Theses and Dissertations (Equine Research Centre)

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    Contribution of respiratory heat loss to heat balance in Thoroughbred horses performing near maximal exercise under thermoneutral and hot-humid conditions
    (University of Pretoria, 2010-10-06) Meintjes, R.; Guthrie, Alan John; upetd@up.ac.za; Lund, Raymond John
    The purpose of the studies presented in this thesis was to develop a greater understanding of the contribution of respiratory heat loss to the thermal balance of exercising horses. In the first experiment the effect of three different warm-up regimens on the thermal balance of Thoroughbred horses was investigated. The experiments showed that a low intensity warm-up was most beneficial aiding heat dissipation during subsequent exercise. The study also showed the heat loss by sweating is not restricted by the rate of sweat production, but by the evaporation rate of the sweat. In the second experiment, horses were exercised to fatigue in thermoneutral and hot-humid environments. The evaporative heat dissipation from sweating and from the respiratory tract was severely impaired during the hot humid exercise protocol. There was a significant increase in the heart rate and the metabolic rate during the hot humid protocol, thus indicating the additional work done by the horse in an effort to dissipate the rapidly accumulating heat. The significantly shorter time to fatigue may be a mechanism to protect the horse from circulatory collapse as the circulatory demands for cardiac output exceed its capacity. In the third experiment adaptations that the horse is able to make to alleviate the compromised evaporative heat loss were identified. These experiments showed that the horse is able to shorten its stride, increase minute ventilation and the velocity of the air in the conducting airways. The results presented also indicate that the horse is able to modify the evaporative area of the airways to enhance evaporative heat loss from the respiratory tract. The experiments also showed that during exercise in hot-humid environments, small changes in the evaporating surface vapour pressure have a significant effect on the vapour pressure gradient thus having a significant effect on the evaporating heat loss. Finally, the lessons gained during the experiments presented in this thesis were used to revise and refine a mathematical model of the thermal balance of exercising horses. The resulting model is more accurate and easier to apply to use in the field.
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    Effect of multiple doses of imidocarb dipropionate on renal and hepatic function of ponies
    (University of Pretoria, 2010-10-06) Van den Berg, Koos; Guthrie, Alan John; upetd@up.ac.za; Meyer, Carla
    Previous studies have shown that four intramuscular doses of imidocarb dipropionate administered at 72-hour treatment intervals are effective in sterilising experimental Babesia equi infections in horses. It has also been documented that imidocarb dipropionate has dose dependent hepato- and nephrotoxic effects in a number of species. The purpose of this study was to examine the clinical and clinicopathological effects of this multiple treatment regime of imidocarb dipropionate in healthy ponies. Specific emphasis was placed on the potential adverse effects on hepatic and renal function in this species. Serum bile acids and serum gamma glutamyltransferase activity were measured to evaluate the effect of this treatment regime on hepatic function. The diffuse hepatocellular necrosis and pronounced periportal hepatocellular swelling and degeneration previously reported as the most consistent hepatic lesions noted in equines following imidocarb treatment were not evident at the dose and dosage interval used in this study. Urinary gamma glutamyltransferase: creatinine ratios (IU/g) and fractional clearance of sodium, potassium and phosphate (%) were calculated as a measure of renal function. Urinary GGT and urinary GGT: creatinine ratios were significantly elevated on Day 5 of the trial and were considered indicative of transient changes in renal function. The rapid return to previous baseline values supported reported observations that changes between 25 and 100 IU/g may be a function of drug excretion and are not necessarily indicative of significant nephrotoxicity. It was concluded that four intramuscular treatments of imidocarb dipropionate at a dose of 4 mg/kg every 72 hours may be a relatively safe method whereby persistent Babesia equi infections can be sterilised.