Flagellate protozoa may have direct life cycles; some have resistant cyst stages. The hemoflagellates have indirect life cycles.
Ichthyobodo necator (also called Costia necatrix) in freshwater and marine fish attaches to skin and gills, feeding on host cells and causing epithelial hyperplasia and goblet
cell destruction. Amyloodinium species is a common dinoflagellate parasite in tropical marine fish that affects both teleost and elasmobranchs, such as
sharks and rays. It is detected by identifying the trophonts in wet mount preparations of the skin or gills. Piscinoodinium species, the freshwater counterpart to Amyloodinium species, contains chlorophyll and causes velvet disease, or rust disease, in tropical pet fish. Fish with heavy infestations
exhibit a rusty or yellow sheen to the affected skin. Hexamita and Spironucleus species in freshwater and marine fish are flagellate protozoa in the gastrointestinal tract of fish that can cause anorexia,
lethargy, and death. Fish exhibiting scant, mucoid feces should be examined for a possible intestinal flagellate infestation.
Massive systemic infections, especially with Spironucleus species, are lethal (Figure 11). Other flagellates, such as Cryptobia species, can also be important pathogens. Trypanosome infections are usually asymptomatic, and their pathogenesis is unknown.
Trypanosomes are incidental findings in blood films or in imprints of tissues such as the kidneys.
Figure 10: Epistylis species is a stalked bell-shaped ciliate protozoan whose stalks tend to coil during contractions in a wet mount (100X, phase
Other protozoal parasites include the microsporidia, such as Plistophora hyphessobryconis in freshwater fish that causes neon tetra disease.10
Myxosporidia species are highly pathogenic, usually intracellular, and involve all organs (Figures 12A & 12B).
Protozoal infestations are usually treated topically with medicated dips or baths. The distinction between a dip and a bath
may vary among authors as does the preference for dosages, but in general a dip is exposure to a medicated solution for less
than 15 minutes, whereas a bath is for a longer period. Prolonged immersion treatments are those that provide a constant exposure
to the medication over several days. Parasitic protozoa can be treated with formalin (0.125 to 0.250 ml 37% formaldehyde/L)
as a one-to 60-minute bath; malachite green (0.1 to 0.15 mg/L) as a prolonged immersion; salt (10 to 30 g/L) as a bath up
to 30 min (a four-to five-minute salt solution dip for freshwater fish or freshwater dip for marine fish); or formalin (0.02
ml 37% formaldehyde/L) plus malachite green (0.1 mg/L) as a prolonged immersion.1,2 Formalin solutions should not be used if they contain white paraformaldehyde precipitates.1
Figure 11: A wet mount teaming with highly motile hexamitid (Hexamita or Spironucleus species) protozoa (5 to 11 µm long) in an intestinal sample from a gourami exhibiting marked weight loss (1,000X).