Gastrointestinal Protozoal Parasites

Basic Information

Definition

Gastrointestinal protozoal parasites that affect fish include Hexamita, Spironucleus, and Cryptobia, all of which are flagellated protozoans.

Synonyms

• Hexamita: three species reported to affect fish: H. salmonis, H. truttae, and H. intestinalis, commonly called “Hex”

• Spironucleus: Spironucleus vortens is reported to be an intestinal parasite in freshwater angelfish.

• Cryptobia: Seven species of Cryptobia have been associated with the gastrointestinal tract of fish; only C. iubilans is reported to be pathogenic and parasitic.

Epidemiology

Species, Age, Sex

• Most common in freshwater fish

• Hexamita/Spironucleus are common in cichlids such as angelfish, discus, oscars, and African cichlids. Hexamita salmonis affects salmonids.

• Cryptobia: reported in African and Central/South American cichlids; recently reported in juvenile discus

Risk Factors

• Poor environmental conditions (crowding, poor water quality) or other stressors (recent shipment/handling, poor nutrition, etc.) may predispose fish to infection.

• Other ectoparasites and endoparasites increase morbidity.

Associated Conditions and Disorders

Other intestinal parasites such as Capillaria and Camallanus may be present.

Clinical Presentation

History, Chief Complaint

• Clinical signs include lethargy, decreased appetite, weight loss, abnormal feces (white, pale, mucosy, etc.), erratic swimming, infertility, decreased hatchability of eggs, death of fry, and chronic low-level mortality.

• Angelfish may have buoyancy abnormalities and lay in lateral recumbency at the top of the water column with a distended abdomen.

• Hexamita/Spironucleus has been implicated as a potential cause for head and lateral line erosion, but its importance in this syndrome is unclear.

• Cryptobia has been implicated as a potential cause for “Malawi bloat,” but its importance in this syndrome is unclear.

• Heavy infestations can cause significant mortality.

Physical Exam Findings

Potential findings include thin/poor body condition, abdominal distention, and abnormal feces.

Etiology and Pathophysiology

• Intestinal protozoans have direct life cycles.

• Transmission likely occurs through the water; infective stages are shed in the feces (fecal/oral transmission).

• Mature trophozoites inhabit the gastrointestinal tract. Cryptobia causes granulomatous gastritis but has also been associated with systemic granulomatous disease (kidney, spleen, liver, etc.).

Diagnosis

Differential Diagnosis

Other intestinal parasites (nematodes, cestodes, etc.), other systemic diseases (mycobacteriosis, etc.), poor nutrition

Initial Database

• The diagnostic approach to a fish with protozoal enteric parasites should begin with complete history, water chemistry (temperature, salinity, pH, ammonia, nitrites, nitrates, alkalinity, dissolved oxygen), and thorough evaluation of the environment/husbandry.

• Direct observation of the fish in the aquarium or pond

• Complete physical examination

Wet mounts of the skin/gills should be performed to rule out ectoparasites.

Direct fecal examination may reveal motile trophozoites. Fecal examination should be performed only on fresh feces.

Necropsy examination with wet mount examination of intestinal contents/feces and squash preparations of the intestines is most accurate for determining the presence and degree of infection. Because trophozoites are small, they are best identified at 400× magnification. Grossly, the stomach (Cryptobia) or intestines (Spironucleus/Hexamita) may be thickened, inflamed, distended, hemorrhagic, and/or edematous.

• Spironucleus/Hexamita: Trophozoites are small (12.5-20 µm in length), flagellated, actively motile, with an ellipsoid to pear shape. Trophozoites of Spironucleus/Hexamita are often localized in the anterior intestinal lumen and therefore may not be present on fecal examination. They have six anterior and two posterior flagella. Species identification requires electron microscopy.

• Cryptobia is most commonly detected by identifying granulomas in squash preparation wet mounts of the stomach. An acid-fast stain should be performed to rule out mycobacteriosis (another common cause of granulomas in ornamental fish). Motile trophozoites are not commonly seen on wet mounts. When present, flagellated trophozoites are elongate (acute infection) to oval/teardrop-shaped (chronic infection) with a characteristic slow undulating movement. The organism has two flagella. Species identification requires electron microscopy.

Advanced or Confirmatory Testing

• Histopathologic examination is not typically required for diagnosis. However, sections of protozoans may be seen on histopathologic examination of the gastrointestinal tract and occasionally other organs (liver, kidney, spleen, etc.). Histologically, intestinal changes may vary from minimal to severe enteritis. Varying degrees of granulomatous gastritis are seen with Cryptobia infestation. Systemic granulomatous disease may also be present (liver, spleen, kidney, etc.) with Cryptobia infestation.

• Definitive species identification requires electron microscopy.

Treatment

Therapeutic Goals

• Improve environmental conditions

• Control parasites

Acute General Treatment

• Husbandry

Correcting environmental abnormalities and removing stressors (poor water quality) is a critical step in decreasing morbidity/mortality and preventing disease.

Thorough cleaning of the gravel/substrate and filter may help reduce environmental contamination.

• Medical treatments

Cryptobia

No effective treatment for Cryptobia has been reported.

Cryptobia may be able to survive intracellularly in phagocytic cells, making treatment difficult.

Sulfadimethoxine is anecdotally reported to decrease mortality but not eliminate the parasite.

Dimetridazole and 2-amino-5-nitrothiazol (additional studies are needed to determine optimal dose and duration) may help to reduce the prevalence of infestation. No significant improvement was illustrated for fish treated with nitrofurazone, primaquine, chloroquine, or metronidazole.

Hexamita/Spironucleus

The treatment of choice for Hexamita and Spironucleus is metronidazole.

Dosage: metronidazole 1.0% (1 gram/100 grams feed) in the feed for 10-14 days; metronidazole can be mixed into a gel diet or top dressed on a commercial diet.

Prolonged bath immersion: 5-6 mg/L (250 mg/10 gal [6.6 mg/L] of water); treat every 24-48 hours for 10-14 days. 50% to 75% water changes should be performed between treatments. Metronidazole can damage the biofilter.

Infestations respond best to oral treatment. The author reserves bath treatments for anorexic fish.

Recommended Monitoring

• Direct fecal examinations should be performed routinely to determine response to treatment. However, trophozoites are not always present on direct fecal examination, and necropsy/wet mounts may be necessary to accurately determine treatment response.

• Water quality should be monitored closely because some treatments can damage the biofilter.

Prognosis and Outcome

• Hexamita/Spironucleus generally responds well to metronidazole if environmental/husbandry corrections are made and infestations are caught early.

• With poor environmental conditions and heavy infestations, significant mortality may occur.

• No effective treatment for Cryptobia is known. Managing concurrent ecto/endoparasites may decrease morbidity/mortality.

Pearls & Considerations

Comments

• Protoopalina is a large, ciliate-like protozoan that is a common, nonpathogenic commensal in discus.

• Coccidia, an intracellular protozoan, can potentially cause enteritis in fish.

Prevention

• Good husbandry and water quality

• Quarantine of new fish for a minimum of 4 weeks. Some aquariums are quarantining cichlids for 60 days to prevent the introduction of Cryptobia.

• Commercial producers should periodically evaluate ornamental cichlids for subclinical infection.