Tandanus Tandanus.
Gunther Schmida

Epizootic haematopoietic necrosis virus (EHNV) is a lethal virus that infects fish through the body surface or gastrointestinal tract. Once in the host, it multiplies in the blood forming organs such as the spleen and kidney and destroys them in the process, ultimately killing the fish. EHNV is only present in Australia, endemic to catchments in south eastern Australia. The disease was first detected in Australia in Victoria during the 1980’s, however the disease is far from idle…

There have been natural outbreaks of the virus in wild causing mass deaths of Redfin perch (Perca fluviatilis) and less serve cases of the disease in captive Rainbow trout (Oncorhynchus mykiss) populations. Recently cases in 2021 recorded at the Lake Hume Resort (-36.103399° 147.038996°) are a cause for concern, reminding anglers to remain vigilant for signs of the virus and report any fish kills in redfin perch or other species.

It is vital to remember that Redfin are a notifiable species in NSW and that it is illegal to move, buy, sell or be in possession of live redfin.

Redfin must either be released at point of capture (which is discouraged) or humanly euthanised.

Juvenile Redfin perch killed by EHNV, note enlarged abdomen. Source: J Humphrey, Australian Fish Health Reference Laboratory.

This killing virus is carried by wild populations of redfin perch, a cause for concern in the aftermath of recent floods. Due to the volume of water travelling through the system, fish are able to move into new habitats. This means that redfin perch may now begin to appear in areas where they may not have previously been identified. Any place where redfin perch appear, there is a risk of an EHNV outbreak. Eastern Mosquitofish, another invasive species, is also known to be susceptible to the virus from laboratory studies. In the upcoming Spring/Summer period, the likelihood of cases of the virus are expected to rise as water temperature increases.

How can I identify EHNV?

Signs of EHNV can be varied depending on species and location.

However, an infected fish may have:

  • Be dead – often fish will die before signs of disease are visible
  • A swollen abdomen
  • Darkened skin colour
  • Disorientated fashion when swimming
  • Petechial (pinpoint) haemorrhages at the base of the fins
  • Haemorrhaging of the gills
  • Enlargement of the kidney, liver and spleen
  • Focal white to yellow liver lesions

Additionally, dead fish may be found on the bank/water surface in small to large numbers. In some cases, hundreds or thousands of dead fish have been found in EHVN related outbreaks. However, even small death event (5-10 fish) should be reported.  

EHNV diagram. Source: Joy Becker.

What do I do?

If you have found a suspected case of EHNV or have encountered a fish kill event, call the Emergency Animal Disease Watch Hotline on 1800 675 888.

What about native fish?

Although natural outbreaks of the EHNV have only been observed in redfin perch and rainbow trout, the potential exists for a natural outbreak of the virus in native fish. An earlier study by Becker et al. (2013), investigated which freshwater Australian fish species can host the virus. Of the freshwater native fish species from southeastern Australia experimentally exposed to the virus, Silver Perch (Bidyanus bidyanus) and Macquarie Perch (Macquaria australasica) were identified as susceptible to EHNV following a bath challenge. In addition, Dewfish (Tandanus tandanus) and Murray–Darling Rainbowfish (Melanotaenia fluviatilis) were identified as being potentially susceptible to EHNV.  

Unspecked Hardyhead (Craterocephalus stercusmuscarum), Carp Gudgeon (Hypseleotris spp.), Trout Cod (Maccullochella macquariensis), Southern Purple-spotted Gudgeon (Mogurnda adspersa), and Southern Pygmy Perch (Nannoperca australis) were potentially not susceptible to EHNV following the bath challenge.

A Macquarie Perch is held gently out of the water by human hands.
Macquarie perch. Source: Renae Ayres & Joanne Sharley

Where are fish affected?

In a 2016 study by Becker et al., the susceptibility of redfin perch from different areas to EHNV was examined under lab conditions using a bath challenge.

A bath challenge is an experiment which exposes fish by adding an appropriate dose of virus to the water. The fish were held in an aquarium style tank with constant water temperature at 21°C) and additional aeration. The fish were monitored once to twice daily for 28-35 days for signs of disease.

They found that initial outbreaks of EHNV in newly impacted areas have a high mortality rate. In areas where the disease has been identified before, such as Blowering Dam, the redfin perch had a notably decreased susceptibility to EHNV with less than 40% becoming infected following a bath challenge. By contrast, redfin perch from nearby water bodies (e.g., Bethungra Dam and Tarcutta Creek) and distant water bodies (e.g., in Western Australia) that had not experienced the virus before, showed moderate to high susceptibility during the bath challenge.

With floods moving redfin perch into new areas and the warming temperatures, the potential for an outbreak of EHNV is increased.

Another study by Becker et al. (2019), found that EHNV was probably confined to the upper Murrumbidgee River and associated impoundments in NSW and the ACT between 2007 to 2011. There is a possibility that EHNV is present in other areas of the MDB but was not detected, as well as locations in VIC where it originated, although these were not sampled in this study.

Not all parts of the upper Murrumbidgee catchment, for example, Cotter Reservoir, are infected, and it would be desirable to maintain this freedom to protect the resident Macquarie perch population there. All reports of fish kills in the upper Murrumbidgee catchment region should be investigated and laboratory tests for EHNV undertaken. Routine surveillance of redfin perch populations in the upper Murrumbidgee catchment should be undertaken to determine the occurrence of EHNV. Policies to exclude recreational use of these areas during EHNV outbreaks, close fisheries, or enable compulsory disinfection should be considered.

Blowering Dam. Source: VisitNSW

Conclusion

Ultimately, EHNV is still active in Australia and vigilance is key. With changing climatic conditions and water management altering flows, cases may rise this upcoming season. Report any suspected cases on 1800 675 888.

Resources

2019 study by Becker et al.

2016 study by Becker et al.

2013 study by Becker et al.

Quarterly Aquatic Animal Disease Report

World Organisation for Animal Health (WOAH) Information

Featured image: Redfin perch

Credit: Gunther Schmida

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