Cryptocaryon irritans is an ectoparasitic protozoan responsible for white spot disease, posing a significant threat to mariculture systems in Vietnam. A research team from the Faculty of Fisheries, Vietnam National University of Agriculture carried out a study to isolate and propagate C. irritans, and to establish an experimental infection model in Trachinotus spp. (5-7cm in length) under laboratory conditions.

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Note: (A) Adult trophont (“white spots”) visible on the host; (B) Trophonts leave the fish body, become tomonts, and form tomocysts; (C) Tomites reproduce within the tomocyst; (D) Free-swimming theronts emerge from the tomocyst and seek a new host.

Figure 1. Life cycle of C. irritans

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 Figure 2. Schematic of C. irritans isolation, propagation, and infection protocol
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 Note: Mean infection intensity (trophonts/fish) in Quang Ninh (A graph with lines and dots AI-generated content may be incorrect.; n = 49) and Khanh Hoa (A graph with lines and dots AI-generated content may be incorrect.; n = 57); and Infection prevalence in Trachinotus spp. from Quang Ninh (A graph with lines and dots AI-generated content may be incorrect.) and Khanh Hoa (A graph with lines and dots AI-generated content may be incorrect.)

Figure 3. The prevalence and intensity of Cryptocaryon irritans infection on Trachinotus spp. in Quang Ninh and Khanh Hoa provinces

Between February and June 2025, a total of 106 fish samples were sourced from aquaculture operations in Quang Ninh and Khanh Hoa, revealing an average C. irritans infection rate of 59.43%, with the highest prevalence observed during March and April. The method for infection and propagation of C. irritans on golden pompano (Trachinotus spp.) juveniles (5-7cm in length) was successfully established, demonstrating the ability to maintain a viable parasite line under laboratory conditions in Vietnam. From 106 fish samples collected from hatcheries and marine farms in Quang Ninh and Khanh Hoa provinces between February and June 2025, 63 samples (59.43%) were positive for C. irritans, with the highest prevalence recorded during the early stocking months (March-April).

Infected fish exhibited clinical signs including skin darkening, mucus loss, white spots on the skin and gills, fin erosion, ulcerations, and tissue damage. The parasite was successfully cultured at the trophont stage and was propagated to the tomont and theront stages at 28-30°C, achieving a conversion efficiency of 61.7% after 48 hours and yielding an average of 150-300 theronts per tomont. These theronts were introduced to Trachinotus spp. at a density of 5,000 theronts per 250L tank (50 fish) at a temperature of 28°C. The infection progressed rapidly, with trophonts observed within 2-3 days and cumulative mortality reaching 100% by day seven.

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Note: (A-C) Hemorrhagic ulceration on the skin and gills (indicated by red circles); (D) Skin darkening and fin erosion; (E-F) White spots on skin and gills (indicated by blue arrows).

Figure 4. Clinical signs observed in Trachinotus spp. infected with C. irritans
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Note: (A) Isolated trophont transitioning into the protomont stage (18-24h post-isolation); (B-G) Tomont division into multiple daughter cells (tomites); (H) Mature cyst rupture releasing theronts after 3-5 days; (I) Free-swimming theront with elliptical shape and active movement via cilia, seeking a host. (40×magnification; Scale bar = 10µm)

Figure 5. The developmental stages of Cryptocaryon irritans
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 Figure 6. The experimental infection of C. irritans to Trachinotus spp. (A) Infection intensity; (B) Survival rate of Trachinotus spp. after infection

The established infection model offers a robust platform for further research in epidemiology, immunology, and therapeutic interventions against C. irritans.

For the full paper, please visit https://doi.org/10.31817/vjas.2025.8.4.04.

 Vietnam Journal of Agricultural Sciences