Diplomonad flagellates (previously called Hexamita sp.) are among the most common infectious agents found in angel fish (Pterophyllum scalare). They are found in 50% of all angel fish examined at the University of Florida fish disease laboratory. Controversy exists as to the clinical significance of the parasite because it is commonly encountered in apparently healthy fish. Post (1987) has correlated the number of Hexamita flagellates found in the gut of infected salmonid fish with severity of clinical manifestations. Using these guidelines, level of infection was quantitated as follows: Negative No organisms seen, Slight 1 to 5 organisms/low power field, Moderate 5 to 15 organisms/low power field, Marked 15 to 30 organisms/low power field, Extreme 30 to 100 organisms/low power field.

Under laboratory conditions, fish with extreme infections but show no other clinical abnormalities. After fish are however, the number of organisms explodes within 24 hours, counts of >500 organisms/ low power field. Experience with clinical cases suggests that failure to control these infections prior to shipping may result in fatalities within one week.

In the present study, angel fish (n=30 fish/treatment) were treated with metronidazole in the food (50 mg/kg x 5 days) or by immersion bath (5 mg/L for 3 hours, repeated daily for 5 days) prior to shipping. Quantitation revealed 0 to 1 and >500 organisms/low power field in treated (medicated feed and bath) and control (non-treated) fish, respectively, at one, two, and seven days post-shipping. No mortality was observed in treated fish, and only one non-treated fish died seven days post-shipping. In addition, intestine was harvested for histologic examination from treated and non-treated fish (n=15 fish/shipping bag) before fish were subjected to a mock-shipping, and one, two, and seven days post-shipping, as well as from selected clinical cases. All tissues were fixed in 10% neutral buffered formalin prior to processing, Relative to treated fish, non-treated controls in the experimental study had slight intestinal dilatation and contained moderate numbers of bacteria and scattered diplomonads. In contrast, the major findings in clinical cases were moderate dilatation and moderate hyperplasia of the gastric mucosa, including an increase in the numbers of mucus-producing cells. These data indicate that (1) diplomonad infections may contribute to the induction of significant intestinal pathology and that (2) preventative treatment with metronidazole may be effective in decreasing the post-shipping morbidity resulting in infection in angel fish.

References

1.  Post, G. 1987. Textbook of Fish Health, second edition. TFH Publications, Neptune City, NJ pp.165-169.