A tick cell line as a powerful tool to screen the antimicrobial susceptibility of the tick-borne pathogen Anaplasma marginale
Abstract Background Anaplasma marginale is the causative agent of the severe bovine anaplasmosis. The tick Rhipicephalus microplus is one of the main vectors of A. marginale in tropical and subtropical regions of world. After the tick bite, the bacterium invades and proliferates within the bovine erythrocytes, causing anemia and impairing milk production and weight gain. In addition, infection can cause abortion and high mortality in areas of enzootic instability. The immunization with live and inactivated vaccines are employed to the control acute bovine anaplasmosis. However, they do not prevent persistent infection. Therefore, infected animals, even if immunized, are reservoirs of the bacterium and contribute to the dissemination of the disease. Antimicrobials are also largely employed for the prophylaxis of bovine anaplasmosis. However, they are often used in subtlethal doses, what can select pre-existing resistant bacteria and induce genetic or phenotypic variations. Therefore, the standardization of an in vitro assay to evaluate the susceptibility of A. marginale strains to different antimicrobials is important to allow the prescription of the more effective treatment, preventing both the selection and spread of resistant strains. Results Initially the antimicrobial susceptibility of two field isolates of A. marginale (Jaboticabal and Palmeira) infecting bovines was evaluated. The least susceptible strain (Jaboticabal) was used for the standardization of an antimicrobial assay using a culture of Ixodes scapularis-derived tick cell line, ISE6. Results showed that enrofloxacin (ENRO) at 0.25, 1 or 4 μg/mL and oxytetracycline (OTC) at 4 or 16 μg/mL are the most efficient treatments, followed by OTC at 1 μg/mL and imidocarb dipropionate (IMD) at 1 or 4 μg/mL. Conclusion In the current study, we present a new in vitro assay using a tick cell line to evaluate the susceptibility of A. marginale to antimicrobials. The maintenance of such culture is much easier than the maintenance of bovine erythrocyte culture, which depends on continuous cell replacement. This assay may be used to guide cattle farmers to the correct use of antimicrobials. The choice of the most suitable antimicrobial is essential to eliminate persistent infections, preventing the spread of resistant strains and helping in the control of bovine anaplasmosis.
