Hexamita spp and Spironucleus spp are common, small (~9 µm), bilaterally symmetric, flagellated (4 pairs) protozoa most frequently found in the intestinal tract and occasionally in skin lesions or degenerating soft tissues of finfish.
These genera are similar but differ slightly in the position and shape of their nuclei (2 within 1 organism). Pathogenicity of these organisms is variable and correlated with the number present. If there is a question as to whether treatment is warranted, the number of organisms present can be assessed in a wet mount of intestine. If <5 organisms per low-power field (LPF) are present, treatment is probably not necessary; if 5-15 organisms/LPF are present, treatment should be administered if fish are in poor condition; and if >15 organisms/LPF are present, treatment is strongly recommended.
The only treatment available for hexamitiasis is metronidazole (use only in ornamental species), which should be given orally but can be administered as a bath if fish are anorectic. The number of organisms present in infested freshwater angelfish increases dramatically after shipping and handling. Chronic problems have been seen in fish maintained in unsanitary or crowded conditions. Preventive treatment of ornamental cichlids is recommended before shipping, and broodstock should be evaluated periodically.
The hatchability of eggs from heavily infested adult angelfish (freshwater) seems to be significantly decreased; resultant fry may be weak with poor longterm survival.