is really non-specific), the Ascarid, migrates from the intestine through the liver, then through the lungs prior to returning to the intestine to become an adult and make more eggs. The parasite that causes primary lung "worm" infection is called Dictyocaulus and is spread to horses from the donkey. Trans-tracheal aspiration can aid in the diagnosis of both of these parasitisms. Trans-tracheal aspiration also can aid in the diagnosis of allergic conditions such as chronic obstructive pulmonary disease or "heaves."

Step-By-Step

The procedure generally is performed in one of two ways. The first is performed with the aid of an endoscope.

The endoscope is passed up the nostril through the larynx and into the windpipe (trachea). The horse is usually sedated and/or restrained with a twitch while this is being performed. After the endoscope is within the upper third of the trachea, a small diameter of sterile plastic tubing is inserted down the inside of the endoscope via a special channel. When the tip of the plastic tube can be seen at the end of the endoscope, a syringe containing 20-30 ml of sterile saline is attached to the other end and the saline is injected into the trachea. (The injection of sterile saline into the trachea, and therefore into the lungs, does no harm, but might make the horse cough.)

When the saline has been injected, the endoscope is advanced forward to the area where the trachea levels out at its entrance into the thoracic (chest) cavity. It is at this point where a "puddle" of the saline (mixed with any abnormal exudate such as pus or phlegm) can be seen. The material then is aspirated back through the tubing into the syringe. It is this material that often reflects what's going on within the lungs and will be evaluated as described later in this article.

Sounds easy, right? It really is relatively straightforward, but does have some potential drawbacks.

First, it requires an endoscope. The endoscope is rapidly becoming a common piece of equipment for many equine practitioners, but still costs upwards of $10,000-$15,000 for the basic set-up. The second drawback is that once the endoscope has been passed through the upper airway, it is no longer sterile. The upper airway has a population of bacteria, some of which are the same as those that can cause lower airway disease. The endoscope's passage through the upper airway can contaminate the tip of the endoscope. As a result, there is the potential to contaminate the tip of the sterile plastic tubing used to collect the sample, and therefore the sample itself.

We do perform many transtracheal aspirates using the endoscope, as it is a simple and relatively non-invasive procedure. Occasionally, however, when bacteria are cultured from the sample, it can be difficult to sort out if it is a contaminant or not--there are special characteristics of the sample that can help with that decision.

The second way of obtaining a trans-tracheal aspirate is via a needle inserted into the trachea. This sounds scary, but it is done frequently with few complications.

Generally, a small three-inch by three-inch area in the middle of the neck (in this area, the trachea can be felt easily through the skin) has the hair clipped off and is prepared with a standard surgical scrub. After scrubbing, a small amount of local anesthetic is injected under the skin (the horse is usually sedated) and a "stab" incision is made in the skin with a scalpel blade. A sterile, hollow needle then is inserted through the incision and into the trachea between the rings of the trachea and directed downward.

A sterile piece of plastic tubing is inserted into the trachea via the needle and advanced down to the level where the trachea flattens out at the thoracic inlet. When the tubing is at the proper level, 20-30 ml of sterile saline is injected and rapidly aspirated bringing back with it any abnormal exudate that was in the trachea. As mentioned above, it is this material that might reflect what is going on deeper within the lungs.

This method is preferred by many clinicians as it can reduce the chance of obtaining a contaminated sample and can be performed easily on the farm without the aid of an endoscope. There generally are few complications resulting from trans-tracheal aspirates obtained in this manner. But, it is slightly more invasive and there is the potential for localized infection at the needle site, which usually can be treated without complication.

Testing The Sample

Now that we have that precious sample, what do we do with it and how does that help your horse? The first thing is to just simply look at it grossly. If the aspirate is clear and is essentially the saline solution that was injected and aspirated, chances are good that other tests will have normal findings.

The two tests that are commonly performed on the sample are the evaluation of what types of cells are present (cytology) and determination if bacteria are present. Sometimes bacteria can be observed when the cytology is performed, especially when the special "Gram" stain is employed.

The mere presence of "free" bacteria (not within white blood cells) is not always a significant finding as they can come from contamination via the upper airway. Sometimes certain cells that are only found in the upper airway are seen on the cytology indicating such contamination. But, if bacteria are seen within the white blood cell, or more specifically neutrophils, the presence of infection is more likely. (A neutrophil is a type of white blood cell that is present when there is inflammation. The neutrophil ingests and hopefully kills the bacteria--a process called phagocytosis.)

To help in determining the presence of bacteria, part of the sample is usually cultured attempting to "grow" the causative bacteria and therefore identify it. The culture then can be tested against antibiotics so the clinician can determine the best antibiotic treatment to use.

The presence of neutrophils does not always mean infection; neutrophils are a hallmark for inflammation, but not necessarily infection. In the horse, a trans-
tracheal aspirate that contains many neutrophils in the absence of any infection (as determined by cytology and culture) is an indication of allergic airway disease. This is slightly different than other species; in people, the hallmark of allergic airway disease is a white blood cell called the eosinophil. An eosinophil is a beautiful cell that when viewed under a microscope resembles a raspberry. The presence of eosinophils in the trans-tracheal aspirate from a horse is typical of a lung parasite infection.

Just to cloud the issue, some horses with allergic airway disease can have secondary bacterial infections and will have bacteria present. As with all of these disease processes, the entire clinical picture must be taken into account utilizing history, physical examination findings, and potentially radiography and/or ultrasonography of the lungs for an accurate diagnosis of more complicated cases.

Trans-tracheal aspiration is not a procedure that needs to be performed every time your horse coughs or sneezes, but should a respiratory problem become more chronic or respond poorly to initial treatment, it may become indicated. A trans-tracheal wash is a relatively uncomplicated procedure that can provide extremely significant information regarding what's going on in your horse's lungs, and therefore help guide your veterinarian toward the most appropriate treatment.

About the Author

Michael Ball, DVM

Michael A. Ball, DVM, completed an internship in medicine and surgery and an internship in anesthesia at the University of Georgia in 1994, a residency in internal medicine, and graduate work in pharmacology at Cornell University in 1997, and was on staff at Cornell before starting Early Winter Equine Medicine & Surgery located in Ithaca, N.Y. He is also an FEI veterinarian and works internationally with the United States Equestrian Team.

Ball authored Understanding The Equine Eye, Understanding Basic Horse Care, and Understanding Equine First Aid, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.

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