Cyprinid herpesvirus 3, CyHV-3: its potential as a biological control agent for carp in Australia

CyHv3Rept_coverCarp (Cyprinus carpio), members of the cyprinid family, were probably introduced to Australia in the 19th Century,  but they did not become a problem until the 1960s when a strain adapted for European aquaculture, known in  Australia as the Boolara strain, was imported. Floods in the mid 1970s resulted in the escape of these fish from  isolated farm dams into the Murray-Darling River system, triggering a major environmental problem. In the mid-2000s, work began on developing an integrated carp control program (Fulton, 2006), including an investigation of cyprinid herpesvirus 3 (CyHV-3), also known as koi herpesvirus, as a potential biological control agent.

There have been only three instances around the world where viral pathogens have been used successfully to counter vertebrate pest species. These include two rabbit viruses, the myxoma virus  and the calicivirus known as rabbit haemorrhagic disease virus, both used in Australia, and feline panleukopaenia virus, a DNA parvovirus that was used as part of a program to eradicate feral cats that were devastating wild seabird colonies on sub-Antarctic Marion Island in the southern Indian Ocean.

In developing a biological control program with CyHV-3 for carp, it is important to consider what can be learned from both the experience with the three previous viruses used in successful biological control programs of vertebrates, and from the epidemiology of the disease in natural populations of wild carp (McColl et al, 2014). Determining the host specificity of a viral biological control agent is clearly important, as is an understanding of the pattern of mortality in the target species. Familiarity with the clinical course of disease in the target species will allow an assessment of the suitability of the agent based on animal welfare considerations, and an understanding of the transmissibility of CyHV-3 from infected to uninfected fish is required in order to assess its suitability as a control agent. The susceptibility of the target species to CyHV-3 must be considered, and it may be dependent on a number of factors, e.g., age-related immunity; passive immunity; the presence of cross-reactive viruses in the target population; ambient temperatures; and the proportion of hybrid individuals in the target population. In summary, the success of a viral biological control
agent is likely to depend upon an intimate knowledge of the biology of the target species, and of the epidemiology of the virus.

While the literature contains a great deal of information on many of these topics, this project aimed to address many of the issues, particularly in an Australian context. In the process, we hoped to provide a solid base of knowledge that would allow a rational decision to be made about the potential use of CyHV-3 as a biological control agent for carp in Australia.

To begin the work, CyHV-3 reference strains and the KF-1 and CCB cell lines were imported into AAHL, and CyHV-3 strains were grown in both cell lines. Eventually, an Indonesian strain of CyHV-3 was also introduced to the lab, and this was chosen for all subsequent experimental work.

In order to work with the virus, assays for the detection of CyHV-3 infections of carp were introduced to AAHL. These included: (i) virus isolation and growth of CyHV-3 in carp cell lines; (ii) several PCRs used by international aquatic animal disease diagnostic laboratories; (iii) development of a generic cyprinid herpesvirus PCR with the potential to differentiate CyHV-1 (carp pox virus), CyHV-2 (goldfish haematopoietic necrosis virus) and CyHV-3; and, (iv) immunoassays for localisation of CyHV-3 in infected fish tissues and in cell cultures.

A crucial step in the project was to test the virulence of CyHV-3 for carp in Australia. In vivo infectivity trials were  undertaken to determine if 4-10 cm carp in Australia are susceptible to infection by CyHV-3, and whether they develop disease and die. The susceptibility of older fish was also examined. Knowledge of the temporal pattern of excretion of virus from infected fish was necessary for an understanding of the epidemiology of the disease caused by CyHV-3 (koi herpevirus disease, KHVD).

The specificity of a potential biological control agent for the target species is, of course, critical. Therefore, a number of non-target species of fish were also tested in susceptibility trials. Furthermore, given that mortality in carp-goldfish hybrids challenged with CyHV-3 is lower than in pure carp, a system to allow differentiation of carp from goldfish and hybrids was considered essential.

In order to ensure that there were no cross-reactive cyprinid herpesviruses in the Murray-Darling Basin (M-DB) that might compromise the efficacy of CyHV-3 were it to be released into the M-DB, an extensive survey of carp from the M-DB was also conducted.

Produced by the Invasive Animals Cooperative Research Centre as part of the PestSmart series.

Secondary title PestSmart
Author Kenneth A McColl and Mark St J Crane
Year 2013
Place published Canberra
Publisher Invasive Animals CRC
Pages 63 pp
ISBN/ISSN Web ISBN: 978-1-921777-38-7
Control method Biological Control
Region Australia - national
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Cyprinid herpesvirus 3, CyHV-3: its potential as a biological control agent for carp in Australia    [1.2 Mb PDF]

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