PROGRESSIVE PRODUCER A Genomic Approach using DNA to Help Combat Foothill Abortion by Bryan Welly, graduate student, University of California, Davis Epizootic bovine abortion (EBA), commonly known as “foothill abortion,” has been a persistent problem in the California beef cattle industry for nearly a century. Responsible for the loss of 45,000 to 90,000 calves per year, EBA is the leading cause of abortion in California cattle.1 The disease was first defined in the late 1950s as a result of its unique fetal pathology. In the 1970s the soft-shelled tick Orntithodoros coriaceus, referred to as the pajaroello tick, was identified as having the same geographic distribution as epizootic bovine abortion and was confirmed to be the vector that transmits EBA.2 EBA is characterized by late term abortion affecting naïve heifers and unexposed cows. EBA has no known effect on the dam, but when it is transmitted to the fetus it multiplies in the fetus, and this leads to a late term abortion or the birth of a weak calf. Once cows have been exposed to a season grazing in pastures infested with the pajaroello tick, they become resistant to foothill abortion.3 Although the tick appears to be spreading due to the increased eastward movement of cattle to neighboring states, to date incidences have only been reported in California, Nevada and Oregon.3, 4 Nearly 30 years after discovering that ticks transmit this disease, the causative agent for EBA was finally discovered to be a novel deltaproteobacteria referred to as “agent of EBA” (aoEBA).1,5 It is not possible to grow this agent of EBA in bacterial media, rather this deltaproteobacteria can only be grown experimentally in immunodeficient mice. This has limited researchers’ ability to fully characterize this bacteria, and constrained the development of effective therapies to counteract foothill abortion. Vaccine development typically consists of multiplying diseasecausing bacteria in a media and then either killing the bacteria with heat or a chemical agent (killed vaccine),
or inactivating (attenuating) the live bacteria to inhibit it from causing the disease while still enabling it to illicit an immune response (modified live). Due to the difficulty of growing aoEBA in culture, researchers have been forced to seek other approaches to vaccine development. University of California, Davis (UC Davis), researcher Jeffery Stott, Ph.D., and his research team have spent several years developing a live bacterial vaccine from aoEBA grown in immunodeficient mice. This vaccine consists of a quantifiable number of aoEBA-infected mouse spleen cells, and promises to be an effective tool to help decrease the incidence of EBA in California. However, the difficulties and costs associated with production of a live bacterial vaccine in mice stimulated researchers to investigate other approaches to help combat Foothill Abortion. The logical long-term solution for EBA abatement, from both a financial and managerial standpoint, is the development of a recombinant vaccine because it would eliminate the need to grow aoEBA in immunodeficient mice and the use of cryopreserved vaccines. A recombinant vaccine for EBA would consist of an unrelated, harmless microbe (virus or bacteria) that expresses an aoEBA gene which encodes an antigenic protein. This modified microbe would act to express the aoEBA antigen in the cow. An antigen is a foreign protein that triggers a host immune response. The major benefit to developing a recombinant vaccine is the ease of production with the ability to produce the vaccine in a laboratory setting utilizing media to grow the bacteria rather than immunodecficient mice. In general, recombinant vaccines have been shown to produce more consistent vaccine titers and are more stable than live bacterial vaccines.6,7 One step required for the development of an effective recombinant vaccine is the knowledge
14 California Cattleman March 2014
of which gene or genes from the aoEBA genome encode antigenic proteins. With that ultimate goal in mind, genetic specialist Alison Van Eenennaam, Ph.D., and her research team at UC Davis embarked on a research project to assemble a reference genome for the novel deltaproteobacterium that is the causative agent of EBA, and identify highly transcribed bacterial genes within the aoEBA genome that could be possible candidates for a recombinant vaccine. The process of sequencing the aoEBA bacterial genome starts with exposing immunodeficient mice to aoEBA to obtain tissue containing high levels of the bacteria. When the mice become highly infected, spleen tissues are collected and DNA is extracted from the aoEBA-infected tissue. The resulting DNA consists of a combination of mouse and bacterial DNA. The DNA can then be sequenced using state-of-theart genomic technology, resulting in millions of 100 base pair (bp) sequences. Specialized software programs are used to sort out millions of sequences from the aoEBA genome. First, the sequences are aligned against the mouse genome and those from mice are removed to yield a higher concentration of sequences from the aoEBA genome. ...Continued on Page 16