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DISEASES OF AQUATIC ORGANISMS Vol. 70: 1–36, 2006 Published June 12 Dis Aquat Org OPENPEN ACCESSCCESS FEATURE ARTICLE: REVIEW Guide to the identification of fish protozoan and metazoan parasites in stained tissue sections D. W. Bruno1,*, B. Nowak2, D. G. Elliott3 1FRS Marine Laboratory, PO Box 101, 375 Victoria Road, Aberdeen AB11 9DB, UK 2School of Aquaculture, Tasmanian Aquaculture and Fisheries Institute, CRC Aquafin, University of Tasmania, Locked Bag 1370, Launceston, Tasmania 7250, Australia 3Western Fisheries Research Center, US Geological Survey/Biological Resources Discipline, 6505 N.E. 65th Street, Seattle, Washington 98115, USA ABSTRACT: The identification of protozoan and metazoan parasites is traditionally carried out using a series of classical keys based upon the morphology of the whole organism. However, in stained tis- sue sections prepared for light microscopy, taxonomic features will be missing, thus making parasite identification difficult. This work highlights the characteristic features of representative parasites in tissue sections to aid identification. The parasite examples discussed are derived from species af- fecting finfish, and predominantly include parasites associated with disease or those commonly observed as incidental findings in disease diagnostic cases. Emphasis is on protozoan and small metazoan parasites (such as Myxosporidia) because these are the organisms most likely to be missed or mis-diagnosed during gross examination. Figures are presented in colour to assist biologists and veterinarians who are required to assess host/parasite interactions by light microscopy. KEY WORDS: Identification · Light microscopy · Metazoa · Protozoa · Staining · Tissue sections Resale or republication not permitted without written consent of the publisher INTRODUCTION identifying the type of epithelial cells that compose the intestine. Generally, identification from sections relies on Classical keys are used extensively to assist with the recognising morphological detail, and determining the identification of whole protozoan and metazoan para- life stage of the parasite in the fish. The size of a parasite sites. However, such keys are of limited use for parasite can be crucial for identification, but shrinkage occurring identification in stained tissue sections as specific detail after fixation may make measurements based on fresh or features may not be present in sections, which are typ- specimens inapplicable to parasites in fixed material. ically 3 to 5 µm thick. References to these taxonomic The host specificity of particular parasites, the habitat keys are generally not cited in this manuscript. Although (e.g. fresh water, brackish water or marine), along with some groups of parasites possess unique features that water temperature and other environmental conditions, enable a diagnosis of whole organisms to be made, many can also provide additional clues to the parasite’s iden- of these features may not be easily observed or identified tity. The location of the parasite in or on the host is also in sections, and the degree of taxonomic detail available important, as some parasites are only found in certain will vary between the major groups. For example, the organs or tissues. For example, ectoparasites such as presence of a good scolex for cestodes may allow identi- certain species of mobile peritrichous ciliates and mono- fication to the order, whereas for nematodes it should be geneans may only occur on the gills, whereas others possible to identify to order, and often to suborder, by parasitise the skin and fins as well as the gills. *Email: [email protected] © Inter-Research 2006 · www.int-res.com 2 Dis Aquat Org 70: 1–36, 2006 Infective agents often cause typical tissue responses carefully removed for fixation in formal saline. The that vary as the infection progresses from acute to demonstration of certain tissue components requires chronic or disseminated phases. Characteristic patho- the use of an alternative fixative such as Bouin’s solu- logical changes (or the lack thereof) associated with tion. Poor handling before fixation can induce tissue certain infestations can contribute to their identifica- changes; for example, the liver serosa in salmonids tion. For example, the liver, gall bladder, kidney and may show tissue changes as a result of handling gonads of fish may be grossly discoloured if infected with forceps including a localised necrosis or capillary with certain protozoa, microsporidia or myxosporea. dilatation. Tissues may contain areas of consolidation, encapsula- Processing. Serial sections can provide further infor- tion, granuloma, inflammation, necrosis or repair asso- mation for identification as diagnostic features used for ciated with parasitic infection. When considered with parasite identification may not be visible in all sections. relevant clinical information, the histological features Ectoparasites and endoparasites that are not firmly can provide sufficient information to confine the search attached to host tissues can easily be lost from the to a particular type of organism or even a specific tissues during fixation and processing and require care entity. In amoebic gill disease (AGD), for example, during processing. In some cases orientation of the amoebae are usually found along hyperplastic gill tis- tissue is important; for example, longitudinal sections sue forming characteristic lesions (Adams & Nowak of gills are usually preferable for examination due to 2001). The condition of the parasite can also affect the the 3-dimensional structure. Lastly, it is important to host response. The host response to certain protozoa correlate findings whenever possible from both gross and to microsporidia that have dispersed from rup- and histological examination. tured xenomas can be severe, as can the response to Staining. A range of staining techniques is used for dead or deteriorating parasites. Conversely, a severe observing parasites in tissue sections and the most reaction of a host to a parasite can even obscure the widely used is the standard haematoxylin and eosin presence of the parasite. stain, commonly called H&E. This stain is often Additional descriptions for recognition of parasites in sufficient for identification of larger parasites such as tissue sections, and the associated histopathological helminths, leeches and crustacea. In the H&E method changes observed in fish and mammalian tissues in- the nuclei of cells are stained by the haematoxylin fected with protozoan and metazoan parasites, are whilst the cytoplasm is coloured by the eosin. In addi- available in publications including Ribelin & Migaki tion, special staining techniques designed to demon- (1975), Ferguson (1989), Lom & Dyková (1992), Bruno strate diagnostic features of organisms can be useful & Poppe (1996), Gardiner et al. (1998), Kent & Poppe for identification (Table 1). For example, Giemsa stain- (1998), Gardiner & Poynton (1999), Roberts (2001), ing is useful for a range of protozoan parasites and Nowak et al. (2002), and Woo et al. (2002). The present some metazoans (Table 1), particularly for distinguish- review features the appearance of finfish parasites in ing myxosporidian spores, as the polar capsules stain colour photographs of histological sections. Special- dark against a pale background. In addition, Feulgen’s ized terms used to describe morphological features nuclear reaction for selective staining of DNA and the and life cycle stages of parasites are defined in the sec- periodic acid-Schiff (PAS) reaction for staining of poly- tions describing specific parasite groups. For addi- saccharides may be helpful for identifying a variety of tional glossaries of parasitological and histopatho- parasites (Table 1). Gram staining or acid-fast staining logical terms, the reader is referred to texts such as (Ziehl-Neelsen) can be used to stain Microsporidian Chitwood & Lichtenfels (1972), Lom & Dyková (1992), and Myxosporidian spores in tissue sections or smears. Bruno & Poppe (1996), Kent & Poppe (1998), and The latter technique is also used for observation of coc- Hoffman (1999). cidian oocysts. A combination of H&E-stained sections and other stains can help to identify some parasite structures. For example, to observe various features of TECHNIQUES amoebae in tissue sections, H&E, Feulgen, and Giemsa stains are often used. Some selective staining methods Fixation. Tissues begin to undergo autolysis rapidly have been developed, such as the method described after death and rapid fixation is essential. Small fish, by Sterba et al. (1989) for selective staining of hooks less than 2.0 cm in length, can be preserved whole in a of echinococci, cysticerci and tapeworms in histologi- fixative such as 10% buffered formol saline, providing cal sections. an abdominal flap has been cut and removed. This Immunohistochemical staining procedures and nucleic practice allows the fixative to penetrate the tissues. acid probes for in situ hybridisation have been devel- Individual tissues from larger fish must be dissected oped for specific identification of some economically using a scalpel or pointed scissors and forceps, and important fish parasites (see e.g. Bartholomew et al. Bruno et al.: Identification of protozoan and metazoan parasites 3 Table 1. Examples of special stains used for identification of parasites in histological sections. PAS: periodic acid-Schiff Stain Parasite group Feature Source Acetic acid /Alum carmine Platyhelminthes Acanthocephala Alcian blue/PAS Neoparamoeba