Bacterial Infection of the Scallop
Bacterial infection of the scallop is a disease that mainly affects larvae and young animals raised in hatcheries or in their natural environment. It is most often caused by bacteria of the genus Vibrio, naturally present in seawater. These...
What is it?
Bacterial infection of the scallop is a disease that mainly affects larvae and young animals raised in hatcheries or in their natural environment. It is most often caused by bacteria of the genus Vibrio, naturally present in seawater. These bacteria take advantage of a weakened animal or poor water quality to multiply and cause disease. It can result in significant losses in farms, particularly among larvae and spat (very young scallops).
Commonly observed signs
These signs are neither exhaustive nor specific. Only a veterinarian can establish a diagnosis.
Related signs — each one links to its page:
Signs are difficult to observe directly on a scallop, unlike in a land animal. However, slowed or stopped swimming can be noticed in larvae, reduced ability to attach or close their shells, stunted growth, and abnormal mortality in a group of farmed animals. In older animals, general weakness, abnormal gaping of the shell, or soft tissues with a dull appearance may be seen. These signs are not specific and require laboratory confirmation.
When to consult without delay
This list does not replace veterinary advice. It gathers situations that justify a prompt consultation.
Causes and risk factors
The disease results from the proliferation of opportunistic bacteria, mainly Vibrio species (such as Vibrio splendidus, Vibrio pectenicida, or related species), naturally present in seawater. These bacteria become pathogenic when the animal is stressed or weakened, or when their concentration in the water increases significantly.
Poor water quality, high water temperature, high farming density, stress from handling or transport, and insufficient microalgae in the diet promote the appearance of the disease. Larval stages and young spat are most vulnerable, since their immune system is still immature.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
Diagnosis is based on analysis in a specialized laboratory: bacterial culture from larvae or tissue samples, identification of the Vibrio species involved, sometimes supplemented by molecular biology techniques. A veterinarian or pathologist specializing in aquaculture should be consulted if unusual mortality occurs in a farm.
Treatment
Treatment on the farm focuses mainly on improving farming conditions (water quality, stress reduction, disinfection). Antibiotic use is strictly regulated in aquaculture and reserved for specific situations under veterinary supervision, due to the risk of selecting antibiotic-resistant bacteria in the marine environment.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Prevention relies on good water quality management in hatcheries, regular disinfection of tanks and equipment, temperature monitoring, and limiting stocking density. Testing the microbial quality of incoming water and maintaining good overall hygiene reduce the risk of introducing pathogenic strains.
Possible complications
Without prompt management, the disease can cause massive mortality of larvae or spat in hatcheries, compromising an entire production cycle. In adults, infection can permanently weaken the animal and make it more susceptible to other pathogens.
When to see a veterinarian
Unusual or sudden mortality observed in a batch of larvae or spat.
Clear slowing of larval growth or swimming activity.
Shells remaining abnormally open or soft.
Suspected deterioration of hatchery water quality.
Prognosis
Prognosis depends on the stage affected and how quickly management begins. In larvae, mortality can be high if nothing is done. Rapid improvement of farming conditions often limits losses in older stages.
Frequently asked questions
Can this disease be transmitted to humans?
No, it is not a disease transmissible to humans. The bacteria involved are adapted to the marine environment and to the physiology of the scallop.
Can scallops be treated with antibiotics when they are sick?
The use of antibiotics in marine environments is highly restricted and closely regulated, as it risks promoting the emergence of antibiotic-resistant bacteria in the environment. Prevention through good water quality remains the best protection.
How is this disease recognized in a livestock operation?
Abnormal mortality or a slowing of larval activity is mainly observed. Only laboratory analysis can confirm the presence of the responsible bacterium.
Does this disease also affect wild scallops?
Yes, but it is more often detected and documented in hatcheries, where high-density conditions favour its spread. In natural environments, mortalities are harder to observe and to attribute to this cause.
References
WOAH — Organisation mondiale de la santé animale (ex-OIE)
FAO — Organisation des Nations unies pour l'alimentation et l'agriculture
Content reviewed by the TunisieVet editorial team · Updated 30 August 2026
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