The present study revealed that black disease mostly occurred during the cold and early rainy seasons. In the rainy season, water quality fluctuated considerably because of flooding, acidic water and eutrophication; thus, fairy shrimp became weak and were easily infected. In this study, a high prevalence of infection was observed in June 2009 because of heavy rainfall which affected the water quality and caused long-term flooding, particularly in the
earthen ponds containing S. sirindhornae. In addition, the severity of disease was observed to be significantly greater in winter (November 2009– January 2010) probably because the low temperatures (20–26 C) are unfavourable for fairy shrimp. Low temperature is also recognized as a stress condition in fish . Stress conditions inhibit the immune systems of aquatic animals resulting in increasing susceptibility to infections. Moreover, overcrowding and dietary deficiency may cause disease in crustaceans. S. sirindhornae was reared in earthen ponds containing organic matter, clay and other particles. The prevalence of infection was higher than that of B. thailandensis which was reared in more favourable conditions. The results strongly suggest that the cultivation of fairy shrimp in winter should be avoided to minimize the risk of
infection and that proper management is strictly required during this period.