Distribution, Seroprevalence, and Factors Associated with Anaplasma marginale Seropositivity in Adult Cattle from Cow-Calf Herds in California

Cases of bovine anaplasmosis, a tick-borne disease caused by Anaplasma marginale , have been reported in nearly all US states. The aims of this study were to investigate the seroprevalence of A. marginale across different regions in California and to analyze the association between A. marginale infection and animal demographics, management practices, and environmental factors. We also examined whether bio-climatic factors were associated with A. marginale infection in cattle between herds. Cohen’s Kappa test was applied to evaluate the agreement between producers' perceptions of infection status and the serology test results. A total of 1,849 blood samples from 69 beef herds were collected between July 2021 and June 2023. The true prevalence of Anaplasma infection in all tested cattle was 62.2% (95% confidence interval (CI): 59.80 – 64.60%); only five herds tested negative. In the GEE model, region, habitat type, wildfire occurrence in the past five years, observed tick infestation, and frequency of needle changes were significantly associated with A. marginale seropositivity. Prevalence peaked at around 25 °C annual average temperature and declined at both lower and higher temperatures. We found only slight agreement between the ranchers’ perception of the herd Anaplasma infection status and the serology test results. The study suggests that Anaplasma infection in cow-calf operations in California is related to management practices, environmental conditions, and bio-climatic features. Moreover, beef producers in California are often unaware of their herd’s infection status.