Detection of Anaplasma marginale (Rickettsiales: Anaplasmataceae) in host-seeking adult Dermacentor variabilis (Acari: Ixodidae) on cattle pastures, Missouri, United States.

Abstract

Bovine anaplasmosis is an economically important and globally distributed disease of cattle caused by a rickettsia, Anaplasma marginale Theiler, which infects bovine red blood cells. In the United States, A. marginale is transmitted by adult male Dermacentor spp. ticks. Our objectives were to estimate the prevalence of A. marginale among host-seeking D. variabilis Say males and describe tick activity on beef cow-calf grazing operations in Missouri. Ticks were collected by dragging a total of 348 750-meter transects on 5 field sites from May 2022 to August 2024. In total, 29,132 ticks were collected: 27,502 Amblyomma americanum Linnaeus, 1,504 D. variabilis, 101 Haemaphysalis longicornis Neumann, and 25 individuals of uncommonly encountered species. A total of 692 adult male D. variabilis were divided into 154 pools of ≤5 ticks/pool for analysis. Anaplasma marginale was detected by quantitative polymerase chain reaction (qPCR) in 1 pool of 5 adult males (0.6%). Illumina sequencing detected an Anaplasma bovis (Donatien and Lestoquard 1936)-like sequence in 38 pools (24.7%). The endosymbionts Francisella spp. and Rickettsia spp. were detected in 100% and 32.5% of pools, respectively. To the best of our knowledge, this is the first study to detect A. marginale in host-seeking D. variabilis collected in the field. Our findings also represent the first reports of H. longicornis, an invasive species, in 4 Missouri counties.