Ich, also known as “Ick” or “White Spot Disease,” is one of the most common and deadly infections in freshwater fish. Caused by the parasite Ichthyophthirius multifiliis, this disease affects both aquarium fish and commercially farmed species. If you have ever wondered what this disease looks like or how to treat it, here you will find all the key information.
This article will provide an in-depth look at what White Spot Disease is, its symptoms, causes, life cycle, treatment, and prevention. Additionally, we will explore whether this disease is dangerous to humans and how you can prevent it from spreading in your aquarium or fish farm.
What is ich, white spot disease, or Ichthyophthiriasis?
White Spot Disease is caused by the parasite scientifically known as Ichthyophthirius multifiliis and commonly referred to as “Ich” or “Ick.” It is an obligate parasite, meaning it cannot survive without a living fish host (Francis et al., 2023). The parasite is highly contagious and can quickly spread throughout an aquarium or pond, leading to significant mortality if left untreated.
Ichthyophthiriasis infects a wide range of freshwater fish species in aquaculture, aquariums, and ornamental fish. The disease is prevalent year-round, thriving in a broad temperature range (11°C to 32°C), suggesting the existence of both tropical and temperate strains of I. multifiliis (Yang et al., 2023).
The name “White Spot Disease” comes from the visible white cysts that form on the skin, fins, and gills of infected fish as the parasite burrows into the tissue. However, not all “white spots” on fish indicate Ich. Other conditions, such as lymphocystis or fungal infections, can cause similar symptoms. Proper diagnosis is essential for effective treatment.
It is also important to note that there is a marine version of White Spot Disease, known as “Marine Ich,” which affects saltwater fish and is caused by the parasite Cryptocaryon irritans.
Key characteristics of Ichthyophthirius multifiliis
- Parasitic nature: Ichthyophthirius multifiliis is a ciliated protozoan parasite.
- Host specificity: Primarily infects freshwater fish.
- Life cycle: The parasite has a complex life cycle, including both free-swimming and host-attached stages.
Taxonomy of Ich
- Domain: Eukaryota
- Clade: Diaphoretickes
- Clade: SAR
- Clade: Alveolata
- Phylum: Ciliophora
- Class: Oligohymenophorea
- Order: Hymenostomatida
- Family: Ichthyophthiriidae
- Genus: Ichthyophthirius
- Species: Ichthyophthirius multifiliis
- Common name in Spanish: Ich, enfermedad del punto blanco
- Common name in English: Ich, Ick, White Spot Disease
What does Ich look like in fish?
One of the most common questions among aquarists is: “What does Ich look like in fish?” The symptoms of White Spot Disease are relatively easy to identify, making it one of the most recognizable fish diseases.
Symptoms of white spot disease in fish
- White spots: The most obvious sign is the appearance of small white spots, resembling grains of salt or sugar, on the skin, fins, and gills of the fish.
- Behavioral changes: Infected fish may exhibit unusual behavior, such as rubbing against objects (flashing), lethargy, or loss of appetite.
- Respiratory distress: As the parasite infects the gills, fish may show signs of difficulty breathing or gasping at the water’s surface.
- Clamped fins: Fish may keep their fins close to their bodies due to irritation and discomfort.
- Cloudy eyes: In severe cases, fish may develop cloudy or opaque eyes.
If you notice any of these symptoms, it is essential to act quickly to prevent the spread of the disease. However, it is important to confirm the diagnosis through a microscopic examination of skin and gill scrapes. It is crucial to distinguish between the immature forms (tomites) of Ichthyophthirius and other protozoa such as Tetrahymina, as the latter generally does not require treatment (Francis et al., 2023).
Recently, Li et al., (2023) presented an artificial intelligence (AI)-based method for the rapid detection of fish parasites, specifically I. multifiliis, Gyrodactylus kobayashii, and Argulus japonicus. Meanwhile, Bonnichsen et al., (2025) developed a real-time AI-based video system for detecting “Ich” in rainbow trout.
The life cycle of Ichthyophthirius multifiliis
Understanding the life cycle of Ichthyophthirius multifiliis is essential for effective treatment. The parasite goes through several stages, each with unique characteristics.
Stages of the Ick life cycle
Jørgensen (2017) reported that the three stages of the Ich disease life cycle are:
- Theront stage: The theront invades the fish’s skin and gills, penetrates the epidermis, and settles on the basal membrane.
- Trophont stage: It feeds on the fish’s tissue until it reaches a size of 0.5 to 1.0 mm and is macroscopically visible as a white spot. The mature trophont exits the fish and transforms into a tomont.
- Tomont stage: After feeding, the parasite leaves the host and forms a cyst on surfaces such as tank walls or substrate. Inside the cyst, it divides into hundreds of daughter cells.
This life cycle can last from a few days to several weeks, depending on water temperature and other environmental factors. Treatment should focus on the tomite stage, as this is when the parasite is most vulnerable.
How does Ich spread?
White spot disease is highly contagious and can spread through:
- Introduction of infected fish: Adding new fish to an aquarium without proper quarantine can introduce the parasite.
- Contaminated equipment: Sharing nets, siphons, or other equipment between tanks can spread the parasite.
Risk factors
- Stress: Poor water quality, overcrowding, or sudden temperature changes can weaken the fish’s immune system, making them more susceptible to infection.
- High fish density: Overcrowded tanks provide an ideal environment for the parasite to spread.
- Poor water quality: The accumulation of ammonia, nitrite, and nitrate can stress fish and promote disease. In this regard, Guang-Ran et al., (2024) found that in ponds with Ichthyophthiriasis outbreaks, nitrate concentrations were negatively correlated with the abundance of I. multifiliis genes.
- Temperature fluctuations: Ich thrives in cooler waters, but rapid temperature changes can also stress fish.
Treatment for Ich (White Spot Disease)
Treating Ichthyophthirius multifiliis requires a comprehensive approach. Below are the most effective treatments:
Increasing temperature
Raising the water temperature to 28-30°C accelerates the parasite’s life cycle, making it more vulnerable to treatments. However, Jørgensen (2017) reported that a temperature of 32°C can destroy the parasite, although its application on a large scale in aquaculture is limited.
Use of chemicals
Some of the most effective treatments include:
- Malachite Green: A classic treatment against external parasites. This chemical targets free-living stages and those within the fish’s epidermis. However, malachite green has been reported to have carcinogenic and teratogenic effects, leading to its ban in food fish in several countries.
- Formaldehyde: One of the most widely used substances in the aquaculture industry to combat I. multifiliis. It is lethal to parasites at concentrations of 50-100 mg/L for 30-60 minutes and can be tolerated by certain fish species such as rainbow trout. However, it has been classified as carcinogenic.
- Copper sulfate: Effective in killing theronts at concentrations of 1 mg/L for 13 minutes, though its toxicity depends on water parameters. Copper sulfate prevents transmission but can be toxic to fish in low-alkalinity waters and is not approved for food fish.
- Chlorine dioxide: In vitro studies demonstrated that 50 mg/L of chlorine dioxide for 1 hour killed theronts, and in vivo, a dose of 25 mg/L for 48 hours reduced parasite load by more than 50% in silver catfish.
- Compounds releasing hydrogen peroxide: Hydrogen peroxide is lethal to parasites at concentrations tolerable to fish. The median lethal concentration (LC50) at 4 hours of peracetic acid (PAA), which decomposes into hydrogen peroxide and acetic acid, against I. multifiliis theronts was 1.076 ppm for tilapia treatment (Abu-Elala et al., 2021).
Francis et al. (2023) recommend that the choice of chemical depends on water quality conditions, the fish species being treated, and the type of system in which the fish are kept.
Drugs
- Toltrazuril: An antiparasitic drug used in other animals that, under the “cascade” principle, can be used in fish if no medications are available for that species. It was found to protect against infection but does not affect already established parasites.
- Plant Extracts: Several extracts have been investigated for their antiparasitic effects.
- Essential oils of Melaleuca alternifolia, Lavandula angustifolia, and Mentha piperita: Showed in vitro toxicity against trophonts. M. alternifolia oil also showed in vivo potential in pacu (Piaractus mesopotamicus); Valladão et al., (2016) reported that 50 μL L−1 had an efficacy of nearly 100% in eliminating parasites from fish skin and gills.
- Extracts of Cynanchum atratum, Zingiber officinale, and Cynanchum paniculatum: Effective in vitro against different parasite stages, although their application through feed had a limited effect in herbivorous carp. Cynatratoside-C, isolated from C. atratum, showed high efficacy in vitro and in vivo in approximately 3 hours at a low concentration of 0.25 mg/L against theronts in herbivorous carp, with low toxicity to the fish (Jørgensen, 2017).
- Gracillin and zingibernsis from Costus speciosus: Effective in vitro and in vivo against Ich in herbivorous carp.
- Extracts from the root bark of Morus alba (white mulberry): Showed in vitro efficiency. Kuwanones G and O, flavonoids from this plant, were lethal to theronts and reduced their infectivity.
- Chelerythrine and chloroxylonine extracts from Toddalia asiatica: Effective in vitro and in vivo in goldfish.
- Pentagalloylglucose from Galla chinensis: Lethal to Ich in vitro and effective in treating catfish and preventing infections in non-infected fish.
- Psoralidin from Psoralea corylifolia: Highly effective in vitro against theronts and capable of disrupting parasite reproduction.
Actinobacteria
- Amphotericin B from Streptomyces sp. strain HL-2-14: Significantly reduced Ich infectivity and intensity in herbivorous carp in vitro and in vivo, although mortality was observed at higher concentrations.
- Nystatin from Streptomyces griseus SDX-4: Effective in vitro against theronts and encysted tomonts and reduced parasite mortality and reproduction in goldfish in vivo. It is important to consider that many of these compounds have antibiotic properties, making them less desirable due to the risk of generating antibiotic-resistant microorganisms.
Biological Control Strategies
- The use of the leopard pleco (Glyptoperichthys gibbiceps) in warm water systems significantly reduced I. multifiliis infections, as these fish feed on biofilms where the parasite settles to encyst.
- Copepods and other filter-feeding microorganisms can ingest tomonts and theronts, reducing infection pressure in the system. Wang et al., (2025) identified six copepod species as predators of I. multifiliis theronts, all belonging to the order Cyclopoida; however, Cyclops vicinus was the dominant predator, representing 82.72% of the total predators.
Use of Aquarium Salt
Adding non-iodized salt at a concentration of 1-3 g/liter can help weaken the parasite and stimulate the production of protective mucus in fish.
Water Changes and Filtration
Frequent water changes and improved filtration help eliminate free-swimming parasites.
Isolation of Infected Fish
If you have a large aquarium, isolating infected fish in a hospital tank can prevent the spread of the disease.
Prevention of white spot disease in fish
To prevent the occurrence of white spot disease in fish, follow these tips:
- Quarantine new fish before introducing them into the main aquarium.
- Maintain good water quality and avoid sudden temperature changes.
- Reduce stress in fish by providing hiding spots and a balanced diet.
- Disinfect decorations and plants before placing them in the aquarium.
- Boosting fish immunity: Jørgensen (2017) reported that immune response studies against I. multifiliis have been conducted mainly in rainbow trout, channel catfish, and common carp. Meanwhile, Buchmann (2020) stated that protective immunity development in fish against Ichthyophthirius multifiliis is based on a combination of strong innate and adaptive immune responses, where specific immunoglobulins, such as IgT, play a crucial role.
Is Ich dangerous to humans?
A common question among aquarists is: Is Ich dangerous to humans? The answer is no. Ichthyophthirius multifiliis is a fish-exclusive parasite and cannot infect humans. However, it is always advisable to wash your hands after handling sick fish or aquarium water.
Conclusion
White spot disease is one of the most feared diseases among aquarists and fish breeders. Knowing its symptoms, life cycle, and treatment methods will allow you to act quickly to save your fish.
Ich is an infection caused by I. multifiliis that, if left untreated, can be fatal. Maintaining good aquarium hygiene and following preventive practices will significantly reduce the risk of infection.
Have you experienced white spot disease in your aquarium? Share your experience in the comments and help other aquarists keep their fish healthy.
Frequently Asked Questions About White Spot Disease (Ichthyophthirius multifiliis)
To further assist aquarists and fish enthusiasts, here are answers to some frequently asked questions about Ich disease:
What is the difference between “Ich” and other fish diseases?
White spot disease is characterized by visible white cysts caused by I. multifiliis. Other diseases, such as fungal infections or velvet disease, may present similar symptoms but require different treatments. Accurate diagnosis is crucial for effective treatment.
Can white spot disease reappear after treatment?
Yes, if the underlying causes (e.g., poor water quality or stress) are not addressed, Ich disease can reappear. Proper tank maintenance and preventive measures are essential to prevent reinfection.
How long does it take to treat Ich disease?
Treatment usually takes between 7 and 10 days, depending on the severity of the infection and the method used. Heat therapy and medications can speed up the process, but patience is key to ensuring the complete eradication of all parasites.
Are some fish species more susceptible to white spot disease?
Yes, certain species, such as angelfish, guppies, and tetras, are more prone to white spot disease due to their sensitivity to stress and water conditions. However, all fish can be affected if exposed to the parasite.
Can I use natural remedies to treat Ich disease?
While some aquarists use natural remedies like garlic extract or tea tree oil, these methods are less reliable than proven treatments such as medications or salt therapy. Always consult a veterinarian or aquatic specialist before trying alternative treatments.
References
Abu-Elala, N. M., Attia, M. M., Abd-Elsalam, R. M., Gamal, A., & Younis, N. A. (2021). Peracetic acid treatment of Ichthyophthirius multifiliis (Ciliophora: Ichthyophthiriidae) and Trichodina spp. Reduces the infection by Aeromonas hydrophila and improves survival in Nile tilapia (Oreochromis niloticus). Aquaculture, 538, 736591. https://doi.org/10.1016/j.aquaculture.2021.736591
Bonnichsen, R., Brink Nielsen, G. G., Dam, J. S., Schrøder-Petersen, D., & Buchmann, K. (2025). AI-Driven Realtime Monitoring of Early Indicators for Ichthyophthirius multifiliis Infection of Rainbow Trout. Journal of Fish Diseases, 48(1), e14027. https://doi.org/10.1111/jfd.14027
Buchmann, K. (2020). Immune response to Ichthyophthirius multifiliis and role of IgT. Parasite Immunology, 42(8), e12675. https://doi.org/10.1111/pim.12675
Francis-Floyd, R., Yanong, R., & Pouder, D. (2023). Ichthyophthirius multifiliis (white spot) infections in fish. 4 p.
Guang-Ran Hu, Run-Qiu Wang, Ke Huang et al. Ecological factors associated with parasite Ichthyophthirius multifiliis gene abundance in the aquatic environment：novel clues for ecological control of Ichthyophthiriasis, 16 January 2024, PREPRINT (Version 1) available at Research Square [https://doi.org/10.21203/rs.3.rs-3859658/v1]
Jørgensen, L. V. G. (2017). The fish parasite Ichthyophthirius multifiliis – Host immunology, vaccines and novel treatments. Fish & Shellfish Immunology, 67, 586-595. https://doi.org/10.1016/j.fsi.2017.06.044
Li, J., Lian, Z., Wu, Z., Zeng, L., Mu, L., Yuan, Y., Bai, H., Guo, Z., Mai, K., Tu, X., & Ye, J. (2023). Artificial intelligence–based method for the rapid detection of fish parasites (Ichthyophthirius multifiliis, Gyrodactylus kobayashii, and Argulus japonicus). Aquaculture, 563, 738790. https://doi.org/10.1016/j.aquaculture.2022.738790
Valladão, G. M., Gallani, S. U., Ikefuti, C. V., Levy-Pereira, N., N Rodrigues, M. V., & Pilarski, F. (2016). Essential oils to control ichthyophthiriasis in pacu, Piaractus mesopotamicus (Holmberg): Special emphasis on treatment with Melaleuca alternifolia. Journal of Fish Diseases, 39(10), 1143-1152. https://doi.org/10.1111/jfd.12447
Yang, H., Tu, X., Xiao, J., Hu, J., & Gu, Z. (2023). Investigations on white spot disease reveal high genetic diversity of the fish parasite, Ichthyophthirius multifiliis (Fouquet, 1876) in China. Aquaculture, 562, 738804.
Wang, L., Xi, B., Chen, K., Xie, J., & Pan, L. (2025). In-Situ Investigation of Copepod Predators of Ichthyophthirius multifiliis Theronts from Fish-Farming Pond. Microorganisms, 13(1), 38. https://doi.org/10.3390/microorganisms13010038
