Infectious stomatitis in crayfish (crayfish plague of the mouth)
Infectious stomatitis, sometimes called crayfish plague of the mouth, is a bacterial infection affecting the mouth and mouthparts of crayfish. It causes inflammation and then necrosis (tissue death) around the mandibles and maxillipeds. Thi...
What is it?
Infectious stomatitis, sometimes called crayfish plague of the mouth, is a bacterial infection affecting the mouth and mouthparts of crayfish. It causes inflammation and then necrosis (tissue death) around the mandibles and maxillipeds. This disease is mainly observed in crayfish kept in aquariums or ponds when water quality is poor. It can affect both the European crayfish (Astacus astacus) and the Louisiana crayfish (Procambarus clarkii).
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:
Early signs are subtle: the crayfish eats less or refuses food and becomes less active.
On close inspection of the mouth, redness, swelling or an abnormal colour (brownish to blackish) can be seen around the mandibles.
Over time, the tissues around the mouth become necrotic: they take on an eroded appearance, as though gnawed away, sometimes accompanied by a whitish deposit or an unusual odour.
The animal has increasing difficulty grasping and chewing its food, leading to gradual weight loss.
In advanced cases, the crayfish remains lethargic, hidden away, and stops eating altogether.
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 is caused by opportunistic bacteria, most often species of the genus Aeromonas, Pseudomonas or Vibrio depending on the environment (freshwater or brackish water). These bacteria are normally present in the water or on the animal without causing problems. They become pathogenic when the crayfish is weakened or injured, or when water quality deteriorates significantly.
Poor water quality (high levels of ammonia or nitrites, insufficient oxygen) is the primary risk factor.
Overstocking in the tank or pond increases stress and injuries from contact between individuals.
Inappropriate or poor-quality diet can weaken the crayfish's immune defences.
Injuries to the mouth parts, such as those from fighting or handling, can allow infection to take hold.
Water temperature that is too cold or too warm for the species' needs, as well as difficult moulting, also favour the development of this condition.
Transmission
Transmission is not strictly direct from one animal to another; it depends mainly on the presence of bacteria in the water and the weakened state of crayfish exposed to the same environment.
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 direct observation of mouth lesions by a veterinarian specialising in aquatic or exotic animals, possibly supplemented by a sample for bacterial analysis. Testing of water parameters is systematically recommended to identify any triggering factor.
Treatment
Treatment typically combines immediate improvement of water quality and, depending on the case, the use of antibacterial or antiseptic agents suited to the aquatic environment, prescribed and dosed only by a veterinarian. Nutritional support with food that is easy to grasp can help the weakened animal.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Prevention relies mainly on maintaining good water quality: effective filtration, regular water changes, and monitoring of ammonia, nitrites and pH.
Overstocking should be avoided, and sufficient hiding places provided to reduce fighting between crayfish.
A varied diet suited to the species strengthens the animal's resistance.
Quarantining any new individual before introducing it to a shared tank or pond helps limit the introduction of pathogens.
Finally, it is advisable to remove promptly any uneaten food waste that pollutes the water.
Possible complications
If necrosis progresses, the crayfish may become unable to feed, leading to severe weight loss and eventually death. In severely weakened animals, the infection can also spread to other parts of the body.
When to see a veterinarian
The crayfish has refused food for several days.
Lesions, abnormal colouration or an unusual odour are visible around the mouth.
The animal remains motionless, hidden, or no longer responds normally.
Several crayfish in the same tank or pond show similar signs.
Prognosis
The prognosis is favorable if the disease is detected early and water quality is quickly improved. However, if mouth lesions are already extensive and prevent feeding, the prognosis becomes guarded.
Frequently asked questions
Can crayfish shell disease be transmitted to other animals in the pond?
The bacteria responsible are often already present in the water. The risk of direct transmission between crayfish exists mainly if water quality remains poor for the entire group.
Can you treat a crayfish with crayfish plague yourself?
It is strongly advised against using antibiotics without veterinary advice. Improving water quality is the first useful step, but specialized advice is necessary for further action.
Does this disease affect only stressed crayfish?
Stress and injuries are important contributing factors, but poor-quality water alone can be sufficient to trigger the disease even in an animal in good initial condition.
Should a sick crayfish be isolated?
Yes, it is recommended to place it in a separate tank to monitor its progress and limit stress from other individuals, while awaiting veterinary advice.
References
WOAH — Organisation mondiale de la santé animale (ex-OIE)
Content reviewed by the TunisieVet editorial team · Updated 29 August 2026
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