Oyster parasite Bonamia found on the Swedish west coast
For the first time, the oyster parasite Bonamia has been found in Sweden. Now, the Swedish Board of Agriculture is imposing restrictions on oyster farms on the Swedish west coast. Pierre De Wit, oyster researcher at the University of Gothenburg, answers questions about the parasite.
What kind of parasite is it?
"Bonamia ostreae is a single-celled parasite that infects flat oysters of the genus Ostrea, which includes our native flat oyster. The parasite can spread freely in seawater, is absorbed by the oysters' gills, infects their blood cells and reproduces there. The disease, known as bonamiosis, is fatal to oysters in the vast majority of cases. The parasite can often lie dormant in oysters for long periods without causing disease, but then quickly break out as disease when the oyster's immune system is weakened due to other stresses, such as heat waves."
"Bonamia is believed to have been brought to Europe from North America in connection with the transport of oysters in the 1970s, causing high mortality rates among oysters on the continent at that time. The parasite also appears to be difficult to get rid of even after the oysters have disappeared; it can lie dormant in the bottom sediments for a long time, so oyster stocks have still not recovered since then. Sweden and Norway have so far been free of the parasite, perhaps because we have not moved oysters around as much here, or possibly because of the colder water up here in the north. However, it has been found in Limfjorden in Denmark for a period of time. A series of cold winters there in the 1980s seems to have caused the parasite to disappear, at least temporarily."
What measures are being taken now?
"On 5 November, Bonamia ostrea was found in oysters at Kalvön in Tanum municipality. The Swedish Board of Agriculture has now issued a restriction zone, where it is not permitted to move oysters or oyster equipment. Hopefully, this will prevent the spread.
What are the consequences of this?
"Restoring the damaged oyster reefs along Europe's coastline is a major issue in many countries right now, but it is often unclear where to obtain disease-free oysters for these projects. However, as recently as June this year, Sweden was officially classified as ‘Bonamia-free’ by the European Commission, which means that oysters can be exported to restoration projects in other European countries. We do not yet know how this will be affected by the discovery of Bonamia here. Future sampling will show whether this was an isolated incident or whether it already exists in other places."
"For the local oyster industry here in Sweden, it would of course also have major consequences if there were an outbreak of disease and large parts of the oyster population were to die. But we don't know anything about that yet."
Is the parasite here to stay?
"We don't know yet! We only have one finding so far, and we hope that the quarantine measures will be enough to prevent it from spreading. And we can only hope for a cold winter that might kill off the parasite. But it's clear that with ongoing climate change, we're likely to see more events like this in the future."
Is it dangerous to humans?
"No, it is not dangerous to humans."
Interview: Karl-Johan Nylén
On 5 November 2025, Bonamia ostreae was detected in wild European oysters at Kalvön in Tanum Municipality. The Swedish Board of Agriculture has decided on restrictions for oyster farms and established a restriction zone to reduce the risk of the infection spreading. Contact tracing and investigation work is ongoing. At present, it is not possible to say whether, or in what way, our disease-free status for Bonamia ostreae may be affected by the infection now detected in wild oysters.
The following applies in the restricted zone:
- Wild oysters may only be collected for handling on land. This means that they may not be moved to other water areas or even within the same water area.
- Harvested oysters and equipment used for harvesting may not be returned to the water. Equipment must be cleaned in a closed drainage system.