4 Temperature

Higher animals (birds and mammals) are homeothermal, i.e. they maintain their internal or deep body temperature independently of that of the environment, within relatively narrow limits, by means of the central thermoregulating mechanism in the anterior hypothalamic area of the brain. Maximal temperature regulation against heat or cold is achieved by the combined action of peripheral thermoreceptors, situated in the skin and certain mucous membranes, and central thermodetectors in the anterior hypo-thalamus. The thermoregulatory mechanisms consist of neural and hormonal components. A subnormal body temperature causes a consi­derable increase in the secretion of adrenaline and of thyroxine, the latter being mediated by release of thyrotropic hormone from the anterior pitui­tary gland. An elevated body temperature has the reverse effect.

Heat production in the body results from intracellular oxidative and other processes, but is added to from the exterior by radiation, conduc­tion and convection. The liver and heart pro­duce heat constantly at a fairly uniform rate, but the muscles, when active, are the site of greatest heat production, contributing some 80% of the total. Insurance against overheating is pro­vided by the physical phenomena of radiation, conduction and convection, as well as by vaporization of water from the skin and respiratory system, and the excretion of faeces and urine.

Internal Temperature

The temperature of the body, as measured by the clinical thermometer, is not indicative of the total amount of heat being produced; it only reflects the balance (steady state) existing between heat production and heat loss. The tem­perature of the surface of the body is usually lower than that of the deeper parts. This tem­perature gradient is important in relation to heat loss.

Although rectal temperature does not in­variably indicate the body temperature in ani­mals, it is the most convenient site at which to obtain this measurement. The determination is made by means of a short blunt-bulb clinical thermometer (see Fig. 190, p. 266) which records the highest temperature reached. Its range should be from about 36°C (97°F) to 42-5°C (108°F). The temperature is taken by first shak­ing the mercury in the column of the thermo­meter down below the lowest point likely to be recorded. This is achieved by means of a wrist flicking action with the thermometer held be­tween the thumb and first two fingers. The bulb end of the thermometer should then be lubricated with soap or petroleum jelly prior to being gently inserted, with a rotary action, through the anal sphincter into the rectum. Care should be taken to ensure that the bulb of the thermometer is inserted to a constant depth in each particular species of animal and that it makes contact with the mucous membrane of the rectum; in order to obtain an accurate determination of the body temperature the instrument should be left in place for about 2 minutes. If there is any doubt about the validity of the reading obtained the temperature should be retaken. In circum­stances where it is considered unlikely that an accurate temperature reading can be obtained from the rectum, e.g. because of atony of the anal sphincter or the presence of large masses of faeces (rectal paralysis), it can be taken, in the case of female animals, in the vagina, where it is approximately 0-5°C (1°F) higher than in the rectum.

The temperature values given in Table 1 are applicable only when the animal is at rest, the environmental temperature and humidity moder­ate and the ventilation satisfactory. The smaller the species the higher the normal temperature of the body. Female, pregnant and young ani­mals have a higher normal temperature than male, non-pregnant and old animals. Some ani­mals appear to possess a protective tolerance to low environmental temperatures, consisting of a mechanism which permits their body tempera­ture to fall, thereby reducing the temperature gradient. Neonatal Iambs, when subjected to cold, have been shown to survive low body temperatures for 48-72 hours, probably by a readjustment of the hypothalamic thermoregu-lator. Young piglets affected with neonatal hypoglycaemia characteristically have subnormal body temperatures. In this case the hypothermia is probably the direct result of inadequate energy supplies.

In all healthy animals the temperature varies during the day, being at its lowest in the early morning, somewhat higher in the middle of the day, and at its peak at about 6 p.m. (up to 0-8°C or 1'5°F higher than in the morning). This di­urnal variation, which appears to have no seasonal trends and the phases of which com­plement the hours of daylight and darkness throughout the year, is most marked in healthy cattle. In certain diseases, notably tuberculosis in the horse, the temperature is higher in the morning than in the evening.

In animals under clinical observation, the temperature is usually taken twice daily (morning and evening). In a healthy animal the difference between the two readings is the daily variation. The readings are recorded on a graph chart and comprise the temperature curve. Physiological rises in temperature, varying up to 1-5°C (3°F), occur after feeding, particularly if excessive, and frequently in dairy cattle, after forced exercise, on the day of parturition (except in the bitch in which case it is usually subnormal), on exposure to very high atmospheric temperature and when the animal is excited. It must not be forgotten that the procedures associated with the clinical examination itself may cause a rise in body temperature (in sheep, nervous dogs and furbearing animals).

Strenuous or prolonged exercise raises the temperature of a healthy animal beyond normal limits, but when the animal is rested the tem­perature should quickly (within 10-20 minutes) return to normal. In a sick animal, even if the disease is non-febrile, exercise causes a greater and more rapid rise and a slower return to the original temperature (within 30-120 minutes). Diagnostic use may be made of this in examining doubtful cases of chronic pulmonary emphysema in horses.

If there is local inflammation of the rectum (proctitis), the thermometer will give a higher reading than the true temperature of the body. This is also the situation when the faeces, as a result of prolonged retention in the rectum, e.g. in paralysis of the rectum, occasionally noted in sows and mares shortly prior to parturi­tion, undergo a spontaneous rise in temperature as a result of bacterial activity (Fig. 18). In acute enteritis with diarrhoea, immediately fol­lowing normal defaecation, or the administration of a cold fluid enema, and when the anal sphinc­ter is flaccid, the thermometer indicates a lower rectal temperature than is representative of the true body temperature. A subnormal tempera­ture (hypothermia) is not uncommon in very old animals and in those that are emaciated because of malnutrition. It also occurs in young lambs and piglets under the circumstances re­ferred to earlier, and in such conditions as shock, parturient hypocalcaemia, acute ruminal impaction in cattle, mulberry heart disease, hypo-thyroidism and just prior to death in most diseases, with some notable exceptions such as tetanus.

Hyperthermia

This is the elevation of body temperature brought about by physical factors such as excessive heat absorption or production or de­ficient heat loss. Excessive heat absorption occurs during exposure to high environmental temperature. The effect of such exposure in animals is likely to be exaggerated by a con­comitant high humidity, muscular exertion, large amounts of body fat, a heavy hair coat or fleece and confinement, particularly when venti­lation is inadequate. Animals in a state of dehydration are slightly more prone to hyper­thermia because heat loss by evaporation of tissue fluids is correspondingly reduced.

The camel possesses a well-known remarkable tolerance to high environmental temperatures mainly by reason of vaporization of sweat at the skin surface rather than on the hair coat itself.

In Friesian type cattle the rectal temperature will begin to rise when the environmental tem­perature reaches 21С. Other effects that are likely to follow include reduction in food intake and in milk production, with little alteration in respiratory rate until the rectal temperature reaches 40°C, when polypnoea often occurs. Sweat gland activity increases relative to the rise in temperature. Jersey cows have been observed to have rises in rectal temperature from 38,3°C at an environmental temperature of 10°C to 39,6°C at an environmental temperature of 35°C. The concomitant respiratory rates were 20 and 90/minute.

The rectal temperature in sheep starts to rise above the normal range when the environmental temperature reaches 32°C. When the body tem­perature rises to 41°C panting respiration begins and this, together with sweating, enables the sheep to survive for hours, even if the environ­mental temperature should rise to 43°C, pro­vided the relative humidity remains below 65 %. In the pig the rectal temperature is affected when the environmental temperature reaches 30°C or so, and at a temperature of 35°C the pig cannot tolerate prolonged exposure to a relative humidity of 65 % or more. Collapse from heat stroke is likely if the rectal temperature reaches 41°C in the pig.

A room temperature of 27°C, or just above, will cause the rectal temperature of the dog to rise above normal. If the environmental tem­perature is much above this point panting type breathing will appear. In the dog thermal equi­librium becomes unstable at a rectal temperature of 41 °C, and collapse is likely if this should rise to 42,5°C.

The critical air temperature for the cat is 32°C, and collapse is likely if it is subjected to an environmental temperature of 40°C and a rela­tive humidity above 65 % for a prolonged period. The cat increases the rate of heat loss by poly­pnoea and by spreading saliva on its coat.

In the majority of circumstances hyperthermia is an undesirable state because the metabolic

rate may be increased up to 50% with a rapid depletion of liver glycogen stores and increased metabolism of endogenous protein as a source of energy. The severity of the metabolic distur­bance is indicated by the degree of hypoglycae-mia and rise in blood non-protein nitrogen which occurs. Dehydration will lead to dryness of the mouth and this, together with the respira­tory embarrassment, will cause anorexia with considerable loss of bodyweight.

The dry state of the mouth causes increased thirst. The peripheral vasodilatation which results from the rise in blood temperature, alter­ing the activity of the thermoregulating centres in the anterior hypothalamus, leads to a fall in blood pressure which itself indirectly increases the heart rate by direct action and the respiratory rate by directly influencing the respiratory centres.

When the body temperature exceeds 41С there is depression of the nervous system which, when marked, presages circulatory failure due to myocardial weakness, the pulse becoming fast and irregular, or respiratory failure preceded by laboured respirations.

Fever

An elevated temperature is one aspect of the clinical syndrome termed fever. Fever is a symp­tom-complex which causes, among other chan­ges, a disturbance of the heat regulating mechanism of the body leading to a state of positive heat balance, and which is not due solely to food, exercise or environment.

Fevers may be caused by specific or non­specific agents, the former group being most common. Specific causal agents include viruses, bacteria, fungi or protozoa. The infective pro­cess may be a localized one, in the form of an abscess or an empyema involving a body cavity, or occur in a generalized form, as in a bacteraemia or septicaemia. Non-specific agents causing fever include foreign proteins, substances which cause tissue damage, protein degradation products, necrotic tissue and damaged blood as in surgical fever.

The mechanisms associated with fever are probably better understood in the case of infec­tive bacteria than for some of the other causal agents. Bacteria are known to produce endo-toxins or exogenous pyrogens, which are capable of inducing a febrile response within one hour after being injected into an animal. This reactivity is lost following repetitive injections of the pyrogen, probably because of developing tolerance. It is considered that the pyrogenic substances do not elicit a febrile response by direct action on the thermoregulatory centres of the brain, but that they influence these centres by causing the release of endopyrogen from the neutrophil leucocytes. Other specific and non­specific agents capable of giving rise to fever are also considered to be capable of producing an exogenous pyrogen-like substance which induces the release of endopyrogen from the granulocytes.

The significance of fever in relation to the efficiency of the defence mechanisms is uncertain, although it has been suggested that antibody production is increased when the body tem­perature is elevated. Artificially elevating body temperature in animals infected with certain micro-organisms has been shown to reduce the severity of the disease.

Clinical Signs of Fever

In addition to a rise in body temperature the other clinical signs of fever are shivering (rigors), increased pulse and respiratory rates, malaise, irregular external temperature, firm faeces, con­centrated urine, etc., the most constant feature being the elevated temperature. An elevated body temperature alone (hyperthermia) is not indi­cative of fever.

Fever is usually divided into three main stages, (a) The increment or onset: although the internal temperature is rising the skin capillaries are constricted with the result that the animal feels cold and shivers (rigors), (b) The fastigium, acme or period of maximum temperature during which the body temperature remains more or less constant and shivering ceases, (c) The decrement or defervescence during which the temperature falls.

A rapid reduction of fever, with the tempera­ture returning to the normal level within a few hours, is known as a crisis (Fig. 19a) and a slow subsidence as a lysis. In both cases the fall in temperature is roughly paralleled by that of the pulse rate. This reduction in frequency, coupled with improvement in quality of the pulse, is an additional indication that the ailing animal has commenced to recover. When the fall in tem­perature is accompanied by signs indicating that death is imminent, the condition is termed col­lapse; here, although the temperature falls, the pulse rate increases and the quality of the pulse deteriorates, indicating incipient cardiac failure. Clearly there has been clinical deterioration, and the animal's life is endangered (Fig. 19b).

In a mild (low) fever the temperature is about 1 °C (2°F) above normal, in a moderately severe fever (pyrexia) l,7°-2,2°C (3°-4°F) and in severe fever (hyperpyrexia) 2,8°-3,3°C (5°-6°F) above normal.

Types of Fever (Fig. 19)

Simple fever. The temperature rises, remains high with variations of less than 1°C (2°F) for several days, and then falls as the animal re­covers or collapses prior to death. When the fever subsides within about 24-48 hours after its development it is described as transient (ephe­meral), as in bovine ephemeral fever.

Continuous fever. The temperature remains high (plateau temperature) for a longer period than in a simple fever. This form of fever is characteristic of tick-borne fever.

Remittent fever. The temperature rises and falls by more than 1°C (2°F) at short and irregu­lar intervals.

Intermittent fever. There are short attacks of fever lasting for 2-3 days, interspersed with non-febrile intervals, usually forming a regular pattern.

Recurrent fever. This takes the form of rela­tively prolonged attacks of fever with non-febrile periods of about similar duration.

Atypical fever runs an irregular course. This is by far the commonest type of fever seen in animals, and it occurs in a great variety of febrile diseases. The form of the fever takes a biphasic pattern in some diseases, e.g. canine distemper, louping ill, strangles and swine erysipelas.

The possibility of inducing a rise in body temperature was formerly of diagnostic value in certain clinically latent diseases (allergic tem­perature reactions). The value of such tests depended upon whether the parenteral introduc­tion of prepared products of bacterial origin (mallein, tuberculin) evoked a characteristic febrile and systemic response in subclinically infected animals.

Temperature of the Skin and its Appendages

The external temperature of the skin is best judged by palpation, passing the palmar surface of the hand from the ears over the horns (in cattle), neck and trunk to the extremities of the fore- and hindlimbs. Skin temperature is depen­dent partly upon the degree of dilatation of the cutaneous capillaries. The environmental tem­perature and functional activity of the heat-regulating areas in the anterior hypothalamus have a significant effect on skin temperature, by influencing the internal temperature. Normally the skin temperature shows regular gradations, the ears, muzzle, feet and root of the tail being cooler, on account of the poorer blood supply, than the neck and trunk. In disease, the external temperature may be variably irregular, and may be generally, or locally, raised or lowered.

Irregular variation of skin temperature occurs in hyperpyrexia, severe illness generally, cardiac insufficiency, collapse, etc. In these conditions, the extremities of the body (ears, feet, horns, vulva) are either cold or abnormally warm. A generalized rise in the temperature of the skin occurs during exertion, after unaccustomed ex­posure to sunlight (heatstroke in pigs), or high environmental temperature, also in the early stages of hyperpyrexia. The temperature over the whole body surface is lowered shortly before death, in extreme emaciation, and following severe haemorrhage, and other forms of vascular shock. A local rise in temperature occurs in the vicinity of localized inflammation of the skin and superficial tissues. A local fall in skin tem­perature occurs where there is a local deficiency in the blood supply (ischaemia). This occurs where there is localized death of tissue (necrosis, gangrene), and also where there is interference with the arterial blood supply on account of a thrombus, or embolus, in the related artery (iliac thrombosis, mesenteric arteritis), or local­ized spasm of the vessel (ergotism).