Swordfish gill mycosis

What is it?

Gill mycosis is an infection caused by filamentous fungi that invade the gills, the respiratory organ of fish. In swordfish, this attack remains poorly documented because this species lives in the open sea and is practically never raised in captivity. The rare descriptions come from observations of fish captured, weakened or temporarily kept in tanks. The fungus disrupts oxygen exchange in the gills, which hampers the animal's breathing.\n\nThis disease is best known in farmed or aquarium fish living in stagnant or poor quality water. Its appearance in a large pelagic such as the swordfish suggests a prior weakening of the animal, through injury, significant stress or environmental pollution.

Commonly observed signs

These signs are neither exhaustive nor specific. Only a veterinarian can establish a diagnosis.

Causes and risk factors

The disease is caused by filamentous fungi, particularly of the Branchiomyces genus, capable of colonizing gill tissue. These fungi develop preferentially in water rich in organic matter, poorly oxygenated or polluted. They invade the blood vessels of the gills, causing local obstruction and death of the surrounding tissue.\n\nIn swordfish, a large, deep-sea oceanic fish, this type of infection remains exceptional. It would especially occur on an already weakened individual, for example after an injury, capture stress, or a prolonged stay in poor quality confined water.

An injured, stressed or weakened fish is more vulnerable. Hot water, low in oxygen or loaded with organic matter encourages the proliferation of the fungus. Containment in a pond or transit tank, a rare situation for swordfish but possible during scientific or rescue studies, also increases the risk compared to life in the open sea.

Diagnosis and care

Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.

Treatment

The treatment of gill fungus uses antifungals used in baths or immersion in fish farms. Improving water quality (oxygenation, reduction of organic matter) is an integral part of the treatment. For swordfish, no specific therapeutic protocol has been established, this situation remaining exceptional and documented only anecdotally.

Prevention

Possible complications

Branchial damage can progress to severe respiratory failure if a large part of the tissue is destroyed. A secondary bacterial infection can be added to the necrotic tissues. In advanced cases, death of the animal may occur due to asphyxiation.

When to see a veterinarian

Prognosis

The prognosis depends on the extent of the branchial damage and the speed of treatment. In farmed fish, early intervention improves the chances of recovery. In swordfish, in the absence of specific data, the prognosis is considered guarded once the infection is identified, due to the difficulty of treating this species in a controlled environment.

Frequently asked questions

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 26 September 2026