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Mouth deformity in koi

This is not uncommon and can be the result of physical injury, bacterial or virus infection, tumours and abnormal bone development. It is often due to localised bacterial ulcerations but occasionally has a sudden onset due to physical bone or joint damage, with the mouth stuck open or closed. The structure is complex, with several bones and joints forming a framework that extends and collapses like a concertina. Close-up examination of the mouth and gill space is required under anaesthetic to determine the cause and treatment options, but recovery is often variable and uncertain.

Koi mouth deformity
Koi mouth deformity

Although the cause is unknown, this adult koi had a long-standing deformity that prevented the mouth from closing completely. There was also some narrowing inside the mouth on the left side but despite this, the koi managed to eat well and continued to grow in size.

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Koi mouth deformity 2
Koi mouth deformity 2

This is the previous fish viewed from below. Although the area has now healed, the lack of a barbel and the asymmetrical shape and distortion on the right-hand side behind the mouth suggests extensive tissue damage in the past. There is also marked curling of the gill cover edges which is often considered to be a genetic defect.

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Spontaneous injury
Spontaneous injury

In some cases, koi are found with their mouth that has stuck open or will not close, making eating and ‘breathing’ difficult. On examination under anaesthetic, it may be possible to identify a dislocated joint or fracture. It is rarely possible to repair this problem easily, but affected fish often survive well with a moderate amount of deformity since feeding relies on suction rather than chewing with their jaws.

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Koi mouth deformity
Koi mouth deformity

Although the cause is unknown, this adult koi had a long-standing deformity that prevented the mouth from closing completely. There was also some narrowing inside the mouth on the left side but despite this, the koi managed to eat well and continued to grow in size.

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