The unrecognised French BSE epidemic

Area of Science:

• Veterinary epidemiology
• Zoonotic disease surveillance
• Food safety science

Background:

• France implemented systematic BSE screening in 2000, complementing passive surveillance of clinical cases.
• Passive surveillance revealed limitations, indicating substantial under-reporting of Bovine Spongiform Encephalopathy (BSE) cases.
• Understanding under-reporting is crucial for reconstructing the French BSE epidemic and its link to variant Creutzfeldt-Jakob disease (vCJD).

Purpose of the Study:

• To estimate age- and year-specific incidence rates of BSE in France.
• To reconstruct the French BSE epidemic pattern, accounting for significant under-reporting.
• To assess the impact of under-reporting on epidemic surveillance.

Main Methods:

• Utilized a back-calculation method based on the principle that clinical cases result from infections after a known incubation period.
• Generalized the model to incorporate cattle mortality, reporting probability, and age-dependent susceptibility/exposure.
• Estimated average BSE incubation period and peak infection risk age in cattle.

Main Results:

• Confirmed an average BSE incubation period of five years.
• Identified peak bovine infection risk between 6 and 12 months of age.
• Estimated 301,200 infected cattle (95% CI [27,600-837,600]) versus 103 reported cases by June 2000, with under-reporting being the most influential parameter.

Conclusions:

• The French BSE epidemic was substantially underreported until the introduction of rapid tests.
• The initial wave of the French BSE epidemic in the late 1980s was largely undetected.
• Systematic screening is essential for accurate epidemic surveillance and understanding zoonotic disease risks.