Sir: The discovery that tonsils and appendices may harbour the organism which causes new variant CJD ("CJD may be transmitted by surgery", 15 January), does not necessarily mean that that person is incubating the disease. It only indicates that they have swallowed the agent which, together with all the nutrients, enters the bloodstream, visiting all the tissues. It has to proliferate in the central nervous system to cause the disease. Furthermore only about 30 per cent of humans are genetically susceptible to CJD.
In 1977 two teenagers in Switzerland developed CJD two years after undergoing surgery for cerebral tumours. Some of the instruments employed in their surgery had previously been used on the brain of a patient known to be suffering from CJD and were of course sterilised in the usual way. The organism which causes these "prion" diseases is virtually indestructible, which is why disposable instruments must be used if CJD-infected material is likely to be encountered.
H C GRANT MD FRCP