Chapter 3 Infrared Thermography in Zoo and Wild Animals
Infrared (IR) thermography is a noninvasive diagnostic screening tool that does not require handling or restraint of an animal. Physiologic or pathologic processes involving changes in surface temperature may be evaluated using this technique. This modern method provides real-time, instantaneous visual images with measurements of surface temperatures over a greater distance.
The first medical application of “thermography” was by Hippocrates (ca. 460-375 bc), who used thin layers of mud for his temperature measurements, similar to modern thermography. An area of great heat emission caused an area of the mud to dry first, and thus a “hot spot” was detected.29 It was not until the mid–eighteenth century, however, that temperature scales were developed by Fahrenheit, Réaumur, and Celsius, and not until 1800 that Sir William Herschel discovered infrared rays distinguishable from visible light. The first detector was constructed in 1830.6
Infrared thermography has been used for skin temperature measurement in human medicine since 1960 and for the early detection of diseases since 1980, mainly pathologic processes such as pain in the lumbosacral region, intervertebral disc prolapse, spinal cord lesion, traumatic lesions, fractures, neuropathology, cardiovascular diseases (especially impairment of blood supply), lateral effects of heat or frost burns, and long-term monitoring of skin transplants. In wildlife biology, IR thermography has been used since the mid-1940s for detecting and monitoring mammal and bird species. To some degree the method could even be used successfully in animal censuses. In veterinary medicine this technique has been used on farm and companion animals since the late 1950s.9 The most advanced field is that of equine medicine.24,28,29 Eulenberger and Kämpfer3 first recommended the use of IR thermography in zoo and wild animal medicine.
Phillips15 performed the first large-scale comparative studies on thermoregulation in zoo animals with the aid of infrared thermography. Both studies employed traditional, carbon dioxide (CO2)–cooled systems, which proved to be difficult to use under routine zoo and wildlife conditions. Hilsberg9 first used IR thermography extensively with modern equipment in zoo medicine.
Infrared thermography makes use of the physical characteristic of bodies or materials to emit electromagnetic waves, and with the aid of a special detector, these rays are visible. Therefore, surface temperatures are measured over a greater distance.6
The advantages of IR thermography compared with other imaging techniques (e.g., ultrasonography, radiography, magnetic resonance imaging, endoscopy) are as follows:
As with other techniques, however, IR thermography presents specific challenges in zoo and wildlife medicine that are not encountered as often in human medicine and classic veterinary medicine. For example, detailed knowledge of the morphology of many different species is required; no control exists over the animal under investigation (e.g., movement, position relative to the sun, muddy or wet surface parts, positioning of animal for best investigation); and no specific examination room in a veterinary clinic with controlled environmental parameters (e.g., temperature) is available.
Using an IR camera or scanner, the heat emitted by every material or object may be detected and made visible through conversion into temperature-associated shades of gray. The warmer areas are colored white or light gray, and the cooler areas are darker gray or black. The system may also use several scales of false-color coding. This means that an image is created in which each temperature is assigned a specific color on a reference scale; the best scale for veterinary diagnostics is the rainbow color scale. The image created can be interpreted and used for diagnostic purposes in medical fields.
The IR camera works similar to a digital video camera, except the lenses possess specific attributes. Because glass hinders the transmission of heat waves, other materials are used as semiconductors, such as germanium-zinc, lead-selenium, or cadmium-mercury-telluride. Each specific mixture of half-metals measures a defined wavelength within the IR spectrum. Each of these wavelength windows possesses specific properties, but also disadvantages, so the industry has tried to optimize the materials used for the required purposes. Gaussorgues6 provides detailed information on the physics behind these systems, with a shorter, more veterinary-oriented version by Hilsberg.9 Before obtaining a system, the clinician must consider the lens specification.
An IR system should be certified by the regional authorities. Only such systems guarantee that the measured temperatures are accurate and that it is legal to use the system; specific regulations exist because of the military use of this technology. Recently, increasing numbers of systems are appearing on the market that are remakes or copies of earlier units. These systems, however, may not be certified and thus may yield false temperature readings. The potential thermographer should consult with engineers or local experts before acquiring such equipment. All the studies described by Hilsberg9 have used the IR systems by the companies AGEMA and later FLIR Systems. With the enormous technical developments achieved in the last decade, this technique should be used throughout veterinary medicine, especially in zoo and wild animal medicine, as an aid in primary diagnostics.
The images captured by the IR detector may be saved and stored on a hard disc or other storage media and viewed and evaluated later on the computer with specialized software. Each false color or gray point in the image is still associated with the originally measured temperature, so the settings of each image may be optimized for evaluation on the computer.
When using this technique, it is important that investigators are aware of the influences on their readings. The animal should be acclimatized to the environment, preferably for 2 hours before thermal imaging. Furthermore, the animal should be clean, dry, and free of dirt; otherwise, artifacts may be created, which may require interpretation. Under certain circumstances it is more advantageous to have the animal wet and not acclimatized, as explained later under the species-specific investigation techniques.
ANIMALS AND ENVIRONMENT
Thermography is best used on animals, or parts of them, without long hair, such as elephants, rhinoceroses, hippopotami, giraffes, zebras/horses, and many larger antelopes. In longer-haired animals such as carnivores, camels with winter coats, and mountain animals, the interpretation of results is more difficult. In these cases the procedure is better done by an experienced thermographer, unless only joints, feet, or parts of the head are evaluated, although even these may create problems. The thermographer must be familiar with the normal skin surface, internal anatomy, and morphology of the animal under investigation. Regional hair length is an important factor for interpretation, as well as the location of blood vessels and the innervation of skin areas under investigation.
SOURCES OF ARTIFACTS
Clipped hair may increase temperature readings. Alcoholic ointments or other surface heat–producing materials also create artifacts in the form of increased heat emission. On the other hand, cold water, dirt, or mud may create an altered heat emission that shows lower temperatures, at least when first applied. Later, this foreign material emits the heat according to its composition. Additionally, uneven pelage creates uneven heat transmission. Strong physical activity of the animal will create local heat production at first, but heat emission from the whole-animal surface may occur later, depending on the type of animal and the type and duration of the activity.
High ambient temperature poses difficulties when looking for smaller temperature differences. Under high ambient temperatures the difference between the animal core and surface temperature decreases. This makes the use of IR thermography more challenging in field investigations than in zoo settings. A good way to address this problem is using the technique in a stable or, for wildlife at night, near a waterhole. The sun itself also creates significant artifacts, and therefore cloudy days are preferred. However, clouds still allow a certain quantity of infrared emission. The effect of the sun is especially visible in giraffes and zebras. In zebras the author found specific skin pattern–related heat radiation when the animals were in their stables at night.1
A brief introduction to these investigations is provided in the later discussion on thermoregulation. Again, the best place for an investigation of a zoo animal is the stable, or the investigations should take place on a cloudy day, after sunset, or before sunrise, if absolute temperatures are required. Otherwise, the investigator should try to lure the animal into a shady part of the enclosure. An experienced thermographer can cope with many artifacts or will do a follow-up investigation a few hours or days later. Artifacts may also result from sources of heat in the housing environment of zoo animals, such as heaters on walls, floor heating, or even heating from ceilings. If not accounted for, these sources may lead to gross misinterpretations, as in pregnancy diagnosis.
When starting to use IR thermography, as just discussed, the best time and place to investigate an animal is the animal’s stable early in the morning. This animal is most likely acclimatized, dry, dirt free, and not stressed or physically exhausted. The investigator should look for signs of scratching on the skin. If the stable has floor heating, the animal must be allowed to stand for at least 1 to 2 hours to prevent false readings from that heat source. If the animal is dirty, hosing it down with medium-temperature to cool water may help. The thermographer can then follow up on the process of warming the skin to look for hot areas. This method is sometimes the best way of investigating elephants and hippopotami.
GENERAL FIELDS OF USE
Thermoregulation: the Basics for Medical Thermography
Before veterinarians can make good use of IR thermography in zoo and wildlife medicine, they must become familiar with the thermoregulatory patterns of each species. This is important because each species presents specific challenges for thermography: color patterns; hair length; thickness of the dermis; location of glands; size of ears, horns, or antlers; location of potential thermal windows on the body itself; and the anatomy of the legs. Thermal windows are areas of increased heat emission; some are facultative and some obligatory (see later discussion).
Because of the lack of hair, elephants (and most rhino species) display a relatively even surface temperature under normal conditions, with only the ears, horns, or tusks showing lesser heat radiation than the body and legs. Mammals with short hair and thin legs (e.g., giraffes, antelopes, zebras) display cooler legs than bodies under normal thermoregulatory conditions and in the shade. Animals with thick hair may display little radiation through the body surface, which may make the use of IR thermography almost impossible. However, some uses may still be possible, such as the diagnosis of inflammatory processes on the legs. The inside of mammalian legs shows a slightly greater heat radiation than the outside because of the more superficial location of blood vessels. When doing close-up views of the ears in both African and Asian elephants, the blood vessels may be located easily. Apart from the blood vessels, ears should display no other source of higher radiation, except at the opening of the ear canal. As an example, normal thermoregulation in elephants is judged by viewing cooler ears than body temperature, as well as measuring the overall average body and leg surface temperatures, which should be relatively constant within a limit of about 1° to 2°C.
The only exceptions from this uniform surface temperature are the obligatory or facultative thermal windows. In mammals the eyes are always obligatory thermal windows, as are the mouth, heart region, and the rectal and vaginal openings, as well as the penis during urination or erection. These are areas where function permits no insulation, or where an opening in the body is connected with the body core. Facultative thermal windows are much more difficult to judge because they may or may not be active, depending on the ambient conditions and the thermoregulatory needs of each animal. These are species specific and may also show individual variations.
Therefore, it is advisable to study many individuals over time before judging pathologic processes. When this is not possible, the investigator should make use of other individuals of the same species in the same environment, or if time permits, investigate the same individual on different occasions under similar conditions. This last approach yields the most accurate investigation technique for an individual. This is the technique used in equine preventive medicine or in racecourse training management, especially in Great Britain. Experienced trainers and veterinarians are able to identify potentially lame animals up to 2 weeks before the animal actually shows clinical signs.13,22,27
General indicators of altered thermoregulation can be physiologic or pathologic, as follows:
As animals without a notable amount of hair on the body, elephants display relatively even heat radiation over their entire body surface when in a thermoneutral zone. Only the ears show less heat radiation than the body, whereas the eyes, mouth, and anus are thermal windows (Figure 3-1). Any other source of heat should be investigated. A thermogram of an elephant feeding on branches may show the “hot” mouth, the hot distal trunk, and the warm tips of the front feet. Thermograms of an African (Loxondonta africana) and an Asian (Elephas maximus) elephant may show heat radiation with specific reference to their ears. In both animals the larger blood vessels are localized.
Intense sunshine creates high temperature readings on the body and outer surface of the ears, especially in African elephants. The underside of the large ears usually remains cool. Ear flapping results in increased convection and saves the ears from collecting further heat. More than 30% of excess heat may be radiated off the ears in African elephants.16 In a group of Asian elephants in a newly built indoor enclosure, the keepers noted that the elephants did not display normal activity patterns but seemed somewhat lethargic. IR thermography revealed an altered thermoregulation, with ears that were the same high temperature as the body. This was noted in all members of the group. The ambient high humidity of 95% was reduced, and the animals were given more frequent access to cool water. Overheating poses a great stress and health risk to captive elephants, especially Asian elephants, and may even cause death during immobilization, if the thermoregulatory influence of a new enclosure is not evaluated; in this case the health of the animals improved.9 Uhlemann25,26 provides similar examples of recent investigations into thermoregulatory behavior of zoo animals involving insight into environmental heat stress caused by enclosure design.
Elephants may also display increased radiation from parts or whole ears caused by psychologic stress. When this occurs, at least some animals in the herd display normal ear radiation and serve as comparisons.9