West Nile Virus Cases Up Sharply

A sharp increase in West Nile virus infections and deaths prompted federal health officials to warn Thursday that this year’s outbreak of the mosquito-borne infection will likely be much more severe than last year’s.

The number of human infections reported has more than tripled, to 204, in the last week, and deaths have doubled, to eight, according to the Centers for Disease Control and Prevention and state health officials. Infections in horses and mosquitoes have exploded, officials said.

“The numbers are starting to change very, very quickly,” portending a much more deadly late summer and fall, CDC Director Dr. Julie Gerberding said. “We are starting the epidemic with more cases and more areas affected than last year.... We could be in for a very serious affliction this summer.”

Half of the deaths -- and more than half of the human infections -- have occurred in Colorado, which was largely unaffected last year, and officials expect a recurrence of the disease in states that experienced major outbreaks last year, such as Ohio and Louisiana. Epidemiologists said that the outbreak was likely to reach the West Coast in full force this fall, but no cases have been reported in the region yet.

In 2002, the United States recorded 4,156 cases in 44 states and 284 deaths. The bulk of the cases, about 65%, occurred in the time period corresponding to the coming six weeks.

The total number of cases is made up of 164 cases in 16 states reported by the CDC, 39 additional infections reported by Colorado authorities on Thursday and one in Georgia that had not yet been included in the CDC’s count.

The new list includes 111 cases in Colorado, 29 in Texas and 15 in Louisiana. That compares with a total of 59 cases reported the week before. At this point last year, 112 cases had been reported in four states.

CDC lists four deaths this year -- two in Texas and one each in Alabama and Colorado. Colorado authorities on Thursday reported three new deaths not listed by CDC, and Georgia officials reported the first death in that state.

It is not clear why Colorado has been so strongly affected, but a wet, cool spring and a hot July have led to a sharp increase in the number of mosquitoes. Gerberding also noted that past history indicated a significant increase in cases in states during their second year of infection, as was the case in Louisiana last year. She noted it was still too early to project how the outbreak would play out in that mountain state.

“I can’t predict what will happen in Colorado, nor can I completely explain why it is happening,” she said.

Gerberding also noted that the average age of patients had dropped, from about 55 last year to about 45 this year. That may be a statistical artifact arising from newly improved diagnostic tests, which are able to detect milder forms of the disease, she said.

Last year, about two-thirds of victims suffered from encephalitis, the more severe form of the disease that usually occurs in older people. Of the eight deaths this year, the youngest victim was 68. So far this year, only about half of the victims have had the more severe form, she said.

The increased detection of milder cases may account for some, but not all, of the additional cases this year, she added.

The West Nile virus is widespread in Africa, East Asia and the Middle East. It is carried by many species of mosquitoes, and it affects birds, mammals -- horses especially -- and humans. About 80% of those infected develop little or no illness and never even know that they are infected. A small percentage, however, develop a severe form of encephalitis or meningitis -- a swelling of parts of the brain -- that can be fatal, especially in the elderly.

The first U.S. case was observed in New York City in 1999. Since then, the virus has been spreading westward and the number of cases, and deaths, has been growing rapidly.

Last year, the disease was observed in every contiguous state except Oregon, Nevada, Utah and Arizona. It is expected to strike all four of those states this year, as well as Alaska.

The virus was not observed in either mosquitoes or horses in California last year, although there was one human case among California residents. A Los Angeles woman who contracted the disease is now thought to have been bitten by a mosquito that arrived on an airplane operated by an overnight delivery service.

The rising incidence of cases makes it important for residents of the most heavily affected states, and especially the elderly, to take protective steps, Gerberding said. That includes wearing long-sleeved shirts and long pants outdoors and protecting exposed areas of skin with mosquito repellents containing DEET.

People should also mosquito-proof their homes by patching or installing screens and by getting rid of objects that can hold standing water, including such things as bird baths, tires and soda cans, she said.