A two-year-old boy, believed to have been killed by enterovirus 71, had developed acute encephalitis but did not show obvious hand-foot-mouth disease symptoms, sources said yesterday. Hand-foot-mouth is an infectious disease caused by several enteroviruses, including EV71. Symptoms include flu, fever, rashes on the palms and soles and mouth ulcers. Sources said the toddler, who attended a Yuen Long nursery, did not show obvious symptoms, which made diagnosis difficult. 'It is not very rare for such a situation to happen,' one doctor said. 'The virus can get in the patient's brain directly without causing any problems to the skin. Parents have to be very careful.' It is understood that medical workers later noticed that the boy's three-year-old sister had a skin rash, leading to suspicions that the boy might have contracted EV71. Tests were conducted on both of them immediately. The Department of Health announced on Friday the boy was believed to have died of EV71 infection on May 14. If confirmed, it will be the first fatal EV71 infection case in the SAR. The boy's sister also contracted the virus, while four other children at the nursery also developed hand-foot-mouth disease but all recovered. Meanwhile, public hospitals are on alert for the peak season of enterovirus infection. Hospital Authority deputy director Dr Ko Wing-man said yesterday guidelines on hand-foot-mouth disease had been distributed to front-line staff. Dr Ko said summer was the peak season for enterovirus infections and medical staff had been asked to be vigilant for complications such as pneumonia. EV71 killed more than 50 people in Taiwan last year. Dr Ko said an authority team visited Taiwan last year and found the virus there was different to that in Hong Kong.