Gastrointestinal parasitism in the megamouth shark (roundworms)
The megamouth shark (Megachasma pelagios) is an extremely rare deep-sea shark species discovered only in 1976. Like many cartilaginous fish, it can harbour parasitic roundworms (nematodes) in its digestive tract. This gastrointestinal paras...
What is it?
The megamouth shark (Megachasma pelagios) is an extremely rare deep-sea shark species discovered only in 1976. Like many cartilaginous fish, it can harbour parasitic roundworms (nematodes) in its digestive tract. This gastrointestinal parasitism corresponds to the presence of these worms in the stomach or intestines of the animal, where they feed at the expense of their host. Knowledge about this condition in this particular species remains extremely limited, due to the very small number of individuals ever observed or studied by humans.
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 cartilaginous fish in general, a light infestation with digestive roundworms may go unnoticed. When it becomes severe, it can lead to progressive weight loss, reduced activity and appetite, as well as digestive disturbances such as abnormal faeces. In the megamouth shark, no detailed clinical observations of these signs have been reported to date, as the species is very rarely encountered alive or studied closely.
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 condition is caused by the presence of nematodes (roundworms) in the digestive tract. These parasites belong to families known in deep-sea sharks and attach to the wall of the stomach or intestines, sometimes using anchoring structures, to feed on the digestive contents or tissues of the host.
The feeding behaviour of the megamouth shark, which feeds by filtering small organisms such as zooplankton and jellyfish in deep waters, exposes it to ingesting parasite larvae present in its prey. A weakened immune system, stress linked to the environment, or an already high parasite burden can promote a more severe infestation.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
Diagnosis is made almost exclusively during a necropsy of a stranded or accidentally captured individual, by direct examination of stomach and intestinal contents for the presence of worms. There is no diagnostic method available for a free-living animal of this species.
Treatment
No treatment is possible for this wild, pelagic species living in the open ocean. The antiparasitic treatments available for farmed or aquarium fish cannot be applied to an animal as rare and impossible to handle in its natural environment.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
In a wild animal living in the deep ocean, no preventive measures can be applied by humans. Parasite regulation occurs naturally through the balance of the marine ecosystem and the animal's own immune system.
Possible complications
Severe, unregulated infestation could theoretically cause marked weight loss, malnutrition, or damage to the digestive tract lining, weakening the animal against other challenges. These complications remain hypothetical due to lack of precise data on this species.
When to see a veterinarian
This situation does not concern private individuals: the megamouth shark is a wild marine species under protection and is never kept in captivity.
Any discovery of a stranded or accidentally captured individual must be reported to the relevant scientific or marine protection authorities.
No individual intervention is possible or recommended on this animal.
Prognosis
Prognosis depends on the intensity of infestation and the animal's overall condition. In most large sharks, a moderate parasite burden is well tolerated and is part of the natural balance with the ecosystem. Without specific data on the megamouth shark, a precise prognosis for this species cannot be established.
Frequently asked questions
Can the megamouth shark be kept as an aquarium fish?
No. It is a wild species from the deep ocean, never maintained in viable captivity. No individual treatment is possible.
Are these roundworms dangerous to humans?
No, this parasitism is specific to the shark's digestive tract and presents no known risk of transmission to humans.
How do we know this disease exists in such a rare species?
The few individuals studied during strandings or accidental captures have made it possible, through autopsies, to discover the presence of nematodes in their digestive system.
What to do if you find a stranded gulper shark?
You should immediately contact the local scientific authorities or marine protection organizations without handling the animal yourself.
References
WOAH — Organisation mondiale de la santé animale (ex-OIE)
Content reviewed by the TunisieVet editorial team · Updated 07 September 2026
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