Essential oils are complex combinations composed of various organic chemical substances, whose constituent composition can be influenced by several factors (Abdoul-Latif et al. 2022). The compounds methyl chavicol and linalool were identified from chemical analysis as the main constituents of the essential oil of O. basilicum. In agreement with these findings, Oliveira et al. (2013), Ventura et al. (2020), and Yigit et al. (2022) observed the predominant presence of both compounds in the essential oil of O. basilicum cultivated in Brazil. Additionally, Costa et al. (2015) reported the predominant presence of these compounds, and other components, in 38 genotypes of basil cultivated in the United States.
Fish undergo hematological variations due to the influence of factors such as temperature, pH, salinity, and dissolved oxygen. Additionally, using anesthetics can influence hematological parameters, resulting in stressful situations for the animals (Ranzani-Paiva et al. 2013). In this study, anesthetic induction in Nile tilapia showed distinct responses for MCV and MHC parameters among groups subjected to different temperatures. Anesthesia with the essential oil of O. basilicum promoted an increase in MCV and MCH parameters at a temperature of 27ºC, with the same parameters showing lower averages among groups tested at a temperature of 23ºC. Tambaquis kept at a temperature of 29,6ºC and anesthetized with clove oil and benzocaine (Pádua et al. 2013) showed the same trend for MCV observed in the present study. Similar results were observed in goldfish (Carassius auratus) anesthetized with propofol and clove oil at a temperature of 22ºC (Gholipourkanani and Ahadizadeh, 2013). Gholipourkanani and Ahadizadeh (2013) observed an increase in MCV value with the use of the synthetic anesthetic propofol and a reduction with the natural anesthetic clove oil, while for MCH, an opposite trend was observed. In rainbow trout (Oncorhynchus mykiss) anesthetized with citronellal at a temperature of 14.2ºC, an increase in the concentration of the essential oil also led to a reduction in these parameters (Hoseini et al. 2021).
Erythrocytes are the most abundant cells in fish, and variations in their count can influence the metabolic rate and hematological parameters of fish (Witeska 2013). These cells play an essential role in the transport of oxygen and carbon dioxide (Ranzani-Paiva et al. 2013). The decrease in the number of RBCs in the present study was observed at a temperature of 27ºC, while at 23ºC there was no difference among the treatments tested. This may explain the mortalities observed during the anesthetic induction procedure performed at 27ºC. The combination of O. basilicum essential oil with an increase in temperature may have caused a considerable level of stress in the fish, leading to an imbalance in the immune system, and resulting in mortality and suppression of RBC count. Similar to the present study, Nile tilapia (O. niloticus) anesthetized with the essential oil of Lippia sidoides at a temperature of 27.67ºC also showed a reduction in the number of RBC (Hashimoto et al. 2016). Anesthetics tend to induce an increase in the number of RBC, as observed in Nile tilapia (O. niloticus) anesthetized with thymol (2-isopropyl-5-methylphenol) and eugenol (Yousefi et al. 2022), and with propofol and eugenol (Zahran et al. 2021), with differs from the present study.
Neutrophils are phagocytic cells that play a role in the host’s defense system (Witeska et al. 2022). An increase in the number of these cells in Nile tilapia when exposed to anesthetic administration could be related to an imbalance in homeostasis as the animal attempts to restore normal conditions. In the present study, neutrophils were influenced by the treatment with water combined with 95% ethanol when compared to the control group with water and essential oil, indicating that the alcohol contained in the essential oil may influence the activity of these phagocytic cells. Ventura et al. (2021b) demonstrated that juvenile pacu (Piaractus mesopotamicus) anesthetized with O. basilicum essential oil at a temperature of 26.4ºC also did not show an increase in neutrophil values when subjected to anesthesia with O. basilicum essential oil. This trend was also observed with the essential oils of melaleuca and clove at a temperature of 28ºC (Santos et al. 2020), as well as in fish subjected to anesthesia with O. gratissimum and Z. officinale at a temperature of 26.3ºC (Silva et al. 2020).
The main blood ions are directly influenced by stressors during the exchange process between the internal and external environments of fish (Silva et al. 2015b). In the present study, the use of O. basilicum essential oil resulted in lower calcium ion levels at a temperature of 27ºC. However, fish exposed to induction with essential oil did not differ from either of the control groups. Many physiological changes are related to exposure to anesthetic, so this result may be associated with the temperature increase during the procedure, as this trend was not observed at a temperature of 14.6ºC which did not show differences between the treatments (Taheri-Mirghaed et al. 2018). In Nile tilapia, it was observed that O. basilicum essential oil prevented calcium ion efflux during the experiment. Osmoregulatory disturbances tend to cause stress in fish, leading to a reduction in ion levels under stressful conditions. In response to these conditions, the animals tend to increase blood flow in the gills and cellular permeability, resulting in a reduction of plasma ions (Becker et al. 2012).