At least 42 people have been killed and around 1,640 others injured in China in an unusual spate of attacks by giant hornets, roughly the size of a human thumb, health officials said on Thursday.

Since July, hornets have invaded schools full of children and descended upon unsuspecting farm workers. Among the 1,640 people injured in the attacks, 206 are receiving treatment in hospital, the National Health and Family Planning Commission said, calling for urgent efforts in treating patients and minimising the number of casualties.

“With the development of air-conditioning, urban landscaping and residential environment, hornets have started to migrate and relocate to cities, which has increased the probability of their hurting people,” the state-run Xinhua news agency reported.          

It quoted Huang Rongyao -- a senior official concerned with pest control in the city of Ankang, which has borne the brunt of the attacks -- as attributing the phenomenon to warmer-than-usual temperatures in the region.

 “Furthermore, hornets are sensitive to bright colours, the smell of human sweat, alcohol, perfume, any specially scented articles and things that are sweet as well as the running of humans or animals,” Huang said.

One of the victims, Mu Conghui, was attacked in Ankang City while looking after her millet crop. “The hornets were horrifying,” she said. “They hit right at my head and covered my legs. All of a sudden I was stung and I couldn’t move.“Even now, my legs are covered with sting holes.”Two months, 13 dialysis treatments and 200 stitches later, Mu still remains hospitalized and unable to move her legs.The influx of venom to the human body can cause allergic reactions and multiple organ failure leading to death.Government officials say these attacks are from a particularly venomous species, the world’s largest hornet, known as the Asian giant hornet or vespa mandarinia.

Hua Baozhen, a professor of entomology at Northwest Agriculture Forestry University, attributed the attacks mostly to a decrease in the number of the hornets’ natural enemies, such as spiders and birds, due to ecological changes.South China’s Guangxi Zhuang Autonomous Region also reported human injuries from hornet bites between July and September, the agency said.