It is almost certain the cluster of three cases of vCJD in the south county Dublin area has arisen by chance, writes Dr Muiris Houston, Chief Medical Correspondent.
The news that the three Irish people with variant Creutzfeldt-Jacob Disease (vCJD) had a geographical link has understandably led to public anxiety that an outbreak of the infectious disease, linked to the consumption of beef, has occurred in the south Co Dublin/north Co Wicklow area.
Dr Michael Farrell, consultant pathologist at the National CJD Surveillance Unit at Beaumont Hospital, told the Dublin County Coroner at the inquest into the death of Jason Moran, from Shankill, Co Dublin, that the identification of three cases of CJD with a geographical link warranted further investigation.
However, this does not mean that a statistical link exists between these cases. In medical terms, the cases are considered to be a cluster. Defined as an aggregation of cases of a disease in a given area over a particular period without regard to whether the number of cases is more than expected, a cluster is a number of cases that look like they are related but are not. Clusters are essentially statistical blips.
It is almost certain that a full investigation of these three cases will not find a common link. Although the two young men involved come from the same geographical area, a common source of beef would have to be proven by detailed examination of eating habits and the purchase of beef by the families concerned.
Meanwhile, the high population density in the area could explain the confluence of cases. The really weak link in this cluster concerns the woman who died from CJD in 1999.
Although originally from the same area as the young men, she lived in the Midlands. Significantly, given the five- to 15-year incubation period for vCJD, the 33-year-old had lived in Britain during the 1980s and early 1990s. It is almost certain that she consumed BSE-infected beef while living in the UK.
While Dr Farrell was correct to call for an investigation, it is extremely difficult to prove a common link in a disease with such a long incubation period.
The small number of cases will also militate against success. Statistically it is almost certain that this cluster of cases has arisen by chance.