White Spot Disease “Ich” in Freshwater and Marine Aquariums

By Michael A Grima (B.Sc. Hons I, GradCertVetStud)

A protozoal disease associated with the formation of small white spots/cysts on the gills and skin of marine and freshwater fish species. White spot disease, in the context of salt and freshwater environments, is not caused by the same species of protozoan, however, clinical signs of infection (symptoms) and pathogenesis (disease development) in both contexts is similar.

Causes:

• Freshwater pathogen - Ichthyophthirius multifiliis.
• Marine pathogen - Cryptocaryon irritans.

Diagnosis:

• Scrapings of gill and/or skin tissue or body slime associated with cysts can be mixed with a drop of aquarium water and mounted on a glass microscope slide and then cover-slipped to be viewed microscopically for the identification of the protozoal parasites. Magnification of 100X–400X is sufficient for optimal diagnosis.
• Ichthyophthirius multifiliis (freshwater “Ich”) – in its mature cystic stage called the “Trophont” is characterised as a large, dark and spherical ciliated protozoan with a characteristic “horseshoe” nucleus and tumbling motion when immersed in water. Immature “Tomites” are smaller, translucent and move more rapidly than mature “Trophonts”.
• Crytopcaryon irritans (saltwater “Ich”) – identification of the “Trophont” from skin, fin or gill tissue is the best means of diagnosis. The organisms appear opaque and are spherical or pear shaped and ciliated. The “Trophonts” exhibit their characteristic “rolling” motion.

Transmission:

• The transmission of the infection usually results from high stocking densities of fish in aquariums.
• Temperature is another important factor to consider in the transmission of this disease, particularly in the transmission of Cryptocaryon in the marine context. The higher the temperature, the increased the rate of lifecycle completion of the parasite.
• Fish may carry the parasite in their body slime or may be mildly infected upon purchase. Once introduced into the aquarium, they transmit the parasite readily leading to infection of the weakest fish which can result in a widespread outbreak. A quarantine aquarium is therefore useful prior to introduction to the display tank.
• Replication of “Ich” parasites is rapid and thus a seemingly minor infestation in an understocked aquarium can quickly become a major issue in a fully stocked aquarium.
• Fish infected with other pathogens—bacterial, fungal, or protozoal—are commonly co-infected with “Ich” pathogens. These other infections may be incidental to the primary “Ich” infection.

Parasitic Lifecycles:

Symptoms:

• Protozoal cystic nodules are visible to the naked eye, typically approximately 1mm in diameter. Fish often appear to have salt sprinkled on them.
• During the cystic stage, fish may display irritation behaviours such as rubbing against ornaments, rocks, or gravel. Irritation may not present during the free-swimming “Theront” stage.
• Degeneration of fins may occur in severe infestations.
• Microscopic examination often shows thickened skin epithelium and excess mucus production. Excess mucus may be visible in severe cases.
• The gills are highly susceptible due to their large surface area. Excess mucus production typically coincides with epithelial hyperplasia.
• The gill operculum is often flared due to respiratory distress; fish may have rapid, shallow respirations.
• Severe infestations may result in lethargy and decreased respiration.
• Burst cysts may form ulcers on gills and skin, predisposing fish to secondary fungal and bacterial infections.

Treatment and Prevention:

Quarantining & Temperature Adjustment

A quarantine period of at least 3–6 weeks at 24–27°C is effective. Longer periods (7–11 weeks) may be necessary. This allows encysted “Tomonts” to release infectious “Theronts”, which perish within 24–48 hours without finding a host.

In reef aquariums, quarantine involves removing all fish without altering display tank temperature. Infected fish can be placed in a well-aerated, bare-bottom quarantine tank at ~27–28°C with reduced salinity (1.015–1.020).

Osmotic Challenge / Hyposalinity

(MARINE)
Hyposalinity: Maintain salinity at 1.015 for 21–30 days in quarantine. Reduce salinity gradually (0.005 per day). Combine with temperature adjustment.

(FRESHWATER)
Hypersalinity: Add 3 teaspoons (15 g) pure sea salt per gallon (3.79 L) to achieve 1/10 salinity of seawater. Introduce gradually over several hours. Treat for one week, then gradually return to normal salinity.

Copper Dosing

The minimum copper treatment period is 3–6 weeks. Maintain 0.15–0.20 mg/L free ionic copper (Cu²⁺), increased gradually over 2–3 days. Monitor copper levels daily. Copper sulfate pentahydrate contains 25.5% active copper.

*IMPORTANT:* Copper should only be used in freshwater and marine FISH ONLY aquariums. It is highly toxic to invertebrates.

Formalin

Formalin (37–40% formaldehyde) is most safely used in a well-aerated quarantine aquarium and can be combined with osmotic challenge.

*IMPORTANT:*

• DO NOT use formalin with elevated temperature—it strips oxygen from water.
• DO NOT use if gill disease is suspected.
• DO NOT use around freshwater or marine invertebrates.
• Formalin is more toxic in soft/acidic water; use a lower dose.

Turn off UV sterilizers and lights during treatment. Bath: 0.15–0.25 mL/L for up to 1 hour, repeat for 3 days. Prolonged immersion: 0.015–0.025 mL/L every 3–4 days, with partial water changes between treatments.

Malachite Green

*IMPORTANT:* Avoid in display aquariums with invertebrates. Mild antibacterial effects may require a bacterial supplement post-treatment.

Bath: 1–2 mg/L for 30–60 minutes, repeat up to 4 days. Prolonged immersion: 0.1–0.25 mg/L every 3 days for 3 treatments, then partial water change.

Chloroquine

Effective for “Ich” and Velvet Disease. Use 10 mg/L chloroquine diphosphate for 2–3 weeks in quarantine with elevated temperature and hypersalinity (freshwater) or hyposalinity (marine).

UV Sterilisation

UV is effective only against free-swimming “Theronts”, not encysted “Tomonts”. Theronts must physically pass through the UV steriliser to be destroyed.

The effectiveness of UV in marine "Ich" is not fully established. Freshwater and marine pathogens are genetically distinct and may not respond identically.