Operational Biosurveillance

Further Challenges to West Nile Surveillance: Concern For Insurability

The latest trend we are observing to dissuade physicians from testing for West Nile infection is concern about insurability of the patient over the long term.  This concern is highlighted by findings of long term sequelae from West Nile infection (see attached for an excellent review). 

What this means is enhancement of a typical pattern in clinical surveillance- that clinicians are more likely to order a diagnostic test for West Nile infection if the patient is admitted to the ICU > admission to the general ward / visit to the emergency department > outpatient setting.  So, the probability of seeing low morbidity confirmations of West Nile infection may be even lower now due to concern now about insurability over the long run for the patient.

Download The Long-Term Outcomes of Human West Nile Virus Infection

West Nile Virus: Loss of a Warning Signature

During the emergence of West Nile virus in the United States from 1999-2006 (which year depended on what part of the country you are in) provided biosurveillance operators with a couple of easy indicators for impending transmission: wild bird die-offs (i.e. corvids) and equine illness / mortality. 

This year, however, in the current surge of West Nile transmission (our local example may be found here), we did NOT see pre-event bird die-offs reported.  In our part of the country there are plenty of corvids to monitor, but presumably thanks to herd immunity we were not provided the benefit of a month or two "heads up".  The fallback position was the early and abrupt WNV-positive mosquito pools noted 6 weeks in advance of the earliest known date of positivity. 

Although our part of the country hosts a substantial equine population, the West Nile vaccine has mitigated much of the illness we would have normally seen.  Indeed, this year it was almost as if human cases were sentinels for equine cases.

This year has indeed been associated with a warmer than usual winter, spring, and summer.  Temperature is a crucial driver for mosquito transmission competency, dramatically shortening the time it takes for a mosquito to go from a viremic avian blood meal to being able to infect another host (less than 24 hrs if incubated from 26-30 degrees Centrigrade in the lab, for instance).  However, as noted above, the loss of the expected signature pattern may present a challenge now for those US locales that have not yet seen a resurgence of West Nile- especially those cities who have deactivated or toned down their mosquito surveillance programs thanks to the poor economy.