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Results of a survey on shellfish A health survey of 1,200 greenshell mussels (Perna health in New Zealand in 2000 canaliculus), 1,050 Pacific oysters (Crassostrea gigas), 300 rock oysters (Saccostrea glomerata), 252 The New Zealand aquaculture industry is based largely on bivalve scallops (Pecten novaezelandiae) and 150 clams molluscs, particularly greenshell mussels (Perna canaliculus) and (Austrovenus stutchburyi), revealed one serious Pacific oysters (Crassostrea gigas). There are also export markets for infection, the presence of Perkinsus olseni in clams. live clams (Austrovenus stutchburyi, also known as cockles), and Mussels and oysters harboured trivial infections and scallops (Pecten novaezelandiae) are prospective candidates for aquaculture. were in good health. The shellfish industry has been under passive disease surveillance remaining meat was incubated in Ray’s fluid thioglycollate medium since 1986 when epizootics caused by the parasite Bonamia first (RFTM) in the dark at room temperature (16°-18°C) for five days, occurred in flat oysters (Ostrea chilensis)(1)(2), virtually destroying the and then Lugol’s iodine was used to stain Perkinsus colonies(16). Foveaux Strait oyster fishery(3). In that time more than14,000 flat Survey results oysters, 4,000 Pacific oysters, 2,000 greenshell mussels and scallops, and 1,000 clams have been examined for parasites and disease. Greenshell mussels These studies have revealed herpesviruses in larval Pacific(4) and The results of examination of mussels from five farms in the flat(5) oysters, viruses associated with digestive tubule epithelial Marlborough Sounds and three farms on the Coromandel degeneration in mussels(6) and scallops(7), as well as some trivial Peninsula are shown in Table 1. infections(8).After Bonamia sp, the only other potentially serious Table 1: Parasites and lesions observed pathogen that has been discovered is Perkinsus olseni, identified in in greenshell mussels (Perna canaliculus) clams from the Mangamangaroa Stream near Auckland in May Coromandel Marlborough Sounds n = 450 n = 750 2000 (Hine PM, unpublished observations). Sloughing of the digestive epithelium <1% A two-year study of commercial shellfish farms is being conducted Gill ciliates 2% <1% and the results of its first year are reported here. The study is part of Apicomplexan/coccidian 5% the active surveillance required by the European Community, Pseudomyicola spinosus 1% following guidelines set down by the Office International des Lichomolgus uncus <1% 1% Paravortex sp 1% Epizooties (OIE)(9). It has also served to clarify the status in New Haemocytosis <1% Zealand of three OIE-listed diseases: winter mortality, P olseni and Sloughing of the epithelium of mussel digestive tubules, involving Bonamia. A study in the early 1970s(10) reported signs resembling large numbers of rounded epithelial cells shed into the lumen, has those of winter mortality, or mikrocytosis, in native rock oysters been associated with putative epithelial viruses(6). The virus-like (Saccostrea glomerata). Winter mortality is known to occur only in particles are probably widespread in mussels but at such low levels Sydney rock oysters(11), which until recently were classified as that epithelial sloughing does not usually occur. Saccostrea commercialis but are now recognised as S glomerata(12). Native rock oysters from Northland were examined in this study. Light infections with gill ciliates were seen rarely in Coromandel Clam samples from the Mangamangaroa Stream were included to mussels, and even less so in Marlborough Sounds mussels, and were determine the incidence of P olseni. As members of the family always without associated pathological changes. (13)(14) Arcidae are also particularly susceptible to Perkinsus ,50 Coccidia-like apicomplexans occurred in the connective tissue of Barbatia novaezealandiae from around Rangitoto Island, Auckland the digestive gland, between the digestive tubules and gonad Harbour, were also examined. Bonamia has been found only in flat follicles, in mussels from two Marlborough Sounds farms. The stage oysters found around the South Island and in Wellington Harbour, observed was 8-10 mm long and 2.5-3.5 mm wide, slipper-shaped, and these are genetically different from North Island flat oyster with a central nucleus, and posterior amylopectin granules. It was (15) stocks . Although it was intended to examine the latter, impossible to determine if they were sporozoites or merozoites, but insufficient stocks could be located. the tight bunching of up to ten organisms suggested they might Survey methods have been sporozoites within sporocysts in an oocyst. In the mussel that was most heavily infected they were associated with focal Sample sizes and locations surveyed are presented in Tables 1-5. destruction of the connective tissue (Leydig) cells. However, they Each bivalve was measured along the axis at right angles to the appeared to be minor pathogens, as they are in other bivalves(17)(18). hinge, opened, and a steak cut through the centre was fixed in 10% filtered (0.22 µm) seawater formalin. Haematoxylin and eosin Two species of copepod were encountered: Lichomolgus uncus on stained sections were prepared by routine methods. Samples of the palps of the mussel, and Pseudomyicola spinosus in the gut, Barbatia novaezealandiae were processed as above and the neither associated with damage to epithelia. Lichomolgus uncus was page 3 Surveillance 29(1) 2002 first described by Jones(10) from mussels in Wellington Harbour Rock oysters between 1973 and1974, with a prevalence of infection of 0-32%. The results of examination of rock oysters from Northland and the Over the same period the prevalence of infection in mussels from Coromandel Peninsula are presented in Table 3. Crail Bay in the Marlborough Sounds was up to 10.4%(10).The The sloughing of diverticular epithelial cells resembled the sloughing prevalence of P spinosus recorded in the present study is lower than reported to be associated with the presence of virus-like particles in the 2-4% infection reported in mussels from Wellington Harbour(10), mussels(6) and scallops(7), but appears to be a natural process. but the prevalence is normally highest in summer, and the samples examined here were collected at the end of the southern autumn in Table 3: Parasites and lesions observed in rock oysters (Saccostrea glomerata) June. Pseudomyicola spinosus is distributed worldwide and normally Northland Coromandel (19) causes no harm to host mussels , although occasionally it may n = 150 n = 150 leave the gut and penetrate the digestive gland(20). Sloughing of the digestive epithelium 3% 1% Paravortex is a turbellarian that may be free-living but is sometimes Adductor bacterial necrosis 1% found in close association with bivalves; it occasionally enters the gut. R-LOs digestive tubule 1% Ciliates on gills 6% 3% Haemocytes had infiltrated the connective tissue in the digestive Ciliates in gut 11% 8% gland of five mussels from the Marlborough Sounds, but the reason Pseudomyicola 5% for this was not apparent. Haemocytosis is a non-specific response Haemocytosis 6% and is not necessarily caused by underlying infection but can be Gut lesion 1% caused by other factors such as absorption of unspawned gametes. One rock oyster from Northland had a necrotic lesion in the adductor muscle associated with short Gram-negative rod-like Overall, mussels in the samples examined appeared to be in good bacteria, but otherwise appeared healthy. health, and this is reflected in the low mortality rates on mussel farms. Rickettsiales-like or Chlamydiales-like inclusions (here referred to as Pacific oysters R-LOs) occurred in the epithelial cells of the digestive tubules of The results of examination of 1,050 Pacific oysters are shown in one rock oyster from the Coromandel Peninsula, and ciliates were Table 2. common on the gills and in the gut of oysters from both regions, There were no apparent pathological changes associated with the but without associated pathology. epithelial rickettsial inclusions, the ciliates or P spinosus infection. Pseudomyicola spinosus was observed in the gut and digestive ducts The overall prevalence of P spinosus (~1%) was low compared with without associated changes in gut epithelium. As noted in a (21) the average one third (16-54%) prevalence reported by Dinamani , previous study(26), there was no evidence of associated lesions, and it but this may be because he examined the pallial cavity fluid where appears that copepods are largely free-living in the mantle cavity the copepod is more frequently encountered. and only occasionally enter the oyster host(21). The prevalence of P Table 2: Parasites and lesions observed in Pacific spinosus infection in this study (5%) is much lower than the 54% oysters (Crassostrea gigas) reported by Jones(10), again explained by the fact that prevalence in Northland Coromandel Marlborough Sounds n = 450 n = 450 n = 150 this study is based on examination of sections, whereas Jones Rickettsia in gills <1% dissected each animal. Rickettsia in digestive tubules 1% The haemocytoses observed were associated with absorption of Ciliates on gills <1% 2% <1% gametes and, despite thorough examination, no microcells Ciliates in gut 3% 1% <1% resembling Mikrocytos roughleyi, the aetiological agent of winter Ciliates on mantle <1% <1% Pseudomyicola <1% <1% mortality, were seen. Spionid polychaetes 63% 41% 39% In one oyster the rear gut lacked an epithelium in an area in which Spionid polychaetes, or mudworms, occurred in the shells of oysters the connective tissue below the epithelial basement membrane was in all three regions, with infestation rates declining southwards. heavily infiltrated with haemocytes. There was no apparent cause Identification of the spionids responsible was not attempted but for the infiltration and the basement membrane was intact. previous studies report four species (Polydora websteri, Polydora Scallops hoplura, Boccardia [Paraboccardia] acus, and Boccardia [Boccardia] The results of examination of scallops from the Coromandel chilensis) infesting Northland Pacific oysters, with P websteri the Peninsula and Marlborough Sounds are shown in Table 4. most common(22).