Parasites are typically introduced by adding new fish or plants without quarantining them, through shared equpment or droplet spread or from contaminated source water.

Symptoms include:

• Flashing - a quick dive down to the gravel (or an object), quickly turning sideways and rubbing against the object as if to scratch an itch
• Flicking the fins as if trying to shake something off
• Yawning
• Gill fluffing (rapidly opening and closing the mouth and gills)
• Hiding
• Darting (while looking like something's after them)
• Scale loss
• Increased mucus production, may look milky
• Gasping or piping at the surface
• Lethargy
• Ulcers
• Weight loss

• Use a glass slide and gently scrape from front to back behind the fins, at the base of the tail or under the chin and belly.
• Place the mucus from the scrape on a slide and add a drop of tank water.
• Place the cover slip on the sample by setting down one edge and then lowering the other side of the cover slip. Press down gently with your fingernail.
• Start with the lowest magnification and scan the slide for movement.

• Obtain a fresh sample of poop. It is important not to use poop that's been sitting around as you are looking for creatures from inside the fish, not from the tank.
• Place the poop on a slide and add a drop of tank water.
• Use the cover slip to mush the poop up in the water.
• Place the cover slip on the sample by setting down one edge and then lowering the other side of the cover slip. Press down gently with your fingernail.
• Start with the lowest magnification and scan the slide for movement.

Gill biopsy

• Sedate fish and use a small pair of scissors to snip the ends of a few primary lamellae.
• Place the gill filaments from on a slide and add a drop of tank water.
• Place the cover slip on the sample by setting down one edge and then lowering the other side of the cover slip. Press down gently with your fingernail.
• Start with the lowest magnification and scan the slide for movement. Also look for clubbing, hyperplasia, hypertrophy, necrosis, lamellar fusion, and telangiectasia.

Ciliated Protozoans

Ich (Ichthyophthirius multifiliis) is the most common protozoan disease. Ich is a large parasite, entirely covered in cilia, moves in a slow-rolling motion, and has a C- or horseshoe-shaped nucleus. The parasite life cycle is direct . Only the free-swimming infective stage (theront) is susceptible to treatment. White salt-like spots on the fish are the encysted feeding stage (trophonts). Trophonts break through the skin and become encysted tomonts with sticky capsules that attach to objects in the environment, including gravel, nets, plants, etc. These tomonts divide, producing tomites that break through the nodule wall to releasing motile, infective theronts. At 25C (77F), infective theronts have 48 hours to find a new host or they die. After finding a host, the theront penetrates the skin and the cycle starts again. The ich life cycle is temperature dependent. and lasts 3 to 6 days at 25C (77F), and 10 days at 15C (59F). Symptoms: white, raised nodules that look like grains of salt (up to 1mm) on the skin and gills,. Symptoms of irritation to include fin flicking, flashing, increased mucus production,etc. The gills are also affected and injury from ich can result in symptoms related to oxygen seeking (gasping, piping, gill huffing, rapid respiration, etc) and issues with osmoregularion (excess fluid, dropsy, eye rings). Severe infections may result in lethargy, difficulty breathing, secondary bacterial or fungal infections, and osmoregulatory compromise due to the skin and gill damage caused by the parasite.

Trichodina and Trichodinella are other cilated parasites that affect the skin and gills. Trichodina is a circular, ciliated parasite with a prominent internal denticular ring that is very active, and can be seen via a microscope rotating, and scooting, erratically around the slide. They typically feed off organic debris in the tank. The life cycles are direct and they reproduce by binary fission. Symptoms: signs of external irritation (flashing, increased mucus production giving a cloudy appearance to the skin,) skin hemorrhages, frayed fins and tail, lethargy, and chronic low level mortalities. With severe gill infestations, respiratory signs may be present (gasping, piping, gill huffing, rapid respiration, etc).

Chilodonella is a flattened pear or an onion shaped ciliated protozoan that moves in a gliding motion or circular motion. It can survive in brackish water. Symptoms include: respiratory distress (gasping, piping, opercular flaring, increased respiratory rate) from hyperplasia and fusion of the lamellae, clamped fins, a ragged appearance to the skin, excess mucus production. Severe infestations may result in skin ulcers, lethargy and death.

Tetrahymena a ciliated parasite that causes external (skin and gill) lesions and internal, systemic infections. It feeds off debris in the aquarium. Symptoms include: small, white patches on the skin, skin hemorrhages, necrosis and sloughing of the skin, and and gill aneurysms. Systemic infections can result in lack of appetite and lethargy. Muscle swelling, lesions around the eyes and keratitis.

Sessile Ciliates

Epistylis, Capriniana piscium, and Ambiphyra are common and are secondary to skin damage. Symptoms: Epistylis -white, fluffy lesions on fins and tail margins, edges of gill cover, and inside the mouth. Capriniana prefers the gills and can cause severe respiratory distress by mechanically blocking the gill tissue.

Flagellated Protozoans

Amyloodinium ocellatum (Velvet) has a life cycle similar to Ich where only the free-living dinospore is susceptible to treatment. This parasite affects the skin and gills.  Symptoms include:  Signs of irritation (e.g. flashing, fin flicking, darting, gill huffing, etc), very small bumps that look like a yellow sheen,. Heavy infestation can result in gill damage to include swelling, hyperplasia, inflammation, hemorrhage, and necrosis of the gill filaments that results in problems maintain fluid balance and oxygen seeking behavior.

Ichthyobodo (formerly called Costia), is a very small, flagellated parasite about the size of a red blood cell , has a direct lifecycle and can survive in temperatures from 2C to 30C. On wet mount Ichthyobodo moves in an erratic spiraling motion. Symptoms: skin irritation, heavy mucus production, flashing, severe respiratory distress, lethargy, anorexia, and death. Deaths may occur before any clinical signs are evident.

Hexamita, Spironucleus, and Cryptobia typically affect the gastrointestinal tract. For Spironucleus or Hexamita, trophozoites are small (12.5–20 mm long) ellipsoid to pear shaped, flagellated (six anterior and two posterior flagella), and are very motile. Trophozoites of Spironucleus or Hexamita are often localized in the anterior intestinal lumen and may not be present on fecal examination. Symptoms include: severe weight loss, lack of appetite, lethargy, abdominal distension, mucoid or pale feces, pop-eye, darkening of the skin, buoyancy disorders, and redness at the vent.

Microsporidians, Coccidia, Cryptosporidium, and Myxozoans

Microsporidians are intracellular parasites with a direct life cycle. Pleistophora causes ‘‘neon tetra disease" that affects affects angelfish, rasboras, barbs, and tetras. Symptoms include: muscle wasting, erratic swimming, color loss, lethargy, secondary bacterial infections when cysts rupture. Cryptosporidium Symptoms include: anorexia, food regurgitation, undigested food in the feces, and weight loss. There are no reported effective treatments for Microsporidians; however, toltrazuril (Baycox Bayer Animal
Health) has shown some efficacy in experimental conditions.

Myxozoans are spore producing parasites with complex life cycles. Hoferellus is a freshwater, myxozaon that causes polycystic kidney disease in koi and goldfish. There is no effective treatment for Myxozoans infections although fumagillin (used to treat a Microsporidean disease caused by Nosema apis in honeybees) and malachite green have been tried.

Monogeneans (flukes)

Flukes are very common in marine and freshwater fish. In freshwater fish, the primary species seen are Dactylogyrus and Gyrodactylus which affect the skin and gills. Flukes predispose fish to secondary ulcers, bacterial diseases, and eventual osmoregulatory compromise due to skin damage created by thei wounds they create by attaching to the skin and gills. Dactylogyrus has two to four prominent anterior eyespots and a four-pointed anterior end. Gyrodactylids have no eyespots, and you can often see an embryo inside the fluke. On a wet mount slide they can be seen stretching and compressing their body. Symptoms include: flashing, rubbing, gasping, lethargy, yawning, clamped fins, excess mucus production, secondary skin ulcerations, and scale loss.

Nematodes

Nematodes are smooth, long worms that affect the intestinal tract. Eustrongyloides, can cause cysts to form in the liver, muscles, and peritoneum. Camallanus has a direct life cycle. Symptoms: red worms protruding from the vent. Eggs with bipolar plugs can be seen on wet-mounts of fresh feces. Symptoms include: anemia, lethargy, poor weight gain, failure to thrive, and reproductive problems. Treatment of nematodes: fenbendazole (25–50 mg/kg in food) or levamisole (1–2 mg/L bath 24h).

Cestodes

Bothriocephalus acheilognathi (Asian tapeworm)is a segmented flatworm that lives in the GI tract. Symptoms include: lethargy, lack of appetite, weight loss, chronic intestinal inflammation, intestinal obstruction, and severe mucosal damage. Treatment: Oral praziquantel 50 mg/kg by mouth for one dose, or 5 to 12 gm/kg of feed every 24 hours for 2 to 3 days. Treatment is recommended to be given in a separate tank to prevent the dispersion of eggs when the cestode dies.

Crustacean Parasites

Argulus (fish louse) is a circular, flattened parasite that can be seen with the naked eye. The wounds it creates open up the fish to infection. Symptoms include: flashing, agitation, focal red skin lesions, and secondary cutaneous ulcers.

Lernaea (Anchor worms). Females attach under the skin of a fish and have two egg sacks that look like a forked "tail". Treatment: Remove carefully and dab the wound with peroxide or iodine to prevent secondary bacterial infection. Observe carefully for signs of internal bacterial infections. Chitin inhibitors such as lufenuron (0.1–0.2 mg/L prolonged immersion) and dimilin.

Summarized from:

Bacterial and Parasitic Diseases of Pet Fish

By Helen Roberts, Brian Palmeiro, and Scott Weber

Veterinary Clinics: Exotic Animal Practice, Vol 12 (2009), pp 609–638

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