Cellular immune response of Amblyomma sculptum and Amblyomma americanum to entomopathogenic fungi: Implications for biological tick control

Abstract

Amblyomma spp. (Acari, Ixodidae) serve as primary vectors for numerous pathogens affecting humans and animals. Unlike other arthropods, ixodid ticks exhibit notably less susceptibility to entomopathogenic fungi, which are widely used as biological control agents. This study aimed to determine whether the dynamics of the cellular immune response to fungal infection differ between Amblyomma sculptum and Amblyomma americanum. Engorged female ticks were treated with conidial suspensions of Metarhizium robertsii ARSEF 2575 or Beauveria bassiana ARSEF 9588. For the survival assay, ticks were immersed in suspensions of 5 × 10⁸ conidia mL⁻¹. Inoculation with 5 × 10⁷ conidia mL⁻¹ was used in assays to evaluate immune responses. Haemolymph was collected 24 h post-treatment, and smears were prepared for microscopy-based characterization of haemocyte responses. Confocal and fluorescence microscopy were used to classify, quantify and visualize the haemocyte types, including granulocytes, plasmatocytes, prohaemocytes, oenocytoids and spherulocytes. Our findings reveal that plasmatocytes dominate the haemocyte population in A. sculptum, whereas granulocytes are predominant in A. americanum, both showing reduced concentrations following B. bassiana exposure. Prohaemocytes and oenocytoids were present in all groups but at lower frequencies. Additionally, B. bassiana was more effective against A. sculptum, whereas M. robertsii exhibited greater efficacy against A. americanum, with phenoloxidase activity of ~29%. This study provides crucial insights into the cellular immune defences of Amblyomma ticks and highlights potential fungal strains that may overcome tick antifungal susceptibility. These findings establish a foundation for future research on haemocyte-mediated immunity in ixodid ticks, a topic with limited existing literature and contribute to the development of targeted biological control strategies.