The number of West Nile virus infections in the United States has jumped more than 60 per cent in the past week in what officials say is one of the country's biggest-ever outbreaks of the disease.
The US Centres for Disease Control and Prevention (CDC) said on Wednesday that 1,118 cases and 41 deaths had been reported so far this year, up from less than 700 cases and 26 deaths a week ago.
That is the highest number of West Nile virus infections reported through the third week of August since the virus was first detected in the US in 1999. The worst US outbreak occurred in 2003, with 9,862 cases and 264 deaths that year.
"We're in the midst of one of the largest West Nile virus outbreaks ever seen in the United States," said Dr Lyle Petersen, head of the CDC's vector-borne infectious diseases unit.
Federal officials are stumped by the severity of the outbreak. Cases usually flare up in the summer because the illness is most often transmitted from infected birds to people by mosquitoes.
Victims may suffer from fever and aches that can become severe or even cause death, especially in the elderly and children. There is no specific treatment for the infection, and many people stricken do not see a doctor, meaning that cases are probably underreported.
More than half of this year's cases are in Texas, but the disease has now been detected in 47 states. Thirty-eight states have reported cases in humans, with only Alaska, Hawaii and Vermont reporting no cases.