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CDC Team Gauges Risk of West Nile Spread Via Blood
By Mary Beth Nierengarten
NEW YORK (Reuters Health) - West Nile virus infection, the mosquito-borne illness now spreading across the US, could be transmitted through blood transfusions, researchers from the Centers for Disease Control and Prevention (CDC) report.
But the risk--while relatively high--would only exist for blood donated during peak times of infection, they conclude.
West Nile virus is carried by certain birds, and can be passed to humans via mosquitoes. The virus--which originated in Africa and was first spotted in the United States in New York in 1999--has quickly spread west and south across the US.
West Nile risk would be highest for blood donated at times and places when the infection was most common--for example the northeastern US in late August, the CDC researchers say. If patients given blood that was donated during such periods exhibit West Nile virus-like symptoms, they add, doctors should consider the possibility that the patient has contracted the infection from the blood donation.
It is particularly important to determine if West Nile could be spread by transfusion, Dr. Brad J. Biggerstaff from the CDC's National Center for Infectious Diseases in Fort Collins, Colorado and Lyle R. Petersen note, because people infected with the virus usually don't have symptoms. But among those who do develop severe illness, fatalities range from 3% to 30%, with the highest death rate among the elderly. There is no cure for the infection.
To investigate the theoretical risk that West Nile virus could be spread by donated blood from infected individuals, Biggerstaff and Petersen conducted a statistical analysis of the 1999 outbreak in Queens, New York. They report the findings in the August issue of the journal Transfusion.
The researchers examined the date of onset for people infected with the West Nile virus during the 1999 epidemic and estimated the number of people who had the virus in their blood throughout the outbreak. To infer the transfusion-transmission risk based on these data, the investigators then estimated the inapparent-to-apparent infection ratio, the proportion of asymptomatic infections, and the size of the population.
The minimum risk of a donor transmitting the West Nile virus through his or her blood during the 1999 outbreak was found to be 1.8 in 10,000 donations, with a maximum risk of 2.7. The risk of transmission was time-limited, peaking during the months of August and September, the period in the northeastern US with the highest rate of infections. Estimates of infection entering the blood supply before August or after September fell to nearly zero.
Elsewhere in the US, the time of highest activity of West Nile virus may start earlier, Biggerstaff noted, "as with the current outbreak in Louisiana. In other parts of the US where West Nile virus is new, we just don't know yet."
Although the estimates indicate a theoretically low risk of transmitting the West Nile virus through transfusion, they are relatively high compared to transfusion-transmitted viruses regularly screened for, such as HIV. Estimates for HIV, for example, range from 1 per 200,000 to 2 million.
"While the estimates we give are relatively high," explained Biggerstaff, "it's important to remember that the average rate we give is during the outbreak only and for the area of the outbreak only, and that the rate is lower outside this time frame and locale."
Implications of this study suggest that doctors should consider the possibility of transfusion-transmitted West Nile virus in patients with unexplained symptoms suggestive of this virus, particularly if the blood donation was obtained when the activity of the virus is highest. "But they should also be aware of the level of risk or chance of such an occurrence when making a clinical decision," Biggerstaff added.
SOURCE: Transfusion 2002;42:1019-1026.
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