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Shellfish Tissues Evaluated for Perkinsus Spp. Using the Ray's Fluid

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Shellfish Tissues Evaluated for Perkinsus Spp. Using the Ray's Fluid Vol. 80: 235–239, 2008 DISEASES OF AQUATIC ORGANISMS Published August 7 doi: 10.3354/dao01944 Dis Aquat Org NOTE Shellfish tissues evaluated for Perkinsus spp. using the Ray’s fluid thioglycollate medium culture assay can be used for downstream molecular assays C. Audemard*, R. B. Carnegie, E. M. Burreson Virginia Institute of Marine Science, College of William and Mary, Route 1208, Greate Road, Gloucester Point, Virginia 23062, USA ABSTRACT: Ray’s fluid thioglycollate medium (RFTM) culture assay is the standard, recommended method for surveillance of Perkinsus spp. infections in marine molluscs. In this assay, shellfish tissues are incubated in RFTM, stained with Lugol’s iodine solution to render Perkinsus spp. cells blue-black, and evaluated microscopically to rate infection intensities. A limitation of this assay, however, is the lack of pathogen species specificity. Generally, identification of Perkinsus spp. requires DNA sequence analysis of parallel or additional samples since the exposure to iodine is believed to ham- per DNA amplification from samples processed by the RFTM assay. However, we show that P. mari- nus DNA can be successfully amplified by PCR from Crassostrea virginica tissues cultured in RFTM and stained with Lugol’s iodine. The beneficial consequence is that, where necessary, DNA sequence data may be obtained from RFTM-cultured tissues, allowing the identification of the Perkinsus sp. responsible for an observed infection. This would obviate further sampling, representing gain of time and reduction in cost, where a Perkinsus sp. is unexpectedly observed in new host(s) or location(s) but where parallel samples are not available for molecular diagnostics. Laboratories without molecu- lar diagnostic tools for Perkinsus spp. may fix presumptive Perkinsus sp.-positive culture material in 95% ethanol for transport to, and subsequent analysis by, a laboratory that does have this capacity. KEY WORDS: Perkinsus · Dermo disease · RFTM · Lugol’s iodine · DNA · PCR Resale or republication not permitted without written consent of the publisher INTRODUCTION Columbia, Canada (Blackbourn et al. 1998); P. chesa- peaki, infecting an array of clams (and C. virginica) in Protistan parasites in the genus Perkinsus infect di- the mid-Atlantic region of the USA (Burreson et al. verse marine molluscs worldwide including oysters, 2005); P. mediterraneus, from the oyster Ostrea edulis clams, scallops, and abalone. Currently, 7 Perkinsus spe- in the Balearic Islands (Spain) of the Mediterranean cies are recognized: P. marinus, infecting the oyster Sea (Casas et al. 2004); P. honshuensis, from Japanese Crassostrea virginica along the Atlantic and Gulf of Manila clams (Dungan & Reece 2006); and P. beihaien- Mexico Coasts of the USA (Mackin et al. 1950); P. olseni, sis, from Chinese Crassostrea spp. oysters (Moss et al. described from the abalone Haliotis rubra in Australia 2008). Among these species, P. marinus and P. olseni but currently known to infect hosts from Asia, are economically significant pathogens listed as ‘notifi- New Zealand, South America, and Europe (Lester & able’ by the World Organisation for Animal Health (OIE). Davis 1981, Elston et al. 2003, Villalba et al. 2004); P. ug- The standard diagnostic method recommended for wadi, in the scallop Patinopecten yessoensis in British surveillance of Perkinsus spp. infections is Ray’s fluid *Email: [email protected] © Inter-Research 2008 · www.int-res.com 236 Dis Aquat Org 80: 235–239, 2008 thioglycollate medium (RFTM) culture assay. This To our knowledge, there have been no attempts to assay was developed in the early 1950s (Ray 1952, PCR-amplify Perkinsus sp. DNA from RFTM-cultured, 1966), after P. marinus was described in Crassostrea Lugol’s iodine-stained material. This may be due to the virginica in the Gulf of Mexico (Mackin et al. 1950). In common belief that the exposure to iodine may com- this assay, pieces of animal tissue, usually mantle promise downstream PCR amplification (Godhe et al. and/or rectum, are incubated in RFTM for several days 2002). This has never been tested, however, in the case during which Perkinsus spp. cells enlarge from of RFTM-iodine processed tissues. If such amplifica- 2–10 µm to 50–70 µm, forming hypnospores. After tions were possible, identification of a Perkinsus sp. incubation, the enlarged cell walls are stained blue- parasite to species would be possible even if parallel black by the addition of Lugol’s iodine, and stained samples were not preserved specifically for molecular Perkinsus spp. cells can be easily observed under a work, and without the time and expense of resampling microscope. This assay presents advantages of low cost a host population. In the reported study, our objective and simplicity relative to histology, and the infection was to determine whether P. marinus in tissues pro- intensity data collected are semi-quantitative. Further- cessed for RFTM assays and stained with Lugol’s more, this diagnostic assay is more sensitive than iodine would be PCR-amplifiable following micro- histological methods (McLaughlin & Faisal 1999). One scopic observation. of the major limitations of the RFTM assay, however, is that it does not allow discrimination among Perkin- sus spp. In fact, even Perkinsus spp. observed histo- MATERIALS AND METHODS logically cannot be identified to species based on morphological characteristics alone. Rather, identi- Samples analyzed. The oyster samples were Cras- fication of Perkinsus spp. requires the analysis of sostrea virginica (n = 67) collected during the fall of DNA sequences, in particular, ribosomal DNA non- 2007 from 2 sites located in the Perkinsus marinus- transcribed spacer (NTS) and/or internal transcribed enzootic Virginia portion of the Chesapeake Bay spacer (ITS) sequences (Goggin 1994, Marsh et al. (USA), Tangier Sound (n = 45) and Pocomoke Sound 1995, Robledo et al. 1998, Casas et al. 2004, Burreson (n = 22). et al. 2005). RFTM assays. Upon arrival at the laboratory, oysters Samples for molecular analysis and DNA-based were processed for RFTM assays as follows. After oys- species identifications by laboratories that have such ters had been cleaned and measured, they were capabilities are generally preserved from recently shucked, with pieces (5 to 10 mm2) of gill, mantle and shucked animals, while separate tissue samples are rectum excised and immersed in RFTM. Antimycotics used for RFTM assay determination of infection and antibiotics comprising 200 units of mycostatin prevalence and intensity; additional tissue samples (Nystatin), 500 units penicillin G, and 500 mg dihydro- may be fixed and processed for histology. It is often streptomycin per ml of media were added before plac- the case with microparasites like Perkinsus spp., how- ing the tubes into an incubator at 22 to 25°C. After 5 to ever, that the few parasite cells present in a very light 7 d, tissues were removed from the RFTM and placed infection may be captured in one tissue sample, but on a glass slide. Then, 1 to 2 drops of 20% (v/v) Lugol’s not in others. Such animals may be, among other pos- iodine solution were added, and tissues were macer- sible outcomes, RFTM-positive for a Perkinsus sp. but ated using a scalpel blade before being coverslipped. histology- and PCR-negative. In such cases, the iden- Scalpel blades were immersed in 95% EtOH and then tity of a Perkinsus sp. observed by RFTM cannot flamed between each sample to prevent cross-contam- definitively be resolved. It is also the case that many ination. The preparation was examined under a micro- laboratories are not equipped for molecular diagnos- scope, and Perkinsus sp. infection intensities were tics, and therefore use only RFTM assays, the OIE- ranked based on the scale of Mackin (Ray 1954): no recommended gold-standard tool, and histology for detection; rare (1 to 2 parasites found in entire prepa- surveillance of Perkinsus spp. infections. While this ration); very light (3 to 10 parasites found in entire tis- may reasonably suffice for routine detection of a sue preparation); light (11 to 100 cells in entire prepa- Perkinsus sp. in a known host from a locality in which ration); light–moderate (some areas free of parasites a particular Perkinsus sp. is known to be enzootic, this but other areas showing localized concentrations of 24 is not adequate for establishing new host records, or to 50 cells, or cells uniformly distributed such that 2 to for drawing conclusions about a parasite’s possible 3 cells occur in each 100× field); moderate (parasites so geographic range expansion. In examples such as numerous that >3 cells are present in all fields at 100×, these, it is unfortunate that definitive parasite species but masses of >50 cells are still more or less localized, identifications cannot be established from RFTM- tissue not showing blue/black color macroscopically); assayed tissue samples. moderate–heavy (parasite cells present in large num- Audemard et al.: PCR analysis of RFTM-iodine processed tissues 237 bers in all tissues, but less than half of the tissue show- Table 1. Results from RFTM assays (number of oysters accord- ing a blue/black color macroscopically); or heavy (par- ing to categories of infection intensity) and subsequent PCR asite cells in enormous numbers, with the major part of assays on the same individuals with the number (percentage) of PCR positive per infection category. Perkinsus chesapeaki tissue appearing blue/black macroscopically). was not detected in any sample DNA extraction. Once the preparations had been examined under a microscope and Perkinsus sp. infec- RFTM infection No. of No. of PCR positive (%) tions had been rated, the coverslip from each sample intensity oysters Perkinsus spp. P. marinus was removed using a scalpel, and tissues were scraped into a tube containing 10 volumes of 95% ethanol per Negative 9 9 (100) 8 (88) 1 volume of tissue. As described earlier, scalpel blades Rare 5 5 (100) 3 (60) Very light 6 6 (100) 6 (100) were flamed between samples to minimize the risks Light 12 12 (100) 11 (92) of contamination from sample to sample.
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