Edwardsiella ictaluri

Edwardsiella ictaluri
Kingdom	Bacteria
Phylum	Proteobacteria
Class	Gammaproteobacteria
Order	Enterobacteriales
Family	Enterobacteriaceae
Genus	Edwardsiella
Species	Edwardsiella ictaluri

Also known as: Enteric Septicaemia of Catfish — Hole in the Head Disease — ESC.

Introduction

Edwardsiella ictaluri belongs to the Enterobacteriaceae family and is a Gram negative, short, pleomorphic rod, measuring 0.75 × 1.5-2.5 µm, which is weakly motile at 25-30°C, but not at higher temperatures. It has peritrichous flagella and occasionally pili that can be seen with a scanning electron micrographs and can have between one to three plasmids depending on their molecular mass. It is generally considered an obligate pathogen, although it can survive in sterilised pond bottom mud for over 90 days but does not compete well with other microbes. The organism is lactose negative, catalase-positive, cytochrome oxidase-negative, glucose fermentative and reduces nitrate to nitrite.

E. ictaluri affects fish species only and causes enteric septicaemia of catfish (ESC) and various other species of fish. ESC is considered one of the most important infectious disease problems in the commercial catfish industry in the USA. Other species of Edwardsiella include E. tarda, which causes septicemia in fish and can affect other animals, whereas E. hoshinae infects birds and reptiles.

Within channel catfish species the bacteria cause two forms of ESC; an acute septicaemia and chronic encephalitis. In the latter form the infection spreads from the olfactory sacs, and migrates along the olfactory nerves to the brain, generating granulomatous inflammation. In the acute form of ESC, the disease is thought to develop from the intestinal mucosa causing a bacteremia.

Signalment

Channel Catfish

Wild hosts include white, bullhead, blue, and wels catfish species and Japanese eel, Glass knifefishes, Tadpole Madtom, Rosy barb (minnow family), and species of carp called Devario devario .

Domestic hosts include white, walking, channel and sutchi catfish species and under experimental setting rainbow trout and chinook salmon.

Clinical Signs

With the chronic form of ESC clinical signs include, altered mentation, listlessness and chaotic swimming with ‘head-up, tail-down’ posture, circling and mortality. In later stages, the dorsum of the head swells and ulcerates revealing areas of the brain (hence the name ‘hole in the head disease’).

With acute forms of ESC you can see petechial haemorrhages around the buccal area, throat, abdomen and the fin base, that progress to depigmented ulcers. Fish generally suffer from moderate pale inflamed gills, exophthalmia, anaemia, haemorrhagic enteritis, systemic oedema, dropsy, ascites and splenomegaly. General behavioural changes include loss of balance, swimming near the surface, lethargy and cessation of feeding.

Epidemiology

The bacteria can survive in pond sediment and once a population of fish have recovered from an infection of ECS, they can become carriers. They can be found in the kidneys of fishes and are thought to be shed in the faeces of fish.

Outbreaks are mainly seasonal and occur within a set temperature range of 18-28°C, primarily in spring and autumn. This temperature limitation precludes the bacterium from being a pathogen for humans or other warm-blooded animals and is not therefore zoonotic. Other environmental factors have been linked to outbreaks and include poor water quality, high stocking density and other stressors.

E.ictaluri can invade the, gill mucosa, olfactory organ and nasal epithelium and nerve, brain meninges, skull and capillaries in the dermis of the skin.

Distribution

E.ictaluri is mainly found in the USA, Asia and Thailand. The continual worldwide dissemination of channel catfish for aquaculture purposes may increase its future distribution.

Pathology

Histological examination reveals a systemic infection of all organs and skeletal muscles, with the most severe changes being diffuse interstitial necrosis of the anterior and posterior kidney and systemic haemorrhages. Focal necrosis in the liver and spleen are also generally seen as pale grey/white lesions.

Skeletal muscle and areas of necrosis within internal organ tissue can be infiltrated with macrophages, that phagocytose the bacteria but do not destroy them , , .

Diagnosis

Clinical signs are quite pathognomonic for ESC but PCR is used to confirm the presence of E. ictaluri in blood and tissues but other methods have been used such as indirect FAT (detecting antibodies) and ELISA test.

The organism is slow growing and forms small, translucent, greenish colonies on Edwardsiella isolation media (EIM), while inhibiting Gram-positive and most Gram-negative contaminating organisms. E. ictaluri can be separated from E.tarda because it is indole-negative and does not produce H₂S on triple sugar iron (TSI) agar.

Treatment

Potentiated sulphonamide, sulfadimethoxine, methoprim or oxytetracycline have been used to treat ESC, but resistance has been recorded.

Control

ESC may be controlled through reducing the amount of stress in fish stocks and cessation of feeding when outbreaks occur. A vaccine called Aquavac ESC is available and administered in water by bath immersion.

References

Edwardsiella ictaluri Learning Resources
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