Guidance

Bonamia exitiosa

Updated 11 September 2024

Applies to England and Wales

1. Contents 

Bonamia exitiosa is a protozoan parasite infecting several oyster species, in particular the native (or flat) oyster (Ostrea edulis).  

Information on susceptible and vector species can be found here: Susceptible and vector species - GOV.UK (www.gov.uk)

Great Britain is free from Bonamia exitiosa, and it is a ‘listed’ (also referred to as ‘notifiable’) disease which you are legally obligated to report immediately to the Fish Health Inspectorate (FHI), Cefas. Fish Health Inspectorate - GOV.UK (www.gov.uk) 

In 2011 Bonamia exitiosa was detected in a sample of native oysters collected from a trader operating in Cornwall. The affected area was subject to controls and extensive surveillance and retesting, all subsequent samples were negative, and controls lifted in 2013. Since then, there have been no further outbreaks of Bonamia exitiosa in the UK. 

It does not affect humans. 

2. How to spot Bonamia exitiosa 

Signs of the disease can include: 

• Increase of dead / empty shells 
• Gaping shells which don’t close when touched or when exposed to air 
• Gills can appear eroded. 

3. How Bonamia exitiosa is spread 

Bonamia exitiosa is spread through movements of and contact with infected shellfish and contact with contaminated equipment or water. Infected shellfish may release infective particles, spreading the parasite directly from host to host, and infected planktonic shellfish larvae may also contribute to the spread of the parasite.   

4. Preventing and controlling Bonamia exitiosa  

There is no treatment for Bonamia exitiosa. 

Prevention is the best approach to disease control. You can protect your shellfish by being aware of the signs of Bonamia exitiosa and other serious diseases and checking stock health regularly. Implement good husbandry and biosecurity practises. 

Further information on biosecurity measures plans, guidance and templates can be found here: Biosecurity Measure Plans - Information and Templates - GOV.UK (www.gov.uk) 

You must be sure any shellfish introduced are Bonamia exitiosa free. Do not buy shellfish of unknown origin. Follow the rules on moving aquatic animals: Aquatic animal health and movements guides - GOV.UK (www.gov.uk) 

You must report it to the FHI immediately if you notice or become aware of shellfish showing any of the signs of Bonamia exitiosa, particularly increased mortality and gaping shells. The FHI will investigate and if required apply disease control measures.   

5. Further information on Bonamia exitiosa 

World Organisation for Animal Health (WOAH) Aquatic Manual chapter Bonamia exitiosa  

6. Legislation relating to Bonamia exitiosa 

The Aquatic Animal Health (England and Wales) Regulations 2009  

The listed diseases are in Regulation 1251/2008 as amended by the Animals, Aquatic Animal Health, Invasive Alien Species, Plant Propagating Material and Seeds (Amendment) (EU Exit) Regulations 2020, at Chapter 2, Annex 1A and Schedule 1 of The Aquatic Animal Health (England and Wales) Regulations 2009. 

7. Who to contact if you suspect Bonamia exitiosa  

If you suspect or know of a listed disease of fish and shellfish in England and Wales, you must report it immediately to the Fish Health Inspectorate (FHI), Cefas. You must also tell the FHI immediately if you know or suspect that increased mortality has occurred.  

Under the Aquatic Animal Health (England and Wales) Regulations 2009 it is a legal obligation to report suspicion of Bonamia exitiosa in any farmed or wild shellfish to the FHI. It is an offence under the Regulations to fail to inform the FHI of suspicion of Bonamia exitiosa. 

• Email: fhi@cefas.gov.uk 
• Telephone: 01305 206700 - Monday to Thursday - 9:00 am to 5:00 pm, Friday 9:00 am to 4:30 pm 
• For emergencies out of hours telephone: 01305 206630 

Fish Health Inspectorate 
Centre for Environment, Fisheries and Aquaculture Science (Cefas)
Barrack Road, The Nothe
Weymouth
Dorset
DT4 8UB