John Thomas Coley qualified as a veterinarian (MRCVS, London) in May 1895 and was appointed as a veterinary-lieutenant in the Army Veterinary Department on 26 February 1896. He came to South Africa in May 1897 and was stationed at Eshowe. During the Anglo-Boer War (1899-1902) he was besieged in Ladysmith. In view of the importance of horses in this war his main responsibility was to look after their health and welfare, leading to an interest in African horsesickness, a disease unknown in Europe. After his return to England in September 1901 he published an article entitled 'Clinical notes on African horse-sickness' in the Journal of Comparative Pathology and Therapeutics (1901) in which he described the symptoms and his unsuccessful attempts to cure the disease by administering turpentine, carbolic acid, potassium iodide and glycerine. A similar article by him appeared in the Veterinary Record (1902).
In August 1903 Coley, then an FRCVS, was promoted to veterinary-captain.