Natural Panola Mountain Ehrlichia infections in cattle in a longitudinal study of Angus beef calves

Panola Mountain Ehrlichia (PME) is an emerging zoonotic pathogen, transmitted by Amblyomma americanum ticks in the southeastern United States. It is closely related to Ehrlichia ruminantium, the causative agent of heartwater disease. Heartwater disease is an often-fatal illness of ruminant livestock present in Africa and the Caribbean. The taxonomic relationship between PME and E. ruminantium has raised concerns about the pathogenicity of PME in livestock. To determine whether cattle could be naturally infected with PME in an endemic area, we conducted a one-year longitudinal study of Angus-breed beef calves in Fayetteville, Arkansas. One hundred seventy-seven calves born between September and October 2022 were sampled for blood and ticks in February, May, and September 2023. Blood and ticks from each animal were tested for bacteria in the family, Anaplasmataceae using quantitative and conventional PCR, and positive samples were sequenced for species identification. Panola Mountain Ehrlichia was detected in 2.34 % of male A. americanum collected in February, and 1.27 % of female, 0.95 % of male, and 0.43 % of nymphal A. americanum collected in May. No PME-positive ticks were collected in September. Active PME infections were detected in two calves: one which tested positive in May 2023 and one which tested positive in September 2023. Neither animal exhibited any signs of disease, and the animal PME-positive in May tested negative in September. Cattle are susceptible to PME, but the pathogen does not appear to cause obvious disease. However, all animals in this study were under one year old, and older animals may be more susceptible. Cattle are at risk of tick-borne illness in the winter as well as spring, and off-season acaricide applications may improve disease management.