Basidiomycosis of the Basking Shark
Gill fungal infection is a rare fungal infection affecting the gills, the respiratory organ of cartilaginous fish such as the basking shark. This large shark, which feeds by filtering plankton through its highly developed gills, depends ent...
What is it?
Gill fungal infection is a rare fungal infection affecting the gills, the respiratory organ of cartilaginous fish such as the basking shark. This large shark, which feeds by filtering plankton through its highly developed gills, depends entirely on this organ for breathing. A fungal infection of the gills can seriously impair gas exchange. This disease remains poorly documented in this species because of its rarity and the difficulty in observing and studying these wild animals, often found in deep ocean waters.
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:
In an animal so difficult to observe in its natural habitat, precise clinical signs are poorly understood. Respiratory distress, slowed or abnormal swimming, lethargic behavior, or rubbing against surfaces may be suspected if the animal is in captivity or stranded. Whitish lesions, filaments, or abnormal gill appearance may be observed on close examination, particularly in a stranded or dead animal during necropsy.
When to consult without delay
This list does not replace veterinary advice. It gathers situations that justify a prompt consultation.
Causes and risk factors
Gill fungal infection is caused by the proliferation of opportunistic fungi on gill tissue. These fungi typically take advantage of prior weakening of the organ, such as injury, significant stress, water pollution, or general decline in the animal's health. In wild fish populations, this type of infection remains occasional and most often affects already weakened individuals.
Factors that weaken the gills favor the establishment of opportunistic fungi. These include poor water quality, local pollution, injuries from fishing nets or collisions with boats, and overall poor health from disease, parasitism, or stress. The basking shark, a protected and vulnerable species, may be particularly susceptible during episodes of environmental stress or habitat degradation.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
Diagnosis is extremely difficult to establish in a wild animal living in open ocean. It is most often based on examination of a stranded, injured, or deceased animal, with direct observation of the gills and possibly laboratory analysis of samples to identify the fungus involved. No routine monitoring exists for this species in the wild.
Treatment
There is no established treatment for this disease in basking sharks in the wild, as the species is not kept in captivity on a sustainable basis. In fish medicine generally, antifungal agents may be used in a controlled setting, but their application to this protected species remains theoretical and undocumented.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
There are no specific preventive measures applicable to this wild species. Preserving the quality of marine waters, reducing pollution, and minimizing ship collisions contribute overall to the health of the basking shark population and limit the occurrence of opportunistic diseases such as fungal infections.
Possible complications
Significant gill involvement can compromise the animal's breathing and, in severe cases, contribute to overall weakening that may worsen its health status, particularly in an individual already weakened by other causes.
When to see a veterinarian
Observing a basking shark showing abnormal swimming or apparent distress near the shore
Finding a stranded animal, living or dead, with visible gill lesions
Any sign of distress in this protected species should be reported to the relevant authorities or local stranding networks
Prognosis
Prognosis is difficult to establish due to lack of data. Mild, localized infection might resolve spontaneously in an animal in good general condition, whereas extensive infection in an already weakened individual could worsen its vital prognosis, especially if it significantly affects respiratory function.
Frequently asked questions
Is branchial mycosis frequent in basking sharks?
No, it's a condition very rarely reported. The basking shark is a wild species that is difficult to observe, so few cases are documented in the scientific literature.
Can this disease be transmitted to humans?
No, there is no evidence that this gill fungus is transmissible to humans or to other terrestrial species.
What to do if you observe a sick basking shark at sea?
It is recommended not to approach or handle the animal, but instead to report the sighting to maritime authorities or to marine protected species monitoring networks.
Is there a vaccine or preventive treatment?
No, there is no vaccine or preventive treatment applicable to this protected wild species.
References
WOAH — Organisation mondiale de la santé animale (ex-OIE)
Content reviewed by the TunisieVet editorial team · Updated 24 August 2026
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