This study compared the immune response to social stress in two groups of young male adults: one group with an urban upbringing in the absence of pets; the other group with a rural upbringing in the presence of farm animals. All of the participants (20 in each group) were between the ages of 20 and 40, were not experiencing physical or mental health problems, had not been traumatized during early life, and shared a number of other socioeconomic situations and characteristics.
All of the participants were individually exposed to the Trier Social Stress Test (TSST) – a laboratory procedure used to induce stress in humans. Immediately before and five times after (at 5, 15, 60, 90, and 120 minutes) the TSST, heart rate and blood pressure were assessed and blood and saliva samples taken. Blood sample analysis included plasma interleukin readings and readings of viable peripheral blood mononuclear cells (PBMCs) – both indicators of immune system functioning. The saliva sample analysis included readings of α-amylase, an indicator of sympathetic nervous system activity. Participants also completed validated questionnaires assessing their subjective strain at different assessment time points after the TSST.
Results showed that significantly more participants with rural upbringing had regular contact with pets and/or farm animals during adulthood than participants with urban upbringing. Findings also showed that TSST exposure increased the number of viable PBMCs in both groups at the 5-minute assessment compared to the pre-TSST assessment. Assessment results for other time-period assessments, however, showed clear differences between the two groups. Of special concern was evidence of a prolonged inflammatory response to socially-induced stress in the urban group. This prolonged response was not evident in the rural group.
Results demonstrated increased immune system activation after a social stressor in participants with an urban upbringing without pets compared to those with a rural upbringing with contact with farm animals, even though various measures demonstrated that the the research protocol was more stressful for those who had a rural upbringing. The authors conclude that the immunoregulatory capacity responsible for adequate resolution of stress-induced immune activation tends to be compromised in urban participants raised without animal contact. These findings are consistent with earlier research suggesting that “the rapid rise in inflammatory physical and mental diseases in modern societies is due in part to a lack of exposure to immunoregulatory microorganisms” (the "old friends hypothesis). The authors also offer another possible explanation for their findings. They note that “natural landscapes provide a stronger positive health effect compared with urban landscapes, resulting in accelerated short-term recovery from stress or mental fatigue, faster physical recovery from illness, and long-term overall improvement on people's health and well-being.”
This research raises some concern about increased vulnerability to psychiatric disturbance for people growing up without contact with animals. This research also supports efforts to provide increased opportunities for children to become actively engaged with the natural environment -- including environments with animals -- on a regular basis. Growing up with animals seems to make a difference for a healthy immune system later in life. This, in turn, plays a role in how one responds to stress.
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