RUDN University veterinarians together with colleagues from Iran, Thailand and Turkey have proved the efficacy of a herbal drug for anesthesia in Nile tilapia. It works no worse than analogues but causes less stress in fish. The results are published in Aquaculture.

To monitor the health of fish in aquaculture, they are periodically weighed, vaccinated, and tagged. This can cause pain and stress to fish. To facilitate these processes, anesthetics are used. However, synthetic drugs can cause side effects, besides they are expensive for aquaculture owners and unsafe for the environment. An alternative to synthetic anesthetics are herbal drugs. RUDN University veterinarians together with colleagues from Iran, Thailand and Turkey have proved that thymol, which is obtained from essential oils, can be an effective drug for fish anesthesia.

Anesthesia is an important procedure in aquaculture that affects aquaculture activities and fish welfare. Synthetic drugs such as MS-222, benzocaine, and phenoxyethanol have been used in aquaculture research and fish farming to attenuate physical damage and stress. However, because of the high cost, undesirable side effects on fish, and environmental concerns of synthetic anesthetics, several plant-based substances have attracted the attention of scientists in the recent years”, said Evgeny Kulikov, an associate professor of the RUDN University’s Department of Veterinary Medicine.

Veterinarians suggested using thymol, which is contained in the essential oil of thyme, oregano and other plants. Its effect was tested in an experiment: 150 tilapia were placed in water with different concentrations of thymol - from 10 to 100 mg per liter. Scientists measured the time after which thymol started to work. The behavior of the fish was constantly monitored. After the experiment, the RUDN University scientists took blood samples from them for analysis. The results of the thymol action were compared with another anesthetic — eugenol.

The third of the four stages of anesthesia (fish laid on lateral side, slightly depressed but regular opercular movement) was achieved after 56-491 seconds, depending on the concentration. Only the smallest concentration of thymol, 10 mg per liter, did not have an anesthetic effect on the fish. After 336-498 seconds, the action of thymol ended, and the normal behavior of the fish was restored. The level of albumin, cholesterol and hemoglobin in the blood did not change, but the level of cortisol, lactate, glucose and some other substances significantly increased. These data suggest that, in general, the action of thymol is similar to eugenol, but it caused less stress. RUDN University veterinarians called the optimal dose of thymol 80 mg per liter. At this concentration, deep anesthesia occurs in a minute, and it takes 300 seconds to recover. Increasing the dose does not significantly accelerate the action but increases the recovery time by 36 seconds.

“Considering the physiological responses, slight but significant elevations in plasma proteins, lactate, lactate dehydrogenase, and malondialdehyde levels attest that thymol may induce less stress, hypoxia, and oxidative stress in the fish than eugenol, but further studies are needed to provide robust and clinically applicable data to support this hypothesis”, said Sergey Seleznev, professor of the RUDN University’s Department of Veterinary Medicine.


Journal Link: Aquaculture, 547 (2022)