WASHINGTON - US federal health officials said on Wednesday that West Nile virus cases rose more than 40 percent over last week, claiming altogether 66 lives in the country so far this year.

According to new numbers announced by the US Centers for Disease Control and Prevention in a telephone news briefing, 48 US states had reported 1,590 human infections of the virus as of Tuesday, including 66 deaths. The figures represent a 42 percent surge in the number of cases and a 61 percent increase in the number of deaths in a week.

The number of human cases further marked the highest case count through the last week of August since the virus was first detected in the country in 1999, said the center.

But as for a whole year record, 2003 saw 9,862 reported cases of West Nile Virus in the United States, and 2002 saw over 4,100 cases including more than 260 fatalities.

Dr. Lyle Petersen, director of the center, said this year's numbers will continue to rise.

Over 70 percent of the cases have been reported from six states, including Texas, South Dakota, Mississippi, Oklahoma, Louisiana and Michigan. Texas is the hardest-hit state, with 45 percent of infection cases nationwide and 31 deaths.

Based on previous experience, federal health officials did not expect Hurricane Isaac, downgraded as a tropical storm Wednesday afternoon, to have a major impact on this year's outbreak.

The West Nile virus is a mosquito-borne illness that is prevalent in temperate and tropical regions and can lead to serious neurological disease in some cases, and its symptoms include sudden onset of fever, headache, nausea, dizziness and muscle weakness.

Scientists say about 80 percent of human infections show no symptoms.