MAIDUGURI (Reuters) - Efforts to contain a cholera outbreak that has struck more than 1,000 people in refugee camps in northeast Nigeria are being hampered because people are failing to report suspected cases to authorities, a United Nations official said.

Health officials in Borno, the northeastern state at the epicenter of both an insurgency by Islamist militant group Boko Haram and the disease outbreak, said the number of suspected cholera cases had jumped to 1,626 as of Sept. 11.

Forty people had died, it said, up from the 23 reported by the U.N. on Sept. 6.

Around 1.8 million have fled their homes because of violence or food shortages, U.N. agencies say. The rainy season has spread disease in densely populated camps where many people live in unsanitary conditions.

Most cholera-related deaths have been recorded at the Muna Garage camp, on the outskirts of Borno state capital Maiduguri.

Speaking from the camp, Souleymane Sow - a United Nations Children Fund (UNICEF) coordinator - said the “main problem” in containing the outbreak was a lack of referrals.

“When the people are sick they don’t proactively report to the clinics,” he said, adding that aid workers were conducting visits to homes in the camps to bring sick people to a treatment center.

Cholera is an acute diarrheal infection spread by contaminated food and water. It can be easily treated with oral rehydration solution if caught early but can kill within hours if left untreated.

Aside from Muna Garage, the nearby camps of Custom House, Ruwan Zafi and Bolori II have also reported cholera cases. Outbreaks have also been reported in the areas of Monguno and Dikwa, northeast and east of Maiduguri.

Borno state said there were 945 suspected cases in the Muna area, 537 in Dikwa and 144 in Monguno, as of Sept. 11.

“The number of cases has increased exponentially in Monguno,” it said in a briefing note, adding that the suspected cases were also rising in Dikwa.

More than 20,000 people have been killed in the Boko Haram insurgency and the U.N. estimates that 5.2 million people in northeast Nigeria do not have secure access to food, with tens of thousands already in famine-like conditions.