Mucous Membranes

Mucous Membranes

As you push on a horse's gums, you are pushing aside blood in that location; the membranes should "pink up" again as blood refills within one to two seconds. This is referred to as the capillary refill time (CRT).

Photo: Alexandra Beckstett, The Horse Managing Editor

When evaluating the overall health of a horse, on first impression we focus on the obvious--the bloom of the hair coat, the horse's attitude, his posture and stance. Beneath this outer appearance, we can find another useful clue about the health of the internal workings of a horse: The mucous membranes.

Mucous membranes are tissues that line body cavities, and they are visible wherever skin interfaces with a body opening, such as the gums, the vulva, the prepuce and penis, inside the nostrils, and the conjunctival sac of the eye. These membranes secrete viscous mucus that keeps them moist and protected, and they are also well-supplied with blood to give us useful information about the circulatory status of the horse.

Mucous Membrane Color

Mucous membranes give  an impression about the efficiency of the heart as a pump and the capacity of vessels to carry the blood circulation to the periphery. When an adequate supply of oxygenated blood is carried to all areas of the body, the color of the gums should be similar to the pink hue that you see beneath your fingernails. Any departure from this pink might mirror an internal change.

A pale color indicates a reduction of red blood cells and/or oxygen in the peripheral circulation. This could result from blood loss, dehydration, anemia (a deficiency of red blood cells), or hemoglobin related to blood loss (acute or persistent), or chronic bacterial, or parasitic infection. Chronic blood loss occurs with leakage from the gastrointestinal tract, such as with bleeding gastric ulcers or with excretion through an inflamed urinary tract. Tissue necrosis related to cancer of internal organs also consumes red blood cells.

A yellow tinge to the mucous membranes reflects jaundice, which might be transient due to diminished appetite and food intake or to a diet rich in alfalfa, but it could be more lasting and serious as a result of liver disease. (Changes in shades of pink are discussed later in this article.)

When assessing mucous membrane color, keep in mind that different lighting might lead to inaccurate interpretation, particularly when using fluorescent lights, a flashlight, or in dim or dusk ambient light conditions. Dane Frazier, DVM, is a practicing equine veterinarian in Missouri and an active representative and veterinary judge of endurance sports at the highest levels in the FEI (Fédération Equestre Internationale). Combining his sport horse experience with 30 years as a clinical practitioner, he has gained insights into the context of using mucous membranes as a reflector of internal health.

The Clinical Picture

Although mucous membranes are telling of the ability of the heart to pump blood and the dispersal of oxygenated blood to the periphery, it is not the only physical parameter used to evaluate the health of a horse. In regard to an exercising horse, Dane Frazier, DVM, an FEI veterinary judge of endurance sports, recommends gathering a full clinical "picture" of a horse's physical status to use in conjunction with evaluation of the mucous membranes.

"No one parameter should ever be used to evaluate the metabolic status of a horse, whether it be the fitness of the horse to accommodate the stress under which it performs or its status during a crisis or illness," says Frazier.

For an exercising horse, he continues, "If only one parameter could be evaluated, it would be the heart rate. Evaluation of the heart rate and its recovery following exercise encompass more physiological parameters that any other single measurement."

Heart rate is also a prognostic parameter useful to assess a horse with colic. Elevations in heart rate occur because of pain, dehydration, endotoxemia, and shock. The higher the heart rate, the worse the prognosis. Frank Andrews, DVM, MS, Dipl. ACVIM, clinical professor at the University of Tennesee, notes that in a colicky horse, there is a 90% survival rate if the heart rate remains below 60 beats per minute (bpm). At heart rates between 60-80 bpm, a horse has 50% likelihood of survival; and at 80-100 bpm, only a 25% chance of survival. A heart rate exceeding 100 bpm is correlated with only a 10% survival rate.

The temperature of a horse's limbs, ears, and muzzle also reflect peripheral blood perfusion; cold extremities and ears and cold, clammy skin are signs that corroborate cardiovascular shock. 

Frazier says other important evaluations of an exercising horse include auscultation (with a stethoscope) of the abdomen in all quadrants to appreciate quality/frequency of gut sounds, palpation of muscle tone in the croup and hamstrings, and assessment of anal tone.

Jugular filling gives a relative impression of blood pressure, as does the quality of the peripheral pulses of other large blood vessels, such as those monitored on the face or along the lower limbs. Frazier remarks on the value of one's overall impression and experience: "A more subjective, but equally important, factor is the horse's demeanor. When you look in its eye, is anybody home? Does the horse want to eat and drink? Does the horse wag its head side to side when asked to trot? Does the horse ‘snirl' its nostrils and/or have a dull, glazed expression?"

Most notably, Frazier observes, "After you have seen 10,000 of these variations from normal, the abnormal gets easier to recognize, but not always easier to describe and explain."

Nancy Loving, DVM

Regardless of activity, whether a horse is a resident on the farm, used in casual recreation, or participates in extreme exercise stress, Frazier notes, "Pre-existing conditions may confound mucous membrane evaluations. Anemic horses (as related to any number of medical problems) will have pale mucous membranes, as will a horse that is dehydrated. A horse that refuses to drink from unfamiliar water sources or due to illness becomes dehydrated."

He points out another issue that makes evaluation difficult: "Some horses object to anyone looking in their mouths and poking around on their gums, making it difficult to accurately assess the character of the mucous membranes."

Capillary Refill Time

As you push on a horse's gums, you are pushing aside blood in that location; the membranes should "pink up" again as blood refills within one to two seconds. This is referred to as the capillary refill time (CRT). What is being measured is the amount of circulating blood volume and the blood pressure. Refill time varies depending on the amount of pressure exerted by one's pressed fingertip.

Frazier comments, "Mucous membrane color evaluated with the CRT is a fundamental hydration factor evaluation that directly correlates to perfusion of other body tissues, especially the bowel."

This has significant consequences to an exercising horse or to any horse suffering from gastrointestinal disease or colic.

In addition, when assessing the mucous membranes, the amount of saliva present on the gums is telling of the degree of hydration in the whole body. A dry, tacky mouth often indicates dehydration.

Reflection of Systemic Health

Whether a horse experiences dehydration as he is participating in an athletic event or transport, or he is suffering from gastrointestinal disease or illness, mucous membranes serve as a monitor of the cardiovascular status of the horse. Frazier's passion for endurance riding has involved him in overseeing the welfare of horses at premier endurance events. In this context, he uses mucous membranes as one parameter in evaluating how well an athletic horse is coping with metabolic stress.

"Performance horses, especially horses that work over extended time and distance, such as the endurance horse, not only contend with fluid shifts to demand tissues (such as heart and muscle for locomotion and brain and kidney for normal function), but they also must contend with fluid loss as sweat to maintain thermoregulation," says Frazier.

He elaborates on what one might see with systemic changes as reflected in the mucous membranes: "The first aberration of the color of the mucous membranes as a horse dehydrates and/or responds to fluid shifts is a reddening of the mucous membranes above the tooth root with a corresponding paling of the rest of the tissue." This is first seen on physical exam as a disparate color along the area where teeth meet gums and appears as a purplish or bluish tinge along the line of the teeth, with the membrane above appearing pale pink or normal pink.

Frazier explains the progression of circulatory compromise: "If these processes advance without compensatory adjustment, the mucous membranes will continue to pale, taking on a bluish tinge, and finally turn a purple, muddy color. Commonly, changes in mucous membrane color (paling) are accompanied by delay in the CRT of greater than one to two seconds. If dehydration and fluid shifts sufficiently compromise the perfusion of the cecum and/or large bowel, endotoxemia may affect the capillary beds and result in a ‘brick red' color to the mucous membranes."

Septic or endotoxic shock causes the peripheral vascular beds to open, with increased filling of blood in the capillaries and small vessels. As circulatory shock progresses, less oxygen is sent to the tissues, so the blood stagnates, loses its oxygen, and the membranes turn purplish or blue-tinged, then eventually turn to a muddy gray.

The Colicky Horse

A horse suffering from systemic illness, such as gastrointestinal disease or colic, is at risk of deterioration of the circulatory system that is reflected by the character of the mucous membranes. Frank Andrews, DVM, MS, Dipl. ACVIM, a clinical professor at the University of Tennessee, has extensive experience with complicated colic cases referred to the Veterinary Teaching Hospital.

"If a colicky horse has pink or cyanotic (blue-tinged) mucous membranes, studies indicate that he has a greater likelihood of survival (55%), whereas a horse with toxic (gray or pale with dark red line at gum/teeth interface) or injected (dark red to ‘muddy') mucous membrane color has a less likely chance of survival (44%)," notes Andrews. "This is not much of a statistical difference, but is more useful following surgery, when membrane color is better correlated with survival probability: If the color is pink after surgery, that horse will likely live."

In determining the prognosis for survival in colic cases, Andrews reports that as a metabolic parameter, CRT has better positive correlation. Rough estimates of survival suggest that with CRT less than two seconds, there is a 90% survival rate; if CRT ranges between 2½ to four seconds, there is a 53% survival rate; while if CRT is greater than four seconds, there is only a 12% survival rate.

Other Disease Reflections

Because mucous membranes are sensitive and informational, they alert us to other disease syndromes that are apparent   if we take the time to look. Blisters, vesicles, or ulcers form subsequent to inflammation or due to a systemic disease process. Examples of pathologic conditions that cause blistering of the mucous membranes of the mouth, in particular, include vesicular stomatitis virus (VS) or blister beetle toxicity. While VS stimulates formation of blisters or ulcers within the tissues of the mouth, nose, and bordering skin, ingestion of even a portion of a blister beetle hidden in hay creates inflammation and necrosis along the entire internal mucosal lining into the bowel and is not isolated only to the oral or external tissues. VS also can elicit vesicles on the mammary gland, prepuce, or penis.

Mucous membranes possess some absorptive capacity that might be useful in administering medications, yet this characteristic can be harmful if membranes come in contact with toxic substances.

Non-steroidal anti-inflammatory drug toxicosis (as for example with phenylbutazone, flunixin meglumine, or ketoprofen) can create oral ulcers concurrent with the development of gastric ulcers.

Take caution when applying caustic or irritant substances such as leg paints and blisters or when painting a fence with chew-stop products; a horse might  have an adverse reaction that could be difficult to interpret without diligent sleuthing. Ingestion of toxic alkaloid-containing plants also can elicit mouth blisters.

Vasculitis describes an illness that is associated with leakage of blood vessels. Collection of red blood cells in the surrounding tissues is often visible as tiny red spots on the mucous membranes. Such blood spots (petechiations) develop from a vasculitis event such as occurs with equine viral arteritis (EVA) or equine rhinopneumonitis virus, or due to purpura hemorrhagica, which is an allergic response subsequent to previous infection with Streptococcus equi bacteria.

Typically, a systemically sick horse demonstrates other abnormal physical signs associated with many of these illnesses, such as an elevated temperature and poor appetite related to pain with eating. Slobbering and excess salivation often accompany mouth pain or irritation. There might be localized swelling, edema, or hives related to some systemic illnesses.

Foxtails and weed seeds can embed in the gum margins and cause an abnormal appearance to the mucous membranes, sometimes accompanied by an unwillingness to eat.

When inflammatory conditions involve the eye's mucous membranes, the horse will be troubled by associated swelling, engorgement of blood vessels, and other signs of ocular inflammation, such as tearing and pain.

Cancer, most commonly squamous cell carcinoma, is a disease process that can develop in mucous membranes. The most common locations of this cancer are the ocular membranes (such as the sclera or the third eyelid), the penis or prepuce of a male horse, or on the vulva of a mare. Squamous cell carcinoma can be quite invasive over time, but topical cancer is often managed well with cryosurgery to freeze affected cancerous tissue.

Mucous membranes lining the stomach and other internal organs are also prone to squamous cell carcinoma. Because this internal form of cancer is more insidious and lurking, it is not readily identified until a horse shows clinical signs of major systemic effects, often beyond the scope of treatment options.

Take-Home Message

Learning how to read the mucous membranes of your horse might help you save his life some day. Mucous membranes are a window into the internal status of the cardiovascular system, and a window that horse owners should know how to look into and interpret.

About the Author

Nancy S. Loving, DVM

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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