Probability of introduction of exotic strains of bluetongue virus into the US and into California through importation of infected cattle

Strategies designed to minimize the probability of bluetongue virus (BTV) introduction to new areas should be based on a quantitative assessment of the probability of actually establishing the virus once it is introduced. The risk of introducing a new strain of bluetongue virus into a region depends on the number of viremic animals that enter and the competency of local vectors to transmit the virus. We used Monte Carlo simulation to model the probability of introducing BTV into California, USA, and the US through importation of cattle. Records of cattle and calf imports into California and the US were obtained, as was seroprevalence information from the exporting countries. A simulation model was constructed to evaluate the probability of importing either a viremic PCR-negative animal after 14-day quarantine, a c-ELISA BTV-antibody-negative animal after 28-day quarantine, or an untested viremic animal after 100-day quarantine into California and into the US. We found that for animals imported to the US, the simulated (best to worst scenarios) median percentage that tested positive for BTV-antibody ranged from 5.4 to 7.2%, while for the subset imported to California, the simulated median percentage that tested positive for BTV-antibody ranged from 20.9 to 78.9%. Using PCR, for animals imported to the US these values were 71.8-85.3%, and for those imported to California, the simulated median that test positive ranged from 74.3 to 92.4%. The probability that an imported animal was BTV-viremic is very low regardless of the scenario selected (median probability = 0.0%). The probability of introducing an exotic strain of BTV into California or the US by importing infected cattle was remote, and the current Office International des Epizooties (OIE) recommendation of either a final PCR test performed 14 days after entry into quarantine, a c-ELISA performed 28 days after entry into quarantine or a 100-day quarantine with no testing requirement was adequate to protect cattle in the US and California from an exotic strain of BTV.