Does improvement in case ascertainment explain the increase in sporadic Creutzfeldt-Jakob disease since 1970 in the United Kingdom?

The aim of this study was to quantify the improvements in case ascertainmen t which are considered to explain the rise in the incidence of sporadic Cre utzfeldt-Jakob disease. The numbers of cases, falling into five 10-year age groups starting at the age of 30 and three calendar periods of report sinc e 1970, were analyzed by Poisson regression, assuming a constant age distri bution. The age-period and age-cohort models were applied and discussed. Th e age-period model showed that underreporting in 1970-1979 was greater amon g patients aged 70 years or older. The age-cohort model indicated that a co hort factor increased over the first half of the 20th century (e.g., the in cidence in the generation born in 1940 was almost twice that in the generat ion born in 1920); this increase was probably an artifact due to the past u nderascertainment pattern. However, from a statistical viewpoint, both mode ls lead to a good fit; the cohort factor may appear to be as relevant as th e period factor in describing the trends in incidence. Thus, one can imagin e an unlikely worst case scenario, assuming that an unknown cohort factor i s involved, in that case, the age-cohort model gives more optimistic predic tions than Neilson's model (BMJ 1996;312:1038-9). These results are consist ent with both interpretations: The rise in incidence is governed by improve ments in case ascertainment, and is greater among old people (the most acce pted interpretation); this rise may depend on a cohort factor as well, whic h may correspond to the zoonotic hypothesis (a totally hypothetical interpr etation). Interpreting the increase of sporadic Creutzfeldt-Jakob disease o ver generations in terms of exposure to putative environmental factors is s till a matter of debate; ongoing epidemiologic surveys may provide more inf ormation. Presently, this increase can be explained as an artifact due to t he past underreporting pattern, with 79% (95% confidence interval: 56, 90) of the cases among persons aged greater than or equal to 70 years being mis sed in 1970-1979.