Risk of BSE from the import of cattle from the United Kingdom into countries of the European Union

A stochastic model of the bovine spongiform encephalopathy epidemic in Canada

Bovine spongiform encephalopathy (BSE) appeared in the United Kingdom in the mid 1980s, and has been attributed to the use of meat and bone meal (MBM) in cattle feed contaminated with a scrapie-like agent. Import of infectious materials from a country where BSE has occurred is believed to be the major factor underlying the spread of the BSE epidemic to other countries. This study presents a new stochastic model developed to estimate risk of BSE from importation of cattle infected with the BSE agent. The model describes the propagation of the BSE agent through the Canadian cattle herd through rendering and feeding processes, following importation of cattle with infectious prions. This model was used estimate the annual number of newly infected animals each year over the period 1980-2019. Model predictions suggested that the number of BSE infections in Canada might have been approximately 40-fold greater than the actual number of clinically diagnosed cases. Under complete compliance with the 2007 ban on feeding MBM, this model further predicts that BSE is disappearing from the Canadian cattle system. A series of sensitivity analyses was also conducted to test the robustness of model predictions to alternative assumptions about factors affecting the evolution of the Canadian BSE epidemic.

Mathematical Models for Estimating the Risks of Bovine Spongiform Encephalopathy (BSE)

When the bovine spongiform encephalopathy (BSE) epidemic first emerged in the United Kingdom in the mid 1980s, the etiology of animal prion diseases was largely unknown. Risk management efforts to control the disease were also subject to uncertainties regarding the extent of BSE infections and future course of the epidemic. As understanding of BSE increased, mathematical models were developed to estimate risk of BSE infection and to predict reductions in risk in response to BSE control measures. Risk models of BSE-transmission dynamics determined disease persistence in cattle herds and relative infectivity of cattle prior to onset of clinical disease. These BSE models helped in understanding key epidemiological features of BSE transmission and dynamics, such as incubation period distribution and age-dependent infection susceptibility to infection with the BSE agent. This review summarizes different mathematical models and methods that have been used to estimate risk of BSE, and discusses how such risk projection models have informed risk assessment and management of BSE. This review also provides some general insights on how mathematical models of the type discussed here may be used to estimate risks of emerging zoonotic diseases when biological data on transmission of the etiological agent are limited.

A quantitative risk assessment for bovine spongiform encephalopathy in Japan

A predictive case-cohort model was applied to Japanese data to analyze the interaction between challenge and stability factors for bovine spongiform encephalopathy (BSE) for the period 1985-2020. BSE risk in cattle was estimated as the expected number of detectable cases per year. The model was comprised of a stochastic spreadsheet calculation model with the following inputs: (1) the origin and quantity of live cattle and meat and bone meal imported into Japan, (2) the age distribution of native cattle, and (3) the estimated annual basic reproduction ratio (R(0) ) for BSE. The estimated probability of having zero detectable cases in Japan in 2015 was 0.90 (95% CI 0.83-0.95). The corresponding value for 2020 was 0.99 (95% CI 0.98-0.99). The model predicted that detectable cases may occur in Japan beyond 2015 because of the assumption that continued transmission was permitted to occur (albeit at a very low level) after the 2001 ban on the importation and domestic use of all processed animal proteins for the production of animal feed and for fertilizer. These results reinforce the need for animal health authorities to monitor the efficacy of control measures so that the future course of the BSE epidemic in Japan can be predicted with greater certainty.

Population trends of grassland birds in North America are linked to the prevalence of an agricultural epizootic in Europe

Globalization of trade has dramatic socioeconomic effects, and, intuitively, significant ecological effects should follow. However, few quantitative examples exist of the interrelationship of globalization, socioeconomics, and ecological patterns. We present a striking illustration of a cascade in which bovine spongiform encephalopathy (BSE; "mad cow disease") outbreaks in Europe exerted pressure on global beef markets, subsequently affecting North American hayfields and grassland bird populations. We examined competing models, which linked the prevalence of BSE in five focal countries, volume of beef exports to those countries from North America, and the amount of hayfield harvested and the abundance of grassland birds in North America. We found that (i) imports from North America increased 1 y after BSE outbreaks; (ii) probably because fewer cattle remained, the hay harvest in North America was reduced 2 y after the outbreak; (iii) the reduced hay harvest yielded a positive response in grassland bird populations 3 y after the outbreak.

Quantitative risk assessment for bovine spongiform encephalopathy in low- or zero-prevalence countries: the example of Norway

A predictive case-cohort model is applied to Norwegian data to analyze the interaction between challenge and stability factors for bovine spongiform encephalopathy (BSE) during the period 1980-2010. For each year, the BSE risk in cattle is estimated as the expected number of cases. The age distribution of expected cases as well as the relative impact of different challenges is estimated. The model consists of a simple, transparent, and practical deterministic spreadsheet calculation model, in which the following country-specific inputs are entered: (i) annual imports of live cattle and meat and bone meal, (ii) age distribution of native cattle, and (iii) estimated annual basic reproduction ratio (R(0)) for BSE. Results for Norway indicate that the highest risk of BSE cases was in 1989, when a total BSE risk of 0.13 cases per year was expected. After that date, the year-to-year decrease in risk ranged between 3% and 47%, except for a secondary peak in 1994 at 0.06 cases per year. The primary peak was almost entirely (99%) attributable to the importation of 11 cattle from the United Kingdom between 1982 and 1986. The secondary peak, in 1994, originated mainly from the recycling of the U.K. imported cattle (92%). In 2006, the remaining risk was 0.0003 cases per year, or 0.001 per million cows per year, with a maximal age-specific incidence of 0.03 cases per million per year in 10-year-old cattle. Only 15% of the cases were expected in imported cattle. The probability of having zero cases in Norway in 2006 was estimated to be 99.97%. The model and results are compared to previous risk assessments of Norway by the EU.

Risk factors for BSE-infections in Lower Saxony, Germany

In order to generate data on the cattle population and farm management in the state of Lower Saxony, Germany, a basic demographic survey was conducted. Afterwards these BSE-free farms as a reference population were compared with the population on BSE farms to identify risk factors for BSE infections. A variety of risk factors for BSE were reviewed, including the import of cattle from the United Kingdom, commercial foodstuff, dairy farming, herd size and cross-contamination with foodstuff for other farm animals. For the basic demographic survey of the reference, a questionnaire was mailed to a representative sample of cattle farms in Lower Saxony where BSE cattle had not occurred before the sample was taken. Distribution of risk factors within this reference population (n=731) and the BSE population (n=49) were compared following the concept of indirect standardisation in stratified populations. The size of farms was used as the stratification variable, with three strata. Under the same rate as in the reference population, the portion of Red Holstein cattle breed was four fold higher than in the BSE population (SER=4.03; p=0.0003). Milk replacer was fed 1.41 times more often on BSE farms (p=0.0478). However, the use of concentrated foodstuff for pigs (SER=0.21) was significantly less frequent in the BSE population than expected (p=0.0001), whereas the husbandry of sheep, goats or game animals seemed to increase the risk of BSE 2.85 times (p=0.0413). There were no significant differences between the two populations concerning the purchase of cattle (p=0.1514) and the use of concentrated feed for calves during the 1990s (p=0.6212). This is an epidemiological indication of increased susceptibility of Red Holstein cattle to BSE. However, this study did not confirm the assumption that the use of commercial foodstuff other than milk replacer or the purchase of cattle increases the risk of BSE infection. It nevertheless remains likely that commercial foodstuffs such as concentrated feed for calves were risk factors in Germany as well.

Brief review on the epidemiology of transmissible spongiform encephalopathies (TSE)

Transmissible spongiform encephalopathies (TSE) form a group of human and animal diseases that share common features such as (a) distinct pathological lesions in the central nervous system, (b) transmissibility at least in experimental settings, and (c) a long incubation period. Considerable differences exist in the host range of individual TSEs, their routes of transmission, and factors influencing the host susceptibility (such as genotype). The objective of this review was to briefly describe the main epidemiological features of TSEs with emphasis on small ruminant (sheep, goats) TSE, bovine spongiform encephalopathy (BSE) in cattle and chronic wasting disease (CWD) in deer and elk.

Population-level retrospective study of neurologically expressed disorders in ruminants before the onset of bovine spongiform encephalopathy (BSE) in Belgium, a BSE risk III country

A retrospective epidemiological study (n = 7,875) of neurologically expressed disorders (NED) in ruminants before the onset of the bovine spongiform encephalopathy epidemic (years studied, 1980 to 1997) was carried out in Belgium. The archives of all veterinary laboratories and rabies and transmissible spongiform encephalopathy (TSE) epidemiosurveillance networks were consulted. For all species, a significantly higher number of NED with virological causes (rabies) was reported south of the Sambre-Meuse Valley. During the period 1992 to 1997, for which the data were complete, (i) the predicted annual incidence of NED varied significantly as a function of species and area (higher numbers in areas where rabies was present) but was always above 100 cases per million, and (ii) the mean incidence of suspected TSE cases and, among them, those investigated by histopathological examination varied significantly as a function of species and area. The positive predictive value of a presumptive clinical diagnosis of NED ranged from 0.13 (game) to 0.63 (sheep). Knowledge of the positive predictive value permits the definition of a reference point before certain actions (e.g., awareness and training campaigns) are undertaken. It also shows the usefulness of a systematic necropsy or complementary laboratory tests to establish an etiological diagnosis. TSE analysis of a small, targeted historical sampling (n = 48) permitted the confirmation of one case and uncovered another case of scrapie. The results of the present study help to develop and maintain the quality of the worldwide clinical epidemiological networks for TSE, especially in countries that in the past imported live animals, animal products, and feedstuffs from countries with TSE cases.

Risk of introduction of BSE into Japan by the historical importation of cattle from the United Kingdom and Germany

All cattle of UK and German origin imported to Japan since 1980 and slaughtered before 2002 were traced (n = 33 and 15 respectively) and the probability that none, one, two or three of these imported cattle had developed BSE (reached the end or last stage of incubation period) at the year of slaughter/death was calculated. The predicted risk that BSE was introduced into Japan by imported cattle was 0.18. Among cattle imported from these countries in various years, cattle imported from the UK in 1987 and 1988 presented the highest risk, while the risk that BSE entered Japan by live cattle imported from the UK in 1982 and from Germany in 1993 was negligible. Because there was no effective system to avoid the recycling of the BSE agent, those infected cattle imported from the UK in 1987 and 1988 most probably entered the feed chain in Japan in 1992 and 1993.

Risk of introduction of BSE into Sweden by import of cattle from the United Kingdom

All cattle of United Kingdom origin imported to Sweden since 1980 were traced (n=94) and the probability that none of these imported cattle had clinical signs of bovine spongiform encephalopathy (BSE) at the year of slaughter (death) was calculated. If BSE had been introduced by live-animal imports, the consequences of such an event also was evaluated. The potential of the recently introduced surveillance system of high-risk cattle to detect such an event also was evaluated. We found that BSE most probably has not been introduced to Sweden by live-animal imports. We also found that, if this event had occurred and assuming a worst-case scenario that the animal was not prevented from being rendered, the rendering system (during certain periods) would not have prevented further spread of infection. Finally, we found that the BSE surveillance of high-risk cattle has not been in place long enough to verify that this event has not occurred (as of December 2001).

Survey of laboratory findings in suspected cases of bovine spongiform encephalopathy in Denmark from 1990 to 2000

A survey of the laboratory findings in suspected cases of bovine spongiform encephalopathy (BSE) in Denmark from 1 June 1990 to 31 December 2000 is presented. During this period BSE was a notifiable disease, and the heads of suspected cases were submitted according to the legislation on BSE. A total of 176 submissions were made, mostly from bovines with neurological disorders and mainly during the last 3 years of this period. Lesions or other laboratory findings consistent with severe neurological disorders were found in 115 cases. The most frequent diagnosis was encephalic listeriosis (35.8% of submissions) followed by other forms of inflammatory lesions. A wide range of lesions were diagnosed less prevalent. BSE was diagnosed twice. The first case occurred in an imported cow in 1992, while the second confirmed case was diagnosed in a native cow in February 2000. A marked increase in the number of submissions occurred following the detection of BSE in February 2000.

Active surveillance of BSE in cattle in The Netherlands

Since January 2, 2001 a large-scale active surveillance programme for BSE started in the Netherlands in addition to the passive surveillance programme of cattle with clinical symptoms compatible with BSE. Based on decisions of the Council of European Ministers of Agriculture, the European Union launched an active surveillance system for BSE in cattle of 30 months and older. Until April 1, more than 100,000 head of cattle were tested in this scheme, including all cattle slaughtered and a large part of the cattle that died on the farm. Four animals were found positive in the active surveillance system and one cow from the passive surveillance tested positive for BSE during the first three months.