Bovine Spongiform Encephalopathy (BSE) was first diagnosed in Ireland in 1989.  The disease was classified as a Class A compulsory notifiable disease in 1989 and a ban on the direct feeding of meat-and-bone-meal (MBM) to ruminant animals was imposed in 1990.  In March 1996, the significance of BSE changed dramatically when the UK announced a link between BSE in cattle and vCJD in humans.  It is thought that humans were exposed to the BSE agent through the consumption of tissue called Specified Risk Materials or SRMs, which have been shown to contain the infective agent.  In the bovine animal the majority of the infectious load is contained within the brain, spinal cord and associated tissues, none of which are traditionally consumed in Ireland.

On foot of the UK announcement in 1996, BSE control measures in place in Ireland were revised.  Strict provisions designed to prevent the cross-contamination of ruminant feeds with MBM intended for use in pig and poultry rations were introduced in October 1996 and Regulations providing for the identification and destruction of SRMs were introduced in February 1997. Tight controls on high-risk rendering plants were also imposed in 1996 and 1997.

Widespread testing of ‘at risk’ animals became possible with the approval of rapid post-mortem tests by the European Commission in the year 2000.  Active surveillance testing began in earnest in January 2001, when it became compulsory to test all cattle over 30 months of age for BSE when presented for slaughter. In July 2001, the programme was extended to include all bovine animals which die on farms and are over 24 months of age.  The introduction of this EU wide active surveillance programme lead to the identification of BSE in countries which heretofore had been considered BSE free. In addition, it led to an increase in the number of cases being identified in countries such as Ireland, which had already experienced indigenous cases. In Ireland’s case the vast majority of these additional cases (average 77%) were identified in bovine animals which had died on the farm.

BSE is a disease with a long incubation period (average approx. five years), which means that it takes a relatively long time to see the effect of any control measures introduced.  In recent years, evidence has emerged, firstly from the age profile of cases and more recently from a reduction in case numbers, to indicate that the additional control measures introduced in Ireland in 1996 and 1997 have been effective at substantially reducing the exposure of bovine animals to the infectious agent.  The percentage of cases diagnosed in animals less than six years of age has declined from 40% in 2000 to 16% in 2001, 2% in 2002 and 0% in 2003 (Figure 1). This trend indicates that younger animals have not been exposed to the agent in the same way as animals born prior to the additional controls.

Figure 1: Temporal Trend in Cases in Animals Less Than Six Years of Age

As can be seen from Figure 2, the age profile of cases diagnosed in recent years is shifting sequentially towards older animals. As the number of older animals in the bovine population continues to decline, it is expected that the incidence of BSE will continue to fall.

 

Figure 2: Percentage of Cases Diagnosed Per Year Per Age Category

Further indications that the additional controls have been effective can now be seen both in terms of case numbers and in the prevalence of test positive animals.  To the end of week 21 in 2004 (week ending May 23) a total of 63 cases of BSE had been confirmed compared to 98 for the same period in 2003 and 158 for the same period in 2002.  Figure 3 depicts the prevalence of test positives among tests carried out for the first 21 weeks of each of the three comparable years.

Figure 3: Prevalence of Test Positives per Test Category per Year

It is now clear that a ban on the direct feeding of MBM to ruminant animals by itself is not sufficient to control BSE.  With the additional measures introduced in Ireland in 1996 and 1997, evidence is now emerging from both the age profile of BSE cases and from the prevalence of test positives that these additional controls have been effective at substantially reducing the exposure of animals to the infectious agent.  It is expected that the incidence of disease will continue to fall as the number of pre-1998 born animals in the population continues to decline.  However, cases may continue to occur for at least the next ten years.

Up to the end of May 2004, there were 1,397 cases of BSE identified in the Republic of Ireland. This compares with approx. 180,000 cases identified in the UK.

There have been 141 cases of vCJD confirmed in the UK and, to date, one case of vCJD confirmed in the Republic of Ireland.

A recent paper, ‘vCJD risk in the Republic of Ireland’, uses an established mathematical model to estimate the total future number of clinical cases of vCJD in Ireland. Irish people could potentially have been exposed to BSE from three sources: eating beef from Irish animals, eating beef imported from the UK and living in and eating beef in the UK during the years 1980 to 1996. Using the mathematical model, the authors estimate one future clinical case of vCJD in the Republic of Ireland.

This research article is available from: www.biomedcentral.com/1471-2334/3/28