Emergency Animal Diseases Bulletin No. 108

Edwardsiella ictaluri – a significant pathogen of fish

Edwardsiella ictaluri, of theFamily Enterobacteriaceae, is a gram negative, rod–shaped bacterium that causes significant disease in a range of fish species. Disease due to E. ictaluri is included on Australia’s ‘National List of Reportable Diseases of Aquatic Animals’1 and is notifiable in each state and territory.

Occurrence overseas

E. ictaluri causes enteric septicaemia of catfish (ESC) or bacillary necrosis of catfish, and significantly impacts on catfish production industries overseas, particularly channel catfish (Ictalurus punctatus) farming in the United States of America (USA) and basa (Pangasius spp.) farming in Vietnam. Of all losses in catfish production in the USA, 60% can be attributed to ESC.2

E. ictaluri primarily affects catfish, which is a generic term used to describe a variety of fishes from different families with the common feature of four or more barbels around the mouth. However, other susceptible species include many ornamental fish species, salmonids (such as rainbow trout) and the Asian fish ayu. E. ictaluri has also been closely associated with ‘pot–belly syndrome’ in barramundi (Lates calcarifer) in Asia.

Occurrence in Australia

In Australia, E. ictaluri was recently reported to have caused clinical disease and mortality in captive native Australian catfish species. The bacterium was isolated from the eel–tailed catfish Anodontiglanis dahli and Neosilurus ater, and from the fork–tailed catfish Neoarius berneyi,held in an aquarium facility. Fish showing clinical disease had been subjected to stresses of capture from the wild and crowding prior to developing the disease. It is suspected that these native Australian catfish were exposed to E. ictaluri via cross–contamination with infected imported ornamental fish held in the same facility. Targeted surveillance of wild fish in the affected and risk areas, and passive surveillance nationally, has not detected evidence of E. ictaluri in wild Australian fish populations.

While E. ictaluri had previously been reported in Australia from live imported fish, particularly zebra danio, Danio rerio, which are tropical freshwater fish used as ornamental and laboratory animals, and rosy barb, Puntius conchonius, which are sub–tropical freshwater fish used as ornamental species, it has never been reported from wild or farmed Australian fish.

Clinical disease

In channel catfish, chronically affected fish may develop encephalitis. Moribund animals develop swelling on the dorsal head, often accompanied by deep ulceration.

Barramundi with ‘pot belly syndrome’ may display abnormal body darkening, poor feeding, emaciation, abdominal distension or ‘pot–belly’ (which may be indicative of peritonitis), and a red, swollen anus.3

Across susceptible species, asymptomatic infections are common. Fish that have recovered from infection are known to become asymptomatic carriers. Disease can reoccur in carrier fish that have been exposed to stress.

Other clinical signs and gross pathology of E. ictaluri infection are described in the ‘Aquatic Animal Diseases Significant to Australia: Identification Field Guide’.4

Diagnosis

As many of the clinical signs are not pathognomonic, infection with E. ictaluri must be definitively diagnosed by laboratory testing. Suitable tests include bacterial culture of tissues such as kidney, liver and spleen; polymerase chain reaction (PCR); indirect immunofluorescence; and, enzyme–linked immunosorbent assay (ELISA).

Treatment

Antibiotics are the primary treatment for disease due to E. ictaluri. However, antibiotic resistance is an increasing problem and multiple antibiotic resistant strains are now common throughout major catfish producing areas.5 As such, diagnosis should be followed by in vitro testing for antibiotic sensitivity to enable successful treatment.

Vaccines, using attenuated or low virulence strains of E. ictaluri, have previously been developed for oral or immersion administration, but their use has been limited either by the duration of immunity or by the levels of protection conferred. They have been improved, with a new vaccine for use in catfish culture released in Vietnam in 2011 for large scale use.6

Implications for Australia

Significant populations of susceptible fish species – including catfish, salmonids and barramundi – are present in Australia and exposure of these populations to E. ictaluri may have significant impacts.

Catfish

There are several families of catfish in Australia. Plotosid or eel–tailed catfish are important elements of the freshwater fish fauna of many northern Australian rivers and are regarded as an excellent table fish. There are 11 species of plotosid catfish in Australian freshwaters and many more in estuarine and marine environments. These catfish breed in the wet season and are collected for export and domestic markets as aquarium fish. Some are also bred in hatcheries and sold for stocking private waters for fishing. They are an important food source for indigenous Australians who prefer them to most other freshwater fish.

Fork–tailed (ariid catfish) inhabit both freshwater and estuary environments. There are five species in fresh waters. They are mainly demersal (mid–water level), carnivorous fish and may be present in very large numbers where suitable food resources (such as herrings) exist. They may be a main food source for some populations of barramundi. Fork–tailed catfish are also the basis for small commercial fisheries, for example ‘golden cobbler’ in Western Australia.

While Australian native catfish species are not considered to be economically important, the impacts of a virulent strain of the bacterium on completely naïve populations could be dramatic. Periodic stress events in some areas of their range (such as cold temperatures) could predispose wild fish to disease if the pathogen is present. As native catfish may be present in very large numbers, mass die offs could have social, visual amenity, water quality and ecological impacts.

It was not previously known that Australian catfish from two very different families (Plotosidae and Ariidae) are susceptible to infection with E. ictaluri. Given the current known host range of E. ictaluri (five families of catfish, salmonids, cyprinids and other fish families), the susceptibility of other Australian native fish species is likely.

Salmonids

A number of salmonid species are susceptible to either natural or experimental infection with E. ictaluriand outbreaks of disease have been seen in farmed rainbow trout overseas.7 Salmonid farming is the largest and most valuable finfish aquaculture industry in Australia. There is also substantial economic activity associated with recreational salmonid fishing in stocked areas. Salmonids are found in southern Australia, where temperatures are generally too low for the expression of clinical disease. However, there are short periods in which temperatures could be warm enough to permit outbreaks of clinical disease in farmed or wild (including stocked) salmonids.

Barramundi

‘Pot–belly syndrome’ in barramundi (Lates calcarifer) in Asia is closely associated with E. ictaluri and may result in mortalities of up to 20%.3 Barramundi farming is the most significant finfish aquaculture industry in northern Australia. Barramundi also support important recreational and commercial fisheries in northern Australia, generating significant income for regional areas. Temperatures in this area are conducive to expression of clinical disease.

If you suspect that you have seen a case of infection with E. ictaluri, you can report it to the 24 hour Emergency Animal Disease Watch Hotline on freecall 1800 675 888.

Brett Herbert
Animal Health Policy
Australian Government Department of Agriculture,
Fisheries and Forestry

References and further reading

  1. Australia’s National List of Reportable Diseases of Aquatic Animals.
  2. Mitchell AJ. Fish disease summaries for the southeastern United States from 1976 – 1995. Aquaculture Magazine 1997;23:87–93.
  3. Gibson–Kueh S. Crumlish M and Ferguson HW. A novel ‘skinny pot–belly’ disease in Asian seabass fry, Lates calcarifer (Bloch). Journal of Fish Diseases 2004;27:731–735.
  4. Aquatic Animal Diseases Significant to Australia: Identification Field Guide.
  5. Dung TT, Haesebrouck F, Tuan NA, et al. Antimicrobial Susceptibility Pattern of Edwardsiella ictaluri Isolates from Natural Outbreaks of Bacillary Necrosis of Pangasianodon hypophthalmus in Vietnam. Microbial Drug Resistance 2008;14:311–316.
  6. PHARMAQ starts fish vaccination in Vietnam.
  7. Noga EJ (2010) Fish Disease. Diagnosis and Treatment. Second edition. Wiley–Blackwell, Ames, USA.