Etiology and pathology of shell disease

The exoskeleton of crustaceans consists of 4 layers. The epicuticle is made up of lipids or proteins and covers the 3 inner chitinous layers--the exocuticle, which is calcified and contains proteins and pigments, the calcified endocuticle, and the innermost layer, the noncalcified endocuticle. Shell erosions result when these layers come under enzymatic attack by microorganisms such as bacteria of the genera Vibrio, Aeromonas , and Flavobacter or fungi of the genus Fusarium . Host responses to invasion by microorganisms include deposition of melanin (antimicrobial defense) and increased chitin formation (shell repair). Molting can rid the host of the diseased exoskeleton except in advanced cases. Physiological disturbances due to overcrowding, poor water quality, unhygienic holding conditions, or possibly pollutants can cause an imbalance in normal chitin repair mechanisms. Decreased chitin formation and mineral deposition coupled with impaired immune function can cause the balance to shift in favor of the microorganisms. Calcified areas eventually become friable and perforate, exposing underlying soft tissues and forming the "ulcerative" stage of this disease.