Gastrointestinal parasitism of sea lions (roundworms)
Gastrointestinal roundworm parasitism is a common condition in sea lions, whether they live in the wild or in care under human supervision (parks, care centers, aquariums). Roundworms called nematodes settle in the animal's stomach and inte...
What is it?
Gastrointestinal roundworm parasitism is a common condition in sea lions, whether they live in the wild or in care under human supervision (parks, care centers, aquariums). Roundworms called nematodes settle in the animal's stomach and intestine. They are often transmitted by fish consumed by sea lions. This disease particularly affects young individuals, who are more susceptible than adults.
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:
The signs vary depending on the number of worms present and the general condition of the animal.
We often observe vomiting, sometimes repeated, and loss of appetite. The sea lion may appear tired, less active, and gradually lose weight.
Digestive problems such as diarrhea or abnormal stools may appear. In more severe cases, blood may be visible in vomit or stool, a sign of irritation or damage to the digestive wall.
Some heavily infested animals may have a swollen belly or visible discomfort after meals. Weakened young animals may develop a dull coat and general poor condition.
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 disease is caused by nematodes, parasitic roundworms that live in the stomach and intestine of the sea lion. The larvae of these parasites are present in many marine fish which constitute the natural diet of the sea lion. By consuming these infested fish, the sea lion ingests the larvae which then develop into adult worms in its digestive tract.
Young sea lions, whose immune systems are still immature, are more vulnerable to severe infestation. A state of stress, poor quality nutrition, or general weakness linked to another illness increase the risk of complications. In the natural environment, the availability and type of prey consumed influence the degree of exposure to parasites.
Transmission
Transmission occurs by ingestion of fish carrying nematode larvae. There is no direct transmission from one sea lion to another through contact. The parasite cycle generally involves several marine intermediate hosts before reaching the sea lion, which is the definitive host in which the parasite becomes an adult.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
The veterinarian relies on the animal's history, observation of digestive symptoms, and above all on a stool analysis to look for parasite eggs under a microscope. In certain cases, additional examinations such as an endoscopy or ultrasound may be offered to assess digestive damage and look for possible lesions linked to the migration of the larvae.
Treatment
Treatment is based on the administration of antiparasitics belonging to the anthelmintic family, chosen and dosed solely by the veterinarian according to the parasitic species identified and the condition of the animal. Digestive support and rehydration may be combined in cases of marked impairment.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
In a care center or in a structure for presentation to the public, prevention is based on regular monitoring of the parasitic state through stool examinations, and on rigorous management of the quality and origin of the fish given to the sea lion.
Prior freezing of fish, practiced in many structures, makes it possible to reduce the viability of parasitic larvae.
In the natural environment, there are no applicable prevention measures, as parasitism is part of the marine ecosystem normal.
Possible complications
A massive, untreated infestation can lead to severe inflammation of the digestive wall, ulcers, and even perforation in extreme cases. The resulting general weakening can make the animal more vulnerable to other infections.
When to see a veterinarian
Repeated or persistent vomiting in sea lions
Presence of blood in vomit or stools
Prolonged loss of appetite
Visible weight loss despite an apparently normal diet
Reduction or unusual drop in activity
Swollen or painful stomach
Prognosis
The prognosis is generally good when the disease is detected early and treated by a marine wildlife veterinarian. Severe cases, particularly in weakened young animals or in cases of digestive complications, require longer treatment and the prognosis becomes more guarded.
Frequently asked questions
Can sea lion roundworms be transmitted to humans?
The risk is considered very low under usual contact conditions. However, any handling of a wild marine animal or one in care must be done with caution and by trained people.
Can a sea lion in the wild live with these worms without being sick?
Yes, a light infestation is often well tolerated and is part of the natural balance between the sea lion and its marine environment. Symptoms appear especially in the event of a significant infestation or weakening of the animal.
How do you know if a sea lion is infested with roundworms?
The definitive diagnosis is based on a stool analysis carried out by a veterinarian, who looks for parasite eggs under a microscope. Symptoms alone cannot confirm the disease.
Is antiparasitic treatment risky for sea lions?
Appropriate treatment, prescribed and monitored by a veterinarian competent in marine wildlife, is generally well tolerated. Self-medication should be avoided because the dosages differ greatly from other species.
References
WOAH — Organisation mondiale de la santé animale (ex-OIE)
CDC — Centers for Disease Control and Prevention
Content reviewed by the TunisieVet editorial team · Updated 30 October 2026
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