A review of the physiology of fever in birds

Behavioral responses during sickness in amphibians and reptiles: Concepts, experimental design, and implications for field studies

In ectothermic vertebrates, behavioral fever, where an individual actively seeks warmer areas, seems to be a primary response to pathogens. This is considered a broad and evolutionarily conserved response among vertebrates. Recent population declines in amphibians are associated with an increase of infectious disease driven largely by climate change, habitat degradation, and pollution. Immediate action through research is required to better understand and inform conservation efforts. The literature available, does not provide unifying concepts that can guide adequate experimental protocols and interpretation of data, especially when studying animals in the field. The aim of this review is to promote common understanding of terminology and facilitating improved comprehension and application of key concepts about the occurrence of both sickness behavior or behavioral fever in ectothermic vertebrates. We start with a conceptual synthesis of sickness behavior and behavioral fever, with examples in different taxa. Through this discussion we present possible paths to standardize terminology, starting from original use in endothermic tetrapods which was expanded to ectothermic vertebrates, particularly amphibians and reptiles. This conceptual expansion from humans (endothermic vertebrates) and then to ectothermic counterparts, gravitates around the concept of 'normality'. Thus, following this discussion, we highlight caveats with experimental protocols and state the need of a reference value considered normal (RVCN), which is different from experimental control and make recommendations regarding experimental procedures and stress the value of detailed documentation of behavioral responses. We also propose some future directions that could enhance interaction among disciplines, emphasizing relationships at different levels of biological organization. This is crucial given the increasing convergence of fields such as thermal physiology, immunology, and animal behavior due to emerging diseases and other global crises impacting biodiversity.

Metabolic trade-offs favor regulated hypothermia and inhibit fever in immune-challenged chicks

The febrile response to resist a pathogen is energetically expensive while regulated hypothermia seems to preserve energy for vital functions. We hypothesized here that immune challenged birds under metabolic trade-offs (reduced energy supply / increased energy demand) favor a regulated hypothermic response at the expense of fever. To test this hypothesis, we compared 5-days old broiler chicks exposed to fasting, cold (25oC), and fasting combined with cold to a control group fed at thermoneutral condition (30oC). The chicks were injected with saline or with a high dose of endotoxin known to induce a biphasic thermal response composed of body temperature (Tb) drop followed by fever. Then Tb, oxygen consumption (metabolic rate), peripheral vasomotion (cutaneous heat exchange), breathing frequency (respiratory heat exchange), and huddling behavior (heat conservation indicator) were analyzed. Irrespective of metabolic trade-offs, chicks presented a transient regulated hypothermia in the first hour, which relied on a suppressed metabolic rate for all groups, increased breathing frequency for chicks fed/fasted at 30oC, and peripheral vasodilation in fed/fasted chicks at 25oC. Fever was observed only in chicks kept at thermoneutrality and was supported by peripheral vasoconstriction and huddling behavior. Fed and fasted chicks at 25oC completely eliminated fever despite the ability to increase metabolic rate for thermogenesis in the phase correspondent to fever when it was pharmacologically induced by 2.4-Dinitrophenol. Our data suggest that increased competing demands affect chicks’ response to an immune challenge favoring regulated hypothermia to preserve energy while the high costs of fever to resist a pathogen are avoided.

Egg-laying increases body temperature to an annual maximum in a wild bird

Most birds, unlike reptiles, lay eggs successively to form a full clutch. During egg-laying, birds are highly secretive and prone to disturbance and predation. Using multisensor data loggers, we show that average daily body temperature during egg-laying is significantly increased (1 °C) in wild eider ducks (Somateria mollissima). Strikingly, this increase corresponds to the annual maximum body temperature (40.7 °C), representing a severe annual thermogenic challenge. This egg-laying-induced rise in body temperature may prove to be a common feature of wild birds and could be caused by habitat-related thermoregulatory adjustments and hormonal modulation of reproduction. We conclude our findings with new perspectives of the benefits of high body temperature associated with egg-laying of birds and the potential effect of heat stress that may occur with the future advent of heatwaves.

Changes in Body Surface Temperature Play an Underappreciated Role in the Avian Immune Response

AbstractFever and hypothermia are well-characterized components of systemic inflammation. However, our knowledge of the mechanisms underlying such changes in body temperature is largely limited to rodent models and other mammalian species. In mammals, high dosages of an inflammatory agent (e.g., lipopolysaccharide [LPS]) typically leads to hypothermia (decrease in body temperature below normothermic levels), which is largely driven by a reduction in thermogenesis and not changes in peripheral vasomotion (i.e., changes in blood vessel tone). In birds, however, hypothermia occurs frequently, even at lower dosages, but the thermoeffector mechanisms associated with the response remain unknown. We immune challenged zebra finches (Taeniopygia guttata) with LPS, monitored changes in subcutaneous temperature and energy balance (i.e., body mass, food intake), and assessed surface temperatures of and heat loss across the eye region, bill, and legs. We hypothesized that if birds employ thermoregulatory mechanisms similar to those of similarly sized mammals, LPS-injected individuals would reduce subcutaneous body temperature and maintain constant surface temperatures compared with saline-injected individuals. Instead, LPS-injected individuals showed a slight elevation in body temperature, and this response coincided with a reduction in peripheral heat loss, particularly across the legs, as opposed to changes in energy balance. However, we note that our interpretations should be taken with caution owing to small sample sizes within each treatment. We suggest that peripheral vasomotion, allowing for heat retention, is an underappreciated component of the sickness-induced thermoregulatory response of small birds.

Preventive Administration of the Heat Shock Protein Hsp70 Relieves Endotoxemia-Induced Febrile Reaction in Pigeons ( ) and Rats

The stress-inducible 70 kDa heat shock protein (Hsp70) can exert a protective effect on endotoxemia and sepsis due to its ability to interact with immune cells and modulate the immune response. However, it remains unknown whether Hsp70 is able to relieve endotoxemia-induced fever. We carried out a comparative study of the effects of preventive administration of the human recombinant Hsp70 (HSPA1A) on lipopolysaccharide (LPS)-induced endotoxemia in pigeons and rats with preimplanted electrodes and thermistors for recording the thermoregulation parameters (brain temperature, peripheral vasomotor reaction, muscular contractile activity). Additionally, we analyzed the dynamics of the white blood cell (WBC) count in rats under the same conditions. It was found that preventive administration of Hsp70 relieves the LPS-induced febrile reaction in pigeons and rats and accelerates the restoration of the WBC count in rats. The data obtained suggest that these warm-blooded animals share a common physiological mechanism that underlies the protective effect of Hsp70.

Sickness behaviors across vertebrate taxa: proximate and ultimate mechanisms

There is nothing like a pandemic to get the world thinking about how infectious diseases affect individual behavior. In this respect, sick animals can behave in ways that are dramatically different from healthy animals: altered social interactions and changes to patterns of eating and drinking are all hallmarks of sickness. As a result, behavioral changes associated with inflammatory responses (i.e. sickness behaviors) have important implications for disease spread by affecting contacts with others and with common resources, including water and/or sleeping sites. In this Review, we summarize the behavioral modifications, including changes to thermoregulatory behaviors, known to occur in vertebrates during infection, with an emphasis on non-mammalian taxa, which have historically received less attention. We then outline and discuss our current understanding of the changes in physiology associated with the production of these behaviors and highlight areas where more research is needed, including an exploration of individual and sex differences in the acute phase response and a greater understanding of the ecophysiological implications of sickness behaviors for disease at the population level.

Regulated hypothermia in response to endotoxin in birds

KEY POINTS

The costs associated with immune and thermal responses may exceed the benefits to the host during severe inflammation. In this case, regulated hypothermia instead of fever can occur in rodents as a beneficial strategy to conserve energy for vital functions with consequent tissue protection and hypoxia prevention. We tested the hypothesis that this phenomenon is not exclusive to mammals, but extends to the other endothermic group, birds. A decrease in metabolic rate without any failure in mitochondrial respiration, nor oxygen delivery, is the main evidence supporting the regulated nature of endotoxin-induced hypothermia in chicks. Thermolytic mechanisms such as tachypnea and cutaneous vasodilatation can also be recruited to facilitate body temperature decrease under lipopolysaccharide treatment, especially in the cold. Our findings bring a new perspective for evolutionary medicine studies on energy trade-off in host defence because regulated hypothermia may be a phenomenon spread among vertebrates facing a severe immune challenge.

ABSTRACT

A switch from fever to regulated hypothermia can occur in mammals under circumstances of reduced physiological fitness (e.g. sepsis) to direct energy to defend vital systems. Birds in which the cost to resist a pathogen is additive to the highest metabolic rate and body temperature (T_b ) among vertebrates may also benefit from regulated hypothermia during systemic inflammation. Here, we show that the decrease in T_b observed during an immune challenge in birds is a regulated hypothermia, and not a result of metabolic failure. We investigated O₂ consumption (thermogenesis index), ventilation (respiratory heat loss), skin temperature (sensible heat loss) and muscle mitochondrial respiration (thermogenic tissue) during T_b fall in chicken chicks challenged with endotoxin [lipopolysaccharide (LPS)]. Chicks injected with LPS were also tested regarding the capacity to raise O₂ consumption to meet an increased demand driven by 2,4-dinitrophenol. LPS decreased T_b and the metabolic rate of chicks without affecting muscle uncoupled, coupled and non-coupled mitochondrial respiration. LPS-challenged chicks were indeed capable of increasing metabolic rate in response to 2,4-dinitrophenol, indicating no O₂ delivery limitation. Additionally, chicks did not attempt to prevent T_b from falling during hypothermia but, instead, activated cutaneous and respiratory thermolytic mechanisms, providing an additional cooling force. These data provide the first evidence of the regulated nature of the hypothermic response to endotoxin in birds. Therefore, it changes the current understanding of bird's thermoregulation during severe inflammation, indicating that regulated hypothermia is either a convergent trait for endotherms or a conserved response among vertebrates, which adds a new perspective for evolutionary medicine research.

Let fever do its job: The meaning of fever in the pandemic era

Although fever is one of the main presenting symptoms of COVID-19 infection, little public attention has been given to fever as an evolved defense. Fever, the regulated increase in the body temperature, is part of the evolved systemic reaction to infection known as the acute phase response. The heat of fever augments the performance of immune cells, induces stress on pathogens and infected cells directly, and combines with other stressors to provide a nonspecific immune defense. Observational trials in humans suggest a survival benefit from fever, and randomized trials published before COVID-19 do not support fever reduction in patients with infection. Like public health measures that seem burdensome and excessive, fevers involve costly trade-offs but they can prevent infection from getting out of control. For infections with novel SARS-CoV-2, the precautionary principle applies: unless evidence suggests otherwise, we advise that fever should be allowed to run its course. Lay summary: For COVID-19, many public health organizations have advised treating fever with medicines such as acetaminophen or ibuprofen. Even though this is a common practice, lowering body temperature has not improved survival in laboratory animals or in patients with infections. Blocking fever can be harmful because fever, along with other sickness symptoms, evolved as a defense against infection. Fever works by causing more damage to pathogens and infected cells than it does to healthy cells in the body. During pandemic COVID-19, the benefits of allowing fever to occur probably outweigh its harms, for individuals and for the public at large.

Metabolic and Hematological Responses to Endotoxin-Induced Inflammation in Chicks Experiencing Embryonic 2,3,7,8-Tetrachlorodibenzodioxin Exposure

Dioxin exposure during bird embryonic development disrupts immunity as well as mechanisms involved in energy metabolism, potentially affecting negatively acute-phase responses to pathogens. Thus, we hypothesized that embryonic exposure to 2,3,7,8-tetrachlorodibenzodioxin (TCDD) changes the metabolism and blood physiology of domestic chicks, affecting their physiological competence for responding to immune challenges. To test this hypothesis, we injected doses of 0, 1.5, and 3 ng TCDD/egg (based on survival experiments) on embryonic day 4 and then measured O₂ consumption and CO₂ production for metabolic rate, ventilation, and body temperature (T_B ) in 5-d-old chicks. Then, chicks were injected with lipopolysaccharide (LPS, endotoxin) or saline prior to repeating the physiological measurements. A second chick group exposed to identical TCDD and LPS treatments had blood partial pressure of oxygen, partial pressure of carbon dioxide, pH, bicarbonate concentration, lactate concentration, osmolality, hemoglobin concentration, red blood cell concentration, and hematocrit, as well as T_B , analyzed at 1 and 5 h after LPS injection. Metabolism in chicks embryonically exposed to 1.5 and 3 ng TCDD/egg was up to 37% higher, whereas body mass of chicks exposed to 3 ng TCDD/egg was approximately 6% lower. Chicks embryonically exposed to 3 ng TCDD/egg challenged with LPS showed a relative persistent hypometabolism accompanied by elimination of the normal hematological and osmotic responses to LPS. We conclude that embryonic exposure to TCDD affects posthatching metabolism as well as impairs metabolic, hematological, and osmotic responses to LPS. Environ Toxicol Chem 2020;39:2208-2220. © 2020 SETAC.

Neurons and astrocytes of the chicken hypothalamus directly respond to lipopolysaccharide and chicken interleukin-6

In 4-5-month-old chicken, intravenous injections of bacterial lipopolysaccharide (LPS) induced a dose-dependent fever response and a pronounced increase of circulating interleukin-6 (IL-6). To assess a possible role for IL-6 in the brain of birds, a hypothalamic neuro-glial primary culture from 1-day-old chicken was established. Each well of cultured hypothalamic cells contained some 615 neurons, 1350 astrocytes, and 580 microglial cells on average. Incubation of chicken hypothalamic primary cultures with 10 or 100 µg/ml LPS induced a dose-dependent release of bioactive IL-6 into the supernatant. Populations of hypothalamic neurons (4%) and astrocytes (12%) directly responded to superfusion with buffer containing 10 µg/ml LPS with a transient increase of intracellular calcium, a sign of direct cellular activation. Stimulation of hypothalamic cultures with buffer containing 50 ng/ml chicken IL-6 induced calcium signaling in 11% of neurons and 22% of astrocytes investigated. These results demonstrate that IL-6 is produced in the periphery and in the hypothalamus in response to LPS in chicken. The observed cellular responses of hypothalamic cells to chicken IL-6 indicate that this cytokine may readily be involved in the manifestation of fever in the avian hypothalamus.

On the Mechanism of Hyperthermia-Induced BRCA2 Protein Degradation

The DNA damage response (DDR) is a designation for a number of pathways that protects our DNA from various damaging agents. In normal cells, the DDR is extremely important for maintaining genome integrity, but in cancer cells these mechanisms counteract therapy-induced DNA damage. Inhibition of the DDR could therefore be used to increase the efficacy of anti-cancer treatments. Hyperthermia is an example of such a treatment—it inhibits a sub-pathway of the DDR, called homologous recombination (HR). It does so by inducing proteasomal degradation of BRCA2 —one of the key HR factors. Understanding the precise mechanism that mediates this degradation is important for our understanding of how hyperthermia affects therapy and how homologous recombination and BRCA2 itself function. In addition, mechanistic insight into the process of hyperthermia-induced BRCA2 degradation can yield new therapeutic strategies to enhance the effects of local hyperthermia or to inhibit HR. Here, we investigate the mechanisms driving hyperthermia-induced BRCA2 degradation. We find that BRCA2 degradation is evolutionarily conserved, that BRCA2 stability is dependent on HSP90, that ubiquitin might not be involved in directly targeting BRCA2 for protein degradation via the proteasome, and that BRCA2 degradation might be modulated by oxidative stress and radical scavengers.

Heart rate reveals torpor at high body temperatures in lowland tropical free-tailed bats

Reduction in metabolic rate and body temperature is a common strategy for small endotherms to save energy. The daily reduction in metabolic rate and heterothermy, or torpor, is particularly pronounced in regions with a large variation in daily ambient temperature. This applies most strongly in temperate bat species (order Chiroptera), but it is less clear how tropical bats save energy if ambient temperatures remain high. However, many subtropical and tropical species use some daily heterothermy on cool days. We recorded the heart rate and the body temperature of free-ranging Pallas' mastiff bats (Molossus molossus) in Gamboa, Panamá, and showed that these individuals have low field metabolic rates across a wide range of body temperatures that conform to high ambient temperature. Importantly, low metabolic rates in controlled respirometry trials were best predicted by heart rate, and not body temperature. Molossus molossus enter torpor-like states characterized by low metabolic rate and heart rates at body temperatures of 32°C, and thermoconform across a range of temperatures. Flexible metabolic strategies may be far more common in tropical endotherms than currently known.

Ecoimmunology and microbial ecology: Contributions to avian behavior, physiology, and life history

Bacteria have had a fundamental impact on vertebrate evolution not only by affecting the evolution of the immune system, but also generating complex interactions with behavior and physiology. Advances in molecular techniques have started to reveal the intricate ways in which bacteria and vertebrates have coevolved. Here, we focus on birds as an example system for understanding the fundamental impact bacteria have had on the evolution of avian immune defenses, behavior, physiology, reproduction and life histories. The avian egg has multiple characteristics that have evolved to enable effective defense against pathogenic attack. Microbial risk of pathogenic infection is hypothesized to vary with life stage, with early life risk being maximal at either hatching or fledging. For adult birds, microbial infection risk is also proposed to vary with habitat and life stage, with molt inducing a period of increased vulnerability. Bacteria not only play an important role in shaping the immune system as well as trade-offs with other physiological systems, but also for determining digestive efficiency and nutrient uptake. The relevance of avian microbiomes for avian ecology, physiology and behavior is highly topical and will likely impact on our understanding of avian welfare, conservation, captive breeding as well as for our understanding of the nature of host-microbe coevolution.

No fever and leucocytosis in response to a lipopolysaccharide challenge in an insectivorous bat

Bat immune systems may allow them to respond to zoonotic agents more efficiently than other mammals. As the first line of defence, the taxonomically conserved acute phase immune reaction of leucocytosis and fever is crucial for coping with infections, but it is unknown if this response is a key constituent to bat immunological success. We investigated the acute phase reaction to a standard lipopolysaccharide (LPS) challenge in Pallas's mastiff bats (Molossus molossus). Challenged bats lost mass, but in contrast to other mammals showed no leucocytosis or fever. There also was no influence on body temperature reduction during torpor. When compared to recent genome-wide assays for constituent immune genes, this lack of a conserved fever response to LPS contributes to a clearer understanding of the innate immune system in bat species and of the coevolution of bats with a wide diversity of pathogens.

Nitric oxide and fever: immune-to-brain signaling vs. thermogenesis in chicks

Nitric oxide (NO) plays a role in thermogenesis but does not mediate immune-to-brain febrigenic signaling in rats. There are suggestions of a different situation in birds, but the underlying evidence is not compelling. The present study was designed to clarify this matter in 5-day-old chicks challenged with a low or high dose of bacterial LPS. The lower LPS dose (2 μg/kg im) induced fever at 3–5 h postinjection, whereas 100 μg/kg im decreased core body temperature (Tc) (at 1 h) followed by fever (at 4 or 5 h). Plasma nitrate levels increased 4 h after LPS injection, but they were not correlated with the magnitude of fever. The NO synthase inhibitor ( NG-nitro-l-arginine methyl ester, l-NAME; 50 mg/kg im) attenuated the fever induced by either dose of LPS and enhanced the magnitude of the Tc reduction induced by the high dose in chicks at 31–32°C. These effects were associated with suppression of metabolic rate, at least in the case of the high LPS dose. Conversely, the effects of l-NAME on Tc disappeared in chicks maintained at 35–36°C, suggesting that febrigenic signaling was essentially unaffected. Accordingly, the LPS-induced rise in the brain level of PGE2 was not affected by l-NAME. Moreover, l-NAME augmented LPS-induced huddling, which is indicative of compensatory mechanisms to run fever in the face of attenuated thermogenesis. Therefore, as in rats, systemic inhibition of NO synthesis attenuates LPS-induced fever in chicks by affecting thermoeffector activity and not by interfering with immune-to-brain signaling. This may constitute a conserved effect of NO in endotherms.

Body temperature changes during simulated bacterial infection in a songbird: fever at night and hypothermia during the day

Although fever (a closely regulated increase in body temperature in response to infection) typically is beneficial, it is energetically costly and may induce detrimentally high body temperatures. This can increase the susceptibility to energetic bottlenecks and risks of overheating in some organisms. Accordingly, it could be particularly interesting to study fever in small birds, which have comparatively high metabolic rates and high, variable body temperatures. We therefore investigated two aspects of fever and other sickness behaviours (circadian variation, dose dependence) in a small songbird, the zebra finch. We injected lipopolysaccharide (LPS) at the beginning of either the day or the night, and subsequently monitored body temperature, body mass change and food intake for the duration of the response. We found pronounced circadian variation in the body temperature response to LPS injection, manifested by (dose-dependent) hypothermia during the day but fever at night. This resulted in body temperature during the peak response being relatively similar during the day and night. Day-to-night differences might be explained in the context of circadian variation in body temperature: songbirds have a high daytime body temperature that is augmented by substantial heat production peaks during activity. This might require a trade-off between the benefit of fever and the risk of overheating. In contrast, at night, when body temperature is typically lower and less variable, fever can be used to mitigate infection. We suggest that the change in body temperature during infection in small songbirds is context dependent and regulated to promote survival according to individual demands at the time of infection.

Mechanisms of fever production and lysis: lessons from experimental LPS fever

Fever is a cardinal symptom of infectious or inflammatory insults, but it can also arise from noninfectious causes. The fever-inducing agent that has been used most frequently in experimental studies designed to characterize the physiological, immunological and neuroendocrine processes and to identify the neuronal circuits that underlie the manifestation of the febrile response is lipopolysaccharide (LPS). Our knowledge of the mechanisms of fever production and lysis is largely based on this model. Fever is usually initiated in the periphery of the challenged host by the immediate activation of the innate immune system by LPS, specifically of the complement (C) cascade and Toll-like receptors. The first results in the immediate generation of the C component C5a and the subsequent rapid production of prostaglandin E2 (PGE2). The second, occurring after some delay, induces the further production of PGE2 by induction of its synthesizing enzymes and transcription and translation of proinflammatory cytokines. The Kupffer cells (Kc) of the liver seem to be essential for these initial processes. The subsequent transfer of the pyrogenic message from the periphery to the brain is achieved by neuronal and humoral mechanisms. These pathways subserve the genesis of early (neuronal signals) and late (humoral signals) phases of the characteristically biphasic febrile response to LPS. During the course of fever, counterinflammatory factors, "endogenous antipyretics," are elaborated peripherally and centrally to limit fever in strength and duration. The multiple interacting pro- and antipyretic signals and their mechanistic effects that underlie endotoxic fever are the subjects of this review.

Surveillance for microbes and range expansion in house sparrows

Interactions between hosts and parasites influence the success of host introductions and range expansions post-introduction. However, the physiological mechanisms mediating these outcomes are little known. In some vertebrates, variation in the regulation of inflammation has been implicated, perhaps because inflammation imparts excessive costs, including high resource demands and collateral damage upon encounter with novel parasites. Here, we tested the hypothesis that variation in the regulation of inflammation contributed to the spread of house sparrows (Passer domesticus) across Kenya, one of the world's most recent invasions of this species. Specifically, we asked whether inflammatory gene expression declines with population age (i.e. distance from Mombasa (dfM), the site of introduction around 1950). We compared expression of two microbe surveillance molecules (Toll-like receptors, TLRs-2 and 4) and a proinflammatory cytokine (interleukin-6, IL-6) before and after an injection of an immunogenic component of Gram-negative bacteria (lipopolysaccharide, LPS) among six sparrow populations. We then used a best-subset model selection approach to determine whether population age (dfM) or other factors (e.g. malaria or coccidian infection, sparrow density or genetic group membership) best-explained gene expression. For baseline expression of TLR-2 and TLR-4, population age tended to be the best predictor with expression decreasing with population age, although other factors were also important. Induced expression of TLRs was affected by LPS treatment alone. For induced IL-6, only LPS treatment reliably predicted expression; baseline expression was not explained by any factor. These data suggest that changes in microbe surveillance, more so than downstream control of inflammation via cytokines, might have been important to the house sparrow invasion of Kenya.

Chicken IL-6 is a heat-shock gene

The febrile response is elicited by pyrogenic cytokines including IL-6 in response to microorganism infections and diseases in vertebrates. Mammalian HSF1, which senses elevations in temperature, negatively regulates the response by suppressing pyrogenic cytokine expression. We here showed that HSF3, an avian ortholog of mammalian HSF1, directly binds to and activates IL-6 during heat shock in chicken cells. Other components of the febrile response mechanism, such as IL-1β and ATF3, were also differently regulated in mammalian and chicken cells. These results suggest that the febrile response is exacerbated by a feed-forward circuit composed of the HSF3-IL-6 pathway in birds.