MFR PAPER 1300 Shellfish Diseases LOUIS LEIBOVITZ ABSTRACT-An overview of commercial bivalve shellfish aquaculture is pre­ many as 120 million offspring from a sented. The advantages and disadvantages of shellfish production as compared single mating. with other forms of food animal production is discussed. The common shellfish There is another unique aspect of diseases are listed andthe known specific etiologic agents are indicated. The latter shellfish production that exceeds the include viral, bacterial, fungal, protozoan, and metazoan parasitic and infectious economic efficiency of other forms of agents. In addition, predators, toxic agents, and fouling organisms produce seri­ animal protein production and that is ous economic losses. free food. Unlike the rising food costs The specialized problems ofshellfish hatcheries are discussed. The importance of other animal feeds, shell fish foods of monitorifJg the qualitative physical, chemical, and bacteriological changes in are naturally generated planktonic shellfish larval cultural media and its ingredients for optimum production is indi­ foods. Since shellfish are an important cated. source of food, we should learn more of A description of a laboratory model for evaluating the pathogenicity of pure their diseases as a part of the technical bacterial cultures for larval shellfish is presented. The experimental optimal and development necessary to increase pro­ lethal concentrations of bacteria for shellfish larvae are defined. An interrelation­ duction. ship between bacteria andprotozoa in the pathogenesis ofshellfish larval diseases The following discussion of shellfish is reported. The shellfish industry has encouraged and supported the reported diseases is an overview and a short con­ research to increase the efficiency of shellfish production by reducing economic sideration of one specific bacterial dis­ losses due to shellfish diseases. ease problem in larval shellfish produc­ tion being currently studied. Less is known about the subject of exposure to urban and industrial pollu­ DISEASES OF SHELLFISH shellfish diseases, and, accordingly, tants discharged into estuarine waters. there is a wider latitude in discussing it. In spite of these hazards, shellfish A list of organisms that cause com­ There are many unique problems, some hold one of the greatest potentials for mon diseases in oysters is shown in of which overlap with fish diseases. the economic production of food pro­ Table I. One problem is that the molluscan tein. Shellfish hold great promise for Viral Diseases bivalves are filter feeders. Their ability the efficient recycling of organic waste to concentrate harmful chemicals and materials, such as agricultural wastes, Of the known reported virus infec­ infectious agents pose serious problems and the capture of energy for food pro­ tions of oysters, "Ovacystis" infection in controlling both shellfish and human duction from thermal effluents, such as is the most common, but it is probably diseases. Since shellfish are estuarine that discharged from atomic power of little economic importance. It can be dwellers, they are subjected to en­ plants. In addition, there are more detected histologically as hypertrophy vironmental variations such as changes species ofshellfish than any other group of the ovarian follicles. The affect of in salinity and temperature, seasonal of animals, with the exception of ar­ this virus upon reproductive. perfor­ tidal variations, and varying degrees of thropods. Genetic selection for greater mance has not been evaluated. food yields from these abundant var­ A herpes virus infection has beende­ ieties should be rewarding. Also, in scribed by Austin Farley (1972) in oys­ Louis Leibovitz is with the Depart­ terms of reproductive potential, there ters. Apparently, expression of the dis­ ment of Avian and Aquatic Animal Medicine, New York State College of are no other food animals that even ap­ ease was temperature dependent and Veterinary Medicine, Cornell Univer­ proach their fecundity. For example, a was found in oysters cultiYjlted in the sity, Ithaca, NY 14853. single pair of oysters can produce as heated effluent of a power plant. When March 1978 61 Table 1.-Causes of common diseases In oysters. pathogen that produces serious eco­ pollution without adequate evidence Group nomic losses in adult shellfish in warm that disease was not responsible. Viral Ovacystis virus climates. Sirolpidium sp. is a common Herpes virus Parasitic Diseases Others infection of hatchery-reared larval Bactenal Achromobacler sp. shellfish. Aeromonas sp. Protozoans Vibrio sp. Myrotomus ostrearum Helminthic Diseases ··Maladie Du Pled" Shellfish protozoan infections are Nocardia sp. Among the helminth parasites of very common. Whether these or­ C/adothrix dichotoma (Actinomycetes) Others shellfish, trematode, cestode, and ganisms are primary infectious agents Fungal Oermocyslidium (Labyrinrhomyxa) nematode parasites may be found. Lar­ is often questionable. This is especially marinum SiroJpldium sp. val forms of trematodes (especially true of the ciliates that are common Others Bucephalus sp.) and cestodes (espe­ inhabitants of shellfish tissues. They Parasite , Sarcodina (Amoeba) protozoan F/abel/u/a sp. cially Tylocephalum sp.) are of become especially active when other 2. Masligophora (Flagellate) Hexamita nelsoni economic importance as shellfish pathogens such as bacterial agents are 3. Sporozoan pathogens that often produce sterility in present. Of the flagellated protozoa, a. Gregarine b. Haplosporidia affected shellfish. Most of the larval Hexamita sp. and the amoeboid pro­ 4. Ciliates-many forms mature in fish which serve as tozoa are pathogenic. When shellfish Helminthic 1. Trematoda (larval) Bucephalus sp. definitive hosts. Some are of public are maintained under adverse condi­ Others health significance. tions, such as extreme temperatures, 2. Cestoda (larval) Tylocepha/um sp. protozoa may actively invade shellfish Arthropods and Others tissues and produce deterioration or Arthropods 1. Copepods Other Organisms Mytilicola intestinalis spoilage. These conditions may also be Others 2. Decapods In addition to helminth parasites, found in "winter-kills" of shellfish Pinnotherid crabs copepod crustacean and polychaete an­ where high mortality associated with 3. Annelids ("Mud blisters") Polydora websteri nelids, during some stage of their life protozoan infections may be found in Others cycles, may parasitize shellfish with re­ sustained low temperature exposures. 4. Sponges ("Bo"ng sponges") Onona celala sultant serious economic losses. Protozoans can be primary shellfish Others A great variety of marine organisms pathogens. The most important single are found in shellfish beds in apparent shellfish pathogen that has produced the symbiotic or commensal relationships greatest economic losses to the shellfish the environmental temperature drop­ to shellfish. Some, as pinnotherid industry is a haplosporidian, Minchinia ped, the disease was not apparent. crabs, enter and leave the pallial cavity nelsoni. This organism has destroyed While there are undoubtedly other of shellfish freely. Crabs may serve as the great oyster industry of the Dela­ shellfish viral diseases present, they the intermediate host for the primitive ware and Chesapeake Bays. Haplo­ have not been defined. gregarine sporozoans (Nemotopsis sp.) sporidians are very poorly understood, The two previous viral diseases men­ whose spores infect shellfish with little poorly classified sporozoans, distinct tioned were demonstrated upon the resultant tissue damage. Macroalgae from myxosporidia, or coccidial or­ basis of diagnostic inclusion bodies and and sponges grow on the surface of ganisms. Their exact taxonomic posi­ electron microscopic demonstration of shellfish. The boring sponges (Cliona tion and life cycles are unknown. In viral particles in affected cells. Virus­ sp.) may damage the external shell and addition to the areas mentioned, M. free molluscan tissue culture systems the shell may then become porous and nelsoni, commonly called MSX, is are needed to isolate and identify mol­ crumble. present in other geographic locations of luscan viruses and human viral patho­ the northeastern U.S. coastline. This Diseases of gens that may be carried by shellfish. organism is apparently salinity­ Unknown Etiology dependent. It is seasonal in its inci­ Bacterial Diseases In addition to the known diseases, dence. There are many other haplo­ Little is known of the bacterial dis­ many unexplained die-offs have been sporidians, of varying pathogenicity eases of shellfish, and the list in Table J reported that have decimated shellfish found as parasites in a variety of aquatic is limited to those that have been de­ populations. Often these populations animals. They are found as hyperpara­ scribed. From the standpoint of human do not recover, and new stock, intro­ sites in trematodes. These organisms health, outbreaks of cholera have been duced to repopulate, are quickly af­ tend to sterilize the trematode host. related to the consumption of shellfish fected and die. Such diseases are often SHELLFlSH HATCHERY in Africa and Italy. named for the locality in which they OPERATION STUDIES occurred, such as "Malpeque Bay" Fungal Diseases and "Denman Island" disease. Often When I began working with the Long Dermocystidium (Labyrinthomyxa) serious losses are attributed to climatic Island shellfish industry, the problems marinum is a very important shellfish conditions, water quality changes, and were overwhelming