Projectleader Dr. Hans Hopster
Institute for
Animal Science and Health (ID-Lelystad)
Division
Animal Sciences, Researchgroup Animal Welfare
PhD-student Drs.
Godelieve Kranendonk
Project period March 2001 – March 2005
Summary
In intensive
husbandry systems, the adaptive capacity of pigs is frequently challenged; high
stocking densities, mixing, transport, etc. force the fig to draw on this
adaptive capacity. There is abundant literature substantiating that there are
different adaptive strategies ('coping strategies') in many species, including
pigs. In several species it has been shown that prenatal stress of the mother
affects the behavioural and physiological adaptive responses of the offspring.
As these responses are of overriding importance for the welfare of pigs, it is
necessary to gain insight in the effects of prenatal stress on the sow and her
piglets, in order to account for these effects when new, welfare-friendly
housing systems for pregnant sows are proposed.
The present
project therefore addresses the following hypotheses: 1) prenatal stress, as
applied by exposing pregnant sows to confinement stress, affects behavioural
and physiological variables (incl. Their 'coping strategy') of the piglets
during the postnatal period; 2) these effects depend on the gestation period in
which the pregnant sow is exposed to the stress; 3) the coping strategy of the
pregnant sow has an influence on the fetus and therefore on these effects
postnatal; 4) these effects also exist in systems that are supposed to be a
good and practical alternative for confinement of pregnant sows, namely large
dynamic groups.
With this
project, an important contribution will be given towards the development of new
housing systems for pregnant sows that take both the pregnant sow and her
offspring into account, optimises their adaptive strategies and, as a
consequence, increases their welfare.
Results
Ample time has
been spent on literature research and on training skills that are relevant in
the project, like the non-invasive measurement of heart rate in piglets, the
performance of the back test, the collection and processing of saliva and urine
samples as well as behavioral observation and analysis. In addition, two experiments have been
carried out. In the first experiment, a novel object test has been pharmacologically
validated for quantifying fear-responses in four-week old piglets. This novel
object test will be used in subsequent experiments to measure differences in
fear responses in piglets born from sows that experienced different levels of
prenatal stress. Briefly, thirty-six
piglets were taken from nine litters and divided into three groups. At testing,
the piglets were four weeks old. Thirty minutes prior to testing, the piglets
were weighed and injected i.m. with either physiological saline (the control
group), 0.4 mg diazepam / kg, or 0.8 mg diazepam / kg. Two minutes before the
test, the piglet was taken from its home pen and was equipped with a heart rate
monitor (Polar Vantage), to continuously measure heart rate during the test.
Subsequently, the piglet was carried to and placed into a familiar testing
arena. After five minutes, a blue bucket (the novel object) was lowered from
the ceiling. The piglet was left with the bucket for another five minutes,
after which the piglet was caught, heart rate equipment was removed and the
piglet was returned to its home pen. The test was recorded on video for further
analysis.
Preliminary
results indicate that mean heart rate is increased in both diazepam-groups,
probably caused by a diazepam-induced decrease of parasympathetic nervous
system activity. Heart rate effects resulting from the sudden appearance of the
bucket did not differ between experimental groups. Latency-time to touch the
bucket was not influenced by treatment, nor was the frequency of touching the
bucket or the mean distance to the bucket. However, diazepam-treated animals
approached the bucket within 50 centimetres more often than saline-treated
piglets did (5 versus 2.2 visits). We speculate that the frequency of
approaching the novel object within 50 centimetres may be a valid measure of
anxiety in piglets.
Regarding
possible mechanisms underlying the effects of prenatal stress on fetal
development, literature research revealed that in rats elevated maternal
peripheral corticosteroid levels are a major cause of disturbed brain
development. In pigs, only two studies are published in this field of research
and both suggested at the most that similar phenomena may play a role in fetal
brain development. Moreover, for the initial idea of inducing long lasting
elevated corticosteroid levels in sows by exposing them to experimental
stressors, no support has been found in the literature.
Because the
development of an experimental model for inducing long lasting adrenocortical
activation in pigs is extremely time consuming and therefore not achievable
within the scope of the current project, we decided to radically adjust the
initial experimental approach. Oral administration of cortisol twice daily
should guarantee a controlled increase in peripheral corticosteroid levels in
sows and should enable us to determine cortisol-induced differences in piglet
behaviour or physiology if present. Our new approach first required a
dose/response experiment to establish a suitable cortisol dose for use in second
parity pregnant sows. Our second question in this experiment concerned the
relation between cortisol concentrations in plasma, saliva and urine after an
artificial increase of cortisol, to enable avoiding blood sampling in future
experiments.
Based on rat
literature, our educated guess was that a two- or threefold increase compared
to baseline was likely to be high enough to bring about measurable effects on
piglet behaviour and physiology. In addition, practical stressors like
isolation, applying a nose-sling and transport are known to induce comparable
cortisol levels in sows. A dose/response experiment has been carried out with
four groups of six second-parity sows. Sows were surgically fitted with a
cannula in the internal jugular vein and housed in farrowing crates. Sows in
different groups received 0 (placebo), 0.1, 0.3 and 0.9 mg/kg cortisol in a
capsule hidden in a candy twice daily at 0830 a.m. and 0830 p.m. At three and
25 days following the start of the cortisol treatment, plasma, saliva and urine
(through a catheter) samples were collected each hour from all sows during 24
hr. Sows were sampled twice weekly in between these 24-hour sampling days.
Preliminary
results indicated that cortisol concentrations in plasma followed a regular
pattern with two peaks at about half an hour after cortisol-administration and
a gradual decrease to baseline after approximately 8 hours. Attained cortisol
concentrations were dose dependent. The 0.3 dose appeared to meet the criterion
of a two- to threefold cortisol-increase. Surprisingly the cortisol
concentration in saliva did not seem to keep pace with plasma cortisol
concentrations during the whole 24 hour. During daytime saliva cortisol
concentrations were significantly lower than during the night, despite comparable
cortisol concentrations in plasma. Analysis of cortisol binding globulin
concentrations and further study of differences in saliva production and
diffusion of cortisol from blood to saliva should elucidate this intriguing
phenomenon.
In total 5 sows
had to be excluded from the experiment because of various health problems. From
the 19 sows that farrowed, three female piglets from each sow were euthanized
and adrenals, thymus and spleen were weighed. Furthermore, their brains were
collected and stored for future analysis of glucocorticoid receptor density in
the central amygdala (mRNA-analysis).
In 2002/2003 a
follow-up experiment has started in second-parity sows to determine the
sensitive period during gestation with regard to cortisol induced effects on
piglet behaviour and physiology. The experiment will be carried out in four
rounds with 36 sows that orally receive the 0.3 mg/kg cortisol dose during
either weeks 3-7, 7-11 or 11-15. A fourth treatment group of 12 sows receives a
placebo during the whole gestation period. The focus in this experiment will be
on neonatal vitality of the piglets, on piglet behaviour in the farrowing
environment and after weaning and on their responses during social mixing,
after exposure to a novel object and after administration of ACTH. In
collaboration with the research group of Dr. Donald Lay (Purdue, USA)
additional measurements in mutual experiments will be considered.