The United States has just had its worst year ever for West Nile virus, according to the Centers for Disease Control and Prevention. As of September 11, more than 2,500 cases have been reported and 118 people have died from the ailment.
West Nile virus is now endemic to the United States. The seasonal epidemic flares up around June and continues until about October.
Dr. Lyle Peterson of the U.S. Centers for Disease Control and Prevention says there have been more than 2,500 cases of the mosquito-born disease so far this year. "The number of people who have become ill with West Nile virus disease continues to go up and we expect the virus to continue until October," he said.
The virus has been reported in every state except Alaska and Hawaii. But Peterson, who heads the CDC’s division of vector-born infectious diseases, says most of the cases have occurred in just a handful of states. "Two-thirds of all cases have been reported from six states: Texas, Louisiana, South Dakota, Mississippi, Michigan and Oklahoma. And 40 percent have been reported from Texas," he said.
The CDC reports that most people who contract the virus will have no symptoms, but some will go on to develop West Nile neuroinvasive disease. In these cases, patients can have seizures, muscle weakness and paralysis. The disease can be fatal.
Peterson said the number of neuroinvasive cases is the highest to date since the virus was first detected in the United States 13 years ago. “Of the 2,636 cases, 1,405 or 53 percent were classified as neuroinvasive disease such as meningitis or encephalitis."
"We consider the number of neuroinvasive disease to be the best indicator of the scope of the epidemic since these cases are most consistently reported," he said.
Although the number of cases is up 35 percent from last week, Peterson suggests the worst is over. "We've turned the corner on the epidemic. West Nile virus outbreaks in the United States tend to peak in mid to late August," he said.
That's when days get shorter and mosquitos become less active.
While the West Nile virus will soon go dormant, scientists will be analyzing data to find out why it was so virulent in Texas. They will look at factors that include the number of mosquitos in an area, how fast the virus replicates in the mosquitos, and they'll also look at climate.
"There's a very complicated relationship of temperature and rainfall and all of these factors in West Nile virus transmission," said Peterson.
So far, scientists have not been able to figure out how those relationships help the virus spread. Peterson says it may take several more years before they uncover the clues.
In the meantime, there is no vaccine to protect people from this sometimes deadly illness.