sentence following a specific reference to BSE, Dealler and hold that transmission by mouth has been shown for zoo Lacey cats, animals, sheep, goats, cows, man, and mice. The inclusion of man is misleading: your correspondents must be referring to kuru and perhaps linking this with Creutzfeldt-Jakob disease (CJD) and then indirectly implying that there is a common pattern of transmission that must include man and BSE. This is the kind of thinking that I sought to challenge. Current evidence does not allow us to link the transmissible spongiform encephalopathies directly in this way. I shall study with care D Her and Lacey’s forthcoming paper. The reason why I respect t views of the Ministry of Agriculture, Fisheries and Food is that "ave a special regard for the care and scientific skill that is evident in their work, and their current views are backed by experienced colleagues who have been advising me on BSE over the past two years. I respect counterarguments too, provided they are scientifically sound and carefully constructed. Mrs Smith unfairly suggests that I overlooked the clustering of CJD in small ethnic group of Libyan Jews domiciled in Israel, but she makes a point that merits discussion. The subject is authoritatively discussed by Kimberlin,! who sets out the evidence against the view that CJD represents scrapie in man. I noted that there is fair)’strong circumstantial evidence that there is no direct causal asst jation between scrapie in sheep and CJD in man, and I drew attention to a good editorial in The Lancet.2 A case control study did not support the suggested link between CJD and consumption of sheep brain. It seems likely that the ethnic group has enhanced genetic susceptibility to CJD, and Kimberlin gives a good account of current theories. If there is a direct link between the aetiologies of scrapie and CJD the evidence is lacking at present, and we would have to set aside or explain the counter-evidence, which is substantial. It seems that an assuredly common factor in the transmissible dementias and spongiform encephalopathies is an aberrant form of prion protein.** We should not be alarmed if immunocytochemical and molecular procedures were to demonstrate that we underestimate the incidence of prion disease and that a substantial part of the formula has a genetic basis.S If the remarkable concept of prion disease does gain ground and is applied in practical diagnostic terms, any attempt to assess the impact of foodborne BSE on the incidence of transmissible dementias in man would have to take account of the improved rate of detection that would result. At that stage, we can anticipate further pressures to control foodstuffs of bovine origin, and this is likely to continue until we have a specific marker for BSE.
Department of Medical Microbiology, University Medical School, University of Edinburgh, Edinburgh EH8 9AG, UK
J. G. COLLEE
1. Kimberlin RH. In. Collier LH, Timbury MC, eds Topley and Wilson principles of bacteriology, virology and immunity, 8th ed, vol IV. London Edward Arnold, 1990. 678-80. 2. Editorial BSE and scrapie: agents for change Lancet 1988; ii 607-08. 3 Taylor DM. Bovine spongiform encephalopathy and human health Vet Rec 1989; 125: 413-15 4. Editorial Pnon disease; spongiform encephalopathies unveiled. Lancet 1990, 336: 21-22. 5 Roberts GW, Collinge J. Bovine spongiform encephalopathy. Br Med J 1990; 300: 943-44.
Anti-IgA screening SIR,-Dr Hunt and Dr Reed (Nov 10, p 1197) confirm the importance of screening for anti-IgA in patients who are to receive intravenous gammaglobulin. However, they also state that the isolation of purified IgA is difficult, when it is one of the easiest normal human serum proteins to purify. The lectin jacalin, present in large amounts in the seed of the jackfruit tree (Artocarpus integrifolia), has been shown to bind, almost exclusively, with the carbohydrate moieties of human IgA.’ In an insoluble form, jacalin provides an extremely easy method for isolation of this immunoglobulin in amounts necessary for the detection of anti-IgA. Although Hunt and Reed prefer agglutination in their screening assay, most modern bloodbanks make extensive use of ELISAs to screen blood for viral markers and so have the ability to apply a technique based on jacalin to anti-IgA detection. Jacalin
insolubilised bind several
beads (Pierce Chemical Company), will of milligrams IgA per millilitre of gel and, in our can be used repeatedly over several years. The experience, availability of polyclonal IgA isolated from normal human serum also avoids IgA paraproteins that may not always be readily available to workers in this field. Finally, this lectin could remove IgA from preparations of intravenous gammaglobulinAnti-IgA testing in patients would then become unneccesary.2 onto agarose
Natal Blood Transfusion Service, PO Box 2356, Durban 4000, South Africa
J. D. CONRADIE
1. Kondoh H, Kobayashi K, Hagiwara K. A simple procedure for the isolation of human IgA of IgA1 and IgA2 subclass by a Jackfruit lectin, jacalin, affinity chromatography. Mol Immunol 1987, 24: 1219-22. 2 Kondoh H, Kobayashi K Elimination of undesirable immunoglobulin contaminants including aggregated IgG from gamma-globulin preparations by jackfruit lectin affinity chromatography Clin Chim Acta 1988; 174: 15-24.
Sydney classification for gastritis SIR,-Dr Caselli and colleagues’ data (Dec 8, p 1445) on adhesion patterns of Helicobacter pylori in chronic gastritis are interesting. However, to suggest that adhesion details, over and above inflammatory activity and numbers of bacteria, should be a major component of the Sydney classification of gastritis shows a misunderstanding of the system’s goal-namely, to provide a simple, logical framework with which to classify all patterns of gastritis, and one which would appeal to gastroenterologist and generalist alike. The imposition of graded details of adhesion patterns of H pylori on such a general framework may prove to be of importance within the confines of Hpylori-associated gastritis but would produce a complexity that would render the system unworkable. Department of Pathology, Northwick Park Hospital and Clinical Research Centre, Harrow HA1 3UJ, UK
ASHLEY B. PRICE
Department of Gastroenterology, Central Middlesex Hospital,
J. J. MISIEWICZ
DNA in Crohn’s
disease tissue SiR,—The hypothesis that Crohn’s disease may result from mycobacterial infection has lately been revived,l and several groups have reported isolating spheroplast forms of Mycobacteria paratuberculosis from colon specimens of patients with Crohn’s disease.2However, blot hybridisation studies on extracts from affected tissue have not revealed M paratuberculosis DNASince the sensitivity of the procedure was about 1 bacterial genome per 100 human cells, it was speculated that the number of organisms was too few to allow detection.6 We have applied a sensitive polymerase chain reaction (PCR) assay to look for mycobacterial DNA in Crohn’s disease. This assay can detect about 30 genomes of mycobacterial DNA, including M tuberculosis, M kansasii, M avium, and M paratuberculosis.’ From the pathology archives of the University of California Medical Center, 20 paraffin blocks of Crohn’s disease colon tissue with at least one well-formed granuloma were selected, as were 10 blocks of normal colon from resections for carcinoma. DNA was extracted from these blocks,8 and 100 ng DNA (about 20 000 human genomes) was amplified with mycobacteria-specific primersIn no specimen was a specific amplified band seen. All gave a strong band with human 0-globin primers,9 demonstrating the suitability of the DNA for amplification. Several blocks of skin and colon with active tuberculosis gave positive bands. Therefore, there must be very few, if any, mycobacterial organisms in tissues with active Crohn’s disease. On the assumption of equal efficiency in extraction of bacterial and human DNA, our limit of sensitivity is about 30 in 10 000 or 1 bacterium per 670 cells. These results make it unlikely that Crohn’s disease represents active mycobacterial infection. It remains possible that the inflammation is a delayed hypersensitivity reaction to mycobacterial antigens