Equine Veterinary Journal Early View September 2015
By Heather Ferguson
Macrolide-induced hyperthermia in foals: Role of impaired sweat responses
Stieler, A.L., Sanchez, L.C., Mallicote, M.F., Martabano, B.B., Burrow, J.A. and MacKay, R.J.
Hyperthermia is a potentially fatal side effect of erythromycin and foals kept outside in warm weather are at particular risk. This study aimed to investigate whether this adverse effect was caused by drug-induced anhydrosis (inhibition of sweating).
Ten pony foals aged 2-3 months received either erythromycin or a control (lactose solution) 3 times daily. For the first 10 days they were kept with their dams in covered shaded stalls and from Day 11 to Day 20 they were kept out at pasture with both shaded and non-shaded areas available. Rectal temperature was measured 3 times daily in stalls and twice daily while turned out. Respiratory rate, heart rate and faecal consistency were recorded and the foals were monitored closely for signs of heat stress.
To investigate the effects of erythromycin treatment on sweat production, intradermal terbutaline sweat tests were performed in both treated and control groups on Days 1, 3, 10 and 20. This involved intradermal injection of serial dilutions of terbutaline in saline at 6 sites on the neck and placement of an absorbent pad over the injection sites. After 30 minutes the pad was removed and the weight change measured to assess the amount of sweat produced.
Terbutaline-induced sweating was significantly lower in erythromycin treated foals at all time points in the study compared to Day 1 levels. At all time points except Day 20, sweat weight was significantly lower in the erythromycin treated group than in the control group. Sweating function partially recovered after 10 days of treatment; however, at Day 20 (10 days following the end of treatment), mean sweat weights were still lower than 50% baseline values, indicating sweating function seems to take longer than this to recover. Maximal rectal temperatures were higher in erythromycin-treated foals than those in the control group, and treated foals appeared more susceptible to higher environmental temperatures.
Hyperthermia in erythromycin-treated foals is related to a drug-induced anhydrosis. This is most prominent in the first few days of treatment, after which sweating function partially recovers, but sweating remains inhibited for more than 10 days following the end of treatment.