Understanding thermal burns in reptile patients (Proceedings)

August 1, 2008

Thermal burns in reptiles are one of the most common injuries seen by herp veterinarians.

Thermal burns in reptiles are one of the most common injuries seen by herp veterinarians. The exact reason why reptiles seem so prone to burns is not understood, but, something about their behavior makes them more susceptible to this type of injury than any other captive animal.

Since reptiles do everything slowly, it is not uncommon for an animal to get burned, but not actually show signs of the injury for several days. This is especially true for minor, or first degree burns. This is significant, since burns, even apparently mild injuries, can have severe consequences if not treated properly. In order to be able to treat burns properly, it is important to understand what causes burns, and how to recognize them in their early stages.

Thermodynamics

As herpetologists, we are all familiar with the importance of providing proper temperatures to the cage environment. Over the years we have seen the evolution of heating devices from the original "hot rocks" to the more advanced, thermostatically controlled environmental chambers.

We have learned that not only must captive reptiles have supplemental heat, but, supplemental heat provided in the proper fashion. For instance, a fifteen foot long python would not fare well with a single, twelve inch hot rock. Likewise, a nocturnal lizard would suffer if its cage were heated with a bright heat lamp.

A look at animals in their natural environment will help us understand the principles of thermodynamics from a practical perspective. As an example, let's evaluate the heating strategies of the ever popular green iguana living in the rain forest.

These animals live for the sun. On an initial glance, it appears that they derive their energy-providing heat from basking in the sunlight. But, on closer inspection, there is a lot more involved than an animal merely perched atop a branch soaking in the sun's rays.

Before we analyze what is happening, let's take a step back and review some of the principles of heat and heat transfer. The study of heat and its properties is called thermodynamics.

In order for an object to get warm, or "heat up," there must be a transfer of heat from some outside source to the object that is being heated. Heat always moves from a warmer area to a cooler area. As the heat leaves the first object and enters the second object, the first object becomes cooler, and the second object becomes warmer. Eventually, the temperatures of the two objects will become equal. In other words, they will equilibrate. Heat will never continue to leave the first object such that it becomes cooler, resulting in the new object becoming the hotter of the two.

There are three ways that an object can gain heat, or become warmed. These are via conduction, convection and radiant heat.

Conduction is the transfer of heat within an object (such as down a long metal pole) or between two objects that are touching each other. A classic example of conduction is a pan on a stove. The burner on the stove heats up. A cold pan is then placed on the hot burner, and the heat then transfers from the hot burner directly to the cooler pan, thereby heating up the pan and the contents inside.

A herpetological analogy here is the use of a "hot rock." Hot rocks, for those not familiar with these items, are a solid, block-like structure, usually made out of brick, concrete, plaster or heavy molded plastic. Imbedded within the rock is some sort of heating coil. When the heating coil is plugged in it generates heat. This heat, in turn, heats up the rock.

If a lizard (or any reptile) crawls up on the rock, the heat from the hot rock will then transfer, via conduction, to the lizard. The path of the heat transfers from the surface of the hot rock, through the feet and belly and tail of the lizard, or whatever parts of the animal are in DIRECT contact with the rock's surface.

Convection, on the other hand, involves the motion of large-scale quantities of matter. In plain words, convection usually involves the movement of either gasses (such as air) or liquids (such as water). Heat is transferred via movement of this matter (e.g. air or water).

For instance, consider the air over a desert. As the surface of the desert heats up from the sun, it warms the air that touches it (this is conduction). This warm air, which is lighter than cooler air, then rises. As it rises, it pushes (displaces) the air above and on the sides away, forcing the cool air back down to the earth. This cool air warms, and then follows the path of the warm air before it, upwards (convection). Hence, you get a warming effect, or a thermal, developing. Birds, hang-gliders and airplanes use these "thermals" to soar high in the sky without using hardly any energy to stay aloft.

Back to our iguana example, in the jungle, where it is very hot, the warm air that blows across the animal while it is basking, exposed, on the end of a branch, is an example of convective heating.

Radiant heat is the last type of heat transfer. Radiation heat transfer involves the flow of thermal energy by electromagnetic waves. In contrast to conductive heating, where objects must be touching, or convective heating, where the matter (gas or water) must touch the object to be heated, radiant heat does not have to have any matter involved (no touching needed for heat transfer).

The classic example here is the fast-food hamburger under the red heat lamp. The lamp does not need to touch the burger to keep it warm, and likewise, there does not need to be a wind current or water flow over the lamp and the burger in order to keep the food hot.

The obvious example here is the iguana basking in the sunlight. It is soaking up the electromagnetic radiation produced by the sun's rays. In captivity, this source of electromagnetic radiation is replaced by any number of artificial means - usually a heat lamp or a ceramic bulb.

So, in review, this apparently simple equation of an iguana sitting on a branch to get warm, really is a lot more complicated than it looks. The iguana in the wild gets its warmth three ways:

1. Radiant heat - The lizard absorbs the electromagnetic radiation from the sun.

2. Convective heat - The warm air that blows across the animal.

3. Conductive heat - from sitting on a branch that has been warmed by the sun. The branch then acts as a conductive heat source, passing the warmth back into the animal resting on it.

The best type of heat source for a captive pet will depend on the type of animal being housed. As mentioned, a small heat rock would be inappropriate for a large snake. These animals do better with convective or radiant heat. Likewise, a big heat lamp would not be proper for an arboreal animal which would normally get its heat from either conduction or convection.

There are several other variables that play important roles in heat transfer and warming. A big factor is humidity. Moist air, or high humidity, tends to hold heat much better than dry heat. When figuring temperatures and thermal gradients careful attention should be paid to humidity. A hot, humid cage will be much more stifling than a hot dry cage.

Ventilation is very important in any cage design. A well designed cage will have good ventilation. Stagnant air, especially in hot humid cages, leads to build up of pathogens, or disease-causing bacteria and fungi. Proper ventilation will dilute out any potential problem before it reaches concentrations that may be dangerous.

However, for all the good that proper ventilation does, it does have its drawbacks. A well ventilated cage tends to lose heat and humidity (it gets exhausted outside). There is really nothing wrong with this, except that the heat and humidity needs to be replaced, and, in the overall scheme of design, it ends up costing more to maintain a steady state of temperature and humidity.

Heat and Tissue Injury

Of the three types of heat transfer, the two that cause the most injuries are radiant heat and conductive heat. Over the years, I have seen several hundred burns caused by hot rocks. These crude heating devices have variable heat output, often unevenly distributed over the surface of the rock, and can reach temperatures that will sear the flesh off any animal that rests on it.

In addition, I have personally seen many hot rocks that have "shorted out.". In one case where a hot rock shorted out, it caught fire, setting the owner's bedroom ablaze.

Humans have a withdrawal reflex. When we touch something hot, without any cognizant thought, we automatically, immediately, withdraw our hand. Even young children and mammals display this reflex. I have touched some brand new hot rocks, and their surface has been so hot, that I have experienced the same withdrawal reflex. They are so hot, I could not physically keep my hand on their surface.

Why then, when a reptile rests on such a "hot" hot rock, don't they also immediately jump off?

Nobody seems to have an easy answer for that. It is not uncommon for a snake to wrap its coils around a bare light bulb because it is attracted to the warmth that the light emits. So, it must feel the warmth, why then, does it not feel the burning heat?

One answer is that the nerve receptors that sense heat and the receptors that sense pain are different. It is possible that, since in the wild, such pain receptors have no evolutionary significance (reptiles do not come into contact with intensely hot objects in the wild). Therefore, evolutionarily, there is no reason that a reptile should have a hot-pain withdrawal reflex.

Other theories put forth suggest that since reptiles do not reason in the same fashion that people do, or other mammals for that matter, even though they may feel pain, they do not associate it with the object that they are touching. Hence, they do not realize that they need to move in order for the pain to subside.

Bottom line is, nobody really knows. So, until we understand why these animals are so prone to burns, the best thing to do is make every effort to prevent the burns in the first place.

Types of Burns

Burns are classified by the type of burn and by the severity of the injury or extent of the body surface area affected. There are three basic classifications of burns in mammals. These categories can also be used in reptiles. Understanding the nature of the burn will help you assess the need for intervention. Is it something that you can handle at home, or, should it be hospitalized for competent medical care?

The extent and severity of a burn is related to several factors. Obviously the temperature of the heat source plays a significant role. Touching a stove burner set on "warm" will result in a far less severe burn than touching the same burner set on "high."

Duration of contact also affects the severity of the burn wound. Again, touching a stove burner set on "warm" only briefly may result in a minor burn. Whereas, holding one's hand on the same burner for several minutes may result in much more severe tissue damage. This is perhaps why we see such intense damage in reptiles that have fallen asleep on what seems like only a mildly hot heating element like a hot rock.

Lastly, the heat conductance characteristics of the material touched also plays a role in the severity of the burn. For instance, touching a hot piece of metal would cause a more severe burn than touching a piece of wood at the same temperature.

Types of burns include thermal, electrical, chemical and radiation. Thermal burns include all the categories that we have discussed so far, such as burns by hot rocks and heat lamps. An electrical burn, although not common in reptiles, can be seen where there is direct contact with an electrical current, such as when there is a short in a wire that has electrical arcing, or when an animal bites through an electrical cord that is plugged into a live socket.

Chemical burns are caused by strong acids or alkalis, such as cleaning supplies like pure bleach (an alkali agent). These are also uncommon in reptile patients, but may occur, especially when chemicals are spilled or agents are not thoroughly rinsed after cleaning a cage.

Radiation burns in reptiles are extremely uncommon. These are usually related to the use of radiation therapy when treating certain types of cancers. Since radiation therapy has been used in reptile patients, this type of burn is possible.

In older terminology, burns used to be classified as first, second or third degree, depending on the severity of the damage. In more recent classification, the terms partial thickness and full thickness burns, are more commonly used. "Thickness" refers to the outer layer of skin.

First degree burns are superficial, or partial thickness injuries that only involve the epidermis (outer skin). These burns are painful. In mammals there may be damage to the hair or fur (singing). The skin is reddened, and in severe first degree burns there may be blisters (such as in a severe sun burn).

In reptiles, you rarely see blisters, although they may occur, and occasionally, you may see singing of the scales, depending on the type of exposure. Usually, you will see reddening of the skin, and often what looks likes "bruises" under the scales, especially in white, pale or clear scales.

These burns usually heal well and rarely leave a scar. In an otherwise healthy reptile, healing takes about one month and a good shed for the burn to completely resolve.

Second degree burns are deeper partial thickness injuries with usually full destruction of the epidermis, and variable damage to the underlying dermis (inner, deep layer). In mammals, although there is more damage to the dermis, the fur or hair may not necessarily be damaged. The burns are very painful, and extensive subcutaneous swelling does occur. In reptiles, blistering and oozing of serum from the burn site are seen. There is extensive bruising and discoloration of the tissue. These injuries lead to the formation of a scab-like covering over the burn.

Healing occurs from the margins of the wound inward. In these wounds, especially in burns that cover a large surface area, healing may be prolonged and significant scarring may occur.

In third degree burns, the entire thickness of the skin is destroyed. The burn is actually painless (all the pain sensing nerves are destroyed), and the tissue takes on either a whitish or charcoal black appearance. In mammals the hair and fur fall out or are destroyed. Third degree burns are four times as serious as second degree burns of similar size.

Healing of third degree or full thickness burns occurs by contraction of the wound (shrinking) and epithelialization (re-growth and migration of new skin from the margins of the wound toward the center). In some cases of third degree burns, skin grafting may be necessary. These burns may take many months (4 - 6) to heal completely. Severe scarring is a hallmark of third degree burns.

The classification of a fourth degree burn is occasionally used to describe a third degree burn that not only involves the full thickness of the skin, but also the underlying tissue such as muscle and bone. These injuries carry a grave prognosis.

Treatment of Burns

Reptiles are amazing animals. They have a penchant for healing far greater than any mammal. Some of the wounds that I have seen in reptiles would have spelled doom for most other animals. Incredibly, I have also seen scars on wild reptiles that bore the legacy of previous severe wounds. Wounds that healed without the benefit of veterinary intervention. That does not mean that we should ignore wounds in captivity, that they will heal without help, but, it does give us hope when we see a bad wound, knowing that with proper care and time, it should heal fine.

Minor burns will often do well with first aid. However, severe burns will require medical attention and possible hospitalization in order to provide pain relief, infection control and treatment for shock.

In general, first degree burns, unless affecting extensive portions of the body, can be treated at home. If the burn is recent, apply cold water rinses or cold compresses (not ice!) for no more than twenty minutes. This helps reduce swelling and pain to the affected area. Applying ice to the tissue can cause frost bite, actually causing freeze damage to the tissue.

If blisters are present, they should NOT be broken. Doing so damages down the body's natural barrier against infection. Let the blisters either resolve or break on their own.

Infection is a common, and potentially serious complication of burns. When the skin is compromised, as with broken blisters, always pay careful attention to contamination, keeping the affected area as clean as possible. A non-irritating antibacterial soap, such as Nolvasan, or even a gentle hand soap, such as Ivory or Dove, will work well.

If there is damaged skin, then application of a topical burn dressing, after gentle wound cleansing, is appropriate. The burn should then be dressed, or covered, with a sterile non-stick bandage. This can be tricky, especially when the burn is on the underside of a snake.

In animals where there are extensive burns, I recommend keeping them in a glass or Plexiglas enclosure without substrate. Even though this is not a natural way to house a reptile, these cages are easy to clean and disinfect, thus minimizing the risk of infection. Their benefits outweigh the disadvantages of the temporary housing.

Second degree burns, because of the extensive tissue damage, need veterinary attention. If for no other reason, pain is a hallmark of second degree burns, and the veterinarian can provide medications for pain relief.

If the animal is in shock, therapy must be initiated to counter the effects. Wounds must be cleaned and debrided. Topical burn creams, such as Silvadene, are applied, and the patient is started on antibiotics to prevent infection at the burn site. Injectable cephazolin or oral cephalexin (20 mg/kg/day) are my first drugs of choice.

Fluids, either intravenous, oral or otherwise, must be administered to counter the effects of shock and fluid loss from the burn. Daily cleansing of the wound and sterile bandage changes are needed to effect rapid healing. These may take several weeks to months to fully heal.

Third degree, or full thickness burns may require intensive care at the outset. For human patients, there are entire burn centers dedicated to the care of burn victims.

Depending on the extent of the burn, the owner of the reptile should be appraised of the potentially grave prognosis. Treatment involves all the care involved for the second degree burn, plus more. As the tissue starts to heal, the pain can be intense. The pain from the daily cleanings and debridements can also intensify as the patient recovers feeling to the burn area.

Antibiotic therapy may be protracted, often lasting for several months. Wound care can get expensive, considering that the bandages may need to be changed daily for many months. As a veterinarian, it is important to consider all the costs of return visits, bandage materials, follow-up laboratory sammpling and other miscellaneous costs when quoting estimates to the clients. Even in the face of such odds, many people do elect to treat their pets that are suffering from extensive burns.

Even some of the fourth degree burns, where not only the skin is destroyed, but also the underlying tissue, has potential to heal if cared for properly. A blue tongue skind that I treated had a full thickness burn through its abdominal wall which left a completefistula into the coelomic cavity. This animal was taken to surgery after it was stabilized and treated for shock. The dead (burned) tissue was removed, and the healthy tissue was attached to the healthy tissue on the opposite side of the burn. There was so much tissue damage that after the dead part was removed, the patient looked like an hour-glass when the sides were pulled together.

After extensive wound care, bandage changes and antibiotic therapy, the patient recovered, nearly six months later. Other than the tremendous scar, the animal was back to normal.

Although not discussed, many patients exposed to heat may not actually be burned, but, may suffer from smoke or other toxic fume inhalation. Many melted glues and plastics produce noxious fumes, as well as smoke that can damage delicate lung tissue. Although initially these patients may appear to be uninjured, they may develop a fatal fluid build-up within their lungs, and could potentially die if not treated properly in a timely fashion.

Summary

Understanding the reason why reptiles are so prone to burn injury remains a mystery. But, understanding the causes of burns and the mechanics of tissue injury will help us prevent such occurrences, and in the unfortunate event that they do happen, better manage their care.

The most important take home message from this article is this - when dealing with burns in reptile patients, be diligent, be clean and, above all, be patient!