Aquatic Animal Diseases Significant to Australia: Identification Field Guide 4th Edition




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Aquatic Animal Diseases Significant to Australia:
Identification Field Guide 4th Edition

Viral haemorrhagic septicaemia (VHS)

EXOTIC DISEASE

VHS in rainbow trout (Oncorhynchus mykiss); note pale colour of stomach region, pinpoint haemorrhages in fatty tissue and pale gills

Source: T Håstein


VHS in rainbow trout (Oncorhynchus mykiss); note swollen stomach and ‘popeye’

Source: T Håstein

Signs of disease


Important: Animals with disease may show one or more of the signs below, but the pathogen may still be present in the absence of any signs.

Disease outbreaks are seen in farmed trout and other salmonids, as well as in farmed turbot and Japanese flounder.



Disease signs of acute infection at the farm, tank or pond level are:

  • rapid onset of high mortality

  • lethargic swimming

  • separation from shoal

  • loss of appetite

  • crowding at pond edges.

Disease signs of chronic infection at the farm, tank or pond level are:

  • significant cumulative mortality (protracted)

  • uncoordinated swimming (ataxia) with rotating movement around body axis (i.e. spinning).

Disease signs of the neurological form of the disease at the farm, tank or pond level are:

  • low mortality

  • severe abnormal swimming behaviour (flashing and spiralling).

General gross pathological signs are:

  • exophthalmos (popeye)

  • haemorrhaging under the skin, around the base of pectoral and pelvic fins and in the eyes

  • swollen abdomen

  • pale gills, with or without petechial (pinpoint) haemorrhages.

Gross pathological signs of acute infection are:

  • slight darkening of the body colour

  • exophthalmos (popeye)

  • bleeding around the eyes

  • bleeding under the skin around the base of the pectoral and pelvic fins

  • skin ulceration

  • swollen abdomen

  • pale gills with pinpoint haemorrhages

  • ascites (fluid in the abdominal cavity)

  • petechial (pinpoint) haemorrhages in the fatty tissue, intestine, gonads, liver, swim bladder and muscle

  • dark-red kidneys.

Gross pathological signs of chronic infection are:

  • often an absence of external signs

  • intense darkening of the skin

  • exophthalmos (popeye)

  • pale gills (anaemic)

  • pale abdominal organs

  • pale and mottled liver (evidence of haemorrhages on surface)

  • pale gastrointestinal tract that is empty of food.

Microscopic pathological signs are:

  • accumulation of erythrocytes in skeletal muscle fibres

  • extensive focal necrosis in the liver, kidney and spleen

  • VHS virus-positive endothelial cells in vascular system evident from immunohistochemistry.

Disease agent


VHS virus is a rhabdovirus of the genus Novirhabdovirus. Several genogroups or genotypes of the virus have been identified from different environments in different parts of the world:

  • type I, continental Europe—freshwater group, trout farms (highly pathogenic to rainbow trout)

  • type II, European marine strain (Baltic sea)—marine strain affecting wild and cultured marine and freshwater species (has low pathogenicity in rainbow trout)

  • type III, north Atlantic marine group (North sea near the British Isles)

  • type IVa, west coast of north America and east Asian group—marine group affecting a range of free-living marine and cultured species (highly pathogenic in Pacific herring; however, rainbow trout appear refractory to infection with this genotype)

  • type IVb, Great lakes region—significant mortalities in wild freshwater species in the Great lakes.

Host range


VHS virus has been isolated from a broad range of marine and freshwater fish in Europe and the north Pacific (including cod, sprats, herring, haddock and turbot).

Species known to be susceptible to VHS are listed below.



Common name

Scientific name

Armoured weaselfish

Hoplobrotula armata

Atlantic cod

Gadus morhua

Atlantic halibut

Hippoglossus hippoglossus

Atlantic herring

Clupea harengus

Atlantic salmon

Salmo salar

Black crappie

Pomoxis nigromaculatus

Black sea bream or black porgy

Acanthopagrus schlegeli

Bluegill

Lepomis macrochirus

Blue whiting

Micromesistius poutassou

Bluntnose minnow

Pimephales notatus

Brook trout

Salvelinus fontinalis

Brown bullhead

Ictalurus nebulosus

Brown trout

Salmo trutta

Burbota

Lota lota

Channel catfish

Ictalurus punctatus

Chinook salmon

Oncorhynchus tshawytscha

Chub mackerel

Scomber japonicus

Coho salmon

Oncorhynchus kisutch

Dab

Limanda limanda

Emerald shiner

Notropis atherinoides

English sole

Parophrys vetulus

Eulachona

Thaleichthys pacificus

European eel

Anguilla anguilla

European seabass

Dicentrarchus labrax

European sprat

Sprattus sprattus

Flounder

Platichthys flesus

Fourbeard rockling

Enchelyopus cimbrius

Freshwater druma

Aplodinotus grunniens

Gilt-head sea bream

Sparus aurata

Gizzard shad

Dorosoma cepedianum

Golden trout

Oncorhynchus aguabonita

Grayling

Thymallus thymallus

Greenland halibut

Reinhardtius hippoglossoides

Haddock

Melanogrammus aeglefinus

Hairtail

Trichiurus lepturus

Hybrid (rainbow trout × coho salmon)

Oncorhynchus mykiss × O. kisutch

Iberian nase

Pseudochondrostoma polylepis

Japanese floundera

Paralichthys olivaceus

Japanese yellowtail

Seriola quinqueradiata

Korean flounder

Glyptocephalus stelleri

Lake trout

Salvelinus namaycush

Lake whitefish

Coregonus clupeaformis

Largemouth bass

Micropterus salmoides

Lesser argentine

Argentina sphyraena

Mullet

Mugil cephalus

Mummichoga

Fundulus heteroclitus

Muskellungea

Esox masquinongy

Norway pout

Trisopterus esmarki

Pacific cod

Gadus macrocephalus

Pacific hakea

Merluccius productus

Pacific herringa

Clupea pallasii

Pacific salmon

Oncorhynchus spp.

Pacific sand eel

Ammodytes personatus

Pacific sand lance

Ammodytes hexapterus

Pacific sardinea

Sardinops sagax

Pacific tomcod

Microgadus proximus

Pikea

Esox lucius

Plaice

Pleuronectes platessa

Poor cod

Trisopterus minutus

Pumpkinseed

Lepomis gibbosus

Rainbow trouta

Oncorhynchus mykiss

Red-spotted grouper or Hong Kong grouper

Epinephelus akaara

River lamprey

Lampetra fluviatalis

Rock bass

Ambloplites rupestris

Rockfish

Sebastes spp.

Round gobya

Neogobius melanostomus

Sablefisha

Anoplopoma fimbria

Sand eel

Ammodytes spp.

Sand goby

Pomatoschistus minutus

Senegalese sole

Solea senegalensis

Shiner perch

Cymatogaster aggregata

Shorthead redhorse

Moxostoma macrolepidotum

Silver pomfret

Pampus argenteus

Silver redhorse

Moxostoma anisurum

Smallmouth bassa

Micropterus dolomieui

Snapper

Pagrus auratus

Splake (lake trout × brook trout)

Salvelinus namaycush × S. fontinalis

Spottail shiner

Notropis hudsonius

Striped bass

Morone saxatilis

Surf smelta

Hypomesus pretiosus

Three-spined stickleback

Gasterosteus aculeatus

Trout-perch

Percopsis omiscomaycus

Tubesnout

Aulorhynchus flavidus

Turbota

Psetta maxima (also known as Scophthalmus maximus)

Walleye pollock or Alaska pollocka

Theragra chalcogramma

White bass

Morone chrysops

Whitefish

Coregonus spp.

White perch

Morone americanus

Whiting

Merlangius merlangus

Yellowback seabream

Evynnis tumifrons

Yellow percha

Perca flavescens

a Naturally susceptible (other species have been shown to be experimentally susceptible)

Presence in Australia

EXOTIC DISEASE—not present in Australia.

Epidemiology


  • Variant strains of the virus are responsible for disease in different geographical locations.

  • Marine and freshwater species are susceptible to VHS virus infection. Younger fish are generally more susceptible to disease.

  • Rainbow trout appear to be less susceptible to infection by marine strains of the virus.

  • Water temperatures in an outbreak are generally near 10 °C. At water temperatures between 15 °C and 18 °C, the disease generally takes a shorter course with a modest accumulated mortality, but transmission can occur at water temperatures up to 22 °C. Mortality and morbidity have rarely been documented when water temperatures are above 18 °C, although VHS virus genotype IV has caused at least one fish kill at 20–22 °C, and some isolates can replicate in vitro at temperatures up to 25 °C.

  • Transmission is horizontal directly through the water, from virus shed in faeces, urine (predominantly) and sexual fluids of clinically infected or carrier fish. The virus can also be spread by birds that have consumed infected fish, via blood-feeding vectors such as leeches, and on equipment that has been in contact with water from infected fish. The virus gains entry via the gills, skin wounds, oral exposure (predation) and possibly through the skin.

  • Once infected, survivors are lifelong carriers of the virus; however, shedding is intermittent.

  • Stressors including overcrowding, extreme temperatures and overfeeding will greatly reduce an animal’s resistance to infection.

  • Mortality rate can range from 10% to 80%, depending on the VHS virus isolate, environmental variables (temperature), age, species, route of exposure and presence of additional stressors (highest mortality rates occur with acute infection, and lowest mortality rates in the neurological form).

  • VHS virus is thought to have existed in the marine environment before its apparent transfer to fresh water, where it first became virulent in trout.

  • It has been suggested that the European freshwater strains of VHS virus originated from fish in the northern Pacific and Atlantic oceans. The mechanism of transfer was possibly through the feeding of marine fish to cultured freshwater species.

Differential diagnosis


The list of similar diseases below refers only to the diseases covered by this field guide. Gross pathological signs may be representative of a number of diseases not included in this guide, which therefore should not be used to provide a definitive diagnosis, but rather as a tool to help identify the listed diseases that most closely account for the gross signs.

Similar diseases


Enteric red mouth disease, epizootic haematopoietic necrosis, epizootic ulcerative syndrome, infectious haematopoietic necrosis, infectious pancreatic necrosis, whirling disease

Sample collection


Due to the uncertainty in differentiating diseases using only gross pathological signs, and because some aquatic animal disease agents might pose a risk to humans, only trained personnel should collect samples. You should phone your state or territory hotline number and report your observations if you are not appropriately trained. If samples have to be collected, the agency taking your call will provide advice on the appropriate course of action. Local or district fisheries or veterinary authorities may also provide advice regarding sampling.

Emergency disease hotline


The national disease hotline number is 1800 675 888. This number will put you in contact with the appropriate state or territory agency.

Further reading


The accepted procedures for a conclusive diagnosis of VHS are summarised in the World Organisation for Animal Health Manual of diagnostic tests for aquatic animals 2011, available at www.oie.int/en/international-standard-setting/aquatic-manual/access-online.

For more information on VHS virus isolates, refer to the European Community Reference Laboratory for Fish Disease at www.fishpathogens.eu/vhsv.



These hyperlinks were correct and functioning at the time of publication



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