Skip to main content

Shellfish disease investigations 2013 and 2014

Posted by: , Posted on: - Categories: Animal Health, Monitoring, SFN 2015, Shellfish News

We've seen many shellfish disease events in 2013 and 2014, and below is a summary listed by species.

Some of the issues were identified as a result of routine inspections by the FHI, but the majority were reported by the farmers involved. This shows the importance of carrying out regular health checks of your stock, and informing the FHI of any unusual or large mortalities.

Native oysters

The screening of native oysters for all diseases listed above is an ongoing programme of work. The FHI collected 20 samples of native oysters from around the coast of England and Wales in 2013 and 2014.

Map of shellfish diseases in England and Wales
Map of confirmed designations for shellfish diseases in England and Wales

The results of these samples confirmed the ongoing presence of Bonamia ostreae in all of the known positive areas. But there were no new areas of infection. However, the positive areas did return differing levels of infection, with prevalence of the parasite in samples varying from 0% to 17%.

The natural (wild) fisheries sampled tended to have the lowest prevalence. Whilst relayed stock tended to have the highest prevalence, reflected in the current guidance on relaying native oysters in Bonamia positive areas. This advises to reduce the density of shellfish lays as much as possible.

Pacific oysters


Once again the main concern for Pacific oyster farmers was oyster herpesvirus (OsHV-1μVar). Especially as two new areas became affected: Poole Harbour in 2013; and the River Crouch in 2014. In addition we also witnessed the spread of virus within the Confirmed Designation in the Blackwater. This was from the initial site, at Goldhanger (first affected in 2012), through to Thirslett, into the creeks at Salcott and further to Mersea Shore. This spread all took place in the summer of 2014.

All of these events, as with previous OsHV-1μVar outbreaks, were associated with noticeable mortalities mostly in juvenile oysters, but still seen in all life stages. Despite the initial impact on these populations, it does appear that it is possible to manage farmed stock with the virus. Although losses can be expected to hit when the water temperature rises in the late spring and early summer. Controls on movements seem to be the only means of stopping the spread of the virus. And new outbreaks are often associated with introductions of oysters from infected areas.

Mikrocytos mimicus

In the spring of 2013, following a particularly prolonged period of cold weather, a number of oyster farmers on the east coast of Norfolk reported unusual levels of mortality in adult oysters during depuration. On investigation it was found that a previously unreported parasite was found in these weakened oysters: now identified as Mikrocytos mimicus. The infection presented mainly in adult oysters after prolonged exposure to the unusually cold weather conditions on the east coast in the winter and early spring of 2013. The farmers reported poor survival through purification, and the main visible sign was the presence of greenish lesions in the adductor muscle.

Green lesion in a Pacific oyster
Green lesions in the adductor muscle of a Pacific oyster (C. gigas)

As well as the loss of stock - due to the infection - farmers also reported loss of revenue as the green lesions made the oysters unsuitable for sale.  Further testing showed that the infection disappeared from affected oysters during the summer of 2013, and has not been found in 2014, despite further testing. The shellfish affected by the infection did not recover well. One farmer reported that large numbers of oysters from the oldest year class did not survive through to market.

Haplosporidium nelsoni

Rounding off what has been an eventful couple of years for farmers of Pacific oysters, the FHI received reports in September 2014 of an unusual mortality event in farmed Pacific oysters in South Devon. This resulted in identifying another parasite: Haplosporidium nelsoni. Haplosporean life stages were seen in histological samples taken from two Pacific oysters (out of 60 tested) from two sites in one estuary. Molecular testing confirmed it as H. nelsoni. Subsequent sampling has not resulted in further positives.

Although this was the first identification of this haplosporidian in England, the parasite has been reported in oysters elsewhere. Most notably causing mortalities in the American oyster C. virginica. It was first identified in Pacific oysters in America during the 1970s, and has been seen more recently in Pacific oysters from France and Ireland.

Although H. nelsoni is not now thought of as a significant pathogen in Pacific oysters, we are keeping a watching brief on the situation. Particularly as the presence of the parasite in oysters seems to be restricted to a small area in one estuary in Devon


There have been no reports of disease-related mortalities in mussels received by the FHI in 2014. Only one event was reported in 2013, when mussels from the Wash were submitted for screening by the Eastern IFCA - following a reduction in the biomass expected on one of the beds within the Wash Fishery Order. But no significant disease pathology was identified.

Outside of the UK, there was a large scale mussel mortality affecting between 10 and 100% of juveniles and adults (but not spat) reported in the Charentais sluice in France in spring 2014. The official report from Ifremer (the EU reference laboratory for mollusc diseases) supported a multifactoral event associated with a pathogenic bacteria, Vibrio splendidus, poor environmental conditions and mussels in spawning condition.

Map of mussel mortalities in France
Map of the Mussel mortalities in France (Image supplied by Ifremer)


There have been sporadic reports of the appearance of dead shell in the scallop fishery in Lyme Bay over the past two years. Most often reported in the spring and early summer, and possibly associated with periods of extreme weather. Samples received at the Weymouth laboratory have not identified any listed diseases. Although low numbers of rickettsia-like organisms (RLOs) were seen in some samples.

RLOs have previously been associated with disease in several different species of mollusc from a number of areas around the world. However the cause of the problem is still under investigation. We’re trying to understand if these RLOs are an underlying issue in what has been observed.

Reporting a mortality event at your site

You must contact the FHI immediately if you notice an unusual pattern of mortality in your stock. We will aim to have an inspector visit and sample the stock as soon as possible.

For updates please sign up to email alerts from this blog, email me or you can follow us on Twitter @CefasGovUK.

Sharing and comments

Share this page