In the December 2017 issue of ATLA, I published an
Editorial, in which I suggested that stress in ani-
mals in laboratories, resulting from their environ-
ment and experimental procedures, was not limited
to welfare concerns (1). It also adversely affects, sig-
nificantly and unavoidably, multiple biological sys-
tems and therefore affects the resultant
experimental data. This exacerbates inter-species
differences and makes extrapolation to humans even
more difficult and unreliable. In an effort to encour-
age and promote discussion of this issue, which is
underappreciated, I am expanding on that Editorial.
Stress may be thought of as “the sum of the biolog-
ical reactions to any adverse stimulus, physical,
mental, or emotional, internal or external, that
tends to disturb the homeostasis of an organism” (2).
It is increasingly acknowledged that diverse species
experience pain, stress and distress (see below), and
experience depression and anxiety disorders (see
Ferdowsian et al. ). This includes fish — notably
the ubiquitous laboratory zebrafish — which show
emotional fever/stress-induced hyperthermia in
response to a variety of stressors, including simple
handling (4, 5). Stress in fish can result in increased
aggressive behaviour (6), elevated anxiety, dimin-
ished weight-gain, and altered levels of dopamine
and serotonin metabolites in the brain (7), as well as
increased brain levels of extracellular adenosine,
which has neuromodulatory effects (8).
While all species experience stressors and stress
in their natural environments, psychological and
physiological problems can arise with exposure to
recurrent stressors, and/or when stress becomes
chronic and too difficult to cope with. This leads to
allostatic overload (excessive wear and tear on the
body), which may manifest in altered physiological
responses, some of which can be harmful (9) — gen-
erally thought of as a state of distress. How ever, the
US National Research Council (NRC) accepts that
there is “confusion in the scientific, regulatory, and
animal welfare communities” concerning the distinc-
tion between stress and distress; and that these
terms are often used interchangeably in animal wel-
Does the Stress of Laboratory Life and Experimentation on
Animals Adversely Affect Research Data? A Critical Review
Cruelty Free International, London, UK
Summary — Recurrent acute and/or chronic stress can affect all vertebrate species, and can have serious
consequences. It is increasingly and widely appreciated that laboratory animals experience significant and
repeated stress, which is unavoidable and is caused by many aspects of laboratory life, such as captivity,
transport, noise, handling, restraint and other procedures, as well as the experimental procedures
applied to them. Such stress is difficult to mitigate, and lack of significant desensitisation/habituation can
result in considerable psychological and physiological welfare problems, which are mediated by the
activation of various neuroendocrine networks that have numerous and pervasive effects. Psychological
damage can be reflected in stereotypical behaviours, including repetitive pacing and circling, and even
self-harm. Physical consequences include adverse effects on immune function, inflammatory responses,
metabolism, and disease susceptibility and progression. Further, some of these effects are epigenetic,
and are therefore potentially transgenerational: the biology of animals whose parents/grandparents were
wild-caught and/or have experienced chronic stress in laboratories could be altered, as compared to free-
living individuals. It is argued that these effects must have consequences for the reliability of
experimental data and their extrapolation to humans, and this may not be recognised sufficiently among
those who use animals in experiments.
Key words: animal welfare, cost–benefit analysis, data accuracy, glucocorticoids, psychological, stress,
translational medical research.
Address for correspondence: Jarrod Bailey, Cruelty Free International, 16a Crane Grove, London,
N7 8NN, UK.
ATLA 46, 291–305, 2018 291
fare literature and that relevant available informa-
tion is “far from complete”, with distress remaining
“a complex and still poorly understood phenomenon”
(10). Nevertheless, it is generally understood that
distress manifests when an individual is unable to
cope and adapt successfully to one or more stressors,
resulting in compromised well-being due to the
inability to return to physiological and psychological
homeostasis (11). Notably, an individual can remain
in distress, even when a stressor is removed.
While different species and individuals have dif-
ferent stressors, variable ranges of stress to which
they can adapt, diverse spectra of tolerance, and
dissimilar manifestations and sequelae of exces-
sive stress, they share the same biological path-
ways and mechanisms that are adversely affected
by stress. Briefly, stressors stimulate the hypotha-
lamic–pituitary–adrenal (HPA) axis, the sympa-
thetic adrenal medullary axis, and the sympathetic
and parasympathetic nerve projections that
directly innervate secondary lymphoid organs (12–
14). Among other things, this results in the eleva-
tion of the ‘stress hormones’, cortisol and
corticosterone (CORT). Raised glucocorticoid (GC)
levels, alongside significant elevations of heart
rate, blood pressure and other hormone levels, are
acknowledged indicators of fear, stress and dis-
tress, and are used as biomarkers for stress in ter-
restrial vertebrates in laboratories (e.g. 15–17).
Direct consequences of these neuroendocrine
changes include deleterious effects on innate and
adaptive immunity, central nervous system
pathology, and cardiovascular and reproductive
perturbations (18, 19), leading to extensive and
diverse adverse health effects. Psychological
trauma, for instance, may result in altered health
or damaging patterns of behaviour. In humans,
immune perturbations manifest in poor responses
to vaccines, increased susceptibility to infections,
and accelerated disease progression (see Gurfein et
al. ). Further, the various mechanisms under-
lying such effects mean that stress has ramifica-
tions beyond the individuals experiencing it.
Successive generations, and individuals who have
experienced prenatal and/or early-life stress, may
be destined to suffer the consequences in adult-
Caveats and Framing the Argument
Chronic or long-term stress is not unique to ani-
mals in laboratories — it is part and parcel of life
for all species in whatever environment. I do not
intend to suggest otherwise, and it is accepted that
animals in the wild experience acute stressors reg-
ularly (such as lack of food and shelter, predation,
disease, and so on); not only is this entirely normal,
but it can be beneficial in terms of building allo-
static capacity. This Comment, however, argues
that stresses resulting from life in the laboratory
often have negative consequences — given the
opportunity, the animals would avoid many labo-
ratory stressors, and so they can hardly be consid-
ered benign. In addition, the degree, type,
frequency and duration of laboratory stressors are
different to those in the wild, and together could be
more chronic. This may result in greater adverse
consequences for animal welfare and for scientific
data quality. Experiencing repeated, unnatural
stressors, such as blood draws and gavage, cannot
be compared to the brief elevations in CORT
resulting from the natural diurnal/nocturnal
adrenal cycle. For instance, the former stressors
are very different from simply waking up, or being
hungry. In well-designed experiments, increases in
stress biomarkers are relative to baseline levels in
any case (see, for example, 16, 20).
That laboratory stress can — arguably, frequently
and unavoidably — have effects that compromise
animal welfare and experimental results is accepted
by the US NRC’s Committee on Recognition and
Alleviation of Distress in Laboratory Animals (10):
“In the longer term… a breakdown in an animal’s
ability to cope with its environment is likely to lead
to adverse emotional states and poor welfare. Some
of these cases may be quite minor and not give rise
to significant ethical concerns; but prolonged or
intense circumstances would compromise the ani-
mal’s welfare enough to warrant concern and also
significantly affect the research results.” Further:
“Strong evidence in rodents has shown that mild
stress of 2–3 months duration — a regimen that pro-
duces no signs of overt distress — alters the animals’
performance in tests of anxiety, depression, and
memory… Other findings indicate that rats’ habitu-
ation to a test environment can dramatically affect
their response to a toxic substance”. Stress can also
be beneficial: “Over a longer time frame, glucocorti-
coid production in response to infection helps restrict
the immune system, thus preventing deleterious
effects of inflammatory factors on tissues” (10).
While this latter statement is true, it also supports
the argument made here, that stress — even when
not ‘bad’ — adversely affects biological systems, and
consequently confounds research data.
Therefore, it is not only distress that leads to bio-
logical consequences affecting data reliability,
quality and relevance. It seems clear that the con-
sequences of stress are also an issue for welfare
and data quality. To quote again the US NRC:
“The impact of distress on both animal welfare and
research results is likely even more pronounced
than that of stress. Animals exposed to prolonged
severe stress experience underlying changes in
physiological functions (e.g. gastric lesions or
immunosuppression) that can interfere with exper-
imental manipulations; alter experimental vari-
ables such as behaviour, drug dosing and
clearance; change the progress of a disease; and
contribute to morbidity and mortality. A variety of
stressors can contribute to unintended distress,
from postoperative pain or infection to barren
housing conditions or the solitary confinement of
an individual of a social species. Stereotypies,
abnormal repetitive behaviours indicative of poor
well-being that are often observed in distressed
animals, are thought to reflect defective brain
function and to be a result of poor animal welfare.
Stereotypies are thus likely to interfere with
behavioural, neuroscience, and pharmacological
studies” (10); and “The impact of stress and dis-
tress on the quality of scientific research can result
in the generation of compromised data, which in
turn necessitates the use of more animals… Both
stress and distress represent potential complica-
tions in a wide range of experiments, and should be
proactively addressed by good experimental
design” (10). While I (and others) disagree that the
use of more animals and the careful design of
experiments can have an acceptably positive
impact on welfare, and on the human-relevance of
the data generated, the main points in this
Comment stand. Efforts to address this issue over
many years are acknowledged, but I argue that,
due to fundamental biology and inter-species dif-
ferences, the improvement of husbandry, veteri-
nary care, regulation, oversight, training, etc. can
never be sufficient to significantly address and
overcome these issues.
Stress Resulting from Laboratory Life
and Research, and its Biological
The issue for animals in laboratories is that life
can be inherently and excessively stressful — per-
haps much more than in their natural environ-
ments, from which laboratory conditions differ
greatly, even when enriched and accounting for
efforts to mitigate stressors. Laboratory conditions
preclude or limit many natural behaviours, hous-
ing tends to be much smaller than the animals’
natural ranges, and the animals are subjected to
frequent manipulations and handling, as well as
other alien factors that they try to resist and avoid
(16, 20). Stressful procedures and environmental
factors are numerous and varied. Briefly, they
include, but are not limited to: general handling
and manipulations, such as weighing and saline
injections (15, 16, 21–23); anaesthesia (15, 24–31);
restraint (21, 32–36); gavage (37–42); blood sam-
pling (15–17, 43–50); food and water restriction
(51, 52); non-natural environment (53–55) and
associated factors, such as noise and light (56–62);
social crowding and/or isolation (63–71); cage con-
ditions/cleaning/changing (18, 58, 72–78); trans-
port (19, 58, 79–83); observing procedures on, and
killing of, other animals (31, 84–87); and even
enrichment itself (17, 88, 89). It has been sug-
gested that captive-bred animals may not know
that these stressors are different to those in the
wild, though there might be some perception that
they are not similar to ‘natural’ stressors such as
limited food, inadequate shelter, predation and so
on. In any case, the point is that they are different,
but more importantly, they are also frequent, reg-
ular and inescapable.
Naturally, this has animal welfare and scientific
implications, acknowledged at least in some quar-
ters. For instance, stress from handling is accepted
as a source of “unexplained variation within and
between animal studies”, as it influences “both the
behaviour and physiology of animals” (23; see also
Balcombe et al.  and Meijer et al. ), and rel-
atively poor caging conditions “may contribute to
problems in translating murine research into
human studies” (18, 77, 78). However, it is acc -
epted by some that these factors are probably
widely underappreciated (90). It should be noted
that some consider enriched environments simply
as ‘less bad’ rather than ‘better’ than those that are
non-enriched. To illustrate, significant numbers of
animals experiencing enrichment still go on to
develop stereotypies (e.g. 91–98).
The Nature of Stress: Biological
Basis/Mechanisms of Stress and its
The physiological consequences of stress are varied
and powerful. This, in itself, is of concern for the
translation of animal data to humans, as they com-
pound and confound existing difficulties in transla-
tion due to species differences. First, however, a
consideration of the underlying mechanisms is
important, to demonstrate their fundamental nature
Primary mediators of stress, such as GCs and cat-
echolamines, are released in response to stressors,
with various biological consequences. Such biological
effects generally go beyond species boundaries/limits
for mammalian species, though the mechanisms and
specific effects differ to varying degrees. These pri-
mary mediators are extremely powerful, because
they ultimately modulate the expression of many
genes. Secondary outcomes have been documented
“…in every physiological system, including the car-
diovascular system, metabolism, the central nervous
system, and the immune system”, and are con-
founded by the characteristics of the stressor(s), as
well as the attributes of the affected individual, such
as age, health, status, genetic background, past
experience, etc. (99).
The potency and ubiquity of the stress response
has been demonstrated in a number of in vitro stud-
ies, revealing that it generally blocks every impor-
tant cellular process, including DNA replication,
transcription, pre-mRNA processing, mRNA export,
and translation, until the cells recover (100).
Therefore, stress exerts its effects via varied molec-
ular mechanisms, with far-reaching consequences.
The principal ones are highlighted below.
— Epigenetic mechanisms (histone acetylation and
DNA methylation) (101–107): Psychological
stress alters gene expression via histone acety-
lation and DNA methylation. Much occurs in
response to environmental triggers, e.g. diet,
drugs, toxins, and psychological stress, e.g. fear
conditioning and maternal care (103). Genes
involved in HPA axis function are especially
susceptible (108), e.g. in suicide victims with a
history of child abuse (109), and post-traumatic
stress disorder (PTSD) is strongly associated
with the epigenetic modification of genes
involved in immune function and inflammation
— Alternative splicing/expression of regulatory
microRNAs: Alternative splicing of gene tran-
scripts and microRNAs (miRNAs), is a powerful
means of altering gene expression that can be
significantly affected by stress (111, 112). For
example, acute stress in humans altered the
splicing of 27 genes in peripheral leukocytes
— Oxidative damage and ageing: Mental stress con-
tributes to oxidative stress in the body, and
therefore to oxidative damage (113). This effect
has been identified in students undergoing aca-
demic examinations (114), and in the lympho-
cytes of psychologically stressed individuals
(115). Oxidative stress is also associated with
PTSD and depression (116), contributes to the
ageing process (117), and is associated with neu-
rodegenerative disease, ophthalmologic disease,
cancer and cardiovascular disease (including
atherosclerosis, hypertension, cardiomyopathy,
chronic heart failure, myocardial ischaemia and
ventricular arrhythmias; 117, 118). There may be
confounding data on the effects of oxidative stress
from different species/strains of mice, which fur-
ther challenges the translation of data across
Direct Physiological Consequences of
The physiological consequences of stress are
numerous and varied. Table 1 shows how promis-
cuous these consequences, effects and manifesta-
tions are in many species, including humans.
Given that these effects involve so many biological
pathways and systems, the effects on experimental
data must be significant.
Habituation/Desensitisation to Stress
Some argue that animals become habituated (120)
and/or desensitised (121) to stress, so implications
for welfare and experimental results may be over-
come (23, 122, 123). For instance, non-human
primates can be trained to approach test environ-
ments, and to present their arms for blood with-
drawal and so on, seemingly voluntarily. However,
it has been shown that repeat exposure to homo-
typic stressors of greater intensity and/or severity
does not result in habituation, and could actually
result in sensitisation of the CORT response (122).
Furthermore, CORT levels might only decrease for
certain types of stressor, persisting for other types
(35); where desensitisation has been shown, it is
only to a modest degree, in a small proportion of
the animals in the studies, and in small sample
sizes (70). While some studies have suggested that
enrichment might decrease stress, others have
shown a paradoxical increase in stress via CORT
levels (18). Similarly, some studies have suggested
that transferring scent-marked materials from old
to new cages reduces stress-related aggression,
while others found that it increases aggression
(74). Furthermore, mice do not habituate to stress
associated with simple handling, and indeed seem
to become sensitised to it (124–126). In instances
where CORT levels decrease, there is increasing
evidence that this does not necessarily mean an
attendant decrease in stress; other indicators, such
as the neutrophil–lymphocyte ratio, might be ‘bet-
ter’ indicators of chronic stress, which can occur in
the absence of increased serum CORT (127).
Adverse Physiological Sequelae of
Psychological Stress are Initiated
Prenatally or in Early Life, and are
Captive-born animals can be exposed to stress pre-
natally via their wild-trapped mothers (128–131),
and as infants in laboratory environments, often
experiencing inadequate maternal contact and
care (132–134; cited by Camus et al. , and
Detter et al. ). If an animal’s parents or grand-
parents lived in laboratories, and/or were born of
parents that lived in laboratories and/or endured
being wild-caught, then their ancestors experi-
enced highly stressful lives and would have been
affected by the adverse consequences described
herein. Even if such an individual was subse-
quently afforded as stress-free a life as possible
(difficult, if not impossible, in a laboratory), the
consequences of their early lives, and the lives of
their ancestors, would lead to the same adverse
effects as if they had continued to experience
excessive stress as adults.
That early-life experiences affect adult psy-
chopathology is widely accepted. As Jean-Paul
Sartre put it, “Childhood decides” (see Murgatroyd
). “A large body of data shows that stress dur-
ing pregnancy causes an increase of GCs in the
blood of the dams and the foetus, leading to alter-
ations of the structure and function of the develop-
ing brain… These alterations result in the
disturbance of the function of the neuroendocrine
system and different kinds of behaviour through-
out life” (137). Early-life/prenatal exposure to
stress leads to altered adrenocorticotrophic hor-
mone responsiveness, dysfunction of the HPA axis
(138, 139), and altered autonomic modulation of
immune function that may begin in utero (103).
Social isolation in several species leads to neuroen-
docrine changes, increased cortisol, and ensuing
behavioural problems (for references, see Cham -
pagne ). Maternal inflammation during preg-
nancy (from infection, or possibly stress) may
increase the risk of neurodevelopmental disorders
such as schizophrenia and cerebral palsy (140).
Physiological sequelae include cardiovascular dis-
ease and metabolic disorders such as diabetes
(141), compromised immune function, including
poor lymphocyte proliferation upon infection and
reduced placental transfer of antibodies during
pregnancy (140), autoimmune disorders, chronic
obstructive lung disease, asthma and obesity (see
Chang ). Pivotal to these adverse outcomes
are the aforementioned stress-related epigenetic
processes and oxidative damage. Methylation of
gene regulatory regions is partly involved (143),
the extent of which may be set during prenatal
development (141). Cord blood samples of infants
of mothers with late-pregnancy depression show
altered methylation of the GC receptor promoter,
which also predicts elevated salivary cortisol in
early life (144). Other modifications are inherited
and transgenerational in nature (145); for exam-
ple, poor prenatal nutrition affects GC receptor
methylation, affecting the growth and metabolism
of first and second-generation offspring (146), and
matrilineal transmission of the effects of diethyl-
stilbestrol (DES) occurs via hypomethylation, lead-
ing to increased cancer risk over two generations
The inherent, multi-faceted stress of laboratory life
— often excessive, relative to the more transient,
acute and ‘natural’ stresses experienced in the wild
— is evidenced by its often negative impact on the
well-being of the animals involved, both psycholog-
ically and physiologically. Because this harm is
mediated via established trans-species biological
mechanisms involving the HPA axis and the sym-
pathetic nervous system, and effected via oxidative
stress and epigenetic mechanisms, which affect
multiple biological pathways and systems, it can
have adverse and confounding effects on experi-
mental results. This modulation of many biochem-
ical pathways and gene expression can result in
downstream effects such as organ damage, cardio-
vascular diseases, attenuated immune function
and autoimmune disorders, premature ageing and
mortality, developmental abnormalities, elevated
tumour initiation and progression, and muscu-
loskeletal atrophy (16).
The absolute degree of translation of animal stud-
ies to humans is debatable, but undoubtedly,
“Studies using animal models are more translatable
to human disease when the animals’ welfare is
maximised” (127). Arguably, it is difficult to “max-
imise” welfare significantly, given the in herent,
widespread, substantial and largely intractable
nature of the stresses involved in animal research
and laboratory life. Notably, habituation and/or
desensitisation to many of the stressors is often not
possible, or at least not significant, and the impact
of these effects on experimental data — and their
extrapolation to humans — is likely to be signifi-
cant. The many sources of stress have been sum-
marised here, along with their effects on multiple
biological and physiological systems. The literature
warns that: “…animals subjected to the environ-
mental changes that occur during transportation…
react with changes in their physiology, such as body
weight, plasma hormonal levels, heart rate and
blood pressure changes… When measurements of
physiological parameters are performed using con-
ventional measurement techniques, the results
must be interpreted with caution as these conven-
tional techniques also have effects on the animals”
(148). Most importantly, “Suffering in animals can
result in physiological changes which may increase
the variability of experimental data” (149). Many
scientists are well aware of these effects and consid-
erations, and have cautioned against disregarding
them (150–152). Yet, while accepting the negative
effects of pain, stress and distress, and their influ-
ence on study outcome, such effects are often not
reported or are under-reported in scientific publica-
The impact of stress on immunological and
inflammatory responses seems particularly preva-
lent, and might be especially critical, seeing as
much animal experimentation involves infectious
agents and/or immune function (for a discussion
and references, see Bailey  and Bailey ).
Crucially, this impact exacerbates and compounds
existing immune differences between humans and
non-humans due to genetic differences. To illus-
trate, genomic duplications — one of the most sig-
nificant causes of genetic variation among
primates (155) and at the root of many aspects of
intra-species and inter-species diversity — differ-
entially affect many genes involved in immune and
Table 1: A summary of the physiological consequences and manifestations of stress, in many species, including humans
Consequences of general stress Notes
Stereotypies (abnormal, repetitive, invariant These correlate with basal plasma cortisol and corticotrophin-releasing hormone (CRH) in cerebrospinal fluid (166).
behaviours with no obvious function; 160, 161)
and self-harm (e.g. 162–165) in non-humans
Wide-ranging physiological symptoms in humans Anger, depression, anxiety, behavioural changes, food cravings, lack of appetite, frequent crying, difficulty sleeping, tiredness, lack of
concentration, chest pains, constipation, diarrhoea, cramps and muscle spasms, dizziness, fainting, nervous twitches, restlessness, sexual
dysfunctions, breathlessness, and a host of diseases and illnesses with probable associated psychogenic (as well as biological) causes (167).
Long-lasting neurophysiological changes These could “…have direct implications for electrophysiological, behavioural, and molecular studies” (see 69).
— Isolated rats show “…structural and functional changes in the mesocorticolimbic dopaminergic system, exhibited hyperlocomotor
activity and impaired sensorimotor gating” (168).
— Isolated pigs show “…sustained changes in behavioural, neuroendocrine and immune regulation” (169).
— Socially isolated humans show increased risk of death, “…genome-wide transcriptional activity of impaired GC response genes and
increased activity of pro-inflammatory transcription control pathways”, and higher risk of developing “…conduct disorders,
personality disorders, major depression, PTSD, schizophrenia, and anxiety disorders” (see 170).
Multi-faceted modulation of the immune system This occurs in many, if not all, mammalian species (140, 171, 172). It is especially problematic for research involving immune function,
infectious diseases, etc. The expression of several hundred genes (many related to immune function) may be perturbed by simple
handling of animals (173; and see 33), which may be affected by the acute or chronic nature of the stressor (see 173). Chronic stress can:
— shift neutrophil–lymphocyte ratios (127, 174);
— affect expression of cytokines and cytokine receptors (175, 176), such as IL-2 and IL-6 (18);
— alter gene splicing in peripheral leukocytes (10);
— alter global and immune gene specific methylation (177); and
— differentially affect brain activity and neurotransmitter release, macrophage activity and antibody production (33).
The same stressor in acute or chronic forms may have different effects:
— In rodents, acute restraint increases delayed-type hypersensitivity (DTH) and leukocyte redeployment, but can increase or decrease
them when chronic (33, 178).
— In humans, chronic, though not acute, stress increases susceptibility to colds (179).
These inconsistencies show that stress is mediated not only by GCs (180), and that accounting for the effects of stress in animal
research must be difficult, if not impossible.
Observed alterations in human immune function These include:
— “attenuated responses to vaccination, poorer wound healing, exaggerated release of inflammatory mediators, & premature aging of
the immune system” (181);
— increased plasma and CNS cytokine levels, impaired natural killer cell activity, lower T-lymphocyte counts (in PTSD/complex
PTSD patients; 182);
— epigenetic changes exerting lifelong impact on immune and inflammatory function (in PTSD/complex PTSD patients; 182); and
— greater inflammatory responses to vaccinations (in depressed humans; 183).
Neuropeptides involved in stress responses may accentuate pathophysiological sequelae in critically ill individuals (184). In healthy
humans, 49 different genetic pathways are affected by stress, including genes associated with immune function (185). Stressed students
undergoing examinations have significantly increased pro-inflammatory cytokines (186).
Activation of the HPA axis This accelerates ageing generally, with adverse effects on brain/central nervous system, immune system, skeletal muscle and bone tissue
(see 187–189). HPA dysregulation may lead to excessive inflammation via increases in the levels of circulatory inflammatory cytokines,
decreases in anti-inflammatory cytokines, and alterations in the expression of genes involved in immune activation of peripheral blood
cells (see 177).
Table 1: continued
Effects of specific stressors Notes
Disease susceptibility An increase in susceptibility has been noted across several species to:
— general disease and somatic disorders (see 100, 103);
— various cancers (190, 191);
— pancreatitis and pancreatic tumours (192, 193);
— gastrointestinal disorders (194);
— thyroid pathology (195);
— multiple sclerosis (171);
— inflammatory bowel disease (142, 196);
— cardiovascular disease (197–199);
— accelerated ageing and age-related disorders (187); and
— musculoskeletal injury (200).
Stressors can increase oxidative stress, triggering inflammatory pathways associated with type-2 diabetes, cardiovascular disease,
osteoporosis, arthritis, some cancers, and susceptibility to some infections (e.g. 201–205).
Handling Causes biological changes, affecting wellbeing and/or experimental results.
Enrichment intended to mitigate stress can substantially alter brain structure, function and physiology.
Light, noise, cage position/changing etc. all affect physiology, behaviour, anxiety and experimental data (206).
Effects of handling stress may often be “missed” by researchers (23).
Early-life stress in various species Results in:
— abnormal brain development, leading to early-life psychopathologies and adult chronic mental illnesses (e.g. 207, 208);
— epigenetic modifications associated with depression and suicidal behaviour in later life (e.g. 102, 209–211); and
— elevated morbidity and mortality from chronic diseases of ageing, including vascular disease, autoimmune disorders, and premature
mortality (e.g. 141, 212).
Stress from maternal deprivation has negative effects on growth rates in rats, and adversely affects circadian clock and stress
Traumatic stress in humans Studies on PTSD patients have shown that:
— acute stress affects glucose metabolism, inflammation and components of the immune system associated with type-2 diabetes (201);
— serious long-term consequences include hypertension, heart attacks and stroke, as well as increased risk of obesity, Alzheimer’s
disease, and AIDS dementia complex (213).
inflammatory responses (156). Indels (genomic
insertions and deletions) also affect major histo-
compatibility complex (MHC) genes, which are
critical to immune responses, and are associated
with differences in response to infections, as well
as susceptibility to autoimmune diseases. These
are further confounded by sex-related and strain-
related differences. It has also been shown that the
susceptibility and responsiveness of mice to stres-
sors varies with the strain (157, 158). Stress also
affects sleep, and conversely sleep perturbations
exacerbate and sensitise individuals to stress, with
all the attendant consequences. These conse-
quences are also strain-dependent, and intimately
linked to the CRH/HPA system (159).
Additionally, the adverse consequences of stress
are multigenerational, as the associated epigenetic
mechanisms affect the germline. This is likely to
have significant consequences for animals, and
their offspring, in laboratories: if an animal’s par-
ents or grandparents experienced a stressful labo-
ratory life and experimental procedures, and/or if
the offspring experienced significant stress in early
life, then this will compound any further stress
that they experience as adults, in turn compound-
ing species differences and the translation of data
Overall, these observations of detrimental phys-
iological effects and the general mechanisms
behind them have been detailed in many species
(including humans), and throughout the evolution-
ary scale from monkeys to rodents. The minutiae of
the genes and biochemical pathways responsible,
and their manifestations, may differ to some
degree, but there are common mechanisms and
adverse effects in all species examined to date. It
must be concluded that laboratory life for animals
used in experiments has serious and intractable
consequences for their welfare, and for the quality
and human relevance of the experimental data
obtained (which, in any case, are already of debat-
able applicability to humans, due to species differ-
Finally, I believe that this issue should be taken
much more seriously by legislators, regulators,
funders, practitioners and advocates of animal
experiments, and urge all involved to do so. The
information presented here could, and should, be a
valuable resource for project licence applicants,
ethics committees and the Home Office Inspect -
orate, for use in experimental design and harm–
benefit analyses, and to aid data interpretation.
Though it argues that relatively little can be done
to minimise many, if not all, stressors and stress,
it could inform attempts to do so — as well as con-
trolling variable factors and mitigating negative
consequences, etc. The information could also be
factored into existing guidance for the strategic
planning of animal experiments, since stress
impacts all areas of this planning, including study
objectives, species/strain selection, experimental
procedures, analgesia, training of staff, and so on.
This review was funded by Cruelty Free
International Trust, London, UK. It was based on
previous (not published) work conducted by the
same author in 2011, funded by the New England
Anti-Vivisection Society (NEAVS), Boston, USA, in
his role as its Science Director.
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