Modelling studies on bovine spongiform encephalopathy occurrence to assist in the review of the over 30 months rule in Great Britain

Evidence for more cost-effective surveillance options for bovine spongiform encephalopathy (BSE) and scrapie in Great Britain

Transmissible spongiform encephalopathies (TSEs) are an important public health concern. Since the emergence of bovine spongiform encephalopathy (BSE) during the 1980s and its link with human Creutzfeldt-Jakob disease, active surveillance has been a key element of the European Union's TSE control strategy. Success of this strategy means that now, very few cases are detected compared with the number of animals tested. Refining surveillance strategies would enable resources to be redirected towards other public health priorities. Cost-effectiveness analysis was performed on several alternative strategies involving reducing the number of animals tested for BSE and scrapie in Great Britain and, for scrapie, varying the ratio of sheep sampled in the abattoir to fallen stock (which died on the farm). The most cost-effective strategy modelled for BSE involved reducing the proportion of fallen stock tested from 100% to 75%, producing a cost saving of ca GBP 700,000 per annum. If 50% of fallen stock were tested, a saving of ca GBP 1.4 million per annum could be achieved. However, these reductions are predicted to increase the period before surveillance can detect an outbreak. For scrapie, reducing the proportion of abattoir samples was the most cost-effective strategy modelled, with limited impact on surveillance effectiveness.

Is there a decline in bovine spongiform encephalopathy cases born after reinforced feed bans? A modelling study in EU member states

Occasional cases of classical bovine spongiform encephalopathy (BSE) still continue to occur within the European Union (EU) for animals born after reinforced feed bans (BARBs), which should in theory have eliminated all risk of infection. The study aimed to determine (i) whether a common rate of decline of BSE infection was evident across EU member states, i.e. to determine whether control measures have been equally effective in all member states, (ii) whether there was any evidence of spontaneous occurrence of BSE in the data and (iii) the expected date for the last BSE case in UK. It was found that there was no significant difference in the rate of decline of BSE prevalence between member states, with a common rate of decline of 33·9% per annum (95% CI 30·9-37%) in successive annual birth cohorts. Trend analysis indicated an ultimate decline to 0 prevalence, suggesting that spontaneous occurrence does not explain the majority of cases. Projecting forward the trends from the back-calculation model indicated that there was approximately a 50% probability of further cases in the UK, and should the current rate of decline continue, there remains the possibility of further occasional cases up until 2026.

A quantitative assessment of the prion risk associated with wastewater from carcass-handling facilities

Wastewater from facilities processing livestock that may harbor transmissible spongiform encephalopathies (TSEs) infectivity is permitted under license for application to land where susceptible livestock may have access. Several previous risk assessments have investigated the risk of bovine spongiform encephalopathy (BSE) associated with wastewater effluents; however, the risk of exposure to classical scrapie and atypical scrapie has not been assessed. With the prevalence of certain TSEs (BSE in cattle and classical scrapie in sheep) steadily in decline, and with considerable changes in the structure of carcass-processing industries in Great Britain, a reappraisal of the TSE risk posed by wastewater is required. Our results indicate that the predicted number of new TSE infections arising from the spreading of wastewater on pasture over one year would be low, with a mean of one infection every 1,000 years for BSE in cattle (769, 555,556), and one infection every 30 years (16, 2,500), and 33 years (16, 3,333) for classical and atypical scrapie, respectively. It is assumed that the values and assumptions used in this risk assessment remain constant. For BSE in cattle the main contributors are abattoir and rendering effluent, contributing 35% and 22% of the total number of new BSE infections. For TSEs in sheep, effluent from small incinerators and rendering plants are the major contributors (on average 32% and 31% of the total number of new classical scrapie and atypical scrapie infections). This is a reflection of the volume of carcass material and Category 1 material flow through such facilities.

Impact of regulatory perturbations to disease spread through cattle movements in Great Britain

During the past decade the British livestock industry has suffered from several major pathogen outbreaks, and a variety of regulatory and disease control measures have been applied to the movement of livestock with the express aim of mitigating the spread of infection. The Rapid Analysis and Detection of Animal-related Risks (RADAR) project, which has been collecting data on the movement of cattle since 1998, provides a relatively comprehensive record of how these policies have influenced the movement of cattle between animal holdings, markets, and slaughterhouses in Britain. Many previous studies have focused on the properties of the network that can be derived from these movements – treating farms as nodes and movements as directed (and potentially weighted) edges in the network. However, of far greater importance is how these policy changes have influenced the potential spread of infectious diseases. Here we use a stochastic fully individual-based model of cattle in Britain to assess how the epidemic potential has varied from 2000 to 2009 as the pattern of movements has changed in response to legislation and market forces. Our simulations show that the majority of policy changes lead to significant decreases in the epidemic potential (measured in multiple ways), but that this potential then increases through time as cattle farmers modify their behaviour in response. Our results suggest that the cattle industry is likely to experience boom-bust dynamics, with the actions that farmers take during epidemic-free periods to maximise their profitability likely to increase the potential for large-scale epidemics to occur.

Estimating the temporal relationship between PrPSc detection and incubation period in experimental bovine spongiform encephalopathy of cattle

This study examines tissues from sequential-kill, time-course pathogenesis studies to refine estimates of the age at which disease-specific PrP (PrPSc) can first be detected in the central nervous system (CNS) and related peripheral nervous system ganglia of cattle incubating bovine spongiform encephalopathy (BSE). Such estimates are important for risk assessments of the age at which these tissues should be removed from cattle at slaughter to prevent human and animal exposure to BSE infection. Tissues were examined from cattle dosed orally with 100 or 1 g BSE-infected brain. Incubation period data for the doses were obtained from attack rate and the sequential-kill studies. A statistical model, fitted by maximum likelihood, accounted for the differences in the lognormal incubation period and the logistic probability of infection between different dose groups. Initial detection of PrPSc during incubation was invariably in the brainstem and the earliest was at 30 and 44 months post-exposure for the 100 g- and 1 g-dosed sequential-kill study groups, respectively. The point at which PrPSc in 50 % of the animals would be detected by immunohistochemistry applied to medulla–obex was estimated at 9.6 and 1.7 months before clinical onset for the 100 g- and 1 g-dosed cattle, respectively, with a low probability of detection in any of the tissues examined at more than 12 months before clinical onset. PrPSc was detected inconsistently in dorsal root ganglia, concurrent with or after detection in CNS, and not at all in certain sympathetic nervous system ganglia.

Exposure assessment of TSEs from the landspreading of meat and bone meal

Recent changes in European legislation have meant that certain processed abattoir waste, which has been appropriately heat treated and ground to a specified particle size, can be spread on nonpasture agricultural land. This has opened the way for the potential landspreading of mammalian meat and bone meal (mMBM) derived from animals slaughtered for human consumption. This article reports on two separate case studies (Study 1 carried out in Great Britain (GB) and Study 2 carried out in Ireland) on the potential exposure to TSE infectivity following the spreading of abattoir waste (derived from animals slaughtered for human consumption) on nonpasture agricultural land. For Study 1, the average TSE infectivity on nonpasture agricultural land per year from sheep with scrapie was found to be higher (five orders of magnitude) than that estimated for BSE in cattle (3.9 x 10(-3) Ovine Oral ID(50)/ton of soil compared to 3.3 x 10(-8) Bovine Oral ID(50)/ton of soil). The mean estimate for BSE in sheep was 8.1 x 10(-6) Ovine Oral ID(50)/ton of soil. The mean level of infectivity in mMBM was assessed to be 1.2 x 10(-5) and 2.36 x 10(-5) ID(50)/ton of mMBM for Study 1 and 2, respectively. For Study 2 the spreading of mMBM was estimated to result in infectivity on nonpasture land of 1.62 x 10(-8) Bovine Oral ID(50)/m(3). The mean simulated probability of infection per year per bovine animal was 1.11 x 10(-9). Given the low infectivity density and corresponding low risks to bovines the spreading of mMBM could be considered a viable alternative for the utilization of mMBM.

Attributable testing for abnormal prion protein, database linkage, and blood-borne vCJD risks

CONTEXT

National prospective collection of tonsillar tissue to be tested anonymously for abnormal lymphoreticular accumulation of prion protein (PrP) was approved to begin in the UK in 2004. The UK is not, however, testing autopsy specimens attributably for abnormal PrP (PrP(SC)) so that recipients at risk after a blood transfusion from, or exposed to surgical instruments from, a deceased carrier of variant Creutzfeldt-Jakob disease (vCJD) can be followed up to quantify transmission risks. In Switzerland, surveillance for subclinical vCJD includes unconsented testing in autopsies: consented testing of tonsillar tissue is potentially attributable to interrupt human-to-human vCJD transmission or treat it.

STARTING POINT

The UK announced its first case of probable blood-borne vCJD transmission in December, 2003, and first detected a case of probable blood-borne subclinical vCJD in July, 2004. To reduce the possible risk of onward transmission to other people, UK patients who had received vCJD-implicated plasma products are being contacted. They, and their general practitioner, are asked to inform anyone giving them medical, surgical, or dental treatment, and the patients must refrain from donating blood, tissues, or organs. WHERE NEXT? Prudent additional surveillance options for human PrP(SC)--particularly at autopsy or to sanction the release of quarantined operation sets pending effective decontamination--can be costed by reference to results for cattle and sheep. Some ethical or legal impediments to the UK's potentially-attributable testing for PrP(SC) may yet be rued.