Spreading Koi herpes virus using angling equipment

Koi herpesvirus (KHV) is a fatal disease of carp which has spread rapidly between UK recreational fisheries. Although live fish movements are the main risk for disease spread, we’ve demonstrated that KHV can be transferred by contaminated angling equipment. 

A fish gill showing necrosis from KHV
Gill necrosis from KHV

Transfer pathway via contaminated angling equipment

For KHV to transfer via angling equipment – eg landing-nets, unhooking mats, weigh slings and keep-nets – a number of steps must happen:

  1. the KHV infected fish – which are shedding virus in their mucus – must be vulnerable to angling, ie feeding to take a bait
  2. the virus must contaminate and remain viable on the equipment until it is used at a different fishery
  3. fish without the virus must be infected through contact with the infected equipment

Although we suspected that this is a disease transfer route, there was little direct proof. So we designed a series of simple experiments to get more data, and thought you’d like to see what goes on behind the scenes at Cefas.

Carp in a net
An example of a net used by anglers to hold carp.

Experiment 1: feeding behaviour versus KHV shedding

To investigate the chance of an angler catching a fish shedding KHV, we examined the feeding of KHV infected fish. If a fish is feeding it can be caught by an angler.

Therefore we infected common carp with KHV, via a bath challenge, and fed them 40 food pellets each day. The feeding rate was monitored by counting uneaten pellets.

Carp were netted from their tank each day and then returned to check the levels of virus in the mucus on the net.

We found that:

  • all of the carp exposed to KHV became infected; 75% developed clinical disease. The remaining 25% survived the infection by creating antibodies.
  • virus shedding started the day after exposure and continued during the trial
  • KHV infection did reduce feeding, but they still ate over 40% of the daily food ration whilst shedding virus.
Graph showing feeding rates of KHV affected fish
This graph shows a time series (from 1-22 days post infection) which plots the percent of food eaten by the fish and the amount (titre) of virus shed in their mucus

Experiment 2: how net storage conditions affect virus survival

This was an in vitro study to assess if KHV can survive on nets, and how storing nets differently would affect the chance of transmission.

Strips of net were coated with KHV-spiked carp mucus, placed in clear plastic zip-lock bags, and kept under one of four conditions:

  1. dark and damp
  2. dark and dry
  3. light and damp
  4. light and dry

The dark samples were kept in an opaque box, whilst light samples were exposed to sunlight. Damp nets were sealed in their plastic bag, whilst bags for the dry samples were split open.

The nets were incubated for 18 hours before the mucus was removed,centrifuged and filtered before it was used to inoculate common carp cell cultures. The cell cultures were then checked to see if KHV had an effect on their structure.

We found that:

  • the virus remained alive in the ‘dark and damp’ conditions, with 100% of the cell cultures were positive for KHV
  • drying reduced KHV infectivity. About 20% of cell cultures were positive for KHV in ‘dark and dry conditions’
  • KHV did not survive exposure to sunlight. There were no positive cells for KHV in both ‘light and damp’, and ‘light and dry’ conditions

Experiment 3: transmission of KHV via anglers’ nets

Next we wanted to check if anglers’ nets transmit the disease.

So we held KHV diseased carp in a fine mesh keep-net overnight at 23°C. These infected carp were then removed, the net stored for 24 hours in a sealed plastic bag, and then we introduced carp without the disease to the net.

We found that these carp displayed clinical signs of KHV disease within 14 days of transfer to the contaminated net.

Dry your nets in the sun to protect fisheries

Hopefully you can see these experiments show that fish shedding KHV virus feed and are therefore likely to be caught by anglers, they can contaminate nets, the virus can survive on these nets, and that KHV can be transmitted to uninfected fish by contact with infected nets.

Put simply, the use of infected nets is a disease risk to fisheries.

Moving fish remains the biggest risk for KHV transmission. But we reviewed popular angling literature and found that 52% of UK fisheries allow the use of keep and landing nets. Anglers’ nets may therefore have helped spread KHVwithin the UK. So a high proportion of carp fisheries are potentially at risk of getting the disease via nets.

To reduce the risk, we always tell anglers to dry and expose their nets to sunlight before leaving a fishery, or going to another one.

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