The myth of maternal transmission of spongiform encephalopathy

Abstract

It has long been accepted that the pattern of occurrence of scrapie-the form of spongiform encephalopathy associated with sheep-is determined mainly by maternal transmission, and this view has had a profound influence on policy decisions in the control of bovine spongiform encephalopathy and on public concern over the risk to human health from this disease. The occurrence of maternal transmission is, however, not predicted by modern knowledge ofthe aetiology of spongiform encephalopathy, and even though claims ofmaternal transmission have been reiterated frequently in the literature, re-examination ofthe source data reveals that these data are extremely scanty, unreplicated, and probably subject to ascertainment bias. The probability of maternal transmission of spongiform encephalopathy in any species should be viewed with the greatest scepticism.

Scrapie-free Merino and Poll Dorset sheep from Australia and New Zealand have normal frequencies of scrapie-susceptible PrP genotypes.

“…RR "&% QQ "(" is a rare genotype and, when it does occur in Britain and Europe, is almost always in scrapie-affected sheep and so it has been suggested that scrapie may be simply a genetic disease (Ridley & Baker, 1995). However, a study of healthy Cheviot and Suffolk sheep born and raised in scrapie-free countries (Australia and New Zealand) has shown that animals of highly susceptible genotypes are present at relatively high frequencies and that VV "$' RR "&% QQ "(" sheep (the most susceptible genotype known) can live up to 8 years of age, well past the usual age-at-death from scrapie (2-4 years) .…”

Big decisions based on small numbers: Lessons from BSE

“…There is no evidence of maternal transmission in the human prion diseases, nor in experimentally transmitted prion disease in any species (37). So, where did this preoccupation with maternal transmission of BSE come from?…”

“…That this is due to maternal transmission depends on there not being a similar heightened risk in the offspring of affected rams. Analysis of the original sources reveals that most of the data fail to take account of the scrapie status of the ram (37). The one study that does provide adequate data on the rams supports a genetic basis for this disease rather than maternal transmission (34).…”

Clinical Implications of Bovine Spongiform Encephalopathy

“…(This last observation has since been explained by persistence of the infectious agent in hay mites in the pastures [3]). Despite a few suggestive studies, in utero transmission of the scrapie agent has not been demonstrated [4]. Studies of naturally infected sheep demonstrate that the infectious agent first appears in tonsils and gut lymphatic tissue when the sheep are very young, which strongly suggests that natural infection occurs by the oral route [5].…”