(CNN) -- Health officials have identified a strain of encephalitis never before seen in the Western Hemisphere in several bird specimens found in the metropolitan New York area.

The discovery of this new strain, known as the West Nile virus, has raised questions about whether 18 recently reported cases of encephalitis stem from the St. Louis variety of the virus or the new one.

The West Nile virus is an arbovirus closely related to the St. Louis encephalitis, but usually causes a milder form of the disease in humans. Both viruses are transmitted through the bite of a mosquito infected by feeding on an infected bird.

Symptoms in the two viruses can be similar with victims experiencing fever, headaches, muscle weakness, and disorientation. The similarities are so acute, said Westchester County Health Commissioner Harold Adel, that West Nile is sometimes called "Old World St. Louis encephalitis."

"The connection between the infections of these birds and human cases of CDC-confirmed St. Louis encephalitis is being investigated by the Centers for Disease Control and we expect to learn more information in coming days," said New York City Health Commissioner Neal L. Cohen in a statement.

Further CDC tests should determine whether patients diagnosed with St. Louis encephalitis are actually carrying the West Nile strain.