White spot syndrome virus (WSSV) is the causative agent of a serious disease of shrimp, causing severe economic impact in the shrimp industry worldwide, with cumulative mortalities up to 100% within 3–10 days (Chou, Huang, Wang, Chiang & Lo 1995; Wang, Lo, Chang & Kou 1998). Immunostimulants include bacteria and bacterial products, complex carbohydrates, nutritional factors, animal extracts, cytokines, lectins, synthetic drugs and plant extracts (Sakai 1999). The immunostimulatory effect of plant extracts has been widely studied in fish and crustaceans (Balasubramanian, Sarathi, Kumar & Sahul Hameed 2007) and Balasubramanian, Sarathi, Venkatesan, Thomas & Sahul Hameed (2008) showed that an aqueous extract of Cyanodon dactylon reduced the mortality of WSSV-infected shrimp to 0%, for 30 days post-infection. Immunological parameters such as prophenol oxidase (proPO), super oxide anion (inline image), nitric oxide (NO), total haemocyte count (THC) and clotting time of the haemolymph were all significantly different (P < 0.05) in the WSSV-infected shrimp treated with plant extract when compared to control groups (Balasubramanian et al. 2008).

It has been reported that Macrophage-Activating Chinese Herb (MACH), which is a preparation made from four plants, namely pumpkin, Cucurbita moschata, seeds, plantain, Plantago asiatica, seeds, honeysuckle, Lonicera japonica, flowers and safflower, Carthamus tinctorius, flowers (Matsuura Yakugyo Co., Ltd), is effective in a wide range of animals (Yoshida, Tanemura, Wakabayashi, Otsuka, Yoshida, Onda, Terao, Shirao, Takagi, Tsumagari & Takeishi 2000; Yoshida, Nakanishi & Yamada 2006a,b), humans (Kaji et al. 2004; Ushiroyama, Yoshida, Tadaki, Ikeda & Ueki 2004a,b) and fish (Chansue, Ponpornpisit, Endo, Sakai & Yoshida 2000; Ponpornpisit, Endo, Murata, Toriumi, Ichikawa, Terao, Yamagiwa, Shimizu & Takeishi 2001). MACH reduced the amount of chronic hepatitis C virus in humans (Kaji et al. 2004) indicating that it can be used for the treatment of viral diseases. In addition, it has been reported that MACH improves the phagocytic activity of macrophages in blood of dogs and cows (Yoshida et al. 2006a,b). This is the first report to investigate the prophylactic effect of MACH against WSSV. Changes in haematological and immunological parameters of kuruma shrimp haemolymph after administration of MACH were also examined.