Fever and survival in the rat. The effect of enhancing fever

Fever and sickness behavior during an opportunistic infection in a free-living antelope, the greater kudu (Tragelaphus strepsiceros)

To study their thermal responses to climatic stress, we implanted seven greater kudu ( Tragelaphus strepsiceros) with intra-abdominal, brain, carotid, and subcutaneous temperature data loggers, as well as an activity logger. Each animal was also equipped with a collar holding a miniature black globe thermometer, which we used to assess thermoregulatory behavior. The kudu ranged freely within succulent thicket vegetation of the Eastern Cape Province, South Africa. The kudu spontaneously developed a bacterial pneumonia and consequent fever that lasted between 6 and 10 days. The fever was characterized by a significant increase in mean 24-h abdominal temperature from 38.9 ± 0.2°C to 40.2 ± 0.4°C (means ± SD, t6 = 11.01, P < 0.0001), although the amplitude of body temperature rhythm remained unchanged ( t6 = 1.18, P = 0.28). Six of the kudu chose warmer microclimates during the fever than when afebrile ( P < 0.0001). Despite the selection of a warmer environment, on the first day of fever, the abdominal-subcutaneous temperature difference was significantly higher than on afebrile days ( t5 = 3.06, P = 0.028), indicating vasoconstriction. Some kudu displayed increased frequency of selective brain cooling during the fever, which would have inhibited evaporative heat loss and increased febrile body temperatures, without increasing the metabolic maintenance costs of high body temperatures. Average daily activity during the fever decreased to 60% of afebrile activity ( t6 = 3.46, P = 0.014). We therefore have recorded quantitative evidence for autonomic and behavioral fever, as well as sickness behavior, in the form of decreased activity, in a free-living ungulate species.

Adjustable set point: to honor Harold T. Hammel

The value of a regulated variable in the absence of external perturbation stabilizes at the set point of the system. This set point is an information input that may be determined by an external signal to which the regulated variable is compared or may be determined by the structural characteristics of the system itself. In the case of temperature regulation the actual internal temperature is compared with the set point "wanted" by the organism. The activating signal for the regulatory responses, the "error signal," is the difference between the actual temperature and the set point. When an error signal is detected, the organism produces the available corrective responses. Yet, the notion of thermoregulatory set point has been challenged recently. Such a questioning entails that both fever and anapyrexia are useless concepts. This minireview examines the available arguments and data and concludes that to abandon the concepts of set point, fever, and anapyrexia is premature, at best.

Fever: causes and consequences

The present review distinguishes pathogenic, neurogenic, and psychogenic fever, but focuses largely on pathogenic fever, the hallmark of infectious disease. The data presented show that a complex cascade of events underlies pathogenic fever, which in broad outline - and with frank disregard of contradictory data - can be described as follows. An invading microorganism releases endotoxin that stimulates macrophages to synthesize a variety of pyrogenic compounds called cytokines. Carried in blood, these cytokines reach the perivascular spaces of the organum vasculosum laminae terminalis (OVLT) and other regions near the brain where they promote the synthesis and release of prostaglandin (PGE2). This prostaglandin then penetrates the blood-brain barrier to evoke the autonomic and behavioral responses characteristic of fever. But then once expressed, fever does not continue unchecked; endogenous antipyretics likely act on the septum to limit the rise in body temperature. The present review also examines fever-resistance in neonates, the blunting of fever in the aged, and the behaviorally induced rise in body temperature following infection in ectotherms. And finally it takes up the question of whether fever enhances immune responsiveness, and through such enhancement contributes to host survival.

The behavior of sick animals

Dogs and cats that are ill with febrile diseases usually are depressed and anorexic. When viewed from the standpoint of the wild ancestors, it is apparent that the behavior of sick animals is adaptive and not necessarily a reflection of debilitation. A wild animal that is acutely ill with an infectious disease often is at a life-or-death juncture, and its behavioral mode may be seen as putting the animal's resources into facilitating the fever response in combating the pathogens.

Increased biogenic amine release in mouse hypothalamus following immunological challenge: antagonism by indomethacin

Stimulation of the acute-phase response in mice by lipopolysaccharide, pokeweed mitogen, concanavalin A or interleukin-1 was associated with increased release of biogenic amines, serotonin and norepinephrine in the hypothalamus as indexed by their primary metabolites, 5-hydroxyindoleacetic acid and 3-methoxy-4-hydroxyphenylglycol, respectively. The increases in norepinephrine and serotonin turnover observed 4 h following systemic administration of interleukin-1 were antagonized by concurrent administration of indomethacin, a potent inhibitor of cyclooxygenase. These data suggest that the increase in norepinephrine and serotonin release in mouse hypothalamus during the acute-phase response to infection is partially mediated by the actions of arachidonic acid metabolites.

Biological basis of the behavior of sick animals

The most commonly recognized behavioral patterns of animals and people at the onset of febrile infectious diseases are lethargy, depression, anorexia, and reduction in grooming. Findings from recent lines of research are reviewed to formulate the perspective that the behavior of sick animals and people is not a maladaptive response or the effect of debilitation, but rather an organized, evolved behavioral strategy to facilitate the role of fever in combating viral and bacterial infections. The sick individual is viewed as being at a life or death juncture and its behavior is an all-out effort to overcome the disease.

The effect of venous blood stream cooling on survival of bacterially infected rabbits

The effect of physical cooling on the mortality rate of rabbits infected with Pasteurella multocida was investigated. Rabbits were cooled for 48 hours after bacterial injection by passing cold fluid through small hollow metal cuffs which had been surgically implanted around the abdominal vena cavae of rabbits. The average body temperatures of the rabbits during the 24-hour period after the intravenous injection of live Pasteurella multocida was 40.92 +/- 0.20 degrees C in control rabbits and 38.98 +/- 0.71 degrees C in cooled rabbits. 90% of physically cooled rabbits survived compared with 46% of control rabbits 48 hours after bacterial injection, suggesting that thermoregulatory effector mechanisms involved in cold defense may enhance survival.

The effect of a fever-like response on the secondary antibody response of the rat

Since fever appears to modulate the primary humoral immune response, a fever-like response was induced in 18 rats by cooling their preoptic areas during the first five days after re-immunization with sheep erythrocytes. The titre of antibodies in these rats was the same as that in 17 control animals, indicating that the febrile response does not influence the magnitude of the secondary antibody response. It is suggested that only those fevers evolving in the early phase of a primary, natural infection may modulate the magnitude of the humoral immune response to the pathogen.

The ventral septal area: electrophysiological evidence for putative arginine vasopressin projections onto thermoresponsive neurons

The present experiments were conducted to identify thermoresponsive neurons in the ventral septal area and to characterize such units with respect to their connectivity to potential sources of arginine vasopressin in this area (the paraventricular nucleus and the bed nucleus of the stria terminalis) and to other brain regions (fornix and amygdala). Single unit in vivo microelectrode techniques were used to classify warm responsive, cold responsive, dynamic, biphasic and phasically active thermoresponsive neurons in the ventral septal area which altered their spontaneous activity in response to thermal stimulation of the scrotal skin. The fornix provided a large number of primarily inhibitory afferents to ventral septal thermoresponsive neurons while the amygdala projection consisted of approximately equal excitatory and inhibitory inputs. Electrical stimulation of the paraventricular nucleus resulted in orthodromic inhibition in 9 of 12 thermoresponsive units while projections from the bed nucleus of the stria terminalis appeared to preferentially inhibit warm responsive neurons in this area. These findings implicate the ventral septal area in thermoregulatory pathways by identifying neurons in this area responsive to peripheral thermal stimulation. Further, evidence in support of arginine vasopressin acting in this area to influence thermoregulation is provided by the identification of the paraventricular nucleus and bed nucleus of the stria terminalis as possible sources of afferents to the ventral septal area, and the characterization of the influence of this afferent input on thermoresponsive neurons in this region.

The effect of intraseptally applied vasopressin on thermoregulation in the rat

The thermoregulatory effects of intraseptal injection of arginine vasopressin were studied in eight rats in which a thermode and a bilateral cannula had been chronically implanted into the preoptic area and lateral septa, respectively. Intraseptal injection of vasopressin completely suppressed the increase in heat production and body temperature elicited by cooling the preoptic area, but did not appear to affect vasomotor tone. Vasopressin also inhibited heat production in a cold environment, and thus induced a marked drop in core temperature; skin temperature did not, however, fall as much as core temperature suggesting that some vasodilatation occurred. At an ambient temperature in the upper range of thermoneutrality vasopressin had no effect on the thermoregulatory variables studied. It is concluded that vasopressin does not reduce the normal set point temperature and that its main effect is to inhibit thermoregulatory heat production. This effect may explain its antipyretic action.

The effect of hypothalamic temperature on the immune response in the rat

To clarify the role of febrile temperatures on the immune system, rats were immunized with sheep erythrocytes and their core temperature was then changed by continuously cooling or heating the preoptic area for five days. Core temperatures of up to 2 degrees C above or below normal were associated with a high titre of antibodies against sheep erythrocytes, whereas larger displacements of core temperature, as well as normal temperature, were associated with a low titre. These results are at variance with the idea that the production of antibodies is proportional to body temperature. It is suggested that the immunostimulation elicited by heating and cooling the preoptic area, and by inference that the immunostimulation associated with fever, could be due to factors other than the change in body temperature.

Endogenous pyrogen activity in human plasma after exercise

Plasma obtained from human subjects after exercise and injected intraperitoneally into rats elevated rat rectal temperature and depressed plasma iron and zinc concentrations. The pyrogenic component was heat-denaturable and had an apparent molecular weight of 14,000 daltons. Human mononuclear leukocytes obtained after exercise and incubated in vitro released a factor into the medium that also elevated body temperature in rats and reduced trace metal concentrations. These results suggest that endogenous pyrogen, a protein mediator of fever and trace metal metabolism during infection, is released during exercise.

Vasopressin: a homeostatic effector in the febrile process

This review compares the physiological changes which accompany infection and fever with the effects of the peptide, arginine vasopressin (AVP). AVP may act as a neuromodulator, a releasing factor, or a hormone to induce responses which are opposite to those homeostatic changes accompanying fever. Since AVP is released into blood and brain during fever, it is hypothesized that AVP contributes to the maintenance of homeostasis in the infected organism.

The effect of whole body heat exposure and of cooling the hypothalamus on antibody titre in the rat

In rats implanted with chronic hypothalamic thermodes and immunized with sheep erythrocytes, body temperature was increased, 4 h per day for 2 weeks, either by exposing the animals to external heat or by cooling the preoptic area. The titre of antibodies against sheep erythrocytes was nearly tripled by preoptic cooling but was drastically decreased by heat exposure. These opposing effects of active and passive increases in body temperature indicate that factors other than the change in body temperature must also have played a significant role in modifying the humoral immune response.