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Canadian Medical Association Journal
March 24'98

Can CJD be transmitted through the blood supply?

CMAJ 1998;158:715-16

© 1998 Canadian Medical Association

Response from: B. Larke, R. Rohwer

In response to: A. Giulivi

See also:
I disagree with the statement that "there is absolutely no evidence that vCJD can be spread through the blood supply." However, I accept Dr. Giulivi's questioning of my statement that "vCJD can be spread through the blood supply." Both are pronunciamentos and have little (or even negative) value. If Giulivi means that there are no reported cases of CJD or vCJD that have been transmitted through the blood supply, then he is right. But this lack of evidence does not mean that such transmission is not occurring. Giulivi is making an all-too- common error, since "the absence of evidence is not evidence of absence."1

Science is more than epidemiology. "Biology is complex, messy and richly various, like real life."2 So what do we know about CJD and vCJD? Spongiform encephalopathies occur in a wide variety of animals. They differ in their histologic features, clinical manifestations (including incubation periods), species preferences and abilities to cross species boundaries and in terms of the inoculum required to cause disease and virulence generally.3­5

First described in the 1920s, CJD is rare (1 to 2 cases per million). But because plasma pools may contain units from about 60 000 donors, it is likely that if the disease could be transmitted through the blood supply, it would have been evident by now (unless the doses of infective agent are low and the incubation periods very long). So it is unlikely that CJD is transmitted through the blood supply. (If this is so, then it would be interesting to know why the Canadian Red Cross is withdrawing blood products donated by people with CJD.)

But vCJD is not CJD. We know little about vCJD. It appears to easily cross species borders (e.g., from cattle to humans) and seems to affect primarily younger people.

Worst-case estimates of the number of people in the UK infected with vCJD range up to 80 000 or about 1 in every 700 blood donors.6 So it is really important to know if vCJD can be spread through the blood supply. Because there is no screening test for vCJD in humans (or animals), our evidence has to come from animal experiments. Waiting for human epidemiologic evidence would be a mistake.

Studies presented at scientific meetings and shortly to be published have shown that in hamsters and mice (species that have a rodent form of scrapie), the infective agent of scrapie is present in blood and can be transmitted from one animal to another within a single species (Dr. Robert Rohwer, Veterans Affairs Medical Center, Baltimore: personal communication, 1998). Also, as Giulivi notes, B cells have a receptor for normal prions — and presumably also for abnormal prions. Here is a possible biological mechanism that would allow prions attached to leukocytes to cross the blood­brain barrier.

So, can vCJD be transmitted through the blood supply? The jury is out, and the regulators and public health officials have a tough job. The point of my editorial was to underline the complexity of these decisions and to support the development of ethical guidelines for regulators. Understanding how these decisions are made and providing ethical support for the people making them is more important than a plethora of pronunciamentos.

I do not believe that my editorials will "set up a chain reaction among physicians, [causing them to] worry and arrive at the wrong conclusions." Our readers are a sceptical lot. Most would agree, I think, that there can be no conclusions in science. But there should be action, and physicians must always give advice to patients that is based on their knowledge of basic science and clinical medicine — not just epidemiology.

John Hoey, MD
Editor-in-Chief
CMAJ

References

  1. Miettinen OS. Evidence in medicine: invited commentary. CMAJ 1998;158(2):215-21 [full text].
  2. Medawar PB. Induction and intuition in scientific thought. Janeway lectures for 1968. Philadelphia: American Philosophical Society; 1969. p. 1.
  3. Cashman NR. A prion primer. CMAJ 1997;157(10):1381-5 [full text / résumé].
  4. Ricketts MN. Is Creutzfeldt­Jakob disease transmitted in blood? Is the absence of evidence of risk evidence of the absence of risk? CMAJ 1997;157(10):1367-70 [full text / résumé] .
  5. Brown P. The risk of bovine spongiform encephalopathy ('mad cow disease') to human health. JAMA 1997;278:1008-11.
  6. Cousens SN, Vynnycky E, Zeider M, Will RG, Smith PG. Predicting the CJD epidemic in humans. Nature 1997;385:197-8.

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