Isolated horses become stressed with weakened immunity

An interesting study has recently been published on the PLOS ONE website. It is called: Single housing but not changes in group composition causes stress-related immunomodulations in horses (link at base of article). On my understanding, the basic conclusion is that when you stable a horse in an isolated manner, they become stressed and their immune system is compromised but changing group composition is not detrimental to their health and welfare. I will expand on this to add some detail.

Horse in stable alone
Horse in stable alone. Photo: Thinkstock.

To use the language of the researchers, “The results of the present study show that relocation to single housing in a stable led to acute stress-induced immune modulations, whereas changes in the group composition did not”. I take that to mean that whereas housing a horse singly, alone, causes stress affecting their immune system, when they are housed in different groups it does not have a detrimental effect on the immune system.

They found that the “immune-modulations” lasted for at least one week and, in addition, the horses demonstrated “disturbed and stereotype-related behaviour patterns”. Clearly, the horses indicated through their behaviour that they were stressed.

They measured stress both in observed behaviour changes and, as mentioned, in the numbers of immune cell types. For example, the number of eosinophils, monocytes and T cells declined and the number of neutrophils increased. “Neutrophils” are the most common or plentiful type of white blood cell. White blood cells are called leukocytes and are a part of the immune system.

They stated that the changes in the composition of the cells of the immune system resembles a well-known pattern in “acute social stress”. There was also an increase in cortisol concentrations. Cortisol is a hormone which is released by the adrenal glands in response to stress.

They state that individual stabling of horses has the potential to cause “chronic immune dysregulation”. That means that the immune system is regulated in a dysfunctional manner due to stress, as I understand it.

And in contrast, as mentioned, a change in “group composition” i.e. a change in the horses in a group with which the other horses interact, did not result in these sorts of indicators of stress. In fact, they found a rise in the number of interactions. In other words, the horses interacted with each other more often and therefore it seems were stimulated by their change in environment.

They state that the horses in new groups were able to consolidate social bonding through increased friendly encounters rather than by fighting over a new hierarchy.

In summary they say that when horses are relocated to individual stabling it led to “acute and lasting alterations in blood counts of various leucocyte types. In contrast, fission of the stable group did not result in behavioral, endocrine or immunological stress responses by the horses”.

The word “fission” in this context means the breaking up of a group of horses and I presume then reforming the horses into a new group. The horses were able to accept this but were unable to accept being alone and isolated. “Endocrine” refers to the hormone system.

Link to the study referred to in the opening paragraph.

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