Gabriele Landolt, DVM, DACVIM

 

Viral respiratory disease, such as equine influenza, remains a commonly occurring respiratory illness of horses despite the wide-spread use of efficacious vaccines. While flu vaccination can substantially lessen the severity of clinical disease, it often does not fully protect your horse from getting infected with the virus. Therefore, even the well-vaccinated equine athlete can sometime catch equine influenza and commingling of horses during horse shows and competitions is a well-documented risk factor for virus transmission. Like humans suffering from the flu, clinical signs of influenza in horses include lethargy, loss of appetite, fever, coughing, and watery nasal discharge. While an uncomplicated course of equine influenza is typically not life-threatening, the development of complications, such as bacterial pneumonia, can have fatal outcomes.

Medical treatment of equine influenza is mainly geared toward easing of disease symptoms, for example by using anti-inflammatory drugs to reduce fever, but they typically do not shorten the course of illness. In addition to these treatments, it is generally recommended that horses with the flu should be rested until the disease has run its course. For anybody that has ever had the flu, this recommendation makes perfect sense, as you typically do not feel like exercising when your chest feels like it is in a vice and it is difficult to catch your breath. Despite this, when your horse is an equine athlete slated to participate in a future competition it can be tempting to ignore this sage advice and continue to exercise the horse to keep it fit even when it has the flu. This raises two fundamental questions; 1) does exercise have a negative effect on the immune system and thereby increases the likelihood of getting infected with the flu and 2) does exercise alter the duration and severity of an equine influenza infection?

The first question has been extensively studied in both humans and animals, including horses. The results of this research have demonstrated that exercise can have both positive and negative effects on immune function and the susceptibility to viral respiratory disease. While data suggest that low to moderate levels of exercise may have a positive effect on immune function, high-intensity exercise appears to impair the function of various immune cells (Ronsen et al. 2001), which results in an increased susceptibility of athletes to minor infections. For example, some human studies have found that high level athletes more commonly suffer from sore throats and flu-like symptoms and that colds may last longer in athletes than in members of the general population (Gleeson 2007). More importantly, intense exercise has been linked to an increased susceptibility of ponies to infection with equine influenza virus and was found to be associated with a significant suppression of immune responsiveness to the virus (Folsom et al. 2001).

To answer the second question of whether exercise alters the course of equine influenza infection, researchers compared two groups of influenza infected horses with one group being exercised on a treadmill five days a week following infection and the second group being stall rested (Gross et al. 2001). While all horses developed clinical signs of equine flu, the symptoms were worse in the exercised group than in the stall rested horses. Moreover, the exercised horses lost substantially more weight than the rested horses and they also appeared to fatigue quickly during and had a delayed recovery after exercise. Despite these findings, there were no differences in the duration of clinical disease or the length of virus shedding. Highlighting the potentially detrimental impact of intense exercise on the course and outcome of the flu, a more recent study performed in infected mice found the previously described increase in susceptibility to infection and severity of symptoms, but they also found a higher death rate in mice undergoing intense, prolonged exercise compared to controls (Murphy et al. 2008).

While low-to-moderate exercise of flu-infected horses likely will not have such detrimental health impacts, it is important to note that studies also did not show that exercise could shorten the duration of illness. Therefore, if your horse contracts the flu, it is best to be cautious and likely safest to not exercise it while it is sick. If you do decide to exercise your horse, it should only perform mild to moderate exercise at the most (i.e. walking or trotting on a lunge line) and you need to allow your horse to rest immediately if any signs of distress become apparent. Bottom line is, it is best to contact your veterinarian if you suspect that your horse has the flu, so that you can discuss the treatment and care for your ill equine companion prior to starting or continuing an exercise program.

 

References:

Folsom RW, Littlefield-Chabaud MA, French DD, Pourciau SS, Mistric L, and Horohov DW. Exercise alters the immune response to equine influenza virus and increases susceptibility to infection. 2001. Equine Veterinary Journal. 33:664-669.

Gleeson M. Immune function in sport and exercise. 2007. Journal of Applied Physiology. 103:693-699.

Gross DK, Hinchcliff KW, French PS, Goclan SA, Lahmers KK, Lauderdale M, Ellis JA, Haines DM, Slemons RD, and Morley PS. 1998. Effect of moderate exercise on the severity of clinical signs associated with influenza virus infection in horses. Equine Veterinary Journal. 30:489-497.

Murphy EA, Davis JM, Carmichael MD, Gangemi JD, Ghaffar A, and Mayer EP. 2008. Exercise stress increases susceptibility to influenza infection. Brain, Behavior, and Immunity. 22:1152-1155.

Ronsen O, Pedersen BK, Oritsland TR, Bahr R, and Kjeldsen-Krag J. 2001. Leukocyte counts and lymphocyte responsiveness associated with repeated bouts of strenuous endurance exercise. Journal of Applied Physiology. 91:425-434.