BSE: Science & research - BSE in cattle - Epidemiology and modelling of the BSE epidemic
Epidemiology is a field of science related to the study of changes in numbers of cases and incidence of disease in relation to time and space. Examining the BSE epidemic in this way forms a core section of Defra’s science and research activities on TSEs.
More information is provided below on some of the epidemiological studies and modelling work on the BSE epidemic.
Early epidemiological studies
After the discovery of BSE in 1986, MAFF funded several epidemiological studies. The objective was to investigate a wide range of different factors which may have been responsible for the BSE outbreak or could have affected the occurrence of the disease. The first results were available at the end of 1987.
The only common feature of all the 200 cases of BSE investigated was the use of commercially produced compound feed containing meat and bone meal (MBM) (1). This conclusion was further supported by evidence that the incidence of BSE in dairy herds (fed a much higher proportion of compound feed) was much greater than it was in beef suckler herds.
Further results included:
- The pattern of BSE cases was typical of an extended common source epidemic (one that involves many individual, independent disease outbreaks, which can each be traced back to a common source);
- There was no evidence found of cattle to cattle transmission;
- There was no evidence of any common exposure to specific pharmaceutical products or pesticides (explicitly organophosphorous compounds);
- There was no evidence that the disease was related to a genetic component in cattle;
- The hypothesis of possible direct transmission of the scrapie agent from sheep to cattle (by either direct or indirect contact) was found to be untenable (there were no sheep present on 20% of the farms affected with BSE).
Subsequently, a formal case-control study of calf feeding practices and MBM inclusion in proprietary concentrates (2) provided further evidence that the development of the BSE epidemic resulted from the use of infectious MBM in cattle feed.
Studies into the maternal transmission of BSE
The early epidemiological studies did not find evidence of cattle to cattle transmission. Additional research was undertaken to investigate further whether BSE could be transmitted from cow to calf (otherwise known as vertical transmission).
Maternal transmission
A long-term cohort study was initiated in 1989 to examine maternally-associated risk factors for BSE. Interim findings were presented to SEAC in July 1996 and SEAC advised that maternal transmission would not sustain the BSE epidemic.
Since 2002, the probability of maternal transmission has been estimated to be approximately 1% in the last six months of the maternal incubation period (3). In 2003, SEAC were advised of the potential future change from culling all offspring born after July 1996, to a cull of offspring born within two years of the clinical onset of disease in the dam. SEAC advised that they saw no scientific grounds for maintaining the policy for culling offspring in the UK in place at that time.
Research has shown that embryos from BSE-infected cows inseminated with semen from BSE-infected bulls are unlikely to carry BSE infectivity even if they have been collected at the end-stage of the disease, when the risk of maternal transmission is believed to be highest (4). This finding contributed to the lifting of the EU ban on the export of embryos from the UK in 2002.
Action taken
Data from the epidemiological studies has been used by Defra to implement BSE and disease control and eradication strategies. As a result, the number of BSE cases in the UK - has declined dramatically.
References
1. Wilesmith JW, Wells GA, Cranwell MP & Ryan JB (1988) Vet. Rec. 123: 638-644.
2. Wilesmith JW, Ryan JB & Hueston WD (1992) Res. Vet. Sci. 52(3): 325-331.
3. Donnelly CA, Ferguson NM, Ghani AC & Anderson RM (2002) Proceedings of the Royal Society of London, Series B 269: 2179-2190. Full text available.
4. Wrathall AE et al. (2002) Vet. Rec. 150(12): 365-378.
Links to other information
- Origin of BSE
- Table of all currently active and recently completed projects
- Veterinary Risk Assessment on BSE Cohort Controls (May 2007,
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Page last modified: 18 March, 2008