187 ANALYZING DISEASE TRANSMISSION RISKS FROM ABATTOIR-DERIVED IN VITRO-PRODUCED BOVINE EMBRYOS

Abstract

While thousands of in vitro-produced (IVP) bovine embryos have been transferred commercially with no reports of disease transmission, such risks must be considered. Due to differences in their zonae pellucidae, the disease risks with IVP embryos are known to be higher than with in vivo-derived embryos. Possible sources of infection include the oocytes, spermatozoa, serum, and co-culture cells. The Terrestrial Animal Health Code of the Office International des Epizooties (OIE, 2003) stipulates that disease risk management should meet standards set by the World Trade Organization. These standards include subjecting the IVP procedures to quantitative risk assessment to evaluate disease transmission risk. The purpose of the present work was to measure the risks of transmitting disease with IVP embryos obtained from abattoir-derived tissues. A simulation model was developed using Microsoft Excel spreadsheets with the Palisade @RISK (London, UK) software program. The model incorporates probability distributions, the shapes of which reflect the random nature of some of the data (e.g. fluid volumes in cultures and washes) and the conjectural nature of some of the scientific information (e.g. on disease agents). The model is adaptable so that, when accurate data or information become available, variability estimates and degrees of uncertainty can be replaced with fixed values. The model assumes: (1) the IVP method is as described in the IETS Manual (1998); (2) there are five possible sources of infection; donor cow, donor bull, fetal calf serum, bovine serum albumin, and co-culture cells; (3) the disease agents can survive and/or proliferate during in vitro maturation, fertilization and culture; (4) fluid volumes in cultures and washes follow “known” normal distributions; (5) uncertainties in current knowledge of IVP embryos and disease agents can be taken into account by use of appropriate probability distributions; (6) different methods of in vitro fertilization do not affect the level of risk; and (7) different methods of in vitro culture can affect the level of risk. The model as constructed fits comfortably into a single workbook with one worksheet allocated for the model itself and another serving to store data on diseases of interest. Data on oocytes, blastocyst numbers, etc., and on media and wash fluid volumes are held within the model while information relating to particular diseases can be selected from a drop-down list at the top of the first worksheet. The relevant data stored in the database are then retrieved and used for modelling, using Monte Carlo simulation. The model estimates the final titer of the disease agent in IVP embryos and the probability of at least one infective transmission to a recipient, expressed as distributions.