Commentary: Age related exposure of patients to the agent of BSE should not be downplayedBMJ 1997; 315 doi: https://doi.org/10.1136/bmj.315.7105.395 (Published 16 August 1997) Cite this as: BMJ 1997;315:395
- Sheila M Gorea, senior statistician ()
There have been two periods of prospective surveillance of Creutzfeldt-Jakob disease in the United Kingdom—during 1980-4, covering England and Wales only, and from 1 May 1990, covering the whole of the United Kingdom. Approximating these, table 1) summarises by age groups the cases reported for 1980-5 and 1990-5, together with the respective reference populations. Case ascertainment in patients under 70 was 20% higher in 1990-5 than in 1980-5 but more than doubled for patients aged 70 and over; in 1990-5, 2.16 cases per million population aged 70 and over were notified yearly. Comparison of rates of Creutzfeldt-Jakob disease among patients aged 70 and over from comparable prospective surveillance studies elsewhere in the European Union and internationally could give reassurance that there were no reasons other than differentially enhanced ascertainment which accounted for the doubling—such as enhanced dietary exposure to the causative agent of bovine spongiform encephalopathy at older ages in the United Kingdom. More generally, because the incidence of Creutzfeldt-Jakob disease varies so greatly with age group, it makes little sense epidemiologically to present international comparisons of incidence unless they are age group specific or age standardised. I therefore endorse the decision in France that routine reporting of sporadic cases and the variant forms of Creutzfeldt-Jakob disease should be by 10 year age groups.1
With only one sporadic case of Creutzfeldt-Jakob disease in a patient under 40 diagnosed yearly in the United Kingdom before 1996, how many suspected cases in patients under 40 were actually referred to the surveillance unit in 1980-5 and 1990-5, of whom 11 were diagnosed as having sporadic Creutzfeldt-Jakob disease? And did the referral pattern change in 1996 because of the emergence of the variant form of the disease? Partial answers are given in a recent letter in the Lancet but this concerns a broader age group (under 50 at referral to the surveillance unit) and gives data for April 1996 to February 1997 only.2
In discussing the 14 confirmed cases of the variant form of Creutzfeldt-Jakob disease, all but one occurring in patients under 40 at the onset of clinical illness, Will et al downplay age related exposure to the agent of bovine spongiform encephalopathy as an explanation for the observed age distribution. They identified no exposure related to the bovine disease that was sufficiently strongly related to age. However, consumption of certain foods is strongly age related (p 404)3. For example, in 1986-7, burgers and kebabs were consumed by 45% of 16-24 year olds but by only 13% of 50-64 year olds. By contrast, the consumption of beef or even sausages is not age related.4
Let us turn now from young adults to farmers. Will et al confirm that the occurrence of sporadic Creutzfeldt-Jakob disease in farmers is more than happenstance.5 Molecular marker studies of bovine spongiform encephalopathy reported by Collinge et al effectively proved that the variant form of Creutzfeldt-Jakob disease is caused by the same agent that causes the bovine disease.6 Hence as well as transmission studies with brain tissue taken at necropsy from farmers dying of Creutzfeldt-Jakob disease, molecular marker studies will be important in determining whether some of the cases in farmers were likely to have been due to occupational exposure to the infectious agent in cattle. It is worth recalling that in July 1996 (that is, before Collinge's work was reported) the degree of belief in the proposition that bovine spongiform encephalopathy causes the variant form of Creutzfeldt-Jakob disease averaged 5.4 (SD 2.8) on a scale of 0 (nil belief) to 10 (absolute certainty).7
Will et al report that in addition to the six cases of Creutzfeldt-Jakob disease in lifelong farmers, there had been three other confirmed cases since 1 May 1990 in people who belonged formerly to the occupational group comprising abattoir workers, butchers, meat cutters, and rendering plant workers. This occupational group together with veterinarians accounted for no more than s=78 000 workers yearly. The “ever employed” person years in this occupational group to be counted in the surveillance period 1990-5 depend on turnover of staff. If all 78 000 remained in the occupational group throughout 1981-95 they would contribute only 6s=6x78 000=0.468 million person years to the surveillance period 1990-5. If two thirds of employees stayed for 15 years, one sixth were replaced every year, and one sixth turned over after three years, then the person years contribution would increase considerably. Serious consideration needs to be given to the proper estimation of “ever employed” and “continuously employed” denominators if full use is to be made of occupational histories collected in cases of Creutzfeld-Jakob diseases.