Surveillance Enhancing Strategies for the Mitigation of Emerging Cattle Parasite Theileria orientalis Ikeda

Abstract

The most virulent genotype of Theileria orientalis, Ikeda, has been spreading through the Eastern and Midwestern United States since its initial detection in Virginia in 2017. Facilitated by its invasive Longhorned tick vector, spread of Ikeda to naïve regions of North America has been characterized by costly clinical outbreaks of Bovine Infectious Anemia, with severe cases resulting in abortion, stillbirth, and death, with still more production costs incurred due to lowered dairy and beef production in surviving animals. As such, Ikeda represent an enormous economic threat to the cattle industry in North America. In the absence of any vaccine or FDA-approved treatment and the infeasibility of vector control, surveillance testing to avoid introducing the parasite to naïve stock is the only mitigation strategy currently available. Expansion of surveillance testing through sample pooling previously infeasible as the high incidence of asymptomatic infection makes prevalence difficult to anticipate with enough confidence to optimize pooling approaches, which can become costly with high misestimation of prevalence. This work demonstrated that the addition of a subsampling test to estimate prevalence within a surveillance submission was sufficient to allow for economical pooling as well as avoid high costs when prevalence exceeded the pooling threshold (32%) when compared to conventional pooling of simulated T. orientalis surveillance herds. Additionally, the resulting prevalence estimation testing (PET) method was shown to be applicable to other surveillance pathogen through Monte Carlo Simulation, with the reduction in test savings not being statistically significantly different from simulated pooling with the true optimal pool size for both bovine viral diarrhea virus, and SARS-CoV-2 (p > 0.05). While PET pooling resulted in additional processing time in most scenarios, it was shown through Discrete Event Simulation to reduce processing time when laboratory capacity was at its minimum and sample intake was high. Otherwise, the primary advantage of PET in other surveillance programs was shown to be reduction in test count through improved optimization, and resource conservation. Further expansion of surveillance with serological methods was also shown in this work to be a feasible option, as 20 recombinantly expressed surface/secreted were shown to be seropositive (OD ratio > 2) across multiple infection types (Chitose, acute Ikeda, and persistent Ikeda). Additionally, 7 antigens showed variable degrees of seropositivity between infection type suggesting there may antigenic diversity between strain and/or infection stage that could allow serological diagnostics that differentiate between strain/infection stage, both of which have major clinical implications. The present work demonstrates the ability to enhance Ikeda mitigation through surveillance by maximizing the effectiveness of currently available rtPCR testing through pooling, as well as the potential to fill currently unmet surveillance needs with improved serological detection.