Arnold Theiler, veterinarian, was the son of Franz Theiler, a teacher with an interest in natural history, and his wife Maria. He completed his study of veterinary science at the Universities of Bern and Zurich in August 1889, in an era of great scientific discovery, especially in the biomedical sciences under the leadership of Pasteur, Robert Koch* and others. After a brief spell in veterinary practice he decided to widen his horizon and emigrated to the South African Republic (ZAR) in 1891. When he arrived in Pretoria he faced a plethora of stock diseases unknown to him, no veterinary services in the two Boer republics, and farmers used to dealing with disease problems in their own way, distrustful of new "foreign" ideas and authorities and with little appreciation for veterinary medicine or scientific research. He started his career as a farm hand where he had ample opportunity to study sick animals but also lost his left hand in a chaff cutter. His first opportunity for public service arose in the medical field. When smallpox broke out in Swaziland in 1892 he anticipated its possible spread to the ZAR and ordered the necessary virus strain and equipment from Switzerland. When the disease was indeed diagnosed in mineworkers in Johannesburg in 1893 he was prepared and offered his services to the authorities. Within months he prepared sufficient quantities of high quality vaccine to bring the outbreak under control.
When news of a devastating cattle disease moving southwards towards the ZAR was received in Pretoria in 1896 he was summoned by President Krüger and sent to Rhodesia (now Zimbabwe) to investigate. His diagnosis of rinderpest was later confirmed, triggering frenetic activity throughout southern Africa in an attempt to control this most dreaded disease of ungulates. The Cape Colony enlisted the help of Robert Koch from Germany whereas the ZAR, in collaboration with Natal, appealed to the Pasteur Institute in France for help. Theiler was joined by H. Watkins-Pitchford* from Natal and together they developed a serum-virus vaccination procedure later refined with the aid of two French scientists, J. Bordet* and J. Danysz*. Koch was first to publish his bile-based vaccine but Theiler's method proved to be safer and more effective. Together they were instrumental in permanently eradicating the disease from South Africa. This was an enormous accomplishment but only achieved after 50% of the cattle population and even more of the susceptible wildlife of the country was lost. The price was high but it served to convince the authorities of the economic importance of stock diseases and the need to support veterinary research. It also established Theiler's reputation as a scientist of the highest order and in 1898 his recommendation that a research institute should be founded was approved and he was appointed as director of the new Bacteriological Institute at Daspoort near Pretoria and as state veterinarian for the state artillery of the ZAR. In the latter capacity he served in the field during the first phase of the Anglo-Boer War (1899-1902) but later returned to Daspoort, mainly to continue his study of horse sickness, a disease of great economic importance at the time in view of the horse's essential role in the war and in general transport. Such was his reputation that the British forces retained his services after the occupation of Pretoria, allowed him to continue his research and consulted him on the control of typhoid and dysentry.
After the war the country was faced with a menacing new disease: East Coast Fever (ECF), which was introduced from neighbouring countries with cattle imported to restock the depleted herds. After two years of research Theiler had proof that the disease was caused by a previously unknown tick-borne organism which was later named Theileria parva in his honour. By means of dipping programmes to control the transmitting tick, ECF was eventually also eradicated from the country. Theiler's success in solving animal health problems facilitated approval of his request for larger and more modern facilities. In 1908 a new institute was founded at Onderstepoort, north of Pretoria. The Onderstepoort Veterinary Institute was destined to become one of the leading veterinary centres in the world under his guidance during the next 20 years and played a vitally important role in the development of agriculture, not only in South Africa but also on the entire African continent and in other parts of the world. Theiler became a world figure in veterinary science with recognised contributions in fields as varied as bacteriology, protozoology, virology, toxicology, nutrition, helminthology, pathology and botany. His most lasting contribution to the development of South Africa, however, is perhaps his legacy of scientific values which inspired generations of scientists that followed in his footsteps. He was largely responsible for the establishment of a Faculty of Veterinary Science at the University of Pretoria and became its first Dean in 1920, a position he held until his retirement in 1927. He continued his research in Switzerland for some time, but in 1934 returned to Onderstepoort until just before his death. Generations of students benefitted from his example as a dedicated scientist. It is fitting that his ashes rest below a statue erected in his honour at Onderstepoort. The University of Cape Town and the University of the Witwatersrand both awarded him an honorary DSc degree in 1935.
Theiler published widely on a variety of subjects. A list of about 240 publications appeared in the Arnold Theiler Memorial Number of the Journal of the South African Veterinary Medical Association, Vol. VII(4), pp. 135-186.