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 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 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 , 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 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. (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 and it was difficult

62 Marine Fisheries Review to select a single starting point. Perhaps BACTERIAL CONCENTRATIONS known dilution of the test substance. In AND EQUIVALENT the most important economic problems DILUTIONS OF INOCULA testing for bacterial pathogenicity, pure were based in shellfish hatchery pro­ IX 107 IX 105 IX 10 3 IX 10' 24-hour broth cultural bacterial isolates duction. If hatchery production could were added to each of the first 3 rows of be increased, and livability of larvae REPLICATE I 0 0 0 0 the plate; from left to right, each well of and juveniles were improved, restock­ each of the first three rows containing REPLICATE II 0 0 0 0 7 3 1 ing and harvesting from shellfish beds approximately 10 , lOS, 10 , and 10 would yield greater production and bacteria per milliliter of well larval sus­ REPLICATE m: 0 0 0 0 efficiency. The techniques of hatchery pension. The fourth row was given the operation are well known, but consis­ MILLIPOREFILTRATE 0 0 0 0 equivalent dilution of bacteria-free tent production of healthy larvae is STERILE filtrate (Millipore filtrate) of the broth difficult. Shellfish larval disease losses PeA BROTH 0000 culture to correspond to the dilutions of are serious hatchery problems, often of LARVAL CULTURE the bacterial suspension wells above AND STERILE epizootic proportions. INSTANT OCEAN 0000 this row. The fifth row received again, Although specific pathogens were equivalent dilutions of sterile culture occasionally responsible for such los­ Figure I.-Pathogenicity model system to broth (plate count agar broth - peA). ses, it became apparent that there were test pure bacterial isolates obtained from lar­ The last (sixth row) received equivalent val cultures. many unexplained phenomena as­ dilutions of synthetic sea salts (Instant sociated with the more common losses. Ocean)1 to the shellfish larval suspen­ In an attempt to resolve these problems, sions. The results of the above test were studies of hatchery media were under­ read at the end of a 24-hour incubation --- BACTERIA taken. These included physical, chemi­ 90 ---- FILTRATE period. The number of alive and dead cal and microbiological examination of ~ --BROTH larvae in each well was counted and the >- .... BO \ ...... INSTANT hatchery water supply, stock algal cul­ ::; OCEAN percentage mortality for each well was ~ tures, pooled algal food cultures, and II: 70 determined. In this manner the effects 0 spawn obtained from hatchery breeding ~ \\ of dilution and comparison of the af­ stock. Each hatchery operation was dis­ ;;-oJ 60 fects of added ingredients could be II: \\ tinctive. Some operated all year, others .. 50 measured to determine their relative limited their operation to warm weather ....-oJ influence on pathogenicity. z \\ w 40 \ only. Hatchery water supply was either u ll: RESULTS W ~, raw bay water, or from deep saltwater Q. 30 wells. Some operations pumped water z The results of the above pathogenic­ w.. 20 '\,, into the plant on demand; others held ~ ity tests (Fig. 2) suggest that almost all water in large storage tanks that was 10 ~, bacterial isolates at high concentrations later gravity fed into the operation on ...... ~ (>IOs/ml) are pathogenic for shellfish 7 demand. Various methods of screen­ 10 10' 103 10' larvae; however, only "true" patho­ 3/ ing, filtration, and centrifugation are BACTERIA INOCULA CONCENTRATIONS-PER ml gens kill at very high dilutions «10 employed for water clarification. In ad­ EQUIVALENT CONTROL DILUTIONS ml). The latter suggests that these true dition some plants utilize ultraviolet pathogens require larvae for growth. treatment of incoming water, or recy­ Figure 2.-Graphic representation of mean Note that the presence of higher con­ percent mortality values of equivalent cled water for disinfection. concentrations of inocula of all tested isolates centrations of even sterile nutritive Physical and chemical examination (35) (each replicated in triplicate), their cul­ broth produces a lethal effect. Accord­ of shellfish larval culture media in­ IUral filtrate, uninoculated broth medium, and ingly, this may suggest that food con­ cluded measurement of pH, salinity, "Instant Ocean" conlrols. centrations, dead or decaying algal chemical oxygen demand, suspended foods, or larvae may aggravate the and total solids. Other tests including pathogenic effect of both extrinsic and nitrogen determinations are currently bacterial isolates obtained from the lar­ intrinsic microbial concentration being explored. Quantitative counts val cultures. The same model could be (within the larvae). Future studies are and identification ofdominant bacterial utilized to test environmental factors, needed. At high concentrations of bac­ populations of the larval culture media drug efficacy, and other factors for their teria and/or equivalent culture media ingredients are also being made. influence in such disease models. This ( 107 or greater), lethal effects are rapid model system was assembled in plastic TESTING BACTERIAL "disposo" trays and consisted of 6 PATHOGENICITY IN LARVAL rows, of 4 wells per row, containing SHELLFISH PRODUCTION precalculated approximate numbers of 'Reference to trade names or commercial firms It became apparent that a pathogenic­ shell fish larvae from 3 to 14 days ofage does not imply endorsement by the National ity model was needed to test the pure (Fig. I). To each row was added a Marine Fisheries Service, NOAA.

March /978 63

I and are not associated with protozoan fected larvae in this bacterial study sup­ to monitor and define hatcheries for op­ activity. However, at levels corres­ port common diagnostic signs and le­ timum performance becomes more ap­ ponding approximately to I05/ml, le­ sions evident in diseased larvae and is a parent. Since individual hatcheries are thal effects are indicated by more separate discussion in other studies. di fferent, each hatchery must be gradual losses and are associated with The results of field studies of bacte­ evaluated for its operational methods intense protozoan proliferation and ac­ rial populations of hatchery media and and equipment. tivity. The latter probably originate its ingredients tend to support the ex­ ACKNOWLEDGMENTS from the normal intrinsic microbial perimental studies. Diseased larval cul­ flora of the shellfish larvae. In fact, if tures are associated with bacterial popu­ This research was sponsored by the one were to observe such cultures with­ lations )07 or greater per milliliter. New York Sea Grant Institute under a out knowing of the presence of the ex­ Further studies will be required to grant from the Office of Sea Grant, Na­ perimental bacterial inoculum, the ag­ define the specific chemical or physical tional Oceanic and Atmospheric Ad­ gressive behavior of the protozoan tests of hatchery media and ingredients ministration, U.S. Department of attack on the shellfish larvae would and their parameters that would be use­ Commerce. suggest that they are the primary patho­ ful in disease detection and diagnosis LITERATURE CITED gen. The mechanism responsible for that could be related to specific Farley, C. A., W. G. Banfield, G. Kasnic, and this phenomenon requires further pathogenic agents. W. S. Foster. 1972. Oyster herpes-type virus. study. Direct observation of the af- As a result of these studies, the need Science (Wash., D.C.) 178:759-760.

MFR Paper 1300. From Marine Fisheries Review, Vol. 40, No.3, March 1978. Copies of this paper, in limited numbers, are available from 0822, User Ser­ vices Branch, Environmental Science Information Genter, NOAA, Rockville, MO 20852. Copies of Marine Fisheries Review are available from the Superin­ tendent of Documents, U.S. Government Printing Office, Washington, DC 20402 for $1.10 each.

64 Marine Fisheries Review