UC Davis

Bovine anaplasmosis, a tick-borne disease caused by Anaplasma marginale, imposes considerable economic burdens on the cattle industry throughout the world. Dermacentor ticks are the most important biological vectors of A. marginale in North America, and the pathogen can also be transmitted mechanically by arthropods, such as horse flies (Tabanidae), or blood contaminated instruments. This dissertation addressed producers’ perceptions of herd infection status, current measures taken for anaplasmosis control and prevention, the seroprevalence and distribution of the disease, the effects of weather, and associated risk factors of A. marginale in California cattle.In chapter 1, a questionnaire was designed and distributed to beef producers in California to better understand the current measures taken for anaplasmosis control and prevention, producers’ perceptions of herd infection status, and reported clinical cases of anaplasmosis. The overall perceived prevalence for A. marginale infection and reported clinical cases of anaplasmosis at the herd level was 26.0% and 17.1%, respectively. Region, herd size, and vaccination were significantly associated with reported clinical cases of anaplasmosis in the previous five years. Chapter 2 utilized test results over a 13-year span from the California Animal Health and Food Safety lab to estimate prevalence and risk factors for Anaplasma infection in both beef and dairy cattle in California. The prevalence of Anaplasma infection at the individual level was 23.7% with a submission-level prevalence of 38.3% in cattle in California. A seasonal change in seroprevalence was observed with lower prevalence for submissions during the wet seasons than during dry seasons. Higher odds of seropositivity were found in older cattle, beef breeds, and those in coastal regions. Reduced A. marginale seropositivity during extreme weather and drought may be due to decreased tick survival and activity. Chapter 3 utilized blood samples collected from 69 cow-calf operations across California to get a more accurate estimate of A. marginale prevalence in beef cattle. The prevalence of Anaplasma infection in all tested cattle was 62.23%, with only 7% herds where all cattle tested negative. Higher odds of seropositivity were observed in cattle located in the Central Coast region, grazing primarily on dry pasture or range areas, with observed tick infestations, and where needles were changed after injecting eleven to twenty animals. We also found that the odds of A. marginale seropositivity decreased as the annual average temperature increased. Moreover, beef producers appear to underestimate the infection status in California.