Significantly fewer cases of West Nile virus infection have been reported so far in 2005 compared with this time last year, but federal officials warn the season is still early.
By mid-July, the Centers for Disease Control and Prevention had received reports of 25 cases nationwide, including 1 death in Missouri.
In contrast, by mid July 2004, the CDC had received reports of 108 human cases, including 2 deaths.
There is no way to project where the disease will hit hardest this year, given the limited experience with the virus in the United States or its track record in Europe, Asia, and Africa, said Theresa Smith, M.D., a medical epidemiologist with the CDC division of vector-borne infectious diseases in Fort Collins, Colo.
“It has a seasonal nature, but not a truly cyclical nature,” she said.
Of the 25 cases reported since Jan. 1, 2005, 15 (60%) were reported as West Nile fever, 9 (36%) were reported as West Nile meningitis or encephalitis (neuroinvasive disease), and 1 (4%) had not been clinically defined at press time.
Testing of potential blood donors identified seven people with presumptive West Nile virus infection: Five occurred in Texas and two occurred in Arizona.
The number of overall cases so far is too small to allow comparison of the epidemiology of the virus this year with last year. But acute flaccid paralysis has been recognized more frequently in the last 2 years, Dr. Smith said. None of the cases reported as of press time in 2005 have involved this paralysis syndrome.
One of the first signs of acute flaccid paralysis can be weakness in one limb, but there also may be no symptoms prior to the onset of paralysis.
As with poliomyelitis, numbness is not a feature of acute flaccid paralysis due to West Nile virus, although pain can be present.
Acute flaccid paralysis affects relatively healthy young people, as opposed to West Nile virus encephalitis or meningitis, which tends to affect older people, she said.
The ages of patients infected with West Nile virus to date in 2005 range from 17 to 80 years, and none is believed to be pregnant, CDC spokesperson Christine Pearson said.
The CDC established a voluntary online registry in 2002 to report women who become infected with the virus during pregnancy. The CDC also established a voluntary birth outcome registry.
The action came after a 27-year-old woman infected with the virus delivered a full-term infant with cystic cerebral tissue destruction, severe chorioretinitis, and laboratory evidence of congenitally acquired West Nile virus infection.
There were no confirmed cases of intrauterine transmission in the 79 pregnancies reported to the registry in 2003–2004, Dr. Smith said.
There were three very early cases of postnatal West Nile virus infections that may have involved transplacental infection or virus transmission during delivery.
There have been nine major birth defects (12%) among the 79 pregnancies. But the birth defects were felt to be “chance occurrences” based on the phenotypic inconsistencies observed, she said.
There was some evidence of a possible pattern of microcephaly, but the numbers were too small to be conclusive.
“The registry is helping us understand what risks are present, and whether pregnancy with West Nile virus has definable, discernible outcomes,” Dr. Smith said.
In February 2004, the CDC issued interim guidelines for evaluating infants born to mothers who acquire infection during pregnancy, but the agency did not advocate screening of asymptomatic pregnant women.
Screening, even in highly endemic areas, is not recommended given the lack of specific treatment for West Nile virus infection, Dr. Smith said, adding that the final infant evaluation guidelines are not expected for some time.
As of mid-July, Colorado had the highest number of cases with seven, followed by South Dakota (five), Arizona (three) and two cases each in California and New Mexico.
In 2004, 2,535 total human cases and 98 deaths were reported to the CDC, with the largest number of cases in Western states.
The highest number of reported cases was in 2003, with 9,862 cases of human illness and 264 deaths.