Polysomnographic measurements were made of 7 healthy horses housed in individual stables over four consecutive nights under standardized conditions as a control group. It was compared to the sleeping behaviour of 35 horses which showed collapses during the resting behaviour and associated scars and injuries, but were otherwise healthy.
The presented data were collected on owner’s request in the context of veterinary diagnostic measurements and informed client consent was obtained for all animals in the study. The polysomnographic measurements are non-invasive and do not cause anxiety, pain, suffering or damage in the examined animals. All examinations were done in compliance with the German animal welfare law and ethical standards.
The device used was the portable polysomnograph “SOMNOscreen™ plus” (SOMNOmedics GmbH, D-97236 Randersacker) which was fixed at the anterior neck of the horses. Special electrodes with longer cables than usually used for humans were attached with electrode paste, superglue and small paddings (Snøgg Animal Polster) to the head of the horses following a previously established diagram (see Figure 1) [5]. The appropriate skin areas were shaved and degreased with alcohol before the application. For better protection the head and neck of the horses were covered with a Sleezy hood (Horse-friends Sleezy, Loesdau GmbH & Co. KG, D-72402 Bisingen). Data were transferred via a radio module attached to the polysomnography. In addition, recordings from an infrared video camera were transferred synchronously to a computer which was set up next to the stalls.
104
R.A Grant et al. (Eds.): Measuring Behavior 2018, ISBN 978-1-910029-39-8 Manchester, UK, 5th-8th June 2018
a) b) c)
d)
Figure 1: a) SOMNOscreenTM plus, b) Electrode localization diagram, c) SOMNOscreenTM plus in protective cover, connected to the electrodes and fixed to the neck of a horse, d) SOMNOscreenTM plus in protective cover, connected to the electrodes and fixed to the neck of a horse covered with a Sleazy hood To evaluate and analyse the data, we used the polysomnography software DOMINO® (SOMNOmedics GmbH, D-97236 Randersacker) which allows a complete automated analysis immediately after data transfer. However, we aimed to retain measurement segments of low certainty and ensure correct assignment of sleep stages according to horse-specific criteria based on the present evaluations. Thus, we refrained from an automatic analysis. The manual analysis was based on the criteria established by Rechtschaffen and Kales[6] and the observations made on the horses. To classify the equine sleep stages, we combined and modulated our own observations. The human sleep stages N1 und N2 were summarized as light sleep, and stage N3 was assigned to slow-wave sleep (deep sleep). Therefore, the results are based on the 4 stages W = wakefulness, LS = light sleep, SWS = slow-wave sleep, and REM = REM sleep. The video recordings where used to determine the body positions.
Statistical analysis was performed using SPSS Statistics software (IBM Deutschland GmbH, D-71139 Ehningen). To indicate the standardized differences of the different sleep stages between the horses with and without collapses we used the Cohen's d with ≤ 0.2 = small, 0.5 = medium and ≥ 0.8 = large effect size. The Cohen´s d was accompanied by a t-test with p < 0.05 showing a significant correlation. To demonstrate the correlation between the amount of REM sleep and the number of collapses we used as well theSpearman's rank correlation coefficient (rho).
Results
The control group (n = 7) showed a quite stable total sleep time of 3.5 hours (211,9 ± 29,2 min) per night. The duration of the different sleep stages also appeared to be relatively constant. REM sleep was detected every night
105
R.A Grant et al. (Eds.): Measuring Behavior 2018, ISBN 978-1-910029-39-8 Manchester, UK, 5th-8th June 2018
in every horse and occurred mainly after midnight. With about 30 min/night and 15 % of the total sleep time REM sleep occupied the smallest amount, slow-wave sleep took up the most time with 65 % of the total sleep time and 40 min/night. The rest of the total sleep time was spent in light sleep. Total and percentage data of sleep duration and sleep stages showed significantly lower intraindividual differences than the times between individuals.
The animals studied mainly lay down after midnight and spent each night a total of about 2.5 hours recumbent in up to 5 phases. REM sleep, which requires complete muscle relaxation, occured only when lying and mostly in sternal recumbency with the head resting on the ground. Slow-wave sleep and light sleep occurred while standing and while lying.
The horses which showed collapses during the resting behaviour (n = 35), suffered from over 60 collapses with up to 199 collapses per day. The individual number of collapses depended significantly on the lying behaviour of each horse. Horses that lay down to sleep showed notably less collapses. Only 2 of the horses that experienced collapses lay down to sleep during the night. On average these 2 horses only slept for 24.5 ± 3.5 minutes in comparison to the horses not experiencing collapses (n = 7), which slept for 134.3 ± 54.5 minutes in a recumbent position. Collapses mainly occured during nighttime hours, especially between 4:00 am and 4:30 am, which corresponds to the period in which horses without collapses spent most of their lying phases and their REM sleep.
The polysomnographic measurements demonstrated that the horses experiencing collapses showed a significantly altered sleeping behaviour compared to horses without collapses. The horses with collapses showed an increased restless sleep profile with a mean value of 0.30 ± 0.09 sleep stage changes per minute. In contrast, this value was almost twice as high as that from the horses with „normal“ sleeping behaviour. Furthermore the horses with collapses spent an increased amount of time in light sleep, less time in slow-wave sleep and significantly less time in REM sleep (see Table 1). The phases of REM sleep of the horses with collapses were significantly shorter with a mean duration of one minute, in comparison to the control horses with a mean duration of more than 4 minutes.
These horses also showed more phases of REM sleep with a mean value of 0.10 ± 0.05 per minute which was almost 3 times the value of the horses without collapses (0.03 ± 0.01). The short phases of REM sleep were almost always associated with the collapses and 86,7 % of the collapses occurred in a time frame that was classified as REM sleep. Hence, a strong correlation can be demonstrated between the amount of REM sleep and the number of collapses (rho = 0.56; p = 0.001).
Table 1: Comparison of the mean sleep stage durations between the horses with (n = 35) and without (n = 7) collapses; * = significant correlations, * = large effect size
Total sleep time (TST)
min min % TST min % TST min % TST
Mean 202.1 22.8 10.8 64.6 32.4 114.7 56.8
Standard
deviation 67.9 16.3 6.1 34.2 13.7 44.6 13.2
Mean 211.9 31.8 15.2 42.3 18.9 137.8 65.8
Standard
deviation 29.2 5.2 2.6 18.2 6.8 16.4 7.0
-0.2 -0.8 -1.0 0.8 1.3 -0.8 -0.9
90.2 % of the horse owners of the horses with collapses reported on injuries, that were directly related to atonic collapses and 19/35 horses showed present injuries at the time of examination. The injuries ranged from abrasions at the dorsal fetlocks to scars and swollen synovial bursae at the carpal and tarsal joints up to head and tail fractures.
Six of the 35 horses showed an altered behaviour since the beginning of the collapses. The examination of the husbandry of these animals revealed a undercut of the recommendations by the German Federal Ministry of Food and Agriculture [7] for the minimal dimension of the lying area in the open stable per horse and accordingly the ground area in a single box in 70.0 % (14/35) of the cases.
106
R.A Grant et al. (Eds.): Measuring Behavior 2018, ISBN 978-1-910029-39-8 Manchester, UK, 5th-8th June 2018
Discussion
The results of this work suggest, that a certain level of stability as well as inter-individual and especially intra-individual repeatability of equine sleep behavior exists under relatively standardized conditions similar to humans.
Additionally, even different “sleep-types” seem to exist, which could lead to different demands on stabling and handling horses.
Furthermore, the examinations of the horses suffering from collapses, associated with sometimes severe injuries and behavioural problems, demonstrated that horses with atonic collapses suffer from a REM-sleep deficiency or rather a recumbent sleep deprivation and a massively altered sleeping behaviour. The affected horses fall into REM sleep stage while in a standing position, where the characteristically decreased muscle tone during this sleep stage causes the more or less severe collapses that range from slight swaying to sudden complete collapse. It is assumed that the observed altered rest and lying behaviour of these horses is caused by either painful disorders or insecurity and discomfort associated with environmental changes or inadequate husbandry conditions.
The serious consequences of this sleep disorder, diagnosed with polysomnographic measurements, demonstrate, that normal resting behaviour is essential for the physical and mental welfare of the horse. Especially when keeping horses in groups, favoured by an increasing number of owners, emphasis is set on the horses needs for movement, social contact, and species-appropriate feeding. However, the need for normal resting behavior including recumbent rest should not be neglected.
References:
1. Rechtschaffen, A., et al., Physiological correlates of prolonged sleep deprivation in rats. Science, 1983.
221(4606): p. 182-184.
2. Bentivoglio, M. and G. Grassi-Zucconi, The pioneering experimental studies on sleep deprivation.
Sleep, 1997. 20(7): p. 570-576.
3. Deboer, T., Technologies of sleep research. Cell Mol Life Sci, 2007. 64(10): p. 1227-1235.
4. Williams, C.D., et al., Qualitative and quantitative characteristics of the electroencephalogram in normal horses during spontaneous drowsiness and sleep. J Vet Intern Med, 2008. 22(3): p. 630-638.
5. Wöhr, A. and M. Erhard, Polysomnographische Untersuchungen zum Schlafverhalten des Pferdes.
Aktuelle Arbeiten zur artgemäßen Tierhaltung, 2006: p. 127-135.
6. Rechtschaffen, A. and A. Kales, A manual of standardized terminology, techniques and scoring system for sleep stages of human subjects. 1969, Washington D.C.: Public Health Service.
7. BMELV, Leitlinien zur Beurteilung von Pferdehaltungen unter Tierschutzgesichtspunkten. 2009:
BMELV, Referat Tierschutz.
107
R.A Grant et al. (Eds.): Measuring Behavior 2018, ISBN 978-1-910029-39-8 Manchester, UK, 5th-8th June 2018