Dietary emamectin benzoate induces dose-dependent variations in haemato-biochemical and erythrocyte-metric parameters of Oreochromis niloticus (L.)

Highlights

• Dietary EB caused a dose-dependent reduction in feed intake and survival.

• EB induced haemato-biochemical and erythrocyte-metric alterations in O. niloticus.

• Most serum biomarkers and erythrocyte-metrics stabilized on day 35 post-dosing.

• O. niloticus was able to mount adaptive responses to the effect of dietary EB.

Abstract

Intensive fish farming often leads to the emergence of infectious and parasitic diseases, which require aquadrugs to control them. The present study evaluated the effects of 1× (50 μg/kg biomass/day) and 3× (150 μg/kg biomass/day) dietary emamectin benzoate (EB)-dosing on healthy Oreochromis niloticus for 7 consecutive days in comparison with control. Dietary EB caused a significant dose-dependent reduction in feed intake and an increase in mortalities. The total erythrocyte counts (TECs), monocyte counts (MCs), haemoglobin (Hb), haematocrit (Ht), and mean cell haemoglobin concentration (MCHC) reduced significantly. Contrarily, the counts of total leukocytes (TLCs), thrombocytes (TCs), lymphocytes (LCs) and neutrophils (NCs) increased significantly within 7 days of EB-dosing (ED) in both groups. The TECs, Hb, TCs and MCs normalized on day 35 post-EB-dosing (PED), while Ht, MCH, LCs, and MCs recuperated on day 14 PED. Both groups experienced a significant increase in serum glucose, alkaline phosphatase, alanine transaminase, aspartate transaminase and creatinine levels as well as a decrease in calcium, chloride ions and acetylcholinesterase levels during the ED period. The alkaline phosphatase and creatinine levels of the 1× group recovered on day 14 and 28 PED, respectively but not in the 3× group. The dosed fish documented reduced erythrocyte cellular and nuclear morphometry like larger axis, minor axis, volume, and surface area, which recovered during the PED period, except for the nuclear volume. The results suggested the tolerability of O. niloticus juveniles and their ability to mount adaptive responses to the effect of EB to recuperate with the suspension of dosing.

Introduction

The aquaculture sector has seen tremendous growth and diversification over the years (FAO, 2020). The rise in fish farming is partly due to the awareness that, when compared to land-based livestock, fish and other aquatic animals are more cost-effective and healthier sources of animal protein. Nile tilapia Oreochromis niloticus is considered to be the most important farmed tilapia species accounting for about 80% of the total tilapia production (FAO, 2020). The aquaculture industry is in a phase of rapid expansion and intensive fish farming often leads to the emergence of infectious and parasitic diseases (St-Hilaire et al., 2021; Arechavala-Lopez et al., 2022). A wide variety of aquadrugs are used to control these diseases (Burridge et al., 2010; Rico et al., 2012; Patil et al., 2022). Among them, emamectin benzoate (EB), belonging to the avermectin family of chemicals, formulated as SLICE® is used to control sea lice Lepeoptherius salmonis and the related Caligus spp. (MSD Animal Health, 2012; Bowker et al., 2013), as well as the freshwater fish lice, Argulus spp. (Hanson et al., 2011) with a therapeutic dose of 50 μg/kg biomass/day for 7 days. The host response to a sea lice infection depends on several variables, including the host species and the kind and stage of the parasite. Besides, inflammation, edema and epidermal pathologies, sea lice may increase the risk of secondary bacterial and viral infections, and osmoregulatory problems (Igboeli, 2013).

The main mechanism of action of avermectins is to disrupt the transmission of impulses in the nervous system as they interact with a large number of ligand-gated chloride channels, increasing the permeability of the cell membrane and causing nervous system malfunction (El-Saber Batiha et al., 2020). Similarly, EB is believed to act by blocking nerve transmission in arthropods resulting in hyperpolarisation of nerve endings, paralysis and consequently death of the parasite (Sahoo et al., 2019). The EB as an in-feed therapeutant is safely and effectively administered and whole culture systems can be medicated in a coordinated manner (Lees et al., 2008; Julinta et al., 2020a, Julinta et al., 2020b; Anandaraja et al., 2020, Anandaraja et al., 2022; St-Hilaire et al., 2021). The EB can be bound to sediments for prolonged periods and potentially toxic to non-target and benthic organisms (Burridge et al., 2010) and crustaceans (Mill et al., 2022).

In India, agricultural producers use EB as a pesticide to manage crop pests (Anon, 2021), but its use is yet to pick up in aquaculture. The use and validation of standardized non-lethal and inexpensive methods to monitor fish health are considered a major issue to ensure the expansion of fish production (Hrubec et al., 2000). Blood analysis is crucial in many fields of ichthyological research, fish farming, the areas of toxicology and environmental monitoring as possible indicators of physiological or pathological changes in fishery management and disease investigation (Shahjahan et al., 2020; Hamed et al., 2021; Sharmin et al., 2021; Singha et al., 2022). Fish erythrocytes are ellipsoidal nucleated cells of different sizes (102–800 fL) and life spans (13–500 days). Erythrocytic morphometric alterations serve as a reliable determinant of fish health (Burgos-Aceves et al., 2019; Sula et al., 2020). Biomarkers are important biochemical indicators of chemical toxicity caused by certain aquadrugs and pesticides to fish (Atli and Canli, 2007; Singha et al., 2022). A proper scientific record on regulated drugs like EB in fish safety is required, as this will aid in resolving the overdose and underdosing of EB in various life phases during the treatment regime. The studies related to the safety aspects of parasitic drugs on commercially important fish in tropical aquaculture are scarce. The present study, thus, assessed the influences of dietary EB at 0, 1 and 3 times the recommended dose on the haematological, serum biochemical and erythrocyte-metric parameters of O. niloticus when fed for 7 consecutive days.