James S.H. Spreull qualified as a veterinarian (MRCVS, Edinburgh) in 1895 and in January 1897 arrived in Cape Town, following his temporary appointment to the Veterinary Service of the Cape Colony for rinderpest control. This campaign took him to Barkley West and various other places in the eastern and northern parts of the territory. At Herschel (Transkei) he was the first to use a defibrinated immune blood vaccine developed by Professor Bordet*, but found it to be ineffective. In 1898 he was transferred to Fort Beaufort where he assisted R.W. Dixon* in his work on the life history of the blue tick. The next year he was stationed in Cape Town and started small experiments on biliary fever in dogs, showing that the disease can be transmitted by inocculation with the blood of sick animals. He sent blood samples to G.C. Purvis*, in which the latter first detected the protozoan parasite causing the disease.
As early as 1901 Spreull conducted valuable research on bluetongue in sheep at Somerset East and was the first person to introduce a method of immunisation, by means of simultaneous inocculations of immune serum and virulent blood. His work was first reported in the Agricultural Journal of the Cape of Good Hope (1902, Vol. 20, pp. 469-477, 530-534), and later in the Journal of Comparative Pathology and Therapeutics (1905). He also worked on Lamsiekte and was in charge of experiments on this disease conducted at Koopmansfontein, Griqualand West, during 1907-1908. The experiments proved that bone meal was effective in preventing the disease.
In 1908, following a study period in England, he was awarded the FRCVS for his thesis Blue tongue in South Africa. After his return he was involved in East Coast fever control at Umzimkulu and in 1911 was appointed senior veterinary officer in charge of the Transkei, stationed at Umtata. He published "East Coast Fever inocculation in the Transkeian Territories, South Africa" in the Journal of Comparative Pathology and Therapeutics in 1914. The paper reported on the immunisation of 283 000 head of cattle between 1911 and 1914, by injecting a few milliliters of "spleen and lymphatic gland pulp" from infected animals, producing full or partial immunity in 70% of cases. From 1916 he served as senior veterinary surgeon in the Transvaal and in 1921 was transferred to the Cape Western Division in the same capacity until his retirement in 1934. After retiring he practiced privately in East London. Spreull was a member of the Cape of Good Hope Veterinary Medical Society from 1907. In 1917 he joined the Transvaal Veterinary Medical Association and served on its council from 1917 until the formation of the South African Veterinary Medical Association in 1920, and then on the first council of the latter organisation.