Friday, January 11, 2019

EID Journal: Cumulative Incidence Of WNV Infection - U.S., 1999-2016

Note: I'm having intermittent internet connectivity today, with service being down about 90% of the time since 6 am.  If this continues, it may prevent me from blogging further today.

We've another EID Journal Dispatch that reminds us of the limits of infectious disease surveillance, and that most infectious diseases are vastly under-reported, even in high resource countries like the United States.
As the CDC pyramid graphic above illustrates, only a subset of people infected with a disease are apt to experience severe enough symptoms to seek medical care - and of those - only a small percentage will be laboratory confirmed and included in officials case counts.
This holds true for everything from common infections - like seasonal influenza and norovirus - to the more exotic avian flu, MERS, and even Ebola.  When case counts are reported by the media, the are often taken at face value, but the truth is - they almost always just represent the tip of the pyramid.
A good example came in 2013, when the CDC revised their Estimate Of Yearly Lyme Disease Diagnoses In The United States, indicating that the number of Lyme Disease diagnoses in the country is probably closer to 300,000 than the 30,000 that are officially reported each year to the CDC

West Nile Virus arrived in North America 2 decades ago, possibly brought in by a viremic visitor, and over a period of a few short years managed to spread across the entire United States and make inroads into Canada.

From the USGS Factsheet on West Nile Virus


While some years are worse than others, each summer thousands of people are infected via mosquito bites in the United States.  Fortunately, about 80% of those infected with WNV experience at worst only mild, or sub-clinical symptoms. Most of the rest may experience a brief febrile illness (West Nile Fever).  Both are likely highly under-reported.
A very small percentage (perhaps 1%) may develop WNV neuroinvasive disease (WNND), a form of encephalitis that can sometimes prove fatal. Those over the age of 50 appear to be the most vulnerable to the most serious form of the illness, and as most of those are hospitalized, those numbers are more solid.
In 2012 – the worst year on record for West Nile deaths thus far – the CDC reported:
Final 2012 West Nile virus update:
In 2012, all 48 contiguous states, the District of Columbia, and Puerto Rico reported West Nile virus infections in people, birds, or mosquitoes. A total of 5,674 cases of West Nile virus disease in people, including 286 deaths, were reported to CDC.
Of these, 2,873 (51%) were classified as neuroinvasive disease (such as meningitis or encephalitis) and 2,801 (49%) were classified as non-neuroinvasive disease. The numbers of neuroinvasive, non-neuroinvasive, and total West Nile virus disease cases reported in 2012 are the highest since 2003.

As bad as the 2012 statistics were, today we've an analysis published in the CDC's EID journal that estimates the true incidence of WNV infection in the United States is likely 140 times higher than has been reported. 
Granted, most of these unreported infections occurred among milder West Nile Fever cases, but if nothing else, it reminds us of the value of protecting ourselves against mosquito bites.
 Some excerpts from this EID report follow, then I'll return with a postscript:

Volume 25, Number 2—February 2019
Cumulative Incidence of West Nile Virus Infection, Continental United States, 1999–2016

Shannon E. Ronca, Kristy O. Murray , and Melissa S. Nolan
Author affiliations: Baylor College of Medicine, Houston, Texas, USA (S.E. Ronca, K.O. Murray, M.S. Nolan); Texas Children’s Hospital, Houston (S.E. Ronca, K.O. Murray, M.S. Nolan) 


Using reported case data from ArboNET and previous seroprevalence data stratified by age and sex, we conservatively estimate that ≈7 million persons in the United States have been infected with West Nile virus since its introduction in 1999. Our data support the need for public health interventions and improved surveillance.

West Nile virus (WNV) is a mosquito-transmitted flavivirus with human health implications. Since its emergence in 1999, WNV has become endemic across the continental United States (1). Seasonal outbreaks occur annually, and large outbreaks occur throughout the country. Infection is commonly asymptomatic; a general febrile illness occurs in ≈ 20% of the population, and < 1% progress to West Nile neuroinvasive disease (WNND), which might include encephalitis, meningitis, and acute flaccid paralysis.

WNV infection can cause permanent sequelae, including physical, neurologic, and cognitive disabilities as well as renal impairment and ocular damage (2). The average annual cost to treat hospitalized WNV patients is ≈ US $56 million, and initial and long-term costs can exceed US $700,000 per patient (3,4).
Considering the clinical and economic impact of acute and long-term WNV outcomes, determining total WNV disease burden in the United States is imperative. ArboNET data indicates that ≈ 40% of WNND cases occurred during 2011–2016, suggesting a need to update the estimated cumulative WNV incidence previously determined by Petersen et al. in 2010 (5). The objective of our study was to estimate total WNV disease burden in the continental US population during 1999–2016.
The Study


We estimate that ≈7 (95% CI 5.7–8.1) million persons in the continental United States were infected with WNV during 1999–2016, more than double the 2010 estimate of 3 million infections. Our estimate highlights the need for improved disease surveillance and reporting.
As the cumulative incidence continues to climb, our findings provide additional support for the economic benefit of insecticide and vaccine interventions, especially in the Midwest, Southwest, and West of the United States; nearly 98% of the US population remains vulnerable to WNV infection.
Dr. Ronca is a postdoctoral associate at Baylor College of Medicine (Houston, Texas) in the Department of Pediatrics, Section of Pediatric Tropical Medicine, where she focuses researching on viral infectious diseases.

While it is easy to lulled into complacency when Saudi Arabia reports only 5 MERS cases in a month (as it did in December) and China reports only 5 H5N6 cases (as it did in 2018), the real incidence could easily be 10, 50, even 100 times higher than is picked up and reported by regular surveillance.

Some earlier blogs on reported vs. estimated incidence of infections include:

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