Research project leads directly to implementation of West Nile virus screening

The virus is bloodborne and can therefore be transmitted via transfusions to patients, who are often particularly vulnerable.

Infections with West Nile virus (WNV) have been occurring in Southern and Southeastern Europe for years. In addition to mosquitoes, WNV is transmissible via blood products. Sanquin has therefore always screened donations from donors who have recently been in an area where human infections have been found. Now that it is getting warmer in the Netherlands, we are also dealing with infections more frequently here. What does this mean for the blood supply, and are we screening in time if we wait for the signal that a person has become ill due to WNV?

Common house mosquito 

The virus, which rarely makes people ill, is transmitted in the Netherlands by the common house mosquito. Mosquitoes become infected when they bite a bird carrying the virus. Subsequently, the mosquito can transmit the virus to other birds, humans, and also horses. Humans and horses cannot then spread the virus any further, even if they are bitten by mosquitoes, though they can still fall ill.

The Blood-borne Infections (BOI) research group at Sanquin Research wanted to know whether infections are already occurring unnoticed in the Netherlands. This knowledge is important because we need to start screening donors in a timely manner to prevent viral contamination of blood products. So the question was whether we can make the blood supply safer by keeping a finger on the pulse. Clinical virologist and researcher at Sanquin, Mariet Feltkamp, explains: “I noticed that we often only heard about WNV-positive mosquitoes, birds, and horses at the end of the mosquito season, rather than during the peak in August-September, which is what you would expect. For us, that actually comes too late, because you would expect the greatest risk of WNV-positive donations precisely during the peak of the season. Furthermore, only a small minority of people, and therefore also of our donors, actually become ill from this virus. By far the majority do not notice anything and simply come to donate, which of course also entails a risk.”

How was the research conducted?

For this 'finger-on-the-pulse' study, donations from donors living in areas of the Netherlands where West Nile virus has previously been detected were, and still are, tested for WNV. In 2025, this was only in animals, and during a small outbreak in 2020, also in humans. The study was funded by a contribution from the Sanquin Research Fund and was awarded to Sanquin researcher Mart Pothast by means of a Young Investigator Award.

Whole blood donors who have given permission for the use of their blood for scientific research are tested for the presence of WNV RNA using a PCR test on plasma from residual material. A pool is created from 24 donations at a time. If a pool tests positive, the individual donations are tested to determine which donor is infected. A positive donation was found as early as the first week of the study. The donor lives in the province of Utrecht and had not recently been abroad.

What happens next

Because an infected donor has been found, Sanquin is immediately switching to screening all donations in the relevant region, in accordance with European regulations. In our case, the province of Utrecht and the Gooi and Vechtstreek region. Meanwhile, the study is also continuing, which will show whether more regions in the Netherlands with infections are found. When asked if Feltkamp is surprised that an infection was found so soon: “Yes, I was. I hadn't expected it so quickly, especially since the mosquito season was slow to get going this year. That seems to be changing now, though, judging by what flies into my home.” 

The research project, which continues until the end of September, is still ongoing in the other areas. Pothast will subsequently work with this data together with Boris Hogema from BOI. He is also developing an AI model that can better predict where and when donors need to be screened, both when staying in areas within the Netherlands and in Europe. This can help in making decisions about when to start screening, how long to continue, and when to stop.