Koi herpesvirus disease - exploiting new sequencing technologies for improved responses and control
Background
Koi herpesvirus disease (KHVD) is a highly contagious viral disease affecting common carp (Cyprinus carpio) and is caused by Cyprinid herpesvirus-3 (CyHV-3). Since 2007, it has been classified as a notifiable disease under the World Organisation for Animal Health (WOAH). There have been reported cases of KHVD in 18 European countries, Asia, South Africa and Iraq.
The disease spreads rapidly when water temperatures range between 17°C and 28°C, with mortality rates reaching 100% during initial outbreaks.
In England, infected fisheries are placed under statutory movement restrictions by the Fish Health Inspectorate (FHI). These restrictions often remain in place for several years, creating operational and financial impacts for owners of fisheries.
Economic impact
While the total financial cost of KHVD to the UK industry is unknown, coarse fisheries had been valued at £2.3 billion in 2001. Also, anglers are spending nearly £2 billion annually. Large scale outbreaks could significantly impact rural economies in the UK. For comparison:
in Germany, outbreaks resulted in losses of €150,000 to €250,000 per fish farm (20 tonnes of fish)
between 2010 and 2016, 138 KHVD outbreaks were recorded across UK fisheries.
Key challenges
Managing KHVD requires answers to a number of scientific questions;
Transmission routes
understanding how the virus spreads between and within fisheries
Virus persistence
assessing how long CyHV-3 remains viable in the environment
Reactivation
assessing the risk of latent virus re-emerging in seemingly healthy fish
Molecular epidemiology of CyHV-3
CEFAS examined CyHV-3 genetic variability using Variable Number Tandem Repeat (VNTR) polymorphisms analysis. The study identified:
75 distinct VNTR profiles were identified in UK samples between 2003 and 2016. The profiles indicated multiple independent introductions. Greater global genetic diversity of CyHV-3 was highlighted, than previously recognised globally
the large number of unique CyHV-3 haplotypes suggests that the importation of infected fish poses a greater risk than local transmission between fisheries
VNTR analysis often links outbreaks to common suppliers, emphasising the role of biosecurity in preventing disease spread.
Scientific innovation
VNTR analysis is now routinely used by FHI to investigate outbreak linkages. However, current methods are time consuming and costly, with only 20% of available cases analysed.
New genomic techniques such as long-read sequencing, could streamline this process, enabling faster and more comprehensive surveillance.
Metagenomic approaches for surveillance
Environmental DNA (eDNA) offers a non-invasive method for KHVD surveillance. However, a key concern with metagenomic sequencing for surveillance when using eDNA, is the inadvertent detection of OIE-listed pathogens.
These pathogens potentially trigger full-scale disease investigations based on a ‘suspicion’ of infection. This is often without clear insight into the infection status inferred from eDNA signals.
CyHV-3-positive ponds provide a natural setting to evaluate eDNA screening, as cyprinid DNA pathogens are limited, reducing the risk of detecting unrelated OIE-listed pathogens. Metagenomic approaches can:
detect low-level CyHV-3 infections in water and sediment samples
assess intra- and inter-sample variation to improve surveillance accuracy
By refining genetic and metagenomic tools, KHVD management can be enhanced, reducing economic losses and improving biosecurity across fisheries.