GUANGZHOU -- At Baiyun International Airport in south China's Guangzhou, four infrared devices on the ceiling read body temperatures, sending signals to a nearby monitor.

China confirmed a second imported Zika case Monday, six days after the first. Both entered the mainland through Guangdong.

"Once abnormal body temperature is detected, the monitor will go red and an alert will be set off. After years of improvement, we can ensure each and every traveller's body temperature gets checked," said Meng Chuanjin, deputy head of the entry-exit inspection and quarantine bureau of Guangdong Province.

Guangdong, south China's province with the metropolitan cities of Guangzhou and Shenzhen, is at the center of exchanges between China and South America where Zika appears to originate. Both patients had stayed in Venezuela.

Guangdong is under great pressure to keep viruses out of the country with the huge amount of people coming and going through the province, nearly 12 million through Baiyun alone in 2015.

More than 30 customs officers have been trained to identify possible infectees even if their temperature is normal, Meng said.

Guangdong started screening for Zika virus in 2014 and examined more than 800 suspected cases, including the two confirmed ones.

As the weather gets warmer and trade with South America picks up pace, it will be even more difficult to keep Zika in check, Meng said.

"We are planning to offer free medical checks for those who have travelled to epidemic areas or were in contact with patients. They can decide on their own whether to take the tests for early diagnosis and treatment," he added.

The second patient was stopped in Baiyun on Friday before being confirmed with Zika on Monday. The first patient was discharged from hospital on Sunday after a full recovery.

Symptoms of Zika, which spreads through mosquito bites, include fever, joint pain, a rash, conjunctivitis, headache and muscle pain