17 Parasitic Diseases of Shellfish Susan M. Bower Fisheries and Oceans Canada, Sciences Branch, Pacific Biological Station, Nanaimo, British Columbia, Canada V9T 6N7 Introduction shrimp) that were not mentioned in Lee et al. (2000) and have unknown taxonomic Numerous species of parasites have been affiliations are discussed prior to presenting described from various shellfish, especially the metazoans that are problematic for representatives of the Mollusca and shellfish. Crustacea (see Lauckner, 1983; Sparks, 1985; Sindermann and Lightner, 1988; Sindermann, 1990). Some parasites have Protozoa Related to Multicellular had a serious impact on wild populations Groups and shellfish aquaculture production. This chapter is confined to parasites that cause Microsporida significant disease in economically impor- tant shellfish that are utilized for either Introduction aquaculture or commercial harvest. These Many diverse species of microsporidians pathogenic parasites are grouped taxonomi- (Fig. 17.1) (genera Agmasoma, Ameson, cally. However, the systematics of protozoa Nadelspora, Nosema, Pleistophora, Thelo- (protistans) is currently in the process of hania and Microsporidium – unofficial revision (Patterson, 2000; Cox, 2002; Cavalier- generic group), have been described from Smith and Chao, 2003). Because no widely shrimps, crabs and freshwater crayfish accepted phylogeny has been established, worldwide (Sparks, 1985; Sindermann, parasitic protozoa will be grouped accord- 1990). The majority of these parasites are ing to the hierarchy used in both volumes detected in low prevalences (< 1%) in wild edited by Lee et al. (2000). In that publica- populations. Although the economic impact tion, Perkins (2000b) tentatively included of most species of Microsporida on crusta- species in the genera Bonamia and Mikrocytos cean fisheries is unknown, some species are in the phylum Haplosporidia. As discussed perceived to have adverse economic impacts. below, subsequent analysis has verified that Also, these parasites occasionally cause Bonamia spp. and Mikrocytos roughleyi are serious disease epizootics in penaeid aqua- Haplosporidia but that Mikrocytos mackini culture (Lightner, 1996). is not and has unknown taxonomic affilia- tions. In addition, several other pathogenic Host range protozoa (three species unofficially grouped as Paramyxea that parasitize oysters; and a Ameson (= Nosema) nelsoni infects at least recently encountered pathogen of pandalid six species of shrimp throughout the Gulf of CAB International 2006. Fish Diseases and Disorders Vol. 1 (ed. P.T.K. Woo) 629 630 S.M. Bower Fig. 17.1. Histological images of plasmodia (p) and developing spores (s) of unidentified Microsporida in crustaceans in British Columbia, Canada. A. Infection congesting the connective tissue between the hepatopancreas tubules (t) of a Pandalus platyceros in which the muscle tissue was not infected. B. Infection replacing the skeletal muscle (m) tissue of a Pandalus jordani in which the hepatopancreas was not infected. C. Similar infection to B in a Cancer magister. All bars = 10 µm. Mexico and north along the Atlantic coast Coast from Bodega Bay, California, to Gray’s of the USA to Georgia (Sparks, 1985). It is a Harbor, Washington, with prevalences low- common pathogen and has caused signifi- est in open oceans (0.3%) and highest in cant financial losses to the bait and food estuaries (usually about 14% but up to shrimp industries (Sindermann, 1990). 41.2% in one location) (Childers et al., 1996). Microsporidosis in captive–wild Penaeus Like A. michaelis, N. canceri was also directly brood stock (infections not apparent at time transmitted to juvenile and adult crabs in of collection) resulted in losses of up to the laboratory by allowing them to ingest 20% (Lightner, 1988). Also, prevalences infected tissue and to megalope and early (16% and 15%, respectively) of A. nelsoni, juvenile crabs by placing them in a suspen- in pond-reared brown shrimp from Texas sion of 106 spores/ml (Olson et al., 1994). and in white shrimp in a net-enclosed bay An unidentified microsporidian in in Florida suggest a potential threat to the hepatopancreatocytes of tiger shrimp shrimp reared in extensive culture (Lightner, (Penaeus monodon) was associated with 1975). low production, slow growth rates and Ameson (= Nosema) michaelis is occasional mortalities in brackish-water widely distributed at low prevalences in pond culture in Malaysia (Anderson et al., blue crab on the Gulf and Atlantic coasts of 1989). Also, unidentified Microsporida have the USA (Sparks, 1985). Diseased blue crabs been presumed to cause high mortalities in (Callinectes sapidus) often inhabit shel- freshwater crayfish in Western Europe and tered areas near the shore and experience England (Pixell Goodrich, 1956). high mortalities when stressed (Overstreet, 1988). However, unlike Ameson in shrimp, Morphology the transmission of A. michaelis is direct, i.e. by ingestion of infected tissue (Sparks, Each species of Microsporida is character- 1985; Overstreet, 1988). Some authors ized by the number of spores per sporont, indicated that this parasite was a significant the spore size, the tissues infected and, factor in blue crab mortality and thus a to some extent, the host species (Sparks, potential threat to the industry. However, 1985; Lightner, 1996). Although the spores more information is needed on pathogenic- of most Microsporida are ovoid and rela- ity, geographical distribution and preva- tively small (about 3 to 5 µm in length), the lence in various populations before its spores of N. canceri are unique in being economic significance can be established exceptionally long (about 10 µm) and needle- (Sparks, 1985). shaped (0.2 to 0.3 µm in diameter), tapering Nadelspora canceri occurs in Dungeness to a posterior pointed end (Olson et al., crab (Cancer magister) along the US Pacific 1994). Parasitic Diseases of Shellfish 631 Host–parasite relationships control microsporidosis in the lucerne weevil) may be suitable treatments for this disease Microsporidians replace host tissue with in penaeid shrimp. However, Overstreet spores as they grow, without invoking host (1975) found that Fumidil B seemed to inflammatory responses. Infected individu- exacerbate A. michaelis infection in blue als exhibit poor stress resistance and poor crabs and benomyl was not as effective as stamina and are thus prone to loss by preda- buquinolate and apparently killed some crabs. tion and to poor survival following capture and handling. Infection of the gonad by some species renders infected individuals sterile and may cause feminization of Alveolates infected male penaeids (Lightner, 1996). Apicomplexa Diagnosis of infection Introduction Infected tissue, especially muscle, is even- tually replaced by spores, giving it an Many species of gregarines and coccidians opaque appearance. Due to this white dis- have been described from shellfish. Because coloration, heavy infections are apparent information to date indicates that most of and justify the common names of ‘cotton’, these species are relatively benign in patho- ‘milk’ or ‘cooked’ shrimp and crabs. In genicity, they will not be mentioned here. addition, the cuticle of some crustaceans However, two species of coccidia have been may have blue-black discoloration due associated with pathology. to expansion of cuticular melanophores Coccidians (family Eimeriidae) have (Lightner, 1996). The fluorescent technique been described from the kidneys and less described by Weir and Sullivan (1989) for frequently other organs of bivalves. Although screening for Microsporida in histological they were all designated as species of sections may be useful for detecting light Pseudoklossia (Upton, 2000), only the first infections. A molecular probe has been (type) species and one other appear to be developed for the detection of Agmasoma heteroxenous. Because the other species sp. in Penaeus spp. (Pasharawipas et al., undergo monoxenous development in their 1994). molluscan host, Desser and Bower (1997) proposed the creation of a new genus, Margolisiella, to accommodate these para- Prevention and control sites. Disease concerns were associated with The only known method of prevention is Margolisiella kabatai (Fig. 17.2A) in Pacific removal and destruction (freezing may not littleneck clams (Protothaca staminea) that destroy spores) of infected individuals were found on the surface of the substrate (Lightner, 1988; Overstreet, 1988). The in Washington State (Morado et al., 1984) intermediate hosts (fin fish) should be and Margolisiella (= Pseudoklossia) haliotis, excluded from culture systems and water which can occur in extremely heavy infec- supplies (e.g. Ameson penaei became infec- tions in the kidneys of abalone (Haliotis tive for pink shrimp following passage spp.) from California (Friedman et al., 1995). through the gut of a shrimp predator, the In addition to gregarines and coccidians, spotted sea trout (Cynoscion nebulosus) Levine (1978) proposed that Perkinsus mari- (Lightner, 1988)). A single treatment of nus (= Dermocystidium marinum = Labyrin- buquinolate (used to treat coccidiosis in thomyxa marina), a pathogen of eastern boiler chickens) prevented microsporidosis (American) oyster (Crassostrea virginica), caused by A. michaelis in most exposed also be included within the phylum blue crab (Overstreet, 1975). Lightner (1988) Apicomplexa. Subsequent taxonomic analy- suggested that Fumidil B (an antibiotic used sis based on nucleotide sequences indi- to control microsporidosis