Gastrointestinal parasitism of the humpback whale (roundworms)
Gastrointestinal parasitism by roundworms (nematodes) is a digestive infestation that affects many cetaceans, including humpback whales. Worms belonging in particular to the Anisakidae family settle in the stomach or intestine of the animal...
What is it?
Gastrointestinal parasitism by roundworms (nematodes) is a digestive infestation that affects many cetaceans, including humpback whales. Worms belonging in particular to the Anisakidae family settle in the stomach or intestine of the animal. This disease is very common in whales, even in apparently good health, but it can become problematic in the event of a high parasite load or general weakening of the animal.
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 the humpback whale, clinical signs are very difficult to observe directly at sea. We assume a significant infestation in a emaciated animal, with an abnormally thin silhouette or a loss of fat visible under the skin. Lethargic behavior, slow swimming or isolation from the group can also be a warning sign. The worms themselves are only visible during a post-mortem examination, where they appear in the stomach or intestine, sometimes in large numbers. In stranded animals, lesions of the digestive wall can be found at 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
The infestation is due to nematodes, notably worms of the Anisakidae family, whose life cycle involves several marine hosts. The whale becomes infested by feeding on prey (krill, small fish) which harbor parasitic larvae. These larvae then develop into adult worms in the cetacean's digestive system.
Young whales, weakened or malnourished animals, and those living in areas with a high density of parasitized prey seem more exposed. A poor general state of health, for example linked to another illness or environmental stress, can encourage greater parasitic proliferation.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
The definitive diagnosis is essentially based on the post-mortem examination (necropsy) of stranded animals, with direct demonstration of worms in the digestive tract. In living animals, the assessment remains indirect, based on observation of body condition, behavior at sea, and sometimes on the analysis of biopsies or samples obtained during supervised scientific interventions.
Treatment
There is no antiparasitic treatment applicable in practice to a wild whale. Individual therapeutic care is not feasible in the living conditions of this species. Only stranded animals cared for in an exceptional care setting could theoretically benefit from antiparasitic treatment, decided by specialized veterinary teams.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
There are no preventive measures directly applicable to a wild animal like the humpback whale. Preserving the quality of marine ecosystems and prey populations indirectly contributes to limiting parasitic imbalances. Monitoring cetacean populations through stranding networks makes it possible to better document the prevalence of this parasitism.
Possible complications
A very high parasitic load can contribute to weight loss, weakening of the digestive wall, or even perforations in extreme cases. Parasitism can also add to other stress factors (pollution, lack of food, injuries) and contribute to a general weakening of the animal, without necessarily being the main cause of stranding or mortality.
When to see a veterinarian
Sighting of an abnormally thin or weakened whale at sea.
Unusual behavior: isolation, erratic swimming, lethargy.
Stranding of an individual, alive or dead, requiring the intervention of a specialized network.
Discovery of digestive lesions during a scientific necropsy.
Prognosis
In the majority of humpback whales, moderate infestation by these nematodes is well tolerated and does not cause apparent illness. The prognosis becomes guarded only in cases of massive parasitic load associated with a general weakening of the animal, where parasitism can aggravate an already fragile condition.
Frequently asked questions
Is this disease dangerous for humans?
No, this parasitism does not present a risk of transmission to humans during whale watching at sea.
Can we treat a whale infested with these worms?
In practice, there is no possible treatment for a wild whale in the wild. Only exceptional cases of stranded animals taken care of by specialized teams could benefit from care.
How do we know that a whale has this type of parasites?
Confirmation is most often made during post-mortem examination of stranded animals, where the worms are found directly in the stomach or intestine.
Is this parasitism common in humpback whales?
Yes, it is considered very widespread among cetaceans in general, often without visible consequences on healthy animals.
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 24 September 2026
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