An analysis of the death of a third patient after a transfusion of infected blood, published yesterday, shows that the disease is very easily transmitted by blood. Nobody knows how many donors may have given infected blood in the past, or may still be giving it today.

That it can be transmitted by infected blood means that there is a serious risk of a self-sustaining secondary epidemic of vCJD. Transmission is much easier than by eating contaminated meat, where a species barrier must be overcome.

The evidence of the three cases is that vCJD can develop in as little as six to seven years if transmitted by blood. The incubation period for vCJD caught from infected beef is significantly longer. The National Blood Service said that it had taken all the precautionary measures it could. “The trouble is that there is no test we can use,” a spokesman said.

At greatest risk are a handful of people known to have had blood transfusions from healthy donors who went on to develop vCJD. There are 24 such recipients still alive, and their risk is substantial, Professor John Collinge, Britain’s leading expert on the disease, said.

The unnamed third patient to catch vCJD from contaminated blood had a transfusion when he was 23. Seven years later he developed symptoms. He opted to join an experimental treatment trial organised by the Medical Research Council in which patients are given the drug quinacrine. He died a year later, aged 32.

He was one of 66 people identified by the National Blood Service as having received blood from a donor who later developed vCJD. Of these, 34 died of other causes within five years of the transfusions. Of the remaining 32, eight have now died, three from vCJD.

Professor Collinge, who reports on the case in The Lancet, said: “That three individuals from this small group of people that we know to have been exposed through blood transfusion have already developed vCJD infection suggests that the infection may be efficiently passed by this route, so the risk to remaining individuals is likely to be substantial.”

So far, about 160 people, mostly young, are known to have died of the disease by eating contaminated beef. The numbers were relatively low because of the “species barrier” between cows and humans.

Measures taken so far to prevent transmission by blood include importing blood plasma from the US, excluding as donors all those who have themselves had a transfusion as well as those whose blood has gone to recipients who have later developed vCJD, and removing white blood cells from all blood components in the belief that they are the most likely carriers of the rogue prions that cause the disease.

But without a test it is impossible to screen all blood donations, as is done for HIV and other diseases. Nor is it yet known how many potentially contaminated donors there are. The incubation period for vCJD acquired from beef is so long that there is ample opportunity for a blood donor to be carrying the rogue prions for years without any knowledge.

Experiments in mice show that such subclinical infections can still act as a source of full-blown infection if transmitted to other mice.

Studies of tonsils removed in routine operations suggest, the Spongiform Encephalopathy Advisory Committee (SEAC) says, that there could be several thousand subclinical carriers of the disease.

They might infect others in two ways: either through blood transfusions, if the precautions prove inadequate, or — more likely in SEAC’s view — through contaminated surgical instruments.

A big potential danger is in dental surgery. “The large number of dental procedures coupled with good patient survival implies that any significant risk via this route could have a major impact on the dynamics of secondary infection,” the committee said.

But nobody yet knows how infective the mouth and gum tissues of vCJD victims are. Dental instruments are sterilised between patients, but routine sterilisation is not enough to destroy the prions that cause the disease.

The Lancet paper says that tests on the tonsils of the patient who died showed that they were infected with prions. Testing tonsil tissue is a way of determining early if there is reason to suspect prion disease, Professr Collinge said.

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