Reported by Dr. Tiffany Chao, ABC News Medical Unit:
Reports of West Nile virus infection in the country now total 1,590, the U.S. Centers for Disease Control and Prevention reported Wednesday - an increase of more than 40 percent in one week alone.
CDC officials also reported during a Wednesday afternoon teleconference that 66 people have died from the disease so far. Of all of the cases reported thus far, 889 - or 56 percent - are classified as neuroinvasive, meaning patients develop meningitis, encephalitis or paralysis.
These figures represent a striking increase from last week's report of 1,118 cases, 629 of which were neuroinvasive, and 41 deaths.
Still, officials say they anticipated that these numbers would go up over time.
"This increase is not unexpected," said Dr. Lyle R. Petersen, director of the division of vector-borne infections diseases from the CDC, during the teleconference. "In fact, the total numbers will continue to rise through October."
He also said that while the overall numbers will continue to accumulate throughout mosquito season, he expects the incidence of infections to have peaked in mid- to late-August. This peak may vary between Northern and Southern states.
CDC officials estimate that, based on current numbers, the final tally of overall U.S. cases will likely be similar to the number of cases seen in 2002 and 2003, during which time more than 3,000 cases of neuroinvasive West Nile Virus and more than 260 deaths were reported.
While all 48 continental states have reported cases of West Nile infections, more than 70 percent of the reported cases are in Texas, South Dakota, Mississippi, Oklahoma, Louisiana and Michigan.
Texas has been hardest hit, with 783 reported cases - nearly half of the total number documented thus far. The health department in Texas has been vigilant about preventing further infections through aerial spraying.
"Assuming a normal progression [of cases], this will be our worst year with West Nile," said Dr. David Lakey, commissioner of the Texas Department of State Health Services. He highlights the grim statistics in Texas. "In 2003, there were 439 neuroinvasive cases, compared 416 already this year. In 2003, there were 40 deaths, and now we're at 31."
Since the CDC does not recommend routine testing of people with West Nile fever, it is likely that there are far more infections than the current numbers suggest. Approximately 80 percent of patients infected with West Nile virus will not develop any type of illness. The remaining 20 percent experience symptoms including fever, headache, malaise, body aches, and occasionally a rash or swollen lymph nodes.
There is no specific treatment for West Nile. The disease itself may run its course in as short as a few days or as long as several weeks.
The virus is most dangerous when it becomes neuroinvasive; in these cases, patients can experience symptoms as severe as coma, seizures, muscle weakness and paralysis. The CDC estimates that only about 1 in 150 people who contract West Nile virus will develop the neuroinvasive form, but nearly all of these patients will require hospitalization.
Of these, those with encephalitis are in the greatest danger, as they are frequently left with severe neurologic deficits. Ten percent of patients who develop encephalitis die. Of the patients who develop paralysis from the disease, one third will recover nearly completely, one third will be left with residual weakness, and one third will not recover.
CDC officials said it is unlikely that Hurricane Isaac will play a role in the severity of the outbreak.
"Based on previous experience, floods and hurricanes do not result in increased transmission of West Nile Virus," said Petersen. He notes that although there will likely be no noticeable effect on the current epidemic, a small increase had been noted in some areas of Louisiana after Hurricane Katrina, likely because of increased outdoor exposure when houses were severely damaged.
In light of the ongoing risk, the CDC encourages everyone to protect themselves from mosquitoes. While many health departments have made mosquito-control efforts, such as aerial and ground spraying, people are encouraged to protect themselves from mosquito bites using basic techniques.
"Use insect repellents, wear long-sleeved clothing during dawn and dusk, install and repair window screens, use air-conditioners when possible, empty standing water, and support local community mosquito control programs," Peterson says.
There is currently no vaccine to protect humans from the West Nile virus, although four effective vaccinations exist for horses. A few of related vaccines are in early clinical trials (Phase I and II) in humans, which have been successful. However, no vaccine has yet been taken to Phase III clinical trials.