"Neon needles" in a haystack: the advantages of passive surveillance for West Nile virus.

Passive surveillance is usually viewed as less efficient for case ascertainment than active surveillance. However, for diseases with nonhuman animal reservoirs, active surveillance can be like looking for a needle in a haystack and may be prohibitively expensive. Fortunately for surveillance of West Nile virus (WNV) in the northeast US, the dead crows have served as "neon needles in a haystack"--indicators of viral activity that call attention to themselves. In 2000, laboratory testing of dead birds, including all species, birds found singly, with signs of trauma, or no compatible pathology, provided the first confirmation of viral activity in most areas. The surveillance factor most closely associated with the number of human cases was the dead crow density. In 2001, dead crow densities will be used as an additional index for monitoring human risk and need for prevention and control activities. If there are few crows in an area, if their case-fatality rate is reduced, or if there is public complacency about reporting dead crow sightings, this passive surveillance indicator may not be helpful in identifying areas likely to have occasional human cases or an outbreak.