Blood donations reveal new estimates of West Nile virus incidence, neuroinvasive disease
A recent analysis of blood donations in the United States showed that the proportion of West Nile neuroinvasive disease cases per West Nile virus infections ranged from 1 in 131 to 1 in 164 from 2010 to 2012.
According to Edouard B.A. Betsem, MD, PhD, MPH, of the Blood Systems Research Institute, San Francisco, and colleagues, the analysis highlights the value using blood donations as a public health tool for West Nile virus (WNV) surveillance, which began in 2003 after reports of transfusion-transmitted infections were confirmed in 2002.
“Blood donors represent a readily accessible sample of the U.S. population that is systematically screened for incident WNV infections,” the researchers wrote in Emerging Infectious Diseases. “This screening provides an approximation of the magnitude of the WNV epidemic each year, and this estimate complements data reported to ArboNET on cases of symptomatic WNV-associated disease.”
Early data from 1999 demonstrated that neuroinvasive disease (NID) develops in less than 1 in 150 WNV-infected individuals; however, revised estimates from 2003 indicated the prevalence was 1 in 256. From 2004 to 2011, reports showed that the incidence of WNV was relatively low but spiked in 2012 during an outbreak in the Midwest. The CDC identified 286 deaths related to NID in 2012 — the highest ever reported to the agency.
To gather more information on the incidence of WNV and the ratio of infections to NID cases, Betsem and colleagues analyzed data obtained from several blood collection agencies, including the American Red Cross, Blood Systems, Inc., the New York Blood Center, Carter Blood Care and One-Blood. The dataset represents approximately 60% of U.S. blood donations from 2010 to 2012.
The analysis included more than 10 million blood samples from donors in Western states (20%), Midwestern states (27%), Southern states (32%) and Northeastern states (21%). Among them, 640 were positive for WNV. The geographic distribution of positive samples showed clustering in states in the Southwest, Central and Northeast regions in 2010 and in the Southwest and Northeast regions in 2011. A more dispersed pattern was observed in 2012, involving the North Central, Southwestern and Northeastern regions.
The seasonal incidence of WNV per 100,000 people was 33.4 (95% CI, 22-45) in 2010, 25.7 (95% CI, 15-34) in 2011 and 119.9 (95% CI, 98-141) in 2012. Cumulative national estimates of WNV infections dropped from 103,450 in 2010 to 76,975 in 2011, but substantially increased in 2012 to 376,612 infections.
Over the 3-year study period, the projected incidence of WNV correlated with the frequency of NID cases. Overall, 1 in 141 WNV cases (95% CI, 118-164) developed NID, which is closer to estimates from 1999 than those in 2003. The annual ratios of NID to WNV rates were 1 in 164 (95% CI, 152-178) in 2010, 1 in 158 (95% CI, 145-174) in 2011 and 1 in 131 (95% CI, 127-136) in 2012.
Since the outbreak in 1999, there has been speculation on the genetic evolution of WNV in the United States, the researchers wrote. However, they did not find evidence supporting a change in virus penetrance that could have contributed to the higher number of deaths in 2012.
“Patterns of WNV activity vary from year to year, exhibiting temporal and geographic variations of incidence, as shown by our data in the blood donor pool and corresponding projections in general population incidence,” Betsem and colleagues concluded. “Fourteen years after their first appearance in the United States, WNV epidemics are still unpredictable and difficult to control, as confirmed by the surge of cases in 2012…after years of relatively mild epidemic years.” – by Stephanie Viguers
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