Gordon Bonnington Willie Messum, chief adviser on medical matters to the government of the South African Republic (Transvaal), was the second son of Julian A. Messum, paymaster in the Royal Navy, and his wife Marion. He was educated in Germany, France, and at Guy's Hospital, London, qualifying as a member of the Royal College of Surgeons (MRCS) of England and a licentiate of the Royal College of Physicians (LRCP) of London. He was a prizeman in both his first and second years of medical study, and held the position of prosector at Guy's Hospital. After gaining experience in surgery there he emigrated to the Cape Colony, where he was licensed to practise on 31 May 1879 and appointed district surgeon at Peddie, in the Eastern Cape. After a few months he moved to King William's Town, where he remained for the next three years, declining an appointment as district surgeon at Maseru, Lesotho, in 1881. In 1883 he was registered as a medical practitioner in the South African Republic and became a partner of Dr John E. Dyer (senior), district surgeon of Pretoria. During the next few years he acted for Dyer several times as district surgeon, until he succeeded to the post in 1888. He also served the government in various other positions: consulting medical officer to the Volks Hospitaal, medical officer to the Staats Artillerie, supervisor of the leper institution, ex officio member of the Health Committee of Pretoria, chairman of the Geneeskundige Commissie (Medical Board), and ex officio member of the republic's Board of Examiners. In 1903, after the Anglo-Boer War (1899-1902), he was offered a seat on the municipal council of Pretoria.
Messum was a founding member of the Pretoria Medical Society, established about 1888, and served on the committee that drew up its rules. The next year he became the central figure in the so-called "Pretoria Hospital Scandal". In January the brother-in-law of Dr James A. Kay* was admitted to the hospital with a compound fracture of the lower leg, following a dynamite accident. Dr Messum probed the wound and dressed it, but nothing more. Upon his return from out of town he found that Dr Kay had taken over the case and a few days later Kay, with the assistance of Dr Davidson, amputated the shattered limb. Three months later Davidson brought a charge of professional negligence against Messum. The hospital board and three medical assessors more or less exonerated Messum, but the case split the medical fraternity of Pretoria into opposing camps and seems to have led to the demise of the Pretoria Medical Society.
In 1890 Messum applied for an appointment as superintendent of the Grahamstown Lunatic Asylum, but then withdrew his application. Early in 1892 he was appointed visiting medical officer to the newly established Krankzinnigengesticht (Lunatic asylum) in Pretoria, later renamed Weskoppies Hospital. He occupied the post until Dr H.A.E. Smeenk was appointed as the institution's full-time medical director in May 1896.
Messum's publications included a paper in the Lancet in 1891 on "A case of Ainhum" - a tropical condition characterised by the development of a groove around the proximal end of the little toe, which eventually drops off. In 1894 he published a paper in the South African Medical Journal in which he supported the communicability of leprosy by contagion and heredity. He was the first president of the South African Republic Medical Association, founded in December 1894, and was still its president by 1897. By 1898 he was a member also of the (second) South African Medical Association. In 1899 he was on leave in England and qualified as Doctor of Medicine (MD) at the University of Durham. He was also a representative of the South African Republic at a congress on bubonic plague in January 1899, returning to Pretoria via Lourenco Marques (now Maputo) in December.
At some time, probably in 1902, he was a clinical assistant in London at the Nose, Ear and Throat Hospital and at Guy's Hospital, and attended clinics on ear, nose and throat diseases, a topic which became his main interest though he did not specialise in it. In 1908 a paper by him on "Acute and chronic suppurative otitis media" was published in the Transvaal Medical Journal (Vol. 4, pp. 15-20). In this paper he reviewed the causes and treatment of infection of the middle ear, drawing also on his own experience with the disease. Earlier he had published "Suppurative cholecystitis [infection of the gall-bladder] associated with enteric" in the South African Medical Record (1905). Subsequently he reported on "Twenty cases of leprosy treated simultaneously with Deycke's Nastin" (Transvaal Medical Journal, 1909/10, Vol. 5, pp. 214-222), but found the remedy to have no effect.
In 1916 Messum was appointed medical examiner under the Miners' Phthisis Act. By 1926 he was still listed as practising in Pretoria, where he was a consultant on ear, nose and throat diseases to Pretoria Hospital. He was survived by his wife, Grace A. Messum, born Tyson, and their four children.