Assessing the susceptibility of previously untested basin fish species to Epizootic Haematopoietic Necrosis Virus (EHNV) and its epidemiology in the wild

Key words: Epizootic Haematopoietic Necrosis, EHN, disease, virus, native fish, Native Fish Strategy

Epizootic haematopoietic necrosis virus (EHNV) is a viral pathogen of international concern. The disease it causes is known from Redfin Perch (Perca fluviatilis) and farmed Rainbow Trout (Oncorhynchus mykiss) in some parts of the upper Murrumbidgee catchment and the lower Murray catchment in NSW, as well as from some parts of Victoria. Outbreaks have been recorded since 1985.

Despite its significance, very little was known about the natural distribution of EHNV in Australia or its real impact on native fish, as no formal surveys had ever been conducted. Although the susceptibility of native finfish species to EHNV had been suspected for some time, no work had been done to confirm this.

Objectives and methods: The objectives of this project were to identify the extent to which EHNV is a risk to native fish in the MDB and to provide scientific knowledge to aid in the development of effective management policy. The specific aims were to;

  1. validate earlier findings of susceptibility of native fish to EHNV,
  2. determine the susceptibility to infection by EHNV of a range of previously untested fish species found in the Basin,
  3. investigate the epidemiology of EHNV in wild populations of priority fish species and,
  4. develop a test to determine exposure of wild populations of priority fish species to EHNV.

Lab tests were undertaken using fish from candidate species separated into treatment and control groups, in which the former were exposed to the virus to identify native fish that are susceptible to EHNV (Fig 1). Subsequent analysis of 3622 tissue and 492 blood samples from fish collected from the field enabled the project team to look for instances of EHNV in the wild. Lastly, a new blood test was developed for Silver Perch (Bidyanus bidyanus), Murray Cod (Maccullochella peelii), Macquarie Perch (Macquaria australasica) and Redfin Perch, to detect antibodies resulting from immune responses to EHNV.

Figure 1, Experimentally infected Redfin Perch showing multiple white spots in the liver. Each spot is an area of dead tissue, where cells have become infected with and killed by EHNV.  (Photo courtesy Richard Whittington.)

Figure 1, Experimentally infected Redfin Perch showing multiple white spots in the liver. Each spot is an area of dead tissue, where cells have become infected with and killed by EHNV. (Photo courtesy Richard Whittington.)

Findings: It was concluded that EHNV is still present in the upper Murrumbidgee River catchment in the Murray-Darling Basin (MDB). During the study two separate outbreaks of disease due to EHNV occurred in juvenile Redfin Perch (at two locations) and a dead Redfin Perch infected with EHNV was detected (at one of the locations of an outbreak). EHNV appeared to be absent from Redfin Perch populations elsewhere in the MDB as there were no reports of disease outbreaks and neither virus nor antibodies against the virus were detected.

EHNV appeared to be absent from other species of fish in the MDB during the study period. Enough data were collected to be 95% confident that EHNV was present in <10% of the population of the following species: River Blackfish (Gadopsis marmoratus), Brown Trout (Salmo trutta), Mountain Galaxias (Galaxias olidus), Eastern Mosquitofish (Gambusia holbrookii), Murray Cod, Silver Perch, Southern Pygmy-perch (Nannoperca australis), Rainbow Trout and Redfin Perch.

The susceptibility of Silver Perch, Macquarie Perch and Eastern Mosquitofish to EHNV after exposure with water was confirmed. Two new susceptible species were identified: Murray-Darling Rainbowfish (Melanotaenia fluviatilis) and Freshwater Catfish (Tandanus tandanus). Species which became infected with EHNV following exposure via water, and in which some individuals survived and appeared to carry the live virus were Silver Perch, Eastern Mosquitofish, and Redfin Perch. Species with resistance to EHNV following exposure via water were Murray Cod, Golden Perch, Un-specked Hardyhead (Craterocephalus stercusmuscarum), Carp Gudgeon (Hypseleotris spp), Southern Purple-spotted Gudgeon (Mogurnda adspersa), Trout Cod and Southern Pygmy-perch.

Redfin Perch generally were highly susceptible to EHNV, but there appeared to be differences in susceptibility between populations. Fish from Blowering Dam in the known endemic region appeared to have a greater degree of resistance than others.

A new blood test was developed during this project to detect antibodies resulting from immune responses in fish against EHNV. A specific test was developed for Silver Perch, Murray Cod, Macquarie Perch and Redfin Perch with results suggesting that the blood test has application in field surveys for EHNV.

Lessons learned and future directions: The findings of this study reinforce a view that EHNV is a factor detrimental to native fish populations in the MDB and policies to reduce the risk of exposure to the disease in the MBD are justifiable and necessary to protect native fish populations.

The blood test developed in this study is versatile and opens up a new range of options to study the health of fish in the MDB. It has application in a much wider range of species, and it can be adapted to detect antibodies against many other pathogens. An advantage of the blood test is that it can reveal past exposure of a population to EHNV.

Stakeholders and Funding bodies: This project was funded through the Murray-Darling Basin Authority’s Native Fish Strategy and undertaken by a collaborative team from the University of Sydney and NSW DPI.

Contacts: Professor Richard Whittington, University of Sydney, Tel +61 2 9351 1619, Email:


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