In 1978 cholera outbreak in Louisiana was attributed to poor sanitary practices in home prepared blue crabs. Ameson michaelis is a microsporidan parasite of blue crabs commonly isolated from crabs in Gulf and Atlantic waters. The muscle tissue of heavily infected crabs has an opaque chalky appearance. This parasite can be used as a spawning history indicator in blue crabs. The branchiobdellid annelid Cambarincola vitreus also infects blue crabs from freshwater and low salinity habitats.
Blue crabs are host to external and internal barnacles.
External barnacles include Chelonibia patula and Balanus niveus that attach to the outer surface of the carapace and appendages. Cypris larvae infect juvenile crabs during early stages of the molting process. The externa contains male and female gonads and serves as a brood pouch for developing larvae. Rhizocephalan infection alters the secondary sex characteristics of the crab by causing the abdomen to appear as that of a mature female. A complex behavioral repertoire has been demonstrated for blue crabs.