Social isolation can have measurable — and negative — effects on us humans. But horses seem to resemble us in that regard. According to new research from the University of Hohenheim in Stuttgart, Germany, horses kept in individual stables have higher levels of stress hormones in their blood and shifts in their immune systems that are further indicative of stress. The latter factor could also make them more vulnerable to pathogens and the diseases they cause.

That being said, control animals housed in social stables with other horses did not show the same markers of stress.

Horsing around

Horse ownership has often been the privilege of the rich and the well-to-do’s, but it is safe to say that this practice has only become rarer in modern times. Horse ownership has been regarded as a privilege for a long time, and in modern times, at least in the developed world, owning a horse can be very expensive. In particular, owning an appropriate plot of land on which to raise these animals is exceedingly rare.

With that being said, horses often need more freedom and social interaction with other horses than they usually get in traditional stabling practices, especially as horses and humans can influence each other in ways that aren’t always easy to understand. Recent studies have even shown that horses are smarter than you think: they remember their owners’ faces and form strong friendships and social relationships. A new study shows that they’re also vulnerable in the face of loneliness.

The findings from the new study are based on 12 German Warmblood castrated male horses, aged 2-3 years old. These were taken from communal stables to separate spaces measuring 3.2 by 3.5 m (10.5 by 11.5 ft). Here, the horses had limited opportunities for social interactions with neighboring horses (through barred windows). They were only let out for 30 minutes a day.

“The results of the present study […] strongly indicate that social isolation is a chronic stressor,” the paper explains.”[A] resulting decrease in immunocompetence might increase disease susceptibility of the horses and thus impair their health and welfare.”

Blood samples were retrieved from the horses before relocation, one day after relocation, and finally, eight day after relocation. Based on these samples, the team analyzed how stress hormone levels varied in their blood and monitored their immune systems.

These samples revealed that the horses were feeling heightened levels of stress after the transition, as measured by their plasma cortisol levels and suggested by their behavioral patterns. Their immune systems were also reeling from this change. One and eight days after relocation to single housing, the mean numbers of eosinophils, T helper cells, and cytotoxic T cells — key white cells — dropped by up to 31%, 20%, and 22% respectively, whereas the mean numbers of neutrophils increased by 25%.

The findings strongly point to certain factors having a significant impact on the health and well-being of horses. That being said, the study does have some serious limitations.

For starters, it’s one of the first studies of its kind, but it only followed the animals for eight days — a very short period. This means that we cannot say for sure whether the changes are temporary or long-lasting.

It also doesn’t separate between the effects caused by changes in stability to those in free time or exercise. Other factors that can impact health and stress levels. Horse nutrition, any allergies each animal might have, or other factors can also influence stress levels and immunity, and these were not detailed in the current study. In other words, although the study did showcase an increase in stress levels and changes in immune potential, it doesn’t prove beyond a doubt that social isolation was the root cause.

One hypothesis as to why horses could be feeling extra stress due to social isolation comes down to how they handle threat detection in the wild. Horses live in groups and use their peripheral vision to keep tabs on their fellows’ body language as they forage. This acts like an alarm system: when one of the horses senses danger and puts its head up, other horses will do the same, and run off if needed. If left by themselves, however, horses cannot rely on this approach for warning against possible dangers and enter a state of prolonged vigilance — stress. Or, at least, that is what the authors believe is happening.

The findings showcase how traditional stabling practices may be quite unhealthy for horses everywhere. Focusing on allowing these animals ample opportunities to spend time with their peers and freely socialize in open areas would go a long way towards improving their quality of life.

The study was published in PLoS.