EFFECT OF EMAMECTIN BENZOATE ON EXPRESSION AND TISSUE DISTRIBUTION OF MULTIDRUG RESISTANCE PROTEINS IN THE DIGESTIVE TRACT OF Caligus rogercresseyi

Caligus rogercresseyi ("Caligus"), is a copepod ectoparasite that strongly affects massive salmon farming of Chile, and like the northern hemisphere sea lice, L. salmonis, currently causes millionaire losses in this industry. Pharmacological treatments against this infestation are based on the use of insecticides, such as emamectin benzoate (EMB), which is delivery orally in food. Due to the increasing drug resistance developed by the parasite since 2007, the effectiveness of EMB has decreased significantly. The aim of this study was to characterize the effect of treatment with EMB on protein expression levels and tissue distribution of two proteins implicated in drug resistance processes, P-glycoprotein (Pgp) and Multidrug resistance-associated protein 1 (MRP1), in the digestive tract of Caligus.

Specimens of Caligus were collected from rainbow trout (O. mykiss) grown on farms in southern Chile, and transported to the laboratory. After 24 h, culture plates with 10 male or female lice, were exposed to seawater (control) or 300 ng/mL EMB in seawater by 24 h. The survivor specimens were fixed in p-formaldehyde for the immunohistochemistry (IHC) and immuno-fluorescence (IF) analysis, or submerged in RIPA buffer and frozen at -20 ºC for Western blot analysis.

Figure 1A shows the tissue distribution of Pgp and MRP1, both in female (top) and male (below), but with higher protein expression levels in females (Fig. 1A). Treatment with EMB caused an increase in the expression levels of both proteins, in a relatively proportional way, maintaining the highest response observed in females (Fig. 1B).

These data support the notion that drug resistance proteins, such as Pgp and MRP1, could be involved in the drug resistance of Caligus to antiparasitic drugs, by affecting the intestinal absorption of these drugs. This information generates fresh perspectives in the search for new strategies to combat this deleterious parasite.

Funding: Fondecyt 1150934 & FONDAP 15110027, Chile.