The aquatic parasite usually classified in six groups include Protozoa (Phylum Rhizopoda, Phylum Mastigophora, Phylum Ciliophora, Phylum Myxozoa, Phylum Microspora, Phylum Apicomplexa), Phylum Platyhelminthes (Turbellaria, Monogene, Digenia, Cestoda), Phylum Nematoda, Phylum Acanthocephala, Phylum Annelida and Phylum Arthropoda (Branchiura, Copepoda, Malacostraca (order Isopoda)). Because of their size are vary from microscopic size (1 µm) to macroscopic type (more than 1 meter in some species), more techniques will need to help scientist to classified them. Histological methods are becoming quite common in diagnostic methodology in aquatic animals for a long time (Klontz, 1985). From 1999 to 2012, the author collected more than 120 species of parasites from wild and cultured fish from freshwater and marine fish in many parts of Thailand; North of Thailand (Chiang Rai province, Payao province), North-east of Thailand (Nakornratchasima province, Kalasin province, Sakonnakorn province), East of Thailand (Chachoengsao province, Chonburi province, Rayong province, Trat province), Middle of Thailand (Suphan Buri province, Ratchaburi province, Phra Nakhon Si Ayutthayaprovince). The specimens from this study werefixed by fixative, dehydrated through a graded ethanol series and embedded in paraffin. Five micrometer thick sections were prepared and stained with Harris' hematoxylin and eosin (H&E).Thismethod advantage to classify the disease because in some situation parasites hide deep in the organ that can not observe by the simple diagnosis such as brain, vertebrae and heart etc.. Moreover, histological studycan explain the effect of parasite in the host and predict for chemical and drug therapy for aquatic animals more precisely.
All groups of aquatic parasiteare explained at below:
The protozoa is a vast assemblage of essentially single celled eukaryotic organisms . Protozoans can be ectoparasites or endoparasites depending on their species. Protozoan ectoparasites are the most common parasites encountered in cultured fish . They are also frequently found in wild fish . Ciliates and flagellates feed on the most superficial skin layer . Parasites cause a reactive hyperplasia of the epithelium and increased mucus production. Hyperplasia appears as a cloundiness to the skin and leads to hypoxia if occur on the gills .Microspora are intracellular and affect a wide variety of vertebrates and invertebrates, Myxosporea are largely intercellular and infect mainly fish .
2.1. Phylum Rhizopoda(amoeba)
Amoeba can infect on the external surface such as gill, skin and internal organ as intestine.
2.2. Phylum Mastigophora (flagellated protozoa)
Aquatic parasites in this subphylum separated in two classes; Phytomastigophora and Zoomastigophora
Parasites of order Kinetoplastida have one or two flagella such as
Parasites of order Retortamonodida are possessing two to four flagella, one turned posteriorly such as genus
Parasites of order Diplomonadida have one to four flagella and two-fold rotational or bilateral symmetry such as
2.3. Phylum Ciliophora
Members of this phylum have cilia in at least one stage of the life cycle. They always have two types of nucleus; micronucleus and macronucleus. Most of fish parasites can separate in three classes; Kinetofragminophorea, Oligohymenophorea and Prostomatea.
Members of class Kinetofragminophorea have oral ciliature slightly distinct from body ciliature such as
Oral apparatus of the members of class Oligohymenophorea usually well defined and oral ciliature is distinct from somatic ciliature such as
Trichodinid species (
The peritrichous ciliate,
2.4. Phylum Myxozoa
Members of this phylum have spores of multicellular origin, with one or more polar capsules and valves . Classification is based on spore morphology . They are divided into two orders; Bivalvulida and Multivalvulida . Spore of members of order Bivalvulida have two valves such as
Parasites in this group settle in muscles and bones of fish hosts lead to fibroplasia and vertebral deformities. In muscle, dermal or sub-dermal cysts are reported. A few myxosporean genera develop intracellularly in skeletal muscle, while others have some development stages that are intracellular .
The pathology of
Myxosporidiosis associated with the genera
2.5. Phylum microspora
The phylum Microspora is comprised of unicellular organisms living as intracellular parasites in a variety of invertebrates and in all five classes of vertebrate hosts . Their extrusion apparatus, always with a polar tube and cap . Microsporidia are currently classified on the basis of their ultrastructural features, including size and morphology of the spores, number of coils of the polar tube, developmental life cycle and host-parasite relationship .
Some species infect the muscle, dermal or subdermal cysts are seen and often rupture through the skin. Some species form cysts within the intestinal wall such as
Many species of microsporidians do not induce hypertrophy of the myocyte but rather replicates within the sarcoplasm, eventually occupying and destroying it such as
The aquatic parasite microsporideans, such as
2.6. Phylum Apicomplexa
Members of this phylum have a unique organelle, the apical complex, visible only with the electron microscope . They are intracellular parasites, the apical complex serves to assist in penetrating the host cell . The genus
3. Phylum platyhelminthes
Members of phylum Platyhelmintheshave a dorsoventrally flattened, bilaterally symmetrical body. This phylum is the first phylum which exhibitthreeprimary germ layers; ectoderm,mesoderm and endoderm. Aquatic parasitology in this phylum includes four classes; Turbellaria, Monogene, Digenia and Cestoda.
Although, most of turbellarian are free-living worms but
Monogenes are common parasites of the skin and gills of both marine and freshwater fish . Monogenes feed on the superficial layers of the skin and gills, causes skin cloudiness or focal reddening resulting from excess mucus production . Large numbers of monogenes can kill small fish . Taxonomic identification of monogenes is based on the morphology of opishaptor (posterior attachment organ), mode of reproduction and presence of eye spot, etc. . However,
In skin area, epithelial hyperplasia or hemorrhage are found in monogene infected area . Heavy monogenean infections by their attachment and feeding can induce a range of histopathological changes to the epithelium , sometimes severe dermatitis with hyperplasia .
Most of monogene infect the gill area (Figure 5), a proliferation of epithelial tissue resulting in the fusion of the secondary lamellae by the attach of the opisthaptor. In some gills, this latter pathology was accompanied by the presence of blood vessel aneurysms (telangiectasis) . Histopathological change by monogene on fish gill was also studied by . In the histological sections of cultured European sea bass,
Another site of infection are reported in
Digeneans have two suckers to attach their host; oral sucker surrounding the mouthand ventral sucker (acetabulum) on the ventralsurface. The life cycle of this group consists of intermediate hosts (one or two types) and final host. They form seven different developmental stages; egg, miracidium, sporocyst, rediae, cercariae, metacercariae and adult. In Aquatic animals, sporocyst stage usually found in infected gastropod (Mollusca: Gastropoda) (Figure 6A). Metacercariae (Figure 6B) and adult may be found as parasites of fish. Metacercariae encyst Metacercariae are found in many sites throughout the host such as gill, fin, bone, muscle, eyeballs, brain, spinal cord, nervous system, intestinal peritoneum, liver, gall bladder, heart ventricle and kidney . Adult trematodes are found in skin, gill, intestine, stomach, pyloric caeca. Sometime other larval stages are reported to effect the aquatic animals such as
Many species of freshwater fishes serve as second intermediate host of digenetic trematode . Five families of digenetic trematodes metacercariae are reported from fresh water fish namely, Clinostomidae, Diplostomidae, Isoparorchiidae, Strigeidae and Heterophyidae [26-27]. Trematodes metacercariae encyst in different organs of fish such as fin, skin, gill, branchial chamber, body cavity and other internal organs (intestinal peritoneum, liver, gall bladder, heart ventricle, on the bulbus arteriosus, kidney) [28-30]. Invasions of metacercariae of numerous trematode species result in pathogenic changes occurring in different organs of the intermediate hosts affected .
Digeneans metacercariae are frequently found in the skin. Metacercaria may or may not evoke a melanin response, if they do, the parasites become a grossly visible as black spot such as
Examples of some metacercariae such as
In kidney, vast necrosis of the endoparenchymal tissue and hemorrhage from the main kidney vein near metacercaria cyst are found . In kidney parenchyma, complex granulation of connective tissue cells and blood morphotic elements are seen. Metacercaria cyst wall with connective tissue fiber layer results in congested and necrotically changed kidney parenchyma. Degeneration necrosis of the kidney tissue are reported. .
In pericardium, inflammational infiltration, destroyed structure and fragmentation of muscle fibers, necrotic lesion, extravasation of erythrocytes, cytolysis of muscle fibers and karyolysis or heteropyknosis of nuclei were found in pericardium when fish are infected by metacercariae this also include losses of muscle tissue . The degenerative and inflammational processes are associated with disturbed circulation . For more cases, metacercariae of digenean
In some case, adult
Cestode (tapeworms) body consists of three regions; scolex, neck and strobila. Cestodes do not have a digestive system
Larval stage of cestodes are also reported as parasite in fish. The larval cestode
Nematodes are invertebrate round worms, theyare elongated, cylindrical formwith unsegmented body. The diseases occur due to the adult and larval nematodes which are very common in marine fishes. The nematode parasites infect various tissues and organs of fish such as stomach, intestine, liver, gonads, visceral mesenteries, peritoneum body cavity, blood vessels, swim bladder, and connective tissues, fin, orbits of the eye and brain . Most species of nematodes in adult stage live in the alimentary canal except the family Philometridae which are found in body cavity, liver and gonads .
Nematode larvae are more harmful than adults and can penetrate into the tissues of various organs, causing severe tissue damage and destruction of the cells of the organ . Ascarididans (Order Ascaridida) are reported to parasitize fish and cephalopods . Their larval stage occurs in the internal organ or maybe digestive tracts of marine fish (Figure 8B). Pathogenesis is a result of their mode of feeding, attachment and movement or migration within the host . Ascarididans genera
In reference  reported the histological change of the intestinal wall of
5. Phylum Acanthocephala
Acanthocephala is a phylum of parasitic worms which have the presence of an evertable proboscis (Figure 9). Body of acanthocephalan consist of two part presoma (proboscis, anterior neck, and posterior neck) and metasoma (trunk). The proboscis extend through the host epithelium into the submucosa with limited hemorrhaging at the point of attachment. The lumen of the host intestine was obstructed, and compressed villi were present . Attachment point of acanthocephalans may be necrotic and can be even perforated, leading to peritonitis .In reference  reported the histopathological changes when the presoma and the anterior part of the metasoma of
In reference  observed heavy infections of acanthocephalan parasite,
6. Phylum Annelida
Annelids are eucoelomate, exhibit metamerism. They are known as segmented worms. The phylum is divided into three classes; Polychaeta, Oligochaeta and Hirudinea. Only class Hirudinea
7. Phylum Arthropoda
The body of arthropods consists of head, thorax and abdomen. They have exoskeleton and have jointed appendages, one pair to each somite, but in parasitic species, the appendages usually reduce or lost. All aquatic parasites are the members of Subphylum Crustacea.Class Branchiura, Copepoda and Malacostraca (order Isopoda) are usually found as parasites in aquatic animals.
7.1. Class Branchiura
The branchiurans are flattened crustaceans. the eyes are sessile, the antennae are very small, the abdomen is small and unsegmented. They do not have gills, most of branchiurans are parasites of fish. A known genus of fish parasite in this group is
The copepod body is short and cylindrical. The head and thorax are fused to form a cephalothorax. Parasitic copepods are increasingly serious problems in cultured fish and can also impact wild populations. Most parasites affect marine fish . Parasitic copepods attached to the host by frontal filament of chalimus larvae. Most sites of infections are gill, fin or skin but sometimes they can attach to oral cavity or eye. Such as Greenland sharks had copepods embedded in both corneas . Histopathological change of fish infected by copepod are epithelial erosions, ulceration around the site of attachment of the parasite’s mouth organs, hemorrhages and around the penetration sites of the claws there occur tissue necrosis, proliferation happens around the site of penetration of antennae . Some copepods may be deeply embedded within the skin and elicit host response, mainly localized in mild dermal fibrosis and epidermal hyperplasia .
7.3. Order Isopoda
Isopod predominantly attach to the body or fins of the fish, although some have been discovered inside gill chambers, buccal cavities and body pouches [51-53]. They occur in marine, estuarine and freshwater habitats, especially in the near shore coastal environment [52,54]. Single isopod can cause damage with their biting and sucking mouth parts. Heavy infestations of parasitic juveniles can kill small fish when they first attach .
Many genera of isopods are reported as parasite of fish both teleost and elasmobranch such as
8. Phylum Mollusca
The larvae of fresh water bivalve molluscs
Although the first step of identifying parasitic specimen is the direct wet mount but in some case the parasites are difficult to distinguish grossly, histology has a role in helping to identify intracellular parasite. Tissue stains not only use for the identification of the parasite but also in the visualization of cellular morphology .Moreover, histopathology can use for examining each organ system for tells tale changes due to pathogenic agents and confirm the diagnosis .
The author would like to thank Associated Professor Dr. Chalor Limsuwan, Mr. Montri Sumontha and Dr. Cheewarut Printrakool for provide their own picture for this chapter and also thanks to Assistant Professor Siriwan Khidprasert for provide fairy shrimp specimens.
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