Fish diseases and parasites
Non-specific defences include skin and scales, as well as the mucus layer secreted by the epidermis that traps microorganisms and inhibits their growth.
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.
Other internal parasites are found living inside fish gills, include encysted adult didymozoid trematodes,[15] a few trichosomoidid nematodes of the genus Huffmanela, including Huffmanela ossicola which lives within the gill bone,[16] and the encysted parasitic turbellarian Paravortex.
The larvae of the Gnathiidae family and adult cymothoidids have piercing and sucking mouthparts and clawed limbs adapted for clinging onto their hosts.
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 Gyrodactylus salaris, Ichthyophthirius multifiliis, cryptocaryon, velvet disease, Brooklynella hostilis, Hole in the head, Glugea, Ceratomyxa shasta, Kudoa thyrsites, Tetracapsuloides bryosalmonae, Cymothoa exigua, leeches, nematode, flukes, carp lice and salmon lice.
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.
[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 cleaner wrasses and other fish which remove parasites from the skin of visiting sunfish.
Sunfish have been reported to breach more than ten feet above the surface, possibly as another effort to dislodge parasites on the body.
[32] One of the more bizarre and recently discovered diseases produces huge fish kills in shallow marine waters.
[34] According to Canadian biologist Dorothy Kieser, protozoan parasite Henneguya salminicola is commonly found in the flesh of salmonids.
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.
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.
According to Klaus Schallie, Molluscan Shellfish Program Specialist with the Canadian Food Inspection Agency, "Henneguya salminicola is found in southern B.C.
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."
Amongst other measures, this requires the total eradication of the entire fish stock should an outbreak of the disease be confirmed on any farm.
The disease can progress slowly throughout an infected farm and, in the worst cases, death rates may approach 100 per cent.
[47] In the wild, diseases and parasites are normally at low levels, and kept in check by natural predation on weakened individuals.
Some of these fish parasites have heteroxenous life cycles (i.e. they have several hosts) among which sharks (certain cestodes) or molluscs (digeneans).
Despite this, many diseases in captive fish can be avoided or prevented through proper water conditions and a well-adjusted ecosystem within the tank.
For example, erythrocytes, macrophages and plasma cells are produced in the anterior kidney (or pronephros) and some areas of the gut (where granulocytes mature.)
They also possess an identifiable thymus and a well-developed spleen (their most important immune organ) where various lymphocytes, plasma cells and macrophages develop and are stored.
Chondrostean fish (sturgeons, paddlefish and bichirs) possess a major site for the production of granulocytes within a mass that is associated with the meninges (membranes surrounding the central nervous system.)
[57] In addition, teleost fish possess a thymus, spleen and scattered immune areas within mucosal tissues (e.g. in the skin, gills, gut and gonads).
Much like the mammalian immune system, teleost erythrocytes, neutrophils and granulocytes are believed to reside in the spleen whereas lymphocytes are the major cell type found in the thymus.
A study in Seattle, Washington showed that 100% of wild salmon had roundworm larvae capable of infecting people.
[71] Infection risk of anisakis is particularly higher in fishes which may live in a river such as salmon (shake) in Salmonidae, mackerel (saba).
Such parasite infections can generally be avoided by boiling, burning, preserving in salt or vinegar, or freezing overnight.
