A team of researchers from the Federal University of Santa Maria in Brazil examined the use of the essential oil derived from Aloysia triphylla (EOAT) in the diet of silver catfish and fish survival from a disease challenge.
The group published its work in the journal of Fish & Shellfish Immunology.
“This study assessed the survival and hematological/biochemical responses of silver catfish (Rhamdia quelen) fed a diet containing EOAT following Aeromonas hydrophila challenge,” said the researchers.
They found that fish getting the higher level of EOAT in their diet had improve survival when compared to fish getting other experimental diets. In a blood-level analysis they compared in many areas to results from a basal group, or the baseline established prior to the feeding trial.
“However, when fish were fed a diet containing 2 mL EO/kg, the values for all parameters (except monocytes count) were similar to those in the basal group, showing a possible potential protective effect of EOAT in A. hydrophila infected fish,” said the researchers. “Fish survival in the group fed 2 mL EO/kg diet was higher than 90% and significantly different from survival in the group infected and fed a control diet (50% survival).”
Why essential oils?
Increasing awareness of antibiotic-resistant bacteria and use of antibiotics has made a priority of finding non-antibiotic or more environmentally-friendly feed additives that can also be used to improve animal health and performance, said the researchers.
In aquaculture, this has generated interest in the use of plant-based products like essential oils, they said.
“Due to the component diversity of EOs, these plant products present diverse and relevant biological activities (e.g. antimicrobial, anti-parasitic, antioxidant, immunomodulatory and anesthetic/sedative),” they said. “EOs affect the host in a number of ways, mainly modulating blood/immune and other physiological responses, showing potential to be applied as new generation compounds for animal health and nutrition.”
In addition to affecting the fish, essential oils also may act upon different negative bacteria cells, said the researchers. “Most of the described antibacterial effects of the EOs and their constituents involve interactions with biological membranes, such as changes in the morphology and lipid profile of bacterial cell membrane, increasing membrane permeability and leading to disruption with cytoplasmic leakage,” they added.
Different EOs also may reduce virulence or limit pathogen resistance, they said. The group examined the use of EOAT because of associated attributes when used with the production of other fish species.
Aloysia triphylla is a shrub that is used for seasoning and sometimes for medicinal purposes, said the researchers. It is considered to offer multiple properties including being antibacterial, anti-stress, antioxidant, anesthetic and improving growth.
Methods and materials
In the study, catfish were given one of three trial diets for 21 days, said the researchers. The diets included a control with no EOAT, that diet with 0.25 or 2.0ml EOAT per kg of diet – the essential oil was mixed in with canola oil.
At the end of the feeding trial, six fish per group were inoculated with A. hydrophila and mortality rates were noted for the next 10 days, they said. Other conditions in terms of tank and feeding were maintained.
After 10 days fish were collected and blood samples were taken for analysis, they said. Hematocrit, hemoglobin, erythroblast percentage, white blood cell count, leukocyte differential and plasma biochemical parameters were established.
Blood samples from select, healthy fish also were collected prior to the start of the feeding trial to establish a basal group, they said. That data was used for an additional comparison.
After the initial feeding trial, fish getting either of supplemented diets had a lower leukocyte count and improved total protein values, said the researchers. After the disease challenge, fish getting the higher level of EOAT saw improved performance when compared to the other groups.
“Lymphocyte and neutrophil counts were significantly lower in fish fed a diet containing EOAT compared to fish of the basal group,” they said. “No significant difference was found between basal and control groups for all parameters before bacterial challenge.”
After the disease challenge both fish from the control group and those getting the diet with the lowest amount of EOAT had lower hematocrit, hemoglobin, erythrocyte, lymphocyte counts and total protein values than the basal group, they said. The challenged fish also had higher leukocyte, neutrophil, monocyte, erythroblast counts and aspartate aminotransferase (AST) values.
“Dietary supplementation with 2.0 mL EOAT/kg diet seems to help silver catfish maintaining the evaluated hematological/biochemical responses similar to basal values, promoting fish survival,” said the researchers. “Considering these results and the diversity of uses/activities reported in fish (antibacterial, antioxidant, anti-stress and growth promoter), EOAT seems to be a plant-based product with great potential to be studied and used in aquaculture.”
However, they added, additional research is needed to more completely understand how EOAT could be used to improve fish response to A. hydrophila.
Source: Fish & Shellfish Immunology
Title: Aloysia triphylla essential oil as additive in silver catfish diet: Blood response and resistance against Aeromonas hydrophila infection
Authors: Alessandro dos Santos, Fernando Sutili, Berta Heinzmann, Mauro Cunha, Isabel Brusque, Bernardo Baldisserotto, Carla Zeppenfeld,