With the number of West Nile Virus (WNV) cases in the United States nearly doubling in the last 12 days, Center for Disease Control (CDC) officials say they are alarmed and don’t know why the disease has suddenly become so problematic.

They say for residents in Texas and across the Southwest, the urgency of control has reached a critical stage. Texas, Oklahoma and now New Mexico have more than half of the 2,192 cases of the virus reported this year and 49 of the 92 deaths attributed to the disease.

“The first known case of West Nile Virus was reported in New York City in 1999, but there were several years that went by when things settled down for a while,” reports CDC’s Janet McAllister. “But this current outbreak is unusual and we don’t know why this is happening.”

In spite of isolated cases of WNV each year, usually in the late summer months when mosquitoes breed heavily, McAllister says until now outbreaks have been limited to mostly southern states. But this year 44 states are reporting WNV cases and she says the unpredictability of the virus has researchers stumped.

“This current outbreak was unexpected. What we know about West Nile Virus is that in 13 years since it was first introduced in the United States, it has now become endemic. But until now, we have never seen such a widespread outbreak, and one that has developed so quickly,” she added.

West Nile Virus is a flavivirus commonly found in Africa, West Asia, and the Middle East. It is closely related to St. Louis encephalitis virus which is also found in the United States. The virus can infect humans, birds, mosquitoes, horses and other mammals.

While most people who contract the virus experience only mild flu-like symptoms including lethargy, weakness, muscle stiffness, malaise and loss of appetite, other and more serious symptoms might develop that include possible neurological signs of encephalitis (inflammation of the brain). The latter might include stumbling, staggering, abnormal posture, disorientation, muscle twitching, seizures, paralysis, and coma. In humans, those with underlying health conditions, including the elderly and extremely young, are at greater risk. In some cases, the disease can be fatal.

Over the last 12 days, the number of WNV cases in Texas alone has risen from 465 to nearly 900 cases. The death count in Texas has risen from 17 related deaths to 40 deaths during the same period.