Abuja, July 18 : Nigerian authorities have activated a health emergency operations centre in Enugu state, following a recent outbreak of cholera that has killed seven persons. Surveillance, contact tracing and risk assessment have also started in a local livestock market where the life-threatening and water-borne disease broke out, the Nigeria Center for Disease Control said in a statement on Saturday. At least 19 sick people were diagnosed with cholera by further tests, said Emmanuel Ikechukwu Obi, the commissioner for health in Enugu, on Saturday, adding that samples were collected for further confirmatory tests, reports Xinhua news agency. Cholera is a highly virulent disease characterised in its most severe form by a sudden onset of acute watery diarrhoea that can kill. The outbreak of cholera in Nigeria has been persistent, occurring annually, mostly during the rainy season, and more often in areas with poor sanitation, overcrowding, lack of clean food and water, and areas where open defecation is common. /IANS