The only known study to examine the effect of leukofiltration on TSE infectivity was recently published by Dr. Brown's group, the committee noted. The NIH researchers used leukofiltration in an attempt to remove TSE infectivity from plasma, which is largely cell-free. There was not a significant reduction in infectivity and, in one of three experiments, infectivity was significantly increased. However, the design of these experiments does not reflect the intended use of leukoreduction which will be applied to whole blood before separation of plasma, the committee noted. In this case, leukocytes would be removed before they may shed prion into the plasma or break down into debris.

"The hypothesis that leukofiltration causes a destruction of leukocytes, thereby probably enhancing TSE infectivity, is neither proven nor disproved," the committee said. It also is unknown whether different filters have differing effects on the removal of TSE infectivity and whether infectivity would be sufficiently reduced to render a presumptively infectious blood unit non-infectious. So far, the available scientific data is ambivalent. In contrast to the classical forms of CJD, small animal experiments have shown that infectivity may be present in peripheral blood of vCJD cases and this infectivity may be predominantly associated with white blood cells. "Therefore, it might be of benefit to remove white blood cells as completely as possible in order to diminish infectivity," the experts said. But they pointed out a number of caveats with respect to vCJD: lack of experimental proof of the effect of leukoreduction on TSE infectivity, lack of knowledge on which cell types carry the infectivity and to which degree they can be removed, unknown effects of different filter types, and lack of validation.

"For this reason, the SCMPMD cannot make a clear-cut recommendation with respect to the general introduction of leukoreduction, unless a number of studies has been performed to answer the open questions," the committee said. They recommended supporting such research but emphasized that it will take at least two years until those studies have been finished.

"In the meantime, it might be advisable to introduce leukofiltration as a precautionary step, as it is assumed that it will contribute to diminishing infectivity in blood," the committee said. "Recommendation for the general use of leukofiltration would be in line with the belief that many if not all transfusion recipients would benefit from the removal of white blood cells for other reasons."

The Ottawa group concluded that the variant form of CJD appears to have crossed the species barrier from cattle. It has an earlier age at onset, and the load of pathological prions is much greater than in classical forms of the disease. The anatomical distribution of nvCJD infectivity is also different, which raises the plausible possibility that it is more likely to have infectivity in the blood. "Therefore, nvCJD is the wild card that warrants special vigilance," the Canadian panel said. Although at present, the disease appears to be isolated to parts of Europe, the number of people in affected countries who are currently incubating the disease is unknown.

The Canadian statement did not take into account that the exclusion of donors may cause a shortage in the supply of blood and blood products as was experienced in the US following the recommendation to withdraw all blood products which were produced from plasma pools to which a donor contributed who was subsequently recognized as suffering from CJD or being at risk for CJD, the EC committee noted last month. Therefore US and Canadian health officials aimed at finding an acceptable balance between reduction of theoretical risk of vCJD transmission and the impact on the blood supply, the EC experts pointed out. The North Americans estimated that excluding potential donors who stayed cumulatively for more than six months in the UK reduced the theoretical overall risk of vCJD transmission by approximately 90 percent while affecting only 2.2 percent of the donor population. Their decision is based on two assumptions: a) The risk of acquiring vCJD (and to be infectious afterwards) is directly proportional to the length of stay in the UK in the period 1980 to 1996; b) The risk of acquiring vCJD exists only in the UK and does not exist in North America.

Currently, all available data support the hypothesis that vCJD is caused by the same agent as bovine spongiform encephalopathy (BSE), the Scientific Committee said last month.

"Therefore, it is plausible to assume that this agent is transmitted to humans by exposure to material derived from subclinically or clinically infected cattle, most probably to food containing bovine-derived material. There is no proof of this hypothesis, but there is no more plausible explanation."

The risk of exposure to contaminated bovine material was definitely high between 1980 and 1990 when additional measures to protect consumers were taken, the Scientific Committee noted. Because these measures were not fully enforced during the early nineties it is reasonable to include the years up to 1996 in the risk period, they said. But there is no information on the infectious dose to humans nor on the amount of infectivity in different types of food; whether the risk was ubiquitous and exposure to contaminated food was inevitable, as to whether infectivity was low and only repetitive exposure led to infection or to whether the probability of exposure to contaminated material per unit time was low and increased with the length of stay in the area of risk.

However, two cases of vCJD now have been confirmed in France and a third case is suspected; none of them had ever traveled to the UK, the EC experts reported. "The French cases demonstrate that there is a risk of acquiring vCJD independent of residence in the UK." If it is accepted that humans are infected through exposure to bovine derived material, "the appearance of vCJD cases outside of the UK is not a surprise," they said. During the eighties and early nineties large amounts of bovine-derived material (including live cattle) were exported from the UK to other European countries and elsewhere. It has been estimated that in the 1980s, five to ten percent of bovine material in French food came from importation from the UK.

"It cannot yet be excluded that there is a substantial risk of vCJD in countries in which significant amounts of this material entered the food chain," the panel said. "However, it may be questionable to assume that the vCJD risk in North America is posed only by travelers to UK," they said. The panel calculated that a 120 months (10 years) stay in France would be equivalent to a six month stay in the UK.

The donor exclusion criteria would have different effects on the reduction of the theoretical risk in North America and Europe, the panel said, since the number of Europeans staying for a short time in the UK is much higher than for US citizens. The adverse effects of any exclusion criterion on the donor population is not only the possibility of a shortage in supply. Excluded donors have to be replaced by new donors, most probably by first time donors. This replacement creates an additional risk as the prevalence of blood borne infectious diseases in first time donors is significantly higher than in repetitive donors. Any new exclusion measure, therefore, has to be balanced against the calculable risk of HIV, HBV and HCV transmissions by window donations of first time donors, the experts cautioned.

"On the basis of these deliberations the SCMPMD recommends a careful consideration whether the exclusion of donors who stayed for a defined period of time in areas with increased risk of exposure to the BSE agent would provide an increase in safety balanced to its negative impact on supply and donor population."

Screening assays.

In March 1999, during a World Health Organization meeting on "Diagnostic Procedures for Transmissible Spongiform Encephalopathies: Need for Reference Reagents and Reference Panels," a new test method was presented, claiming to be able to detect prion protein in the blood of infected animals. "This test stimulated great interest, particularly regarding the question as to whether or not it could be used for the screening of blood donors," the EC panel noted.

At the WHO meeting, M.J. Schmerr presented results of a new assay that may be able to detect the pathological form of prion protein in blood of animals in the clinical as well as in the preclinical phase of TSE (scrapie in sheep and chronic wasting disease in deer). The basic reaction is the competition of the proteinase K treated sample with a labeled peptide derived from the sequence of prion protein binding to an antibody raised against this peptide. The test material is extracted from buffy coat prepared from a sample of peripheral blood. From the relation between free and bound peptide as determined by capillary electropheresis the amount of competing protein (protease resistant prion protein) is calculated.

"In contrast to other tests used for the detection of TSE infected animals or for confirmation of CJD/vCJD in humans the assay proposed by Schmerr uses for the first time as test material a body fluid, namely blood, which is easily available," the panel said. "However, it has to be stressed that this assay is far from being validated either in animals or in humans. On the contrary, preliminary tests with human material performed in several laboratories have not yet validated this test," the panel said. There is no doubt that the assay has the potential to be developed into a screening assay, but this development will need a number of carefully designed studies, they cautioned.

ABC members and Newsletter subscribers may request a copy of the EC Scientific Committee's updated recommendations on vCJD from Tammy Walters at the ABC office. Fax: (202) 393-1282; E-mail: twalters@americasblood.org. The document is 11 pages. Please specify fax or e-mail (PDF format).

EC Experts Recommend Leukoreduction as Prudent Precaution against vCJD in Blood Products; Address Cases Reported from France (PDF: 57KB)