Cases of West Nile virus are up 25% over the past week and are expected to rise further over the next several weeks, the U.S. Centers for Disease Control and Prevention said Wednesday.
As of Tuesday, 1,993 reported cases of West Nile virus have been reported in humans, including 87 deaths. Last week, the CDC said there were 1,590 cases with 66 deaths.
"As we expected, the number of humans with West Nile virus disease continues to rise in the United States," said Dr. Lyle Petersen, director of the CDC's Division of Vector-Borne Diseases. "We expect this increase to continue for the next several weeks, probably until October."
The number of West Nile cases is the highest reported to the CDC through the first week in September since 1999, when the mosquito-borne virus was first detected in the United States.
More than 70% of cases are in six states: Texas, South Dakota, Mississippi, Michigan, Oklahoma and Louisiana, officials said. Texas bears the brunt of the outbreak, with nearly 45% of cases.
Wednesday's briefing focused on how Hurricane Isaac may have affected the outbreak and how Texas is managing its high number of cases.
"As of now, 2012 is the worst year in the state of Texas for West Nile disease," said Dr. David L. Lakey, commissioner of the Texas Department of State Health Services. He says that before this, 2003 was the worst year for the virus.
Although Petersen did not give any specific numbers associated with states affected by Hurricane Isaac, he stressed that the CDC is working with state and local officials in those areas to offer assistance on managing infestations.
Experience with natural disasters such as hurricanes has shown that West Nile virus cases do not tend to increase in their wake, he said. However, people who spent more time outdoors because they were displaced from their homes or during recovery efforts may be more likely to contract the virus.
Aerial spraying has sometimes been used to control mosquitoes after hurricanes, Petersen said.
In Texas, spraying has been effective in areas that utilized it, Lakey said.
A 93% reduction was seen in the density of mosquitoes in areas that received two consecutive sprays, he said. Unsprayed areas showed increases in virus-carrying mosquitoes, and places that were not sprayed consecutively showed smaller reductions in mosquitoes.
Protection against mosquito bites is the best defense against West Nile virus, Petersen said. He recommended wearing long pants and sleeves during dusk and dawn -- prime mosquito times -- to prevent bites.
In addition, he said, wear insect repellent when you go outdoors; install or repair screens on doors and windows; use air conditioning if possible; empty standing water from outdoor containers and objects that collect water; and support local mosquito control programs.