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Fish crustacean parasites

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Introduction

Although parasitic crustaceans are not as numerous as protozoans or helminths, some of them are important pathogens and diseases caused by them may result in considerable economic losses. Most of theem are copepods. Parasitic crustaceans are considered "macroparasites", visible to the naked eye. Three genera, Argulus, Ergasilus, and Lernaea, are commonly found in freshwater fish. In marine systems, Gnathia sp. larval stages are parasitic isopods that can damage host tissue and even kill small fish.

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Sea lice (genera Lepeophtheirus and Caligus)

Taxonomy

cellular organisms - Eukaryota - Fungi/Metazoa group - Metazoa - Eumetazoa - Bilateria - Coelomata - Protostomia - Panarthropoda - Arthropoda - Mandibulata - Pancrustacea - Crustacea - Maxillopoda - Copepoda - Neocopepoda - Podoplea - Siphonostomatoida - Caligidae -

Brief facts

Sea lice is the term used to describe several species of ectoparasitic copepods of the genera Lepeophtheirus and Caligus that parasitize cultured fish and may cause diseases with damage to the epidermis and in severe cases death through osmoregulatory failure or secondary infections. Lepeophtheirus salmonis is now recognized as one of the most serious pathogens of marine farmed Atlantic salmon. This species and Caligus elongatus have economic impact on farmed salmonids in the northern hemisphere; other caligids pathogenic to cultured or wild fish are C. patulus, C. orientalis, C. epidemicus, and Pseudocaligus apodus.

Treatment and prevention

Formaldehyde, malathion and natural compounds show either poor efficacy or unsuitable therapeutic margins. Pyrethroids are at present the most used therapeutic against sea lice. Diflubenzuron and teflubenzuron added to feed are also used in significant amounts. Carbaryl and diflubenzuron are effective but the compounds make them unsuitable due to undesirable environmental toxicological characteristics. Despite these problems, chemotherapy remains an important component of control strategies.

Lepeophtheirus elegans adult female
Lepeophtheirus elegans Gusev, 1951. Adult female: A, habitus, dorsal; B, antennule; C, antenna; D, mandible; E, postantennary process; F, maxillule; G, maxilla; H, maxilliped; I, abdomen, ventral. Scale bars: A = 0.4 mm; B–I = 0.1 mm.
Venmathi Maran BA et al. Parasite. 2013; 20: 15.

sea+lice+fish in PubMed

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Lernaea

Taxonomy

cellular organisms - Eukaryota - Fungi/Metazoa group - Metazoa - Eumetazoa - Bilateria - Coelomata - Protostomia - Panarthropoda - Arthropoda - Mandibulata - Pancrustacea - Crustacea - Maxillopoda - Copepoda - Neocopepoda - Podoplea - Cyclopoida - Lernaeidae - Lernaea

Brief facts

Infestation by Lernaea spp. is one of the most common and often harmful parasitic diseases of fish in warm water habitats, both in farms and in the wild, in Asia, Africa, and America. Economic losses result not only from the direct harm to the fish, but also from disfigurement that renders the grown food and ornamental fish unsuitable for market. Lernaea cyprinacea is the most ubiquitous species with a distribution range approximating that of the common carp, i.e. in cold water as well as warm climatic geographical regions. L. cyprinacea is, nevertheless, entirely nonspecific in its choice of hosts and may be found in any freshwater fish as well as tadpoles.

Pathology

Both copepodites parasitizing the gills and females infecting any site on the fish integument may harm the fish. Heavy infestation of gills by copepodites of L. cyprinacea may damage tissue that is in immediate contact with the parasite, thus stimulating extensive hyperplasia of the entire gill epithelium. Copepodite infestations are only potentially lethal to small fish, while large fish (such as Bagrus) will tolerate over 1,000 copepodites per fish. The attachment of the adult females to the integument stimulates an intense inflammatory response, and the tissue around the anchor turns into a granuloma or necrotic lesion that later transforms into a fibrotic encapsulation. Infestation by 1-3 females is damaging or even deadly to <40-mm-long fish. Parasites may anchor in the liver, the intestine, or the brain, causing physical damage to vital organs. The skin lernaeids, such as L. cyprinacea and L. hardingi and some populations of L. barnimiana, cause severe damage only in heavy infestations. Parasite species with a higher predilection for a particular organ or location on the fish body may induce severe damage even at lower levels of infestation. For example, L. polymorphs in bighead carp anchor around the eye, and in advanced stages the lens is destroyed resulting in blindness. Damage to specific organs has also been observed in infestations by species preferentially aggregating in the mouth or on the lips, such as L. barnimiana, L. bagri and others.

Biology and Life Cycle

The life cycle has been studied in detail in populations of L. cyprinacea and L. barnimiana parasitizing cyprinids, cichlids, and Nile perch, Lates nilotica, in the Great Lakes of East Africa. The latter species follows a "two-host cycle" of development in which larval development from metanauplii through copepodite stages takes place on the gills of Bagrus docmac (in Lake Victoria and Lake George) and B. bayad (in Lake Albert) while adult females parasitize other host fish. Copepodites can be induced to develop on the same fish species (such as cichlids) that simultaneously host the adult stage. The length of the life cycle including egg development, metamorphosis of nauplii, and successive copepodite stages last about the same time in the tropical L. barnimiana as in the L. cyprinacea. Temperature plays an important role in the length of the life cycle. The upper limit for optimal development seems to be 30 °C and development becomes prolonged at lower temperatures. Egg development also seems to be affected by other factors, such as dissolved oxygen. Other variations may be related to the nature of the interaction between the parasite and the host. Repeated or prolonged infestation accelerates the host's inflammatory response and intense ulceration occurs, which, in turn, denies anchored females firm attachment and nutrition. L. cyprinacea was accidentally introduced into Indonesia from Japan in 1953, causing severe epizootics in farmed common carp, Puntius javanicus, and several species of guramies (Trichogaster). Thirty percent of hatchery production (a total of 1.48 billion fish fry) in Java, South Sumatra, and North Sulawesi was destroyed.

Lernaea in PubMed

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Argulus

Taxonomy

cellular organisms - Eukaryota - Fungi/Metazoa group - Metazoa - Eumetazoa - Bilateria - Coelomata - Protostomia - Panarthropoda - Arthropoda - Mandibulata - Pancrustacea - Crustacea - Maxillopoda - Branchiura - Arguloida - Argulidae - Argulus

Brief facts

Argulid parasitism is considered one of the more serious threats to farmed fish in warm water systems. The most widespread argulid species in warm water culture systems are the carp-associated species Argulus foliaceus and A. japonicus, which, through the introductions of carp and goldfish, have become transcontinentally widespread. Although they are primarily associated with carp, these species are opportunistic in their choice of hosts and also parasitize non-cyprinid fish. Since 1980, A. japonicus, as well as L. cyprinacea, became rare in farmed carp. This may have resulted from the intensive and frequent application of insecticides to fish ponds. The average level of infestation found in both hosts is about six per fish, occasionally, however, C. gariepinus hosts have been found to harbor up to 35 parasites. Although no record exists of argulids infesting farmed fish, the severe pathology seen on fish infested by these argulids suggest a potenially serious threat to farmed fish.

Pathology

Argulids infesting the fish's skin or mouth induce severe local damage to the integument. The damage is caused by the piercing proboscis stylet that enters deep into the dennal layer and secretes lytic and toxic substances resulting in acute hemorrhagic inflamed wounds. Crawling and attachment to the skin also causes irritation and abrasions leading, first to proliferation, and later to desquamation and erosion of the epithelium. Damage is quickly alleviated when parasites can actively change attachment sites. When a shift of attachment site is slowed or prevented by overcrowding, as in heavy infestations (particularly true of site-specific parasite species) chronic inflammation develops, the infected surface of the integument ulcerates, the epithelium is destroyed, and the dermis becomes exposed. Particularly deep ulcers are formed by site-restricted species, such as A. bicolor, which infests its host at the base of the tail. Persistent irritation, as caused by heavy infestations, may affect appetite resulting in anorexia and cessation of growth. Frequently the parasite-induced wounds become secondarily contaminated by bacteria and fungi (Saprolegnia). Another aspect of Argulus infestation is their possible role as vectors of viral infections. For example, Spring viraemia of carp can be transmitted via A. foliaceous.

Argulus foliaceus on goldfish
Argulus foliaceus on the tail fin and head of goldfish
Noaman V et al. Iran J Parasitol. Jun 2010; 5(2): 71-76.

Argulus sp., fish louse taxonomy, facts, pathogenicity, bibliography at MetaPathogen

 

Argulus in PubMed

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References