West Nile virus infects humans across the country

So far this year, most states west of the Mississippi are reporting higher numbers of West Nile virus cases than those in the East.

The following incidence data for 2007, as reported to the Centers for Disease Control and Prevention through the ArboNET system, include both the mild and severe human cases of West Nile virus occurring from Jan. 1 to Aug. 14. (Case reports are still being collected for the rest of the calendar year.) ArboNET is a national, electronic surveillance system established by the CDC to assist states in tracking West Nile virus and other mosquito-borne illnesses. Thus far, 31% of the total 444 cases were neuroinvasive disease. More details and updates are available online (link).

Why so serious

The CDC notes that the high proportion of neuroinvasive diseases cases among reported cases of West Nile virus disease reflects surveillance reporting bias. Serious cases are more likely to be reported than are mild ones. Also, the system is not designed to detect asymptomatic infections. Data from population-based surveys indicate that among all people who become infected with West Nile virus, fewer than 1% will develop severe neuroinvasive disease. The breakdown:

West Nile encephalitis and meningitis: Severe forms of the disease that affect a person's nervous system have occurred most often in California (32 cases), South Dakota (19), Colorado (10) and Arizona (10).

West Nile fever: This typically less-severe form shows no evidence of neuroinvasion. It is still considered a notifiable disease; however, the number of reported cases may be limited by whether the affected people seek care, whether lab diagnosis is ordered and the extent to which physicians report cases to health authorities. Highest tallies are California (51), Colorado (62), South Dakota (43) and North Dakota (44).

Other clinical/unspecified cases: In some fairly rare instances, West Nile virus does not manifest in either of the two forms described above. The category of "other clinical" includes cases that take forms such as acute flaccid paralysis. "Unspecified" covers those cases for which sufficient clinical information was not provided. Cases in this category were reported by Arizona (2), California (3), Georgia (1), Illinois (1) and Wyoming (2).

Comparison with 2006

Last year, 34% of the 4,269 total cases occurring between Jan. 1, 2006, and Dec. 31, 2006, were neuroinvasive -- encephalitis or meningitis. Sixty-one percent were reported as West Nile fever. The remaining 5% were clinically unspecified.

Source: Division of Vector-Borne Infectious Diseases, Centers for Disease Control and Prevention, Aug. 14