Seroprevalence and risk factors for Anaplasma marginale seropositivity in cattle in California between 2010 and April 2023

• Study compares Anaplasma infection between beef and dairy production systems in California. • A. m arginale infection was detected in cattle in over 70 % of counties in California. • A. marginale infection was less prevalent under extreme weather conditions. • Beef cattle had higher odds of Anaplamsa seropositivity compared to dairy cattle. The objectives of this study were to estimate prevalence of A. marginale infection in cattle in California and to identify potential risk factors for infection including weather effects. We considered individual-level prevalence the percentage of positive individual test results and submission-level prevalence the percentage of submissions from the same owner at the same time with at least one animal testing positive for Anaplasma . A total of 3684 Anaplasma test results from 773 submissions to the California Animal Health and Food Safety Laboratory between January 2010 and April 2023 were used in this study. Time series analysis and generalized estimating equations (GEEs) were used to analyze the data. Abundance of Dermacentor ticks in California was mapped. The overall true prevalence of Anaplasma infection at the individual level was 23.7% (95% CI: 22.3 – 25.2%) with a submission-level prevalence of 38.3% (95% CI: 34.9 – 41.8%) in cattle in California. A seasonal change in seroprevalence was observed in time series analysis, with lower seroprevalence in wet seasons than in dry seasons. Higher odds of Anaplasma positivity were found in cattle two years of age or older, beef cattle, with lower average ambient temperatures in the dry season, and higher average ambient temperatures in the wet season in GEEs.