Fish diseases and parasites

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microsporidian fungal parasite in its intestines.[2]

Like humans and other animals,

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

koi herpes virus in koi.[5][6]

Some commercially important fish diseases are

whirling disease
.

Parasites

parasitic crustacean which enters a fish through its gills and destroys the fish's tongue.[7]
Velvet disease
velvet disease in fish [8]
Adult Siamese fighting fish
(Betta splendens) with velvet disease

Parasites in fish are a common natural occurrence. 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.[9]
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.

threespine sticklebacks. When that happens, the females reject them, suggesting a strong mechanism for the selection of parasite resistance.[11]

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.[12] 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.[13]

Parasites can be internal (

protists and Myxosporea are also parasitic on gills, where they form cysts
.

Fish gills are also the preferred habitat of many external parasites, attached to the gill but living out of it. The most common are

Isopod fish parasites are mostly external and feed on blood. The larvae of the Gnathiidae family and adult cymothoidids have piercing and sucking mouthparts and clawed limbs adapted for clinging onto their hosts.[20][21] Cymothoa exigua is a parasite of various marine fish. It causes the tongue of the fish to atrophy and takes its place in what is believed to be the first instance discovered of a parasite functionally replacing a host structure in animals.[22]

Other parasitic disorders, include

carp lice and salmon lice
.

Although parasites are generally considered to be harmful, the eradication of all parasites would not necessarily be beneficial. Parasites account for as much as or more than half of life's diversity; they perform an important ecological role (by weakening prey) that ecosystems would take some time to adapt to; and without parasites organisms may eventually tend to asexual reproduction, diminishing the diversity of sexually dimorphic traits.[23] Parasites provide an opportunity for the transfer of genetic material between species. On rare, but significant, occasions this may facilitate evolutionary changes that would not otherwise occur, or that would otherwise take even longer.[24]

Below are some life cycles of fish parasites:

Cleaner fish

Main article: Cleaner fish
wrasses, Labroides phthirophagus, servicing a goatfish
, Mulloidichthys flavolineatus

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.[26] 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.[27]

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.[28][29]
In temperate regions, drifting kelp fields harbour
breach more than ten feet above the surface, possibly as another effort to dislodge parasites on the body.[30][31]

Mass die offs

The complex life cycle of Pfiesteria piscidica. Red = toxic stages, yellow = possibly toxic stages, blue = passive stages
See also: Fish kill and Harmful algal bloom

Some diseases result in mass die offs.

land runoff increases.[34]

Wild salmon

See also: Diseases and parasites in salmon
Henneguya salminicola
, a parasite commonly found in the flesh of salmonids on the West Coast of Canada. Coho salmon

According to Canadian biologist Dorothy Kieser, protozoan parasite

Henneguya salminicola
is commonly found in the flesh of salmonids. It has been recorded in the field samples of salmon returning to the Queen Charlotte Islands. The fish responds by walling off the parasitic infection into a number of cysts that contain milky fluid. This fluid is an accumulation of a large number of parasites.

Henneguya and other parasites in the myxosporean group have a complex lifecycle where the salmon is one of two hosts. The fish releases the spores after spawning. In the Henneguya case, the spores enter a second host, most likely an invertebrate, in the spawning stream. When juvenile salmon out-migrate to the Pacific Ocean, the second host releases a stage infective to salmon. The parasite is then carried in the salmon until the next spawning cycle. The myxosporean parasite that causes whirling disease in trout, has a similar lifecycle.[35] However, as opposed to whirling disease, the Henneguya infestation does not appear to cause disease in the host salmon — even heavily infected fish tend to return to spawn successfully.

According to Dr. Kieser, a lot of work on Henneguya salminicola was done by scientists at the Pacific Biological Station in Nanaimo in the mid-1980s, in particular, an overview report[36] which states that "the fish that have the longest fresh water residence time as juveniles have the most noticeable infections. Hence in order of prevalence coho are most infected followed by sockeye, chinook, chum and pink." As well, the report says that, at the time the studies were conducted, stocks from the middle and upper reaches of large river systems in British Columbia such as Fraser, Skeena, Nass and from mainland coastal streams in the southern half of B.C. "are more likely to have a low prevalence of infection." The report also states "It should be stressed that Henneguya, economically deleterious though it is, is harmless from the view of public health. It is strictly a fish parasite that cannot live in or affect warm blooded animals, including man".

Henneguya salminicola
, caught off Haida Gwaii, Western Canada in 2009

According to Klaus Schallie, Molluscan Shellfish Program Specialist with the Canadian Food Inspection Agency, "Henneguya salminicola is found in southern B.C. also and in all species of salmon. I have previously examined smoked chum salmon sides that were riddled with cysts and some sockeye runs in Barkley Sound (southern B.C., west coast of Vancouver Island) are noted for their high incidence of infestation."

Pacific coast of Canada, the louse-induced mortality of pink salmon in some regions is commonly over 80%.[44]

Farmed salmon

Atlantic salmon
See also:
Aquaculture of salmon § Issues, and Diseases and parasites in salmon

In 1972, Gyrodactylus salaris, also called salmon fluke, a monogenean parasite, spread from Norwegian hatcheries to wild salmon, and devastated some wild salmon populations.[45]

In 1984,

infectious salmon anemia (ISAV) was discovered in Norway in an Atlantic salmon hatchery. Eighty per cent of the fish in the outbreak died. ISAV, a viral disease, is now a major threat to the viability of Atlantic salmon farming. It is now the first of the diseases classified on List One of the European Commission’s fish health regime. Amongst other measures, this requires the total eradication of the entire fish stock should an outbreak of the disease be confirmed on any farm. ISAV seriously affects salmon farms in Chile, Norway, Scotland and Canada, causing major economic losses to infected farms.[46] As the name implies, it causes severe anemia of infected fish. Unlike mammals, the red blood cells of fish have DNA, and can become infected with viruses. The fish develop pale gills, and may swim close to the water surface, gulping for air. However, the disease can also develop without the fish showing any external signs of illness, the fish maintain a normal appetite, and then they suddenly die. The disease can progress slowly throughout an infected farm and, in the worst cases, death rates may approach 100 per cent. It is also a threat to the dwindling stocks of wild salmon. Management strategies include developing a vaccine and improving genetic resistance to the disease.[47]

In the wild, diseases and parasites are normally at low levels, and kept in check by natural predation on weakened individuals. In crowded net pens they can become epidemics. Diseases and parasites also transfer from farmed to wild salmon populations. A recent study in

sea lice from river salmon farms to wild pink salmon in the same river.[48] The European Commission (2002) concluded "The reduction of wild salmonid abundance is also linked to other factors but there is more and more scientific evidence establishing a direct link between the number of lice-infested wild fish and the presence of cages in the same estuary."[49] It is reported that wild salmon on the west coast of Canada are being driven to extinction by sea lice from nearby salmon farms.[50]
Antibiotics and pesticides are often used to control the diseases and parasites.

Coral reef fish

Nematoda), a roundworm parasitic of the ovary of the blacktip grouper
Monogenean parasite on the gill of a grouper

parasites and their various and numerous hosts. Numerical estimates of parasite biodiversity have shown that certain coral fish species have up to 30 species of parasites.[52][53]
co-extinction. Results obtained for the coral reef fish of New Caledonia suggest that extinction of a coral reef fish species of average size would eventually result in the co-extinction of at least ten species of parasites.[52]

Aquarium fish

Main article: Disease in ornamental fish
Nitrogen cycle in a common aquarium.

Ornamental fish kept in aquariums are susceptible to numerous diseases
.

In most

stresses on ornamental fish in a tank. Despite this, many diseases in captive fish can be avoided or prevented through proper water conditions and a well-adjusted ecosystem within the tank. Ammonia poisoning
is a common disease in new aquariums, especially when immediately stocked to full capacity.

Due to their generally small size and the low cost of replacing diseased or dead aquarium fish, the cost of testing and treating diseases is often seen as more trouble than the value of the fish.

Immune system

Immune organs vary by type of fish.

hemopoietic
organ; where erythrocytes, granulocytes, lymphocytes and macrophages develop.

Like chondrostean fish, the major immune tissues of bony fish (or

T cells accumulate while waiting to encounter an antigen.[60]

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.[61] 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.

Eating raw fish

See also: Sushi
Differential symptoms of parasite infection by raw fish: Clonorchis sinensis (a trematode/fluke), Anisakis (a nematode/roundworm) and Diphyllobothrium a (cestode/tapeworm),[62] all have gastrointestinal, but otherwise distinct, symptoms.[63][64][65][66]

Though not a health concern in thoroughly cooked fish, parasites are a concern when human consumers eat raw or lightly preserved fish such as sashimi, sushi, ceviche, and gravlax. The popularity of such raw fish dishes makes it important for consumers to be aware of this risk. Raw fish should be frozen to an internal temperature of −20 °C (−4 °F) for at least seven days to kill parasites. It is important to be aware that home freezers may not be cold enough to kill parasites.[67][68]

Traditionally, fish that live all or part of their lives in

Seattle, Washington showed that 100% of wild salmon had roundworm larvae capable of infecting people. In the same study farm-raised salmon did not have any roundworm larvae.[69] Historically, parasite infection of humans eating raw fish has been rare in the developed world, though a 2020 meta-analysis of available data shows that since 1980 there has been a sharp increase of parasites in the types of marine fish that are eaten uncooked.[70]

There are three main kinds of parasites:

ikura (salmon roe), and even if they seem raw, these foods are not raw but are frozen overnight to prevent infections from parasites, particularly anisakis.[citation needed
]

See also:
salmon poisoning disease

Below are some life cycles of fish parasites that can infect humans:

See also

Citations

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General and cited references

Further reading

External links

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