Some medicinal plants as immunostimulant for fish

Abstract

Immunostimulant effects of the dietary intake of various medicinal plant extracts on fish, rainbow trout (Oncorhynchus mykiss), were investigated. For this purpose fish were fed with diets containing aqueous extracts of mistletoe (Viscum album), nettle (Urtica dioica), and ginger (Zingiber officinale). Food containing lyophilized extracts of these plants as 0.1 and 1% was used at a rate of 2% of body weight per day for three weeks. At the end of the experimental period, various parameters of non-specific defence mechanisms, including extracellular and intracellular respiratory burst activities, phagocytosis in blood leukocytes and total plasma protein level were examined. Specific growth rates (SGRs) and condition factors (CFs) of the fish were also measured.

Plant materials tested for immunostimulatory food additives caused an enhanced extracellular respiratory burst activity (P<0.001) compared to the control group. Especially the rainbow trout fed with a diet containing 1% aqueous extract of powdered ginger roots for three weeks exhibited a significant non-specific immune response. Phagocytosis and extracellular burst activity of blood leukocytes were significantly higher in this group than those in the control group. All plant extracts added to fish diet increased the total protein level in plasma except 0.1% ginger. The highest level of plasma proteins was observed in the group fed with 1% ginger extract containing feed.

Introduction

The outbreak of diseases is a limiting factor in fish culture. At many fish farms and hatcheries several antibiotics, vaccines, and chemotherapeutic agents as well as some immunostimulants have been used to prevent viral, bacterial, parasitic, and fungal diseases.

Like man, fish rely on both specific and non-specific mechanisms to protect themselves against invading pathogens. In fish, the primary lines of non-specific defences are the skin and mucus. When pathogens enter the body, cellular and humoral non-specific defence are mobilized. Phagocytosis is one of the main mediators of non-specific immunity to pathogens including bacteria, viruses, and parasites in fish. The most important cells involved in this defence are the phagocytes. These are supported by several soluble factors, such as complement and lysozyme (Dalmo et al., 1997, Verlhac and Gabaudan, 1999, Yano, 1996). It is also well known that the innate immune system in fish can be triggered by many immunostimulants such as levamisole (Siwicki, 1987, Siwicki, 1989, Siwicki et al., 1990, Jeney and Anderson, 1993), glucan (Engstad et al., 1992, Jorgensen and Robertsen, 1995, Chen and Ainsworth, 1992, Ainsworth, 1994, Jeney et al., 1997), glucan plus vitamin C (Verlhac et al., 1996), yeast RNA (Sakai et al., 2001), lipopolisaccharide (Dalmo and Seljelid, 1995, Solem et al., 1995), growth hormone (Sakai et al., 1995, Sakai et al., 1996), zeranol (Keleş et al., 2002) and kitosan (Siwicki et al., 1994). However, some of the immunostimulants could not be used because of various disadvantages, such as high cost, limited effectiveness upon parenterally administration, etc.

On the other hand, a large number of plants have been used in traditional medicine for the treatment and control of several diseases (Duke, 1987). Three of such plants are mistletoe (Viscum album), nettle (Urtica dioica), and ginger (Zingiber officinale). Some of the medicinal plants have been used as the phytogenic basis immunostimulatory preparations. Such preparations have been used, as such as adjuvant therapy, in cancer and AIDS (Mentle et al., 2000, Zarkovic et al., 2001, Verpoorte et al., 1999). Especially mistletoe and nettle have been stated to possess immunomodulatory activity. On the other hand, powdered ginger root is used as spice as well as an antiemetic (Phillips et al., 1993) or anticancer agent (Yusof et al., 2002).

The aim of the present work was to examine if mistletoe, nettle, or ginger extracts would influence various parameters of non-specific defense in rainbow trout including induced respiratory burst activity (extracellular and intracellular), phagocytosis in blood leukocytes, and total plasma protein level.