Fish Diseases and Parasites

White Spot (Ichthyophthirius multifiliis) photo by Thomas Kaczmarczyk Public domain 14/03/11
White Spot (Ichthyophthirius multifiliis) photo by Thomas Kaczmarczyk

Like humans and other animals, fish suffer from diseases and parasites. Fish defences against disease are specific and non-specific. Non-specific defences include skin and scales, as well as the mucus layer secreted by the epidermis that traps microorganisms and inhibits their growth. If pathogens breach these defences, fish can develop inflammatory responses that increase the flow of blood to infected areas and deliver white blood cells that attempt to destroy the pathogens.

Specific defences are specialised responses to particular pathogens recognised by the fish's body, that is immune responses. In recent years, vaccines have become widely used in aquaculture and ornamental fish, for example vaccines for furunculosis in farmed salmon and koi herpes virus in koi.

Some commercially important fish diseases are VHS, ich and whirling disease.


All fish carry pathogens and parasites. Usually this is at some cost to the fish. If the cost is sufficiently high, then the impacts can be characterised as a disease. However disease in fish is not understood well. What is known about fish disease often relates to aquaria fish, and more recently, to farmed fish.

Disease is a prime agent affecting fish mortality, especially when fish are young. Fish can limit the impacts of pathogens and parasites with behavioural or biochemical means, and such fish have reproductive advantages. Interacting factors result in low grade infection becoming fatal diseases. In particular, things that causes stress, such as natural droughts or pollution or predators, can precipitate outbreak of disease.

Disease can also be particularly problematic when pathogens and parasites carried by introduced species affect native species. An introduced species may find invading easier if potential predators and competitors have been decimated by disease.

Pathogens can cause fish diseases such as:

  • viral disorders
  • bacterial infections, such as Pseudomonas fluorescens leading to fin rot and fish dropsy
  • fungal infections, such as saprolegnia
  • mould infections, such as oomycete and saprolegnia

Koi herpes virus

Koi herpes virus (KHV) is a viral disease that is very contagious to the common carp Cyrpinus carpio and has a very high mortality rate. It is most common found in ornamental koi and carp fishing venues. The first case of KHV was reported in 1998, but not confirmed until later in 1999. KHV is a notifiable disease in the UK. KHV is listed as a non-exotic disease of the EU and is therefore watched closely by the European Community Reference Laboratory for Fish Diseases.

KHV is a DNA-based virus. After discovery, it was identified that KHV is indeed a strain of herpesvirus. Like other strains, KHV stays with the infected fish for the duration of their life, making the recovered and exposed fish potential carriers of the virus. Koi fish infected with KHV may die within the first 24-48 hours of exposure.

Symptoms of KHV
Symptoms of KHV include:

  • Gill mottling
  • Red and white patches appearing on gills
  • Bleeding gills
  • Sunken eyes
  • Pale patches
  • Blisters


Saprolegnia is a genus of freshwater mould often called a "cotton mould" because of the characteristic white or grey fibrous patches it forms. Current taxonomy puts Saprolegnia as a genus of the heterokonts in the order Saprolegniales.

Saprolegnia, like most water moulds, is both a saprotroph and necrotroph. Typically feeding on waste from fish or other dead cells, they will also take advantage of creatures that have been injured or compromised eggs. When they inhabit a live animal, they exhibit as a fungal infection known as mycoses.

Saprolegnia is tolerant to a wide range of temperature, 3°C to 33°C, but is more prevalent in lower temperatures. While it is found most frequently in freshwater, it will also tolerate brackish water and even moist soil.

Saprolegnia filaments (hyphae) are long with rounded ends, containing the zoospores. Saprolegnia generally travels in colonies consisting of one or more species. They first form a mass of individual hyphae. When the mass of hyphae grows large enough in size to be seen without use of a microscope, it can be called a mycelium. Colonies are generally white in color, though they may turn grey under the precesence of bacteria or other debris which has become caught in the fibrous mass.

It has a diploid life cycle which includes both sexual and asexual reproduction. In the asexual phase, a spore of Saprolegnia releases zoospores. Within a few minutes, this zoospore will encyst, germinate and release another zoospore. This second zoospore has a longer cycle during which most dispersal happens; it will continue to encyst and release a new spore in a process called polyplanetism until it finds a suitable substrate. When a suitable medium is located, the hairs surrounding the spore will lock onto the substrate so that the sexual reproduction phase can start. It is also during this stage of polyplanetism that the Saprolegnia are capable of causing infection; the most pathogenic species have tiny hooks at the end of their hairs to enhance their infectious ability.

Once firmly attached, sexual reproduction begins with the production of male and female gametangium, antheridia and oogonium respectively. These unite and fuse together via fertilization tubes. The zygote produced is named an oospore.

Characteristics of Infection
Saprolegnia is generally a secondary pathogen, though in the right circumstances, it can act as primary. It most frequently targets fish, both in the wild and in tank environments. Through cellular necrosis and other epidermal damage, Saprolegnia will spread across the surface of its host as a cotton-like film. Though it often stays in the epidermal layers, the mould does not appear to be tissue specific. A Saprolegnia infection is usually fatal, eventually causing haemodilution, though the time to death varies depending on the initial site of the infection, rate of growth and the ability of the organism to withstand the stress of the infection.

The extensive mortalities of salmon and migratory trout in the rivers of western Europe in the 1970's and 1980's in the UDN outbreak were probably almost all ultimately caused by the secondary Saprolegnia infections.

DNA studies suggest the Saprolegnia species affecting Australian freshwater fish is an introduced strain, presumably imported in the 1800s with exotic fish species.

Fin rot

Fin rot can be the result of a bacterial infection (Pseudomonas fluorescens, which causes a ragged rotting of the fin), or as a fungal infection (which rots the fin more evenly and is more likely to produce a white 'edge'). Sometimes, both types of infection are seen together. Infection is commonly brought on by bad water conditions, injury, poor diet, or as a secondary infection in a fish which is already stressed by other disease or parasites.

Fin rot starts at the edge of the fins, and destroys more and more tissue until it reaches the fin base. If it does reach the fin base, the fish will never be able to regenerate the lost tissue. At this point, the disease may attack the fish's body directly.


  • Fin edges turn white
  • Fins fray
  • Base of fins inflamed
  • Entire fin may rot away or fall off in large chunks


  • Test the water quality and make improvements by doing water changes, reducing feed and cleaning filters etc.
  • Treat with antibacterials such as Melafix or Acriflavin
  • Use salt baths


  • Ensure the water quality is good
  • Do not overstock the pond or tank
  • Do not overfeed the fish
  • Maintain constant water temperature


Dropsy is a common disease among fresh-water aquarium and pond fish. It is characterized by a swollen or hollow abdomen. The name is from an old name for Edema in humans.

Goldfish with dropsy Creative commons Attribution-Share Alike 3.0 14/03/11
A goldfish with dropsy. Photo by Taipan198

In dropsy, a concentration of fluid in the body tissues and cavities causes the fish's abdomen to become swollen and appear bloated (Ascites). Swollen areas may exhibit a 'pine-cone' appearance caused by the fish's scales sticking out. Fish may also stop feeding, appear off-color, become listless and/or lethargic, have sunken eyes, and hang at the top or stay at the bottom of the aquarium. The condition affects the fish's internal organs, ceasing proper function.

The cause of fish dropsy can be difficult to diagnose. The main cause is bacterial infection. The causative agent may be introduced through poor water quality. Kidney failure or excess fluid (ascites) due to liver or heart failure are other possible causes. Some household (pet) fish may produce a red film in their urine during pregnancy or change of habitat fish tank relocation after only a weeks time.

Because dropsy is a symptom of an illness, it may or may not be contagious. However, it is standard practice to quarantine sick fish to prevent stress among the other fish in the tank community. This extra stress may make the others vulnerable to dropsy or other forms of disease.

Most cases of dropsy are fatal. By the time the fish has swollen up enough that the scales begin to raise, the internal damage may be too extensive to repair.

Water quality is an important factor in prevention of fish disease. Water changes will dilute existing disease agents, and reduce stress on the tank occupants.


Parasites in fish are a natural occurrence and common. Parasites can provide information about host population ecology. In fisheries biology, for example, parasite communities can be used to distinguish distinct populations of the same fish species co-inhabiting a region. Additionally, parasites possess a variety of specialized traits and life-history strategies that enable them to colonize hosts. Understanding these aspects of parasite ecology, of interest in their own right, can illuminate parasite-avoidance strategies employed by hosts.

Usually parasites (and pathogens) need to avoid killing their hosts, since extinct hosts can mean extinct parasites. Evolutionary constraints may operate so parasites avoid killing their hosts, or the natural variability in host defensive strategies may suffice to keep host populations viable. Parasite infections can impair the courtship dance of male threespine sticklebacks. When that happens, the females reject them, suggesting a strong mechanism for the selection of parasite resistance.

However not all parasites want to keep their hosts alive, and there are parasites with multistage life cycles who go to some trouble to kill their host. For example, some tapeworms make some fish behave in such a way that a predatory bird can catch it. The predatory bird is the next host for the parasite in the next stage of its life cycle. Specifically, the tapeworm Schistocephalus solidus turns infected threespine stickleback white, and then makes them more buoyant so that they splash along at the surface of the water, becoming easy to see and easy to catch for a passing bird.

Argulus (Fish Louse)

Branchiura, commonly called carp lice or fish lice are a group of parasitic crustaceans of uncertain position within the Maxillopoda. Although they are thought to be primitive forms, they have no fossil record. Almost all are ectoparasites on fish, a few on amphibians.

Fish Louse (Argulus) Creative Commons Attribution-Share Alike 3.0 14/03/11
photo by Michal Grabowski

Branchiurans have a flattened, oval body, which is almost entirely covered by a wide carapace. Their compound eyes are prominent, and the mouthparts and the first pair of antennae are modified to form a hooked, spiny proboscis armed with suckers, as an adaptation to parasitic life. They have two pairs of thoracic appendages, which are used to swim between different hosts. They leave their hosts for up to three weeks in order to mate and lay eggs, and reattach behind the fish's operculum, where they feed on mucus and sloughed-off scales, or pierce the skin and feed on the internal fluids. The eggs hatch into parasitic postnauplius larvae.

The most effective treatments against Argulus were organophosphates but they are now banned in the UK for use in fish treatments. There is no really effective treatment avaiable now.

Gyrodactylus salaris (Skin fluke)

Gyrodactylus is a small monogenean ectoparasite (about 0.5 mm long) which mainly lives on the skin of freshwater fish, especially Atlantic salmon. It's common name is Skin Fluke.

The parasite attaches to the fish by a large specialized posterior attachment organ, the (haptor) which has sixteen sharp hooks located around its margin. The parasite cannot be seen with the naked eye, but it can be seen with a hand held lens. When feeding, the parasite attaches its anterior end to the fish with cephalic glands. It everts its pharynx through the mouth and releases a digestive solution with proteolytic enzymes which dissolves the salmon skin. Mucus and dissolved skin are then sucked into the gut. Attachment of many parasites can cause large wounds and the epidermis of the host fish can be damaged which allows in secondary infections. The parasites gives birth to live young nearly as big as themselves and at this time, a further generation is already growing inside the neonates.

Chemical treatments include Malachite & Formalin, Chloramine-T, Potassium Permanganate, Flubenol and Superverm. Always check if the treatment is safe for the fish to be treated.

  • Superverm must not be used with any of the goldfish family (goldfish, shubunkins, comets etc).
  • Formalin, Potassium Permanganate, Copper sulphate (CuSo4), Organophosphates or any treatments that state not to be used with Orfe or Rudd must not be used with sturgeons.
  • A strong salt bath is the safest treatment for sturgeons.

Dactylogyrus (Gill fluke)

Dactylogyrus are oviparous monogeneans that have two pairs of anchors. These anchors can be used to latch onto the gills of a host, particularly freshwater fish such as carp. In heavily infected fish, Dactylogyrus can also be found on the buccal cavity. Other characteristics of the Dactylogyrus include the appearance of four eye-spots, 14 marginal hooks, one to two connective bars and two needle-like structures and spindle-shaped dactylogyrid-type seminal vesicles.

Life cycle
The Dactylogyrus life cycle is direct, having no intermediate host. The hermaphroditic adults are oviparous and produce eggs into the water which hatch prior to attaching to the gills of a fish host and developing into an onchomiricidium.

After the eggs hatch, water currents aid the free-swimming ciliated larva in reaching its host. The time required for egg maturation into the adult form is temperature dependent. Water temperatures of 72 - 75°F allow life cycle completion in a few days, whereas temperatures of 34 - 36°F extend the generation time to five or six months.

Dactylogyrus is a monogean parasite that is usually found on the gills of Cyprinidae fishes. The prevalence of Dactylogyrus infection on fish differ depending on the seasons. It was found that Dactylogyrus infections are at their greatest during late autumn or early winter. Correlation has also been found between the temperature of the water and the intensity of Dactylogyrus infection.

Cyprinidae that are infected by Dactylogyrus may have symptoms that include inflamed gills, excessive mucous secretions and accelerated respiration. The infected fish also becomes lethargic, swims near the surface, and its appetite decreases. Additionally the infected fish may hold its gill covers open and scratch its gills on rocks.

In severe infections, Dactylogyrus can cause hemorrhaging and metaplasia of the gills which can lead to secondary bacterial infections and death. Heavily infected fish are also anorexic and can be found gasping for air and exhibiting abnormal behavior such as jumping out of the water.

See Gyrodactylus salaris (Skin fluke) above.

Ichthyophthirius multifiliis (Whitespot or Ich)

Ichthyophthirius multifiliis (commonly known as freshwater white spot disease, freshwater ich, or freshwater ick) is a common disease of freshwater fish. It is caused by the protozoa Ichtyopthirius. Ich is one of the most common and persistent diseases. The protozoan is an ectoparasite. White nodules that look like white grains of salt or sugar of up to 1 mm appear on the body, fins and gills. Each white spot is an encysted parasite.

Clown fish with White Spot (Ichthyophthirius multifiliis) photo by ML5 Public domain 14/03/11
Clown fish with White Spot (Ichthyophthirius multifiliis) photo by ML5

I. multifiliis is one of the most prevalent protozoan parasites of fish and is an important pathogen of ornamental and farm-raised food fish species when reared under intensive conditions. Wild fish populations are also susceptible and outbreaks are occasionally seen. There are few aquarists that have not met it on one or more occasions.

The ich protozoa goes though the following life stages:

  • Feeding stage : The ich trophozoite ( a protozoan in active stage of life ) feeds in a nodule formed in the skin or gill epithelium.
  • After it feeds within the skin or gills, the trophozoite falls off and enters an encapsulated dividing stage (tomont). The tomont adheres to plants, nets, gravel or other ornamental objects in the aquarium.
  • The tomont divides up to 10 times by binary fission, producing infective theronts, thus dividing rapidly and attacking the fishes.

This life cycle is highly dependent on water temperature, and the entire life cycle takes from approximately 7 days at 25 °C (77 °F) to 8 weeks at 6 °C (43 °F).

Marine ich is a similar disease caused by a different ciliate, Cryptocaryon irritans.

Predisposing factors
There is no dormant stage in the lifecycle. Ich does not lie in wait for a weakened fish to infect. However, any factor that reduces immunity like changes in water temperature and quality may, in a subclinically infected fish, accelerate an outbreak of Ich. The presence of ammonia, nitrite and high levels of nitrate in water does not in itself cause clinical cases of Ich. However, poor water quality will stress fish, allow an outbreak to spread rapidly and increase mortality rates in infected fish.

Typical behaviours of clinically infected fish include:

  • Anorexia (loss of appetite, refusing all food, with consequential wasting)
  • Rapid breathing
  • Hiding abnormally/ not schooling
  • Resting on the bottom
  • Flashing
  • Rubbing and scratching against objects
  • Gill infection will cause breathing at the surface and fast respiration. Gill examination may show numbers of such white spots. Wet mount of a Gill Biopsy may show I. mutifiliis trophozoites.

A subclinically infected fish will not show any of these signs. For example, a healthy fish with a newly attached trophozoite will not yet have clinical disease. The trophozoite will not become visible to the naked eye until it has fed on the fish and grown to one or two millimetres. A trophozoite attached to the gills usually is not readily seen. A subclinically infected fish may initially only have a single trophozoite.

Chemical treatments available for white spot include Formalin, Malachite Green, Chloramine T and Potassium Permanganate. Chloramine T or Malachite Green can be used with sturgeon but do not use a Malachite and Formalin mixture. There are also a large number of proprietary treatments available for the treatment of white spot, and the related Oodinium (velvet disease) most of which are based on the chemicals mentioned above. A strong salt bath is the safest treatment for sturgeons.

Cryptocaryon (Marine Whitespot or Ich)

Cryptocaryon irritans is a species of ciliate protozoa that parasitizes marine fish, and is one of the most common causes of disease in marine aquaria. The symptoms and life-cycle are generally similar to those of Ichthyophthirius in freshwater fish, including white spots, on account of which Cryptocaryon is usually called marine ich. However, Cryptocaryon can spend a much longer time encysted.

Infections can be extremely difficult to treat because of other creatures, such as corals and other invertebrates, which will not survive standard treatments. Ideally fish with Cryptocaryon are quarantined in a hospital tank, where they can be treated with a copper salt or using hyposalinity. The display tank needs to be kept clear of fish for 6-9 weeks, the longer the better. This gives time for the encysted tomonts to release infectious theronts, which die within 24-48 hours when they cannot find a host.

Cryptocaryon irritans was originally classified as Ichthyophthirius marinus, but it is not closely related to the other species. It belongs to the class Prostomatea, but beyond that its placement is still uncertain.

Chemical treatments for Cryptocaryon irritans are copper solutions, formalin solutions, and quinine based drugs (such as Chloroquine Phosphate and Quinine Diphosphate).

Oodinium (Velvet Disease)

Oodinium (also known as Piscinoodinium) is a genus of microscopic parasitic dinoflagellates. They live on salt and fresh water fish, causing a type of fish velvet disease (also called gold dust disease).

A velvet infected fish. Creative commons Attribution-Share Alike 3.0 14/03/11
A velvet infected fish. Photo by Tze Sin, Tan

The host typically develops small yellow or gold dust scattered on its head, fins and body. At this stage, the infestation is already severe. The attack usually starts at the gills at which stage it is difficult to notice. The host is irritated and often swims in fuzziness while rubbing itself against rocks. The yellowish spots are more vivid under sunlight or flashlight. It is very similar to Ichthyophthirius, though the oodinium spots are yellowish and smaller.

The life cycle of oodinium starts as a dinospore that swims in the water to look for a suitable host. As it attaches itself onto the host skin, it forms a hard shell protecting itself against the outside environment while it is eating the fish skin cells. This is the cyst stage seen as dust covering the fish skin. After few days, the cyst sinks to the bottom, freeing new generation of dinospores. And the cycle repeats. The dinospore must find a host within 48 hours, otherwise the dinospore would die.

The most effective treatments are copper or formalin. Free swimming dinospore is extremely vulnerable to copper. Bringing the water temperature to 30°C helps to release the dinospore from cyst.

Cleaner fish

Some fish take advantage of cleaner fish for the removal of external parasites. The best known of these are the Bluestreak cleaner wrasses of the genus Labroides found on coral reefs in the Indian Ocean and Pacific Ocean. These small fish maintain so-called "cleaning stations" where other fish, known as hosts, will congregate and perform specific movements to attract the attention of the cleaner fish. Cleaning behaviours have been observed in a number of other fish groups, including an interesting case between two cichlids of the same genus, Etroplus maculatus, the cleaner fish, and the much larger Etroplus suratensis, the host.

More than 40 species of parasites may reside on the skin and internally of the ocean sunfish, motivating the fish to seek relief in a number of ways. In temperate regions, drifting kelp fields harbour cleaner wrasses and other fish which remove parasites from the skin of visiting sunfish. In the tropics, the mola will solicit cleaner help from reef fishes. By basking on its side at the surface, the sunfish also allows seabirds to feed on parasites from their skin. Sunfish have been reported to breach more than ten feet above the surface, possibly as another effort to dislodge parasites on the body.

Pond and Aquarium fish

Ornamental fish kept in ponds and aquariums are susceptible to numerous diseases.

In most ponds and aquariums, the fish are at high concentrations and the volume of water is limited. This means that communicable diseases can spread rapidly to most or all fish in the pond or tank. An improper nitrogen cycle, inappropriate aquarium plants and potentially harmful freshwater invertebrates can directly harm or add to the stresses on ornamental fish. Despite this, many diseases in captive fish can be avoided or prevented through proper water conditions and a well-adjusted ecosystem. Ammonia poisoning is a common disease in new ponds and aquariums, especially when immediately stocked to full capacity.

Spreading disease and parasites

The capture, transportation and culture of bait fish can spread damaging organisms between ecosystems, endangering them. In 2007, several American states, including Michigan, enacted regulations designed to slow the spread of fish diseases, including viral hemorrhagic septicemia, by bait fish. Because of the risk of transmitting Myxobolus cerebralis (whirling disease), trout and salmon should not be used as bait. Anglers may increase the possibility of contamination by emptying bait buckets into fishing venues and collecting or using bait improperly. The transportation of fish from one location to another can break the law and cause the introduction of fish and parasites alien to the ecosystem.