¡¡¡¡NEW YORK, Oct. 12 (Xinhuanet) -- The New York City Health Department has issued an alert after five residents in Staten Island, one of the five boroughs of the city, were hospitalized with what the department called a mystery illness.

¡¡¡¡Four of the five patients, ranging in age from 22 to 54, are incritical condition at Staten Island University Hospital. Doctors said they are suffering from symptoms of encephalitis, a rare swelling of the brain, but they said they do not know at this stage what is causing the disease.

¡¡¡¡Encephalitis is a symptom of the West Nile virus, but preliminary tests for the West Nile virus have been negative. Doctors are considering a strain of another virus that causes encephalitis and is also carried by mosquitoes and ticks.

¡¡¡¡The department has sent a medical alert to physicians, laboratory directors and health-care providers, asking them to report any patients exhibiting symptoms consistent with encephalitis, an inflammation of the brain tissue.

¡¡¡¡Symptoms include fever, headache, fatigue and malaise. Some of the hospitalized patients have also had seizures and exhibited confusion.

¡¡¡¡There were nearly 200 cases of encephalitis in New York City in 2001. Enditem