Will Australia's common carp (Cyprinus carpio) populations develop resistance to Cyprinid herpesvirus 3 (CyHV-3) if released as a biocontrol agent? Identification of pathways and knowledge gaps.
Due to its significant impacts on the environment and their overabundance in multiple waterways, common carp (Cyprinus carpio) has long been a target for control in southeastern Australia. Local eradication and site-based actions have had limited impacts, instigating national efforts to examine alternatives for effective long-term control, including the release of the exotic pathogen Cyprinid herpesvirus-3 (CyHV-3) as a biological control measure. First described in Germany and Israel in the late 1990s, CyHV-3 causes a serious clinical disease in carp but it is a species-specific virus with no evidence of it causing clinical or histological signs of disease in non-target species. Detailed modelling studies predict that in the short term this virus would effectively suppress high density populations of carp in the rivers and waterbodies of southeastern Australia, particularly in the southern Murray-Darling Basin (MDB). However, the long-term efficacy of CyHV-3 as a biocontrol remains more uncertain, owing to the possibility that the host will develop resistance allowing populations to recover. Here we review the literature to identify the factors that could influence the evolution of host resistance following the release of CyHV-3 in southeastern Australia and suggest follow-up strategies to address knowledge gaps. One of the factors identified as a risk is the influence of common carp hybrids, and the introgression of resistant genes from goldfish (Carassius auratus). Carp × goldfish hybrids exist in southeastern Australia, and even though hybrids are generally not as ecologically fit as parental taxa, the impact of hybrids on the efficacy of CyHV-3 as a biocontrol at a population level needs further evaluation. Australian carp populations are most likely of western European ancestry, with no evidence suggesting that they bear genetic relatedness to resistant carp strains from the Amur river in northeastern Asia, but this needs to be confirmed through genetic comparison. Finally, considering that resistance to CyHV-3 is complex and polygenic, Amur-resistance alleles are absent in the Australian carp populations, and selection pressure on the host due to case fatality would be moderate (60-80% mortality), we predict that the evolution of host resistance arising from a random mutation in the genome will develop at a slow pace and resistance might take decades to impact on CyHV-3 field effectiveness.