Histometric and morphological damage caused by Serratia marcescens to the tick Rhipicephalus microplus (Acari: Ixodidae).

Rhipicephalus microplus tick is the ectoparasite causing the greatest economic losses in the livestock industry. Multi-resistance in ticks is increasing, generating the inefficiency of traditional ixodicides, for which biological control has been proposed as an alternative. In this work, we analyze the histomorphological damage caused by the bacterial strain EC-35 on Rhipicephalus microplus. The ixodicidal effect of EC-35 total protein was evaluated on larval or adult ticks comparing with the commercial ixodicide coumaphos 0.02% as a control. Female ticks were processed using the paraffin-embedding technique and stained with hematoxylin-eosin. Also, the pathogenicity of EC-35 was evaluated by capillary feeding and coelom inoculation tests. The identification of the bacterium was performed using the molecular markers 16S RNA and rpoB, by PCR and sequencing technique, and the evolutionary distance was analyzed by Bayesian phylogenetic inference. No differences were observed in the perimeter and area of larvae treated with EC-35 or Coumaphos. The thickness of the integument decreased a 65% with the EC-35 treatment (6.01 ± 0.6 µm) and of 30% in coumaphos (12.04 ± 1.2 µm) in larvae compared with the control group (18.41 ± 2 µm), while no difference was found in adult ticks. The capillary feeding test and coelom inoculation with EC-35 showed an inhibition of reproductive potential of 99.8 ± 7 and an oviposition Inhibition 97 ± 3.02%. The EC-35 strain was genetically related to Serratia marcescens, concluding that these bacteria caused high mortality, oviposition Inhibition, and integument thinning and drastic loss of histoarchitecture in R. microplus tick larvae.