In the United States, outbreaks of West Nile virus disease occur each summer, but this year some areas are experiencing earlier and more cases, officials say.
Officials at the Centers for Disease Control and Prevention said to date, 42 states have reported West Nile virus infections in people, birds or mosquitoes.
The CDC said it had recorded a total of 241 West Nile cases, including four deaths -- the highest number since 2004.
Almost 80 percent of the cases were reported from three states: Texas, Mississippi and Oklahoma.
West Nile virus is transmitted to people by infected mosquitoes and most are infected from June through September, but peaking in mid-August.
"It is not clear why we are seeing more activity than in recent years," Dr. Marc Fischer, medical epidemiologist with CDC's Arboviral Diseases Branch, said in a statement.
"There are no medications to treat, or vaccines to prevent, West Nile virus infection. People with milder illnesses typically recover on their own, although symptoms may last for several weeks," CDC officials said in a statement. "In more severe cases, patients often need to be hospitalized to receive supportive treatment, such as intravenous fluids, pain medication and nursing care. Anyone who has symptoms that cause concern should contact a health care provider."
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