West Nile virus has never struck the U.S. as hard as it is striking now.
The number of confirmed cases now reaches 1,118 – including 41 deaths, all from the neuroinvasive form of the disease. The number has risen dramatically in recent weeks, leading federal health officials to call the outbreak the worst since the virus arrived in the United States in 1999.
"This is the highest number of cases and deaths that has ever been reported in the period of the first 3 weeks of August," said Dr. Lyle R. Petersen, director of the CDC’s Division of Vector-Borne Infectious Diseases. By the third week in July, only 25 cases had been identified, he added.
Human cases occurred in 38 states; 47 states have found the virus in birds, mosquitoes, or in humans, Dr. Petersen said during a press briefing.
Most troubling is that 56% of the cases (629) have been the more aggressive neuroinvasive disease, he said. The remaining 44% were the noninvasive type characterized by fever and myalgia.
Neuroinvasive cases are somewhat easy to track, since most people who develop that form end up in a hospital. The number of fever-related cases is almost certainly a vast underestimate.
"We think that only 2%-3% of all people with [nonneuroinvasive disease] actually get reported. The reasons are very simple. Many people with milder disease don’t go to the doctor or the doctor does not recognize it as a case. There’s no specific treatment for it, and even during an outbreak, routine testing is not recommended," Dr. Petersen said.
Most of the cases (75%) have occurred in Texas, Mississippi, Louisiana, and Oklahoma, with 50% in Texas alone. "People are understandably very concerned, especially in the hardest-hit areas," Dr. Petersen said.
The case load is particularly high in the city of Dallas and its northern suburbs, said Dr. David Lakey, commissioner of Texas Department of State Health Services.
"We are talking about a disease that’s impacting the lives of hundreds – even thousands – of people," Dr. Lakey said at the briefing. "As of right now we have confirmed 586 cases and 21 deaths. ... I know of at least four more," that will be included in the next Texas case update.
From 2003 to 2011, there were 10 deaths. "So now we have many more cases" than in all the previous years of West Nile combined. "This will certainly be our worst season ever." Cases will probably continue to rise as the West Nile season wears on, Dr. Petersen said.
"The peak usually occurs in mid-August, but it takes a couple of weeks before people become sick, go to the doctor, and get diagnosed and reported. So we expect many more cases to occur and the risk of infection will be with us through the end of September."
A mild winter, early spring, and exceptionally hot summer have probably contributed to the high West Nile activity. "It’s a complicated cycle within the interaction of birds, mosquitoes, and people," Dr Petersen said. "Hot weather seems to promote major outbreaks. The biggest outbreaks in Europe and Africa have occurred during periods of abnormally hot weather. And we know from lab experience that heat can increase transmissibility through mosquitoes."
The CDC is investigating whether the virus has mutated into a more dangerous form, but so far, Dr. Petersen said, there is no evidence to support this.
The blood donor screening program has identified West Nile-infected blood in 26 states. "All donors are now screened for West Nile virus, and by deferring those, we have taken an important step in protecting the blood supply," he said.