Research into How Short-term Transport Stress Affects Immune Function in Older Horses

Horse transport ready to roll on a cloudy day.
Horse transport ready to roll on a cloudy day. Serge Melki

Newsdate: Friday, April 19, 2024 - 11:30 am
Location: BETHESDA, Maryland

Horses are frequently transported for sporting and breeding purposes. Often, horse owners and veterinarians report “shipping fever” or pleuropneumonia after horses are transported. These cases of pleuropneumonia are generally of bacterial etiology, and Streptococcus equi spp. zooepidemicus, a commensal bacterium of the equine lung, is frequently isolated.

Horse with a cough a sign of respiratory distress.

Horse with a cough a sign of respiratory distress.

Study indicates that older horses being transported for a short distance and/or duration may have altered immune responses, which could increase their susceptibility to transport-related illnesses.
© 2017 by S. Hanusch/

Studies have shown that altering conditions within trailers, e.g., restraint length, head position, and water/hay access, can reduce the risk of transport-related illness.

However, management changes do not eliminate the risk entirely, indicating that immune function is altered because of transport, and thus, bacteria, such as S. zooepidemicus, can establish and induce pleuropneumonia.

Alterations in immune function can further compromise the tissue, which may allow for greater risk of viral respiratory illness as well. Because horses are often shipped in the company of other horses and tend to then reside in new stable environments, it is concerning that immune function may be compromised during transport.

One recent study showed that approximately half of 167 horses that had recently undergone lengthy travel while being imported to the U.S. from Europe were positive for equine herpesvirus (EHV) and/or 5.

This further suggests that transport-induced alterations in immune function allow for recrudescence of latent viral infection and potential transmission to other horses in new locations, and it is particularly concerning.

While long-term transportation (i.e., ≥ 24 hr duration) is known to affect immune function in horses and other species, the extent to which short-term transport (i.e., ≤ 8 hr duration) impacts both local and systemic immune function remains unclear, although similar physiological and endocrine factors are affected regardless of transport length.

Additionally, many studies have involved transporting horses to new environments, which makes it more difficult to isolate the effects of transport versus the effects of moving horses to new locations with new exposures.

Horse owners often transport their horses locally and return home within the same day, which makes it important to understand whether shorter trips also impact immune function. Therefore, the first objective of this study was to examine the effects of short-term transport (less than 2 hr) on immune function in aged horses who were returned home afterwards.

The second objective was to investigate whether an antioxidant supplement formulated to help support airway function would enhance immune function overall and confer protective responses during and after transport.

Long-distance transport is associated with stress-related changes in equine immune function, and shipping-associated illnesses are often reported. Horses are frequently transported short distances, yet the effects of short-term transport on immune function remain largely unknown.

Twelve horses, aged 15–30 yr, were assigned to either the control (n = 6) or treatment (n = 6) groups; treatment horses received a daily antioxidant supplement 3 weeks before and after transport. All horses were transported for approximately 1.5–2 hr on Day 0.

Blood was collected via jugular venipuncture at 15-min pre- and post-transport and on Days –21, 1, 3, 7, 14, and 21. Body temperature, heart rate, body weight, total cortisol, and gene expression of IFNγ, IL-1β, IL-2, IL-4, IL-6, IL-8, IL-10, IL-12α, IL-17α, SAA1, and TNFα in whole blood were measured. Peripheral blood mononuclear cells were isolated, stimulated with PMA/ionomycin, and stained for IFNγ and TNFα before analysis via flow cytometry. Statistical analyses were performed with significance set at P < 0.05 (SAS 9.4).

Transport and supplementation did not appear to affect body weight, heart rate, IL-4, IL-8, IL-12α, IL-17α, change (Δ) in the % and mean fluorescence intensity (MFI) of IFNγ+ lymphocytes after stimulation, or Δ in the % and MFI of TNFα+ lymphocytes after stimulation. Supplementation decreased IL-1β and SAA1 expression. Transport increased total cortisol concentration, body temperature, and IL-2, IL-6, and IL-10 expression but decreased IL-1β, TNFα, and IFNγ expression.

Short-term transportation affected physiological, endocrine, and immune responses; supplementation may ameliorate inflammation in aged horses. Immune responses were most altered at 15-min post-transport and typically recovered by Day 1, suggesting that horses may be vulnerable to disease during and almost immediately after short-term transport.

Overall, the results of this study demonstrate that short-term transport significantly affects numerous aspects of equine immune function, particularly within the first 24 hr of transport, and may reflect altered activation of the acute phase and pro-inflammatory responses to transport stress.

This is potentially concerning given both the frequency with which horses are transported across short distances as well as the consistency of the results with other research findings that have demonstrated that immune function is affected by long-term transportation.

In this study, supplementation also decreased IL-1β and SAA1 expression, which was consistent with the supplement’s anti-inflammatory properties. These findings demonstrate that supplementation can successfully alter inflammation in horses and could potentially be used to modulate the immune response.

Considering the findings in this study and in other published work, further research into the immunological effects of transport stress in horses is warranted, especially for shorter transport durations and distances.

Future work should focus on the first 24 hr after transport initiation to determine how immune function responds during this window.

Additionally, investigations of the effects of age and trip frequency are also necessary to better understand the potential risks that may accompany transport.

This study indicates that older horses being transported for a short distance and/or duration may have altered immune responses, which could increase their susceptibility to transport-related illnesses.

Although further work is needed to confirm this, additional caution should be taken in the meantime when transporting these horses, such as by ensuring adherence to general biosecurity precautions and including increased monitoring for any potential adverse effects.

Press release by HHExpress

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