WEDNESDAY, Aug. 29 (HealthDay News) -- One of the worst outbreaks of West Nile virus to ever hit the United States continues to expand, with 66 deaths and 1,590 illnesses reported as of Tuesday, according to the U.S. Centers for Disease Control and Prevention.
Cases have jumped 40 percent nationwide since just last week, the agency added.
Cases have now reached their highest level since the mosquito-borne virus was first found in the United States in 1999, agency officials said in a Wednesday press briefing.
While almost all states have reported at least one case of West Nile illness, over 70 percent of cases have come from six states --- Texas, South Dakota, Mississippi, Oklahoma, Louisiana, and Michigan.
The outbreak has hit hardest in Texas, where nearly half (45 percent) of the total U.S. cases have been reported.
"The number of people reported with West Nile virus continues to rise," said Dr. Lyle Petersen, director of the CDC's Division of Vector-Borne Infectious Diseases. "We have seen this trend in previous West Nile epidemics, so the increase is not unexpected," he added. "In fact, we think the reported numbers will get higher through October."
According to Peterson, of the cases reported so far, 56 percent are what is called neuroinvasive disease, when the virus enters the nervous system causing conditions such as meningitis or encephalitis. The remaining reported cases (44 percent) are non-neuroinvasive.
"These numbers represent a 40 percent increase of last week's report of 1,118 total cases and 41 deaths," Petersen said.
These numbers can be somewhat misleading since most cases of West Nile are non-neuroinvasive and are mostly unreported, the CDC said. That means that the number of unreported cases probably far exceeds reported ones.
Neuroinvasive disease is the most serious for of West Nile infection and these patients usually are hospitalized, Petersen said. The size of the outbreak is based on these cases since they are the ones easily identifiable, he added.
The only states that have not reported cases are Alaska and Hawaii, he said.
"Based on current reports, we think the number of cases may come close to, or even exceed, the total number reported in the epidemic years of 2002 and 2003, when more than 3,000 cases of neuroinvasive disease and more than 260 deaths were reported each year," Petersen said.
The reasons for a major outbreak this year aren't clear, Petersen said. The drought in Texas may have played a role, but there were probably other factors as well, he added.
The best way to avoid the virus is to wear insect repellant and support local programs to eradicate misquotes, Petersen said.
There is currently no treatment for West Nile virus and no vaccine to prevent it, he added.
Speaking at the press conference, Dr. David L. Lakey, Commissioner of the Texas Department of State Health Services said that, "As I look at the data, I am not convinced that we have peaked."
Since last week, there have been 197 new cases and 10 more deaths in Texas, Lakey said. "Those numbers will continue to go up," he added.
Generally speaking, 80 percent of people who are infected with West Nile virus develop no or few symptoms, while 20 percent develop mild symptoms such as headache, joint pain, fever, skin rash and swollen lymph glands.
Less than 1 percent will develop neurological illnesses, such as encephalitis or meningitis, and develop paralysis or cognitive difficulties that can last for years, if not for life.
People older than 50 and those with certain medical conditions, such as cancer, diabetes, hypertension, kidney disease and organ transplants, are at greater risk for serious illness, according to the CDC.
There are no specific treatments for West Nile virus; the greatest risk for infection with West Nile virus typically occurs from June through September, with cases peaking in mid-August.
And because reporting lags behind actual infections, "we expect many more cases to occur and the risk of West Nile infection will probably continue through the end of September," said Petersen.
Although most people with mild cases of West Nile virus will recover on their own, the CDC recommends that anyone who develops symptoms should see their doctor right away. The best way to protect yourself from West Nile virus is to avoid getting bitten by mosquitoes, which can pick up the disease from infected birds. The CDC recommends the following steps to protect yourself:
For more information on West Nile virus, visit the U.S. Centers for Disease Control and Prevention.
SOURCES: Aug. 29, 2012, press conference with: Lyle R. Petersen, M.D., M.P.H., director, Division of Vector-Borne Infectious Diseases, U.S. Centers for Disease Control and Prevention; David L. Lakey, M.D., Commissioner, Texas Department of State Health Services
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