Metazoan Parasites CHARACTERISTICS OF METAZOAN PARASITES IN TISSUE SECTIONS
David Bruno FRS Marine Laboratory Aberdeen, Scotland
This report was prepared for the EAFP histopathology workshop in Dublin, September 2001 and is designed to assist histologists to identify metazoan parasites in tissue sections.
Metazoan Parasites: Introduction Metazoans are multicellular organisms and can be found in all organ systems. During necropsy or biopsy procedures these parasites can be provisionally identified as they usually look different than the tissue and they move, however in tissue sections identification requires some knowledge of morphology and life cycle. In this review the following groups of parasites will be examined: trematodes, cestodes, nematodes, acanthocephalans, molluscs and arthropods each with examples and notes on each group to help in diagnosis.
The examination of haematoxylin and eosin (H&E) stained sections provide clues for diagnosis and is based upon tissue reaction to a suspected infection. Tissue sections may contain areas of consolidation, necrosis, inflammation, encapsulation, granuloma or repair associated with parasitic infection. Infective agents often cause typical tissue responses that may vary as the infection progresses from acute to chronic or disseminated phases. When considered with relevant clinical information, the histological features may provide sufficient information to confine the search to a particular type of organism or even a specific entity. Each metazoan group has characteristics in common with other groups, but they possess some unique features that enable a reasonable diagnosis to be made. With H&E staining, organisms often show basophilic staining characteristics, in contrast to viral inclusion bodies that are often eosinophilic. Larger parasites may be readily visible at low magnification and show differential staining of their structures and ova. More often, however, routine staining or poor fixation merely provides a diagnostic puzzle that may only be resolved by the use of special staining techniques, designed to demonstrate types of organisms and occasionally specific entities or further sections to reveal diagnostic features.
Within the metazoa many organs are hollow e.g. intestines, and many parasites do not have the ability to attach to those tissues. Thus, parasites maybe lost from tissues during histological processing and therefore it is important to correlate what is seen grossly with what is seen histologically. Many sections of parasites do not allow identification since salient features, needed in the identification process, are absent. Cross sections are usually preferable for study, although serial sections can be requested to gain more information.
The body cavity may be present or absent in parasites. If it is lacking, the parasite is said to be acoelomate. If present, it may be a coelom or a pseudocoelom (however the latter terms cannot be differentiated in histological sections). To identify the body cavity the muscles should be located. These structures stain eosinophilic with haematoxylin and eosin. If muscles are present they usually support a body wall. Thus, if there is a body wall, there should be an associated cavity. Overall the characters used to identify whole parasites are different from those used to identify those in sections. In sections the most useful features are integument, musculature, digestive system, body cavity, reproductive systems and special structures.