Brucellosis is a serious infectious disease of cattle, caused by Brucella abortus, which can cause devastating economic losses to cattle industry. California has been classified 
a fre
e of bovine brucellosis since 1997 and, according to a report from the USDA-APHIS 2017, all the United States are now free of brucellosis in cattle. However, brucellosis is still endemic in free ranging elk and bison populations in the Greater Yellowstone Area (GYA), which includes parts of Montana, Idaho, and Wyoming. The prevalence is estimated to be up to 60% in bison populations and 40% in elk populations, and it seems to be increasing. Therefore, the risk of transmitting brucellosis from these wildlife populations to domestic cattle still exists. Transmission among wildlife, mainly between elks, seems to be occurring particularly in winter when they gather together in the feed-grounds. However, transmission from wildlife to cattle is likely to occur mainly in late winter or early spring when bison, elk and cattle graze and share territory in low elevation grasslands outside Yellowstone. The cattle industry is a highly valuable industry for California and has high socio-economic impact in the overall US economy with revenue of $3.4 billion generated in 2015. California is the 4th state with the highest number of cattle (5.15 million heads), which implies frequent movement of cattle and livestock products with other states, including those where brucellosis is present in wildlife. For example, during 2015 cattle moving from Montana, Idaho, and Wyoming represented 10% of the total imported cattle into California. Therefore, introducing brucellosis into California with cattle imported from the GYA remains a concern. California maintains a brucellosis vaccination program, despite its high cost, to prevent the severe consequences of introduction of the disease from other states through movement of cattle. The vaccination program has provided good disease control, and has helped maintain California’s brucellosis-free status for nearly 20 years. However, now that the disease is in the final stage of eradication from domestic populations, the cost-effectiveness of the vaccination program is questionable. The money that livestock producers spend on the vaccination program may reduce the profits in the cattle industry, compared to states that have ceased the program. Some studies assessed the risk of disease transmission from the GYA to outside of designated surveillance areas in Montana, Idaho, and Wyoming.2,6 However, no study has been done on the risk of introducing brucellosis into California and the cost-effectiveness of the vaccination program to better inform policies to continue or cease the vaccination program.
