Forty-two people are dead and and more than 200 are hospitalized in China after an unusual spate of attacks by Asian giant hornets.

The hornet attacks happened in Shaanxi province in northwest China, the state-run Xinhua news agency reported, adding that more than 1,640 people have been injured in the rash of attacks. Of the 206 people still hospitalized by hornet attacks, 37 remain in critical condition, according to the BBC.

Ankang City was reportedly the worst hit: at least 18 people there have died from the hornet attacks, which have taken place since July. The cities of Hanzhong and Shangluo were also badly hit.

The cause of the attacks remains unclear, though Huang Rongyao, an insect control expert at the Forestry Bureau in Ankang, told Xinhua that increased local vegetation growth has provided the hornets with more territory and two months of continous hot weather had made the hornets more active.

Another explanation, provided by Hua Baozhen, a professor of entomology at Northwest Agriculture & Forestry University, is that ecological changes have resulted in a decrease of the hornets' natural enemies, including birds and spiders.

Asian giant hornets (Vespa mandarinia) are suspected in the attack. Not only are Asian giants the world's largest hornet species, they have highly toxic stings. Multiple stings to a human victim can result in death if gone untreated. The insect is a vicious predator of insects, including honey bees.

A Chinese medical official in the city of Ankang said people should seek medical assistance if they are stung more than 10 times and warned that emergency treatment is required for anyone stung 30 times or more.

Last month, a 55-year-old woman told a provincial newspaper that she was stung more than 200 times, the South China Morning Post reported, adding that another sting victim was hospitalized for acute renal failure after hornets reportedly chased him and stung him on the head and legs.