Anti-Ehrlichia properties of the dichloromethane fraction of Ageratum conyzoides associated with doxycycline: In vitro study

: The increasing number of cases of canine ehrlichiosis caused by Ehrlichia canis in hospitals and veterinary clinics has demonstrated the need for a new drug protocol for this disease. Doxycycline is used to treat ehrlichiosis, but the resistance of the microorganism to this treatment protocol, as well as the various side effects to the animals, has become a concern. Several studies have shown a positive interaction with extracts of plants and drugs, which allow for the reduction of the concentration necessary to produce the desired effect, minimizing adverse effects. This study determined the efficiency of the combination of the dichloromethane (DCM) fraction of Ageratum conyzoides L. with anti-Ehrlichia activity and doxycycline by using the checkerboard assay. Plant material was collected in São Luís, northeastern Brazil, followed by extraction in MeOH: H2O (8:2) and partitioning of the DCM fraction. After determining the Minimum Inhibitory Concentration (MIC) of the fraction under study against DH82 cells infected with Ehrlichia canis, it was combined with doxycycline to derive the Fractional Inhibitory Concentration Index (CIF Index). A reduction of 5.83 times the doxycycline minimum inhibitory concentration was observed, showing that this fraction of A. conyzoides composed predominantly by the class of lignans, identified by mass spectrometry notably intensified the activity of doxycycline against E. canis, resulting in a synergistic effect.