GUANGZHOU, Feb. 17 -- The number of human H7N9 cases in south China's Guangdong Province is now 51, with a new case reported Monday, local health authorities said Tuesday.

A 33-year-old pregnant woman from the capital city of Guangzhou was diagnosed with the disease on Monday. She is in critical condition, the provincial health and family planning commission said in a press release.

Winter and spring see the most new cases of bird flu.

In Guangdong Province and Hong Kong, thousands of chickens have been culled this year. Several cities in Guangdong have banned the trade of live birds.

Shanghai, Fujian, Jiangxi, Guizhou and Zhejiang have also reported human cases of H7N9 this winter.

H7N9 was first reported among humans in March 2013.