Stephen A. Smith Fish can serve as definitive, intermediate, or paratenic (transport) hosts in the life cycle of many species of protozoan, metazoan, and crustacean parasites. Most of these parasites can be readily identified grossly or microscopically, and as with mammalian parasites, the correct identification and an understanding of their life cycle are important in the prevention or management of an outbreak of disease due to parasites. Protozoan parasites probably cause more disease in both ornamental and cultured fish than any other group of parasites. An example of a common protozoan disease in fish is white spot disease, or “ich,” caused by Ichthyophthirius multifiliis in freshwater fish or by Cryptocaryon irritans in marine species. Other protozoan parasites that commonly occur on fish include Tetrahymena spp., Trichodina spp., Trichophyra spp., Amyloodinium spp. and Ichthyobodo spp. Metazoan parasites can be found as larval or adult forms in almost every tissue of fish. Most can be grossly identified as monogeneans, digenetic trematodes, nematodes, cestodes, acanthocephalans, or crustaceans, but specific identification generally involves special staining techniques or clearing of specimens. Examples of common fish helminths include monogeneans on the gills and skin; larval digenetic trematodes (metacercariae) in the eyes, skin, musculature, and abdominal cavity; larval cestodes and nematodes in the visceral organs and abdominal cavity; and an assortment of adult trematodes, nematodes, cestodes, and acanthocephalans in the lumen of the gastrointestinal tract. In addition, a number of arthropod parasites and leeches can be found occurring on or attached to the skin and fins of fish. A variety of nonlethal techniques that include skin, fin, and gill biopsies have been developed for the diagnosis of the common external parasites of fish. Most of these biopsy techniques can be performed on live fish without the use of anesthesia, although light sedation often simplifies the procedure and makes it less stressful for the fish. Fig. 6.1 Collection of mucus sample via a skin biopsy from the side of a fish for external‐parasite examination. The skin is the primary target organ for many of the external fish parasites. Therefore, a biopsy of the skin (Fig. 6.1) is one of the most useful and common samples for diagnosing ectoparasitic problems. This biopsy is performed by gently scraping a small area on the surface of the fish with a scalpel blade or the edge of a microscope coverslip in a cranial to caudal direction. Care should be taken to use only a minimal amount of pressure to obtain this superficial scraping, since damage to the skin may result in secondary bacterial and fungal infections or osmoregulatory imbalance in the fish. The mucus from the skin scraping should be transferred immediately to a drop of aquarium, tank, or pond water (either fresh or salt water, depending on the species of fish, but not city tap water or distilled water) on a glass microscope slide and a coverslip carefully applied. This wet mount should then be examined under the compound microscope for the presence of free‐swimming, attached, or encysted protozoan or metazoan parasites. A fin biopsy (Fig. 6.2) is obtained by snipping a small piece of tissue from the peripheral edge of one of the fins. This procedure is more traumatic to the fish than a skin biopsy, since a physical wound is produced. The fin snip should be transferred immediately to a drop of aquarium, tank, or pond water on a glass microscope slide, spread to its full extent, and a coverslip carefully applied. This wet mount should then be examined under the compound microscope for the presence of protozoan or metazoan parasites. A gill biopsy (Fig. 6.3) is obtained by inserting the tip of a pair of fine scissors into the branchial cavity behind the operculum (gill cover) and cutting off the distal ends of several of the primary gill lamellae attached to the gill arch. Since only the tips of the primary lamellae are removed, minimal bleeding should occur. The gill tissue should be transferred immediately to a drop of aquarium, tank, or pond water on a glass microscope slide; the individual lamellae separated; and a coverslip carefully applied. This wet mount should then be examined under the compound microscope for the presence of protozoan or metazoan parasites. Fig. 6.2 Collection of a fin biopsy by clipping a small portion of the distal tip of the pectoral fin. Fig. 6.3 Collection of a gill biopsy by lifting the operculum (gill chamber cover) and removing the distal tips of several filaments (lamellae) of the gill. Examination of fish feces for the presence of internal parasites is accomplished with the same techniques as those used for mammals and birds. A fresh fecal sample is collected with a pipette either from the bottom of the aquarium or as it hangs from the vent of the fish. If an appropriate sample cannot be acquired from the environment or if examination of a specific individual is desired, the application of gentle pressure on the sides of a netted fish often produces the desired sample. The fecal specimen is then processed by standard flotation, sedimentation, or direct smear techniques and evaluated for the presence of parasite eggs and larvae. Though specific parasite identification is generally impossible, fecal examination does provide useful information about the types of parasites (nematodes, trematodes, cestodes, acanthocephalans) that may be present in a fish. PARASITE: Ichthyophthirius multifiliis (Figs. 6.4 and 6.5) Common name:“Ich” or freshwater white spot disease. Taxonomy: Protozoa (ciliate). Geographic Distribution: Freshwater fish worldwide. Location in Host: Within the surface epithelial layer of the skin, fins, and gills. Life Cycle: This parasite has a direct life cycle, with free‐swimming, ciliated tomites (theronts) in the water invading the skin, fins, and gills of fish. The tomites penetrate into the epithelial tissues and form large, feeding trophozoites (trophonts) that eventually excyst from the host and enter the water, where each develops into a cyst. The cyst stage then undergoes multiple divisions, producing numerous infective tomites that are released to the environment to infect other fishes. Laboratory Diagnosis: This large, holotrich ciliate has a characteristic C‐shaped macronucleus and is detected in wet mounts of skin biopsies and gill and fin snips. Clinical Importance: The parasite causes small, raised, white lesions on skin, fins, and gill tissue. Penetration into and excystation out of the epithelial tissue by the parasite causes loss of integrity of external tissues and results in disruption of normal homeostatic osmoregulatory processes. PARASITE: Cryptocaryon irritans (Fig. 6.6) Common name:Marine white spot disease. Taxonomy: Protozoa (ciliate). Geographic Distribution: Marine and brackish water fishes worldwide. Location in Host: Within the surface epithelial layer of the skin, fins, and gills. Life Cycle: The life cycle is similar to that of freshwater Icthyopththirius multifiliis:free‐swimming, ciliated tomites in the water invade the skin, fins, and gills, penetrate into the epithelial tissues, and form large, feeding trophozoites that eventually excyst from the host into the water and develop into cysts. The cyst stage then undergoes multiple divisions, producing numerous infective tomites that are released to the environment to infect other fishes. Laboratory Diagnosis: This holotrich ciliate, which does not have a C‐shaped nucleus like I. multifiliis, is detected in wet mounts of skin biopsies and gill and fin snips. Clinical Importance: Cryptocaryon irritans causes small, raised, white lesions on the skin, fins, and gill tissue. Penetration into and excystation out of host epithelial tissue by the parasite causes loss of integrity of external tissues and results in disruption of normal homeostatic osmoregulatory processes. Fig. 6.4 Ichthyophthirius multifiliis trophozoite. This ciliate, which produces small, raised, white lesions on the skin, fins, and gills of fish, is commonly called “ich” or “white spot disease.” This freshwater parasite varies in size, ranging from 100 to 1000 μm, depending on the stage of maturation, available nutrition, and host species. Fig. 6.5 Encysted trophozoite of Ichthyophthirius multifiliis embedded in the epithelial tissues of a gill filament of a goldfish (Carassius auratus). Fig. 6.6 Cryptocaryon irritans trophozoite. This ciliated parasite, which has a similar life cycle to Ichthyophthirius multifiliis, occurs in marine fish species. PARASITE: Tetrahymena spp. (Fig. 6.7) Taxonomy: Protozoa (ciliate). Species include T. corlissi and T. pyriformis. Geographic Distribution: Freshwater fish worldwide. Location in Host: Trophozoites are generally found on surface epithelial layers of the skin and fins but may occasionally be found in deeper skin tissues, muscle, and abdominal organs. Life Cycle: This parasite forms reproductive cysts in the freshwater environment in which two to eight infective tomites are produced. Laboratory Diagnosis: Small, cylindrical‐ to pyriform‐shaped ciliates can be found in wet mounts of skin biopsies. Clinical Importance: This facultative parasite can become histophagous and invade the skin, muscle, and internal organs causing osmoregulatory problems and body system dysfunction. PARASITE: Uronema spp., including U. marinum (Fig. 6.8) Taxonomy: Protozoa (ciliate). Geographic Distribution: Marine and brackish water fishes worldwide. Location in Host: Surface epithelial layers of skin and fins of marine fish, but may also invade deeper skin tissues, muscle, and abdominal organs. Life Cycle: This parasite (similar to freshwater Tetrahymena spp.) forms reproductive cysts in the marine environment in which several infective tomites are produced. Laboratory Diagnosis: Small, cylindrical‐ to pyriform‐shaped ciliates can be found in wet mounts of skin biopsies. Clinical Importance: This facultative, histophagous parasite (similar to freshwater Tetrahymena spp.) can invade the skin, muscle, and internal organs, causing osmoregulatory problems and body system dysfunction. Fig. 6.7 Trophozoites of Tetrahymena spp. in a skin biopsy of a hybrid striped bass (Morone saxatilis × M. chrysops). This small ciliate also infects many species of ornamental aquarium fish. Fig. 6.8 Tissue section showing numerous trophozoites of Uronema spp. invading the deeper tissues of a summer flounder (Paralichthys dentatus) species. This marine organism causes pathology similar to that of Tetrahymena spp. of freshwater species of fish.
CHAPTER 6
Parasites of Fish
TECHNIQUES FOR RECOVERY OF ECTOPARASITES
Skin Biopsy (Mucus Smear)
Fin Biopsy (Fin Snip)
Gill Biopsy (Gill Snip)
RECOVERY OF ENDOPARASITES
PARASITES OF FISH
Size:
Trophozoite in tissue
up to 1 mm in diameter
Tomite in water
25–50 × 15–22 μm
Size:
Trophozoite in tissues
up to 1 mm in diameter
Tomite in water
25–50 × 15–22 μm
Size:
Trophozoite
55 × 30 μm
Size:
Trophozoite
50 × 30 μm
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