Anthelminthic efficacy of Cymbopogon citratus essential oil (Poaceae) against monogenean parasites of Colossoma macropomum (Serrasalmidae), and blood and histopathological effects

This study investigated the in vitro and in vivo anthelminthic efficacy, histopathological and hematological effects of Cymbopogon citratus essential oil (EO) in Colossoma macropomum. The major compounds of the essential oil were geranial (45.7%) and neral (33.9%). Essential oil was assayed at concentrations of 100, 200, 300, 400 and 500 mg L-1 for in vitro efficacy against monogeneans Anacanthorus spathulatus, Mymarothecium boegeri and Notozothecium janauachensis of C. macropomum. Two controls were considered for in vitro assays, one with the use of cultivation tank water and the other with cultivation tank water +70% alcohol. The concentration of 500 mg L-1 of the EO in vitro assays showed 100% efficacy against the parasites within 5 min of exposure, causing structural damages to their tegument. The 400 and 300 mg L-1 concentrations were 100% effective with 10 min and 30 min of exposure, respectively. The 200 mg L-1 concentration in vitro assays was also 100% effective within 30 min and the 100 mg L-1 treatment was 100% effective within 100 min of exposure. Total parasite mortality in both control groups occurred within 7 h. Efficacy was 47.1% after therapeutic baths of 20 min for three consecutive days with 60 mg L-1 of this EO. In addition, 60 mg L-1 of EO was the highest concentration tolerated by the fish and it had an anesthetic effect, and it caused increase in plasma glucose levels and decrease in leukocytes and lymphocytes number, as well as hyperplasia, lamellar fusion, detachment and aneurysm in C. macropomum gills. A low efficacy against the monogeneans was shown perhaps due to the low concentration of this EO tolerated by C. macropomum and the strategy used for the therapeutic baths, which may be corrected with a lower concentration of EO and more consecutive days for therapeutic baths.