The affected prawns show brown to black spots on the gills. In acute cases gills may completely become brown or black with atrophy and necrosis. 

CAUSE 

A number of causes have been assigned to this disease. Presence of excessive levels of toxic substances like nitrate, ammonia, acids, crude oils, potassium premanganate, copper, cadmium, ozone, etc. in the culture water may lead to black gill disease. Presence of 2-3 ppm nitrite leads to black gill condition resulting in low levels of mortalities. High organic matter content and highly reducing conditions in soil also cause black gills. Infection with infectious hydrothermal and haematopoietic necrosis (IHHN) virus, bacteria (Vibrio, Cytophaga, Flexibacter) and fungi (Fusarium and Halithoros) also leads to black gill syndrome. 

Based on the signs and symptoms and histopathology. Black gill condition due to microbial etiology may be confirmed by standard microbiological methods. 

PREVENTION 

Maintain good water quality and avoid overfeeding. 

CONTROL 

Treatment of this disease depends upon cause of the disease. When the disease is due to pollution of pond water with toxic substances, the water quality must be improved by sufficient water exchange and aeration. If the disease is due to microbial infection, antibiotic treatment may be given after knowing the in-vitro sensitivity of the pathogen.