Restraint increases afebrile body temperature but attenuates fever in Pekin ducks (Anas platyrhynchos)

Characteristics of Functional Hyperthermia Detected in an Outpatient Clinic for Fever of Unknown Origin

Background: Functional hyperthermia (FH) is characterized by hyperthermia resulting from sympathetic hyperactivity rather than inflammation, and it is frequently overlooked by medical practitioners due to the absence of abnormalities in a medical examination. Although FH is an important differential diagnosis for fever of unknown origin (FUO), the literature on FUO cases in Japan lacks information on FH. In this study, we aimed to uncover the population of FH patients hidden in FUO cases. Methods: An outpatient clinic for FUO was established at Okayama University Hospital, and 132 patients were examined during the period from May 2019 to February 2022. Results: A diagnosis of FH was made in 31.1% of the FUO cases, and FH predominantly affected individuals in their third and fourth decades of life with a higher incidence in females (68.3%). The frequency of a history of psychiatric illness was higher in patients with FH than in patients with other febrile illnesses. Although the C-reactive protein (CRP) is generally negative in FH cases, some obese patients, with a body mass index ≥ 25 had slightly elevated levels of CRP but were diagnosed with FH. Conclusions: The results showed the importance of identifying FH when encountering patients with FUO without any organic etiology.

Pekin ducks are motivated to lay in their preferred nest substrate

Nest design is one factor contributing to floor-laying in farmed poultry. We investigated: (i) if ducks (Anas platyrhynchos) prefer a particular nest substrate; and (ii) how important that preference is to them, indicated by stress-induced hyperthermia, egg albumen corticosterone, and behaviour. Twelve female ducks that were trained in a push-door task had temperature data loggers implanted. Preference testing identified the most and least preferred nest substrates between sawdust, astroturf, and hemp fibres. A behavioural demand test then required the ducks to use push-doors to access nests containing either the most or least preferred substrate. The preferred substrate door was loaded with increasing weight (0-120% of bodyweight, four nights per workload) and eventually blocked to prevent nest access. The least preferred substrate door remained unweighted. The overall rank order of substrate preferences was sawdust > hemp > astroturf. Six of the 12 birds pushed all workloads and attempted to push the blocked door. The area under the curve (AUC) of hyperthermia was larger when the preferred substrate door was blocked compared with 0%. The AUC did not differ between nights 2-4 of the blocked door compared with night 1. Egg albumen corticosterone was unaffected. We conclude that laying Pekin ducks prefer manipulatable nest substrates and accessing one is important enough to pay a cost. The results indicate that a manipulatable substrate should be provided to commercially farmed nesting ducks.

Eye Region Surface Temperature and Corticosterone Response to Acute Stress in a High-Arctic Seabird, the Little Auk

Measuring changes in surface body temperature (specifically in eye-region) in vertebrates using infrared thermography is increasingly applied for detection of the stress reaction. Here we investigated the relationship between the eye-region temperature (TEYE; measured with infrared thermography), the corticosterone level in blood (CORT; stress indicator in birds), and some covariates (ambient temperature, humidity, and sex/body size) in a High-Arctic seabird, the Little Auk Alle alle. The birds responded to the capture-restrain protocol (blood sampling at the moment of capturing, and after 30 min of restrain) by a significant TEYE and CORT increase. However, the strength of the TEYE and CORT response to acute stress were not correlated. It confirms the results of a recent study on other species and all together indicates that infrared thermography is a useful, non-invasive measure of hypothalamic-pituitary-adrenal (HPA) axis reactivity under acute activation, but it might not be a suitable proxy for natural variation of circulating glucocorticoid levels.

Pekin ducks are motivated to access their nest site and exhibit a stress-induced hyperthermia when unable to do so

The origins of floor-laying in ducks could be low motivation for a nest, or stress related to difficulties with accessing a nest (e.g. competition). Using a behavioural demand test, we investigated if increasing the work required to access their nest impacted ducks' behaviour and two indicators of stress: egg corticosterone concentration and elevation of core body temperature (stress-induced hyperthermia (SIH)). Twelve laying Pekin ducks previously trained in an operant push-door task were required to use a push-door to access their nest. The door was loaded with increasing weight (0-160% of individual BW, four nights per workload) and eventually blocked to prevent nest access. Before testing, temperature data loggers were implanted in the abdomen. Eggs were collected daily to measure corticosterone concentrations. Behaviour towards the push-door was quantified. Three birds were excluded from the experiment at an early stage. Five of the nine remaining birds pushed all workloads up to 160% BW and attempted to pass the blocked door, with another two birds pushing up to 80 and 140% BW. For those that pushed at all workloads (n = 5) the area under the curve (AUC) of hyperthermia was larger at workloads of 80% (P < 0.001), 120% (P < 0.01), 140% (P < 0.001), 160% (P < 0.001), and when the door was blocked (P < 0.001), compared with 0%. On the first night when the door was blocked, all five birds pushed more at the door, but no attempts were made to push on the following 3 nights, yet the AUC of hyperthermia did not differ between nights 2-4 of the blocked door, compared with the first night that the door was blocked. Increasing workload and inability to access the nest had no effect on corticosterone in egg albumen. It was concluded that laying Pekin ducks were motivated to access a nest. Although it was not possible to differentiate metabolic from psychogenic stress on the first night that nest access was denied, we suggest that the occurrence of hyperthermia on the subsequent nights was due to SIH resulting from frustration at their inability to use their preferred nest. Floor-laying therefore is unlikely due solely to low nest-seeking motivation. Egg corticosterone was not a relevant indicator of acute stress. Strategies to improve nest availability (e.g. decreasing competition) could improve the welfare of commercial ducks.

Ambient temperature effects on stress-induced hyperthermia in Svalbard ptarmigan

Stress-induced hyperthermia (SIH) is commonly observed during handling in homeotherms. However, in birds, handling in cold environments typically elicits hypothermia. It is unclear whether this indicates that SIH is differently regulated in this taxon or if it is due to size, because body temperature changes during handling in low temperatures have only been measured in small birds <0.03 kg (that are more likely to suffer high heat loss when handled). We have therefore studied thermal responses to handling stress in the intermediate-sized (0.5-1.0 kg) Svalbard ptarmigan (Lagopus muta hyperborea) in 0°C and -20°C, in winter and spring. Handling caused elevated core body temperature and peripheral vasoconstriction that reduced back skin temperature. Core temperature increased less, and back skin temperature decreased more, in -20°C than in 0°C, probably because of higher heat-loss rate at the lower temperature. Responses were qualitatively consistent between seasons, despite higher body condition/insulation in winter and dramatic seasonal changes in photoperiod, both of which could possibly affect stress responsiveness. Our study supports the notion that SIH is a general thermoregulatory reaction to acute stressors in endotherms, but also suggests that body size and thermal environment should be taken into account when evaluating this response in birds.

Stress-induced hyperthermia and hypothermia

Stress affects core body temperature (T_c). Many kinds of stress induce transient, monophasic hyperthermia, which diminishes gradually if the stressor is terminated. Stronger stressors produce a longer-lasting effect. Repeated/chronic stress induces anticipatory hyperthermia, reduces diurnal changes in T_c, or slightly increases T_c throughout the day. Animals that are exposed to chronic stress or a cold environment exhibit an enhanced hyperthermic response to a novel stress. These changes persist for several days after cessation of stress exposure. In contrast, long-lasting inescapable stress sometimes induces hypothermia. In healthy humans, psychologic stress induces slight increases in T_c, which are within the normal range of T_c or just above it. Some individuals, however, develop extremely high T_c (up to 41°C) when they are exposed to emotional events or show persistent low-grade high T_c (37-38°C) during or after chronic stress situations. In addition to the nature of the stressor itself, such stress-induced thermal responses are modulated by sex, age, ambient temperature, cage mates, past stressful experiences and cold exposure, and coping. Stress-induced hyperthermia is driven by mechanisms distinct from infectious fever, which requires inflammatory mediators. However, both stress and infection activate the dorsomedial hypothalamus-rostral medullary raphe region-sympathetic nerve axis to increase T_c.

Fish can show emotional fever: stress-induced hyperthermia in zebrafish

Whether fishes are sentient beings remains an unresolved and controversial question. Among characteristics thought to reflect a low level of sentience in fishes is an inability to show stress-induced hyperthermia (SIH), a transient rise in body temperature shown in response to a variety of stressors. This is a real fever response, so is often referred to as ‘emotional fever’. It has been suggested that the capacity for emotional fever evolved only in amniotes (mammals, birds and reptiles), in association with the evolution of consciousness in these groups. According to this view, lack of emotional fever in fishes reflects a lack of consciousness. We report here on a study in which six zebrafish groups with access to a temperature gradient were either left as undisturbed controls or subjected to a short period of confinement. The results were striking: compared to controls, stressed zebrafish spent significantly more time at higher temperatures, achieving an estimated rise in body temperature of about 2–4°C. Thus, zebrafish clearly have the capacity to show emotional fever. While the link between emotion and consciousness is still debated, this finding removes a key argument for lack of consciousness in fishes.

Thermal Imaging to Study Stress Non-invasively in Unrestrained Birds

Stress, a central concept in biology, describes a suite of emergency responses to challenges. Among other responses, stress leads to a change in blood flow that results in a net influx of blood to key organs and an increase in core temperature. This stress-induced hyperthermia is used to assess stress. However, measuring core temperature is invasive. As blood flow is redirected to the core, the periphery of the body can cool. This paper describes a protocol where peripheral body temperature is measured non-invasively in wild blue tits (Cyanistes caeruleus) using infrared thermography. In the field we created a set-up bringing the birds to an ideal position in front of the camera by using a baited box. The camera takes a short thermal video recording of the undisturbed bird before applying a mild stressor (closing the box and therefore capturing the bird), and the bird’s response to being trapped is recorded. The bare skin of the eye-region is the warmest area in the image. This allows an automated extraction of the maximum eye-region temperature from each image frame, followed by further steps of manual data filtering removing the most common sources of errors (motion blur, blinking). This protocol provides a time series of eye-region temperature with a fine temporal resolution that allows us to study the dynamics of the stress response non-invasively. Further work needs to demonstrate the usefulness of the method to assess stress, for instance to investigate whether eye-region temperature response is proportional to the strength of the stressor. If this can be confirmed, it will provide a valuable alternative method of stress assessment in animals and will be useful to a wide range of researchers from ecologists, conservation biologists, physiologists to animal welfare researchers.

Psychogenic fever: how psychological stress affects body temperature in the clinical population

Psychogenic fever is a stress-related, psychosomatic disease especially seen in young women. Some patients develop extremely high core body temperature (Tc) (up to 41°C) when they are exposed to emotional events, whereas others show persistent low-grade high Tc (37-38°C) during situations of chronic stress. The mechanism for psychogenic fever is not yet fully understood. However, clinical case reports demonstrate that psychogenic fever is not attenuated by antipyretic drugs, but by psychotropic drugs that display anxiolytic and sedative properties, or by resolving patients' difficulties via natural means or psychotherapy. Animal studies have demonstrated that psychological stress increases Tc via mechanisms distinct from infectious fever (which requires proinflammatory mediators) and that the sympathetic nervous system, particularly β3-adrenoceptor-mediated non-shivering thermogenesis in brown adipose tissue, plays an important role in the development of psychological stress-induced hyperthermia. Acute psychological stress induces a transient, monophasic increase in Tc. In contrast, repeated stress induces anticipatory hyperthermia, reduces diurnal changes in Tc, or slightly increases Tc throughout the day. Chronically stressed animals also display an enhanced hyperthermic response to a novel stress, while past fearful experiences induce conditioned hyperthermia to the fear context. The high Tc that psychogenic fever patients develop may be a complex of these diverse kinds of hyperthermic responses.

Prior chronic stress induces persistent polyI:C-induced allodynia and depressive-like behavior in rats: Possible involvement of glucocorticoids and microglia

When animals suffer from viral infections, they develop a set of symptoms known as the "sickness response." Recent studies suggest that psychological stress can modulate the sickness response. However, it remains uncertain whether acute and chronic psychosocial stresses have the same effect on viral infection-induced sickness responses. To address this question, we compared changes in polyI:C-induced sickness responses, such as fever, change of body weight and food intake, mechanical allodynia, and depressive-like behavior, in rats that had been pre-exposed to single and repeated social defeat stresses. Intraperitoneal injection of polyI:C induced a maximal fever of 38.0°C 3h after injection. Rats exposed to prior social defeat stress exhibited blunted febrile responses, which were more pronounced in the repeated stress group. Furthermore, only the repeated stress group showed late-onset and prolonged mechanical allodynia lasting until 8days after injection in the von Frey test and prolonged immobility time in the forced swim test 9days post-injection. To assess the role of glucocorticoids and microglia in the delayed and persistent development of these sickness responses in rats exposed to repeated stress, we investigated the effect of pretreatment with RU486, a glucocorticoid receptor antagonist, and minocycline, an inhibitor of microglial activation, on polyI:C-induced allodynia and depressive-like behavior. Pretreatment with either drug inhibited both the delayed allodynia and depressive-like behavior. The present study demonstrates that repeated, but not single, social defeat stress followed by systemic polyI:C administration induced prolonged allodynia and depressive-like behavior in rats. Our results show that even though a single-event psychosocial stress does not have any effect by itself, animals may develop persistent allodynia and depressive-like behavior when they suffer from an infectious disease if they are pre-exposed to repeated or chronic psychosocial stress. Furthermore, this study suggests that stress-induced corticosterone and microglial activation play a pivotal role in this phenomenon.

Surface temperature changes in response to handling in domestic chickens

Stress-induced hyperthermia (SIH) occurs in numerous species and is characterised by an increase in core body temperature, and a decrease in surface temperature, of between 0.5 and 1.5°C within 10 to 15 min of the onset of "emotional stress". The aim of the current study was to ascertain whether the husbandry-relevant procedure of handling resulted in measurable changes in surface body temperature in chickens, as measured using infrared thermography. Baseline temperatures for 19 domestic hens were compared to temperatures immediately, and up to 20 min following handling (catching and brief restraint by a human). Surface head, eye and comb temperatures were plotted to investigate the pattern of temperature change. In response to handling, comb temperature decreased significantly, showing a rapid 2°C drop. Eye temperature showed an initial decrease then rose to levels significantly higher than handling. Head temperature increased over the 20 min post-handling period, to reach levels significantly higher than baseline. It can be concluded that surface temperature changes assessed using infrared thermography, in particular of the hen's comb, are sensitive to husbandry procedures such as handling and represent a potentially useful method for assessing stress-induced hyperthermia in chickens.

Differential effects of early- and late-life access to carotenoids on adult immune function and ornamentation in mallard ducks (Anas platyrhynchos)

Environmental conditions early in life can affect an organism’s phenotype at adulthood, which may be tuned to perform optimally in conditions that mimic those experienced during development (Environmental Matching hypothesis), or may be generally superior when conditions during development were of higher quality (Silver Spoon hypothesis). Here, we tested these hypotheses by examining how diet during development interacted with diet during adulthood to affect adult sexually selected ornamentation and immune function in male mallard ducks (Anas platyrhynchos). Mallards have yellow, carotenoid-pigmented beaks that are used in mate choice, and the degree of beak coloration has been linked to adult immune function. Using a 2×2 factorial experimental design, we reared mallards on diets containing either low or high levels of carotenoids (nutrients that cannot be synthesized de novo) throughout the period of growth, and then provided adults with one of these two diets while simultaneously quantifying beak coloration and response to a variety of immune challenges. We found that both developmental and adult carotenoid supplementation increased circulating carotenoid levels during dietary treatment, but that birds that received low-carotenoid diets during development maintained relatively higher circulating carotenoid levels during an adult immune challenge. Individuals that received low levels of carotenoids during development had larger phytohemagglutinin (PHA)-induced cutaneous immune responses at adulthood; however, dietary treatment during development and adulthood did not affect antibody response to a novel antigen, nitric oxide production, natural antibody levels, hemolytic capacity of the plasma, or beak coloration. However, beak coloration prior to immune challenges positively predicted PHA response, and strong PHA responses were correlated with losses in carotenoid-pigmented coloration. In sum, we did not find consistent support for either the Environmental Matching or Silver Spoon hypotheses. We then describe a new hypothesis that should be tested in future studies examining developmental plasticity.

The development of endotoxin tolerance, and the role of hypothalamo-pituitary-adrenal function and glucocorticoids in Pekin ducks

Endotoxin tolerance represents a state of abated immunological responsiveness to pyrogens, which, in mammals, leads to the decline or abolition of the fever response. The development of endotoxin tolerance in birds is not well understood; consequently, the impact of repeated pathogenic exposure on the avian febrile response, and thus on the ability of birds to fight recurrent infection, is not known. We determined the effect of repeated injections of lipopolysaccharide (LPS) on the febrile response of Pekin ducks. We gave ducks five injections of LPS, spaced 1, 4 or 10 days apart, and recorded their core body temperature with abdominally implanted temperature data loggers. Once we established that Pekin ducks developed endotoxin tolerance, we investigated the effect of repeated injections of LPS on the central and peripheral segments of the hypothalamo-pituitary-adrenal (HPA) axis in an attempt to elucidate the role of glucocorticoids in the modulation of the febrile response during the tolerant period. When our ducks became tolerant to LPS, they had significantly higher basal levels of plasma corticosterone (CORT, the principal glucocorticoid in birds), and their HPA response to treatment with LPS was blunted. We propose that the augmented levels of basal plasma CORT resulted from sensitized HPA function, and this, in turn, contributed to the development of endotoxin tolerance. Regulation of the circulating level of CORT might be a possible target for the re-establishment of appropriate immune responses in birds.

Brain IL-6- and PG-dependent actions of IL-1β and lipopolysaccharide in avian fever

There is no persuasive evidence of a correlation between proinflammatory cytokines and avian fever. In this study, for the first time, we use avian cytokines to investigate a role for proinflammatory cytokines in the central component of avian fever. IL-1β and IL-6 injected intracerebroventricularly into Pekin ducks ( n = 8) initiated robust fevers of equal magnitude and duration, although there was a significant difference in the latency to a febrile response. In addition, the IL-1β-induced fever could be abolished with an intracerebroventricular injection of antibodies to avian IL-6 or an oral administration of a PG synthesis inhibitor. Our findings indicate the following sequence of events within the central component of the avian febrile mechanism: IL-1β gives rise to bioactive IL-6, which stimulates an accelerated synthesis of PGs, and these PGs then adjust the sensitivity of warm-sensitive neurons in the avian brain stem to mediate fever. Yet PGE2 was not upregulated in the cerebrospinal fluid of ducks made febrile with LPS. We conclude that IL-1β and IL-6 may well mediate fever by instigating an accelerated synthesis of brain-derived PG, of a class other than PGE2, or that IL-6 serves as one of the terminal mediators of the avian febrile response.

Is there an energetic-based trade-off between thermoregulation and the acute phase response in zebra finches?

There has been recent interest in understanding trade-offs between immune function and other fitness-related traits. At proximate levels, such trade-offs are presumed to result from the differential allocation of limited energy resources. Whether the costs of immunity are sufficient to necessitate such energy reallocation remains unclear. We tested the metabolic and behavioural response of male zebra finches (Taeniopygia guttata) to the combined effects of thermoregulation and generation of an acute phase response (APR). The APR is the first line of defence against pathogens, and is considered energetically costly. We predicted that at cold temperatures zebra finches would exhibit an attenuated APR when compared with individuals at thermoneutrality. We challenged individuals with bacterial lipopolysaccharide (LPS), an immunogenic compound that stimulates an APR. Following LPS injection, we measured changes in food intake, body mass, activity, and resting and total energy expenditure. When challenged with LPS under ad libitum food, individuals at both temperatures decreased food intake and activity, resulting in similar mass loss. In contrast to predicted energetic trade-offs, cold-exposed individuals injected with LPS increased their nocturnal resting energy expenditure more than did individuals held at thermoneutrality, yet paradoxically lost less mass overnight. Although responding to LPS was energetically costly, resulting in a 10% increase in resting expenditure and 16% increase in total expenditure, there were few obvious energetic trade-offs. Our data support recent suggestions that the energetic cost of an immune response may not be the primary mechanism driving trade-offs between immune system function and other fitness-related traits.