Gastrointestinal parasitism of elephant seals (roundworms)
Gastrointestinal parasitism caused by roundworms (nematodes) is a very common condition in elephant seals, particularly young ones. Worms called Contracaecum and Anisakis live attached to the animal's stomach, where they feed and cause loca...
What is it?
Gastrointestinal parasitism caused by roundworms (nematodes) is a very common condition in elephant seals, particularly young ones. Worms called Contracaecum and Anisakis live attached to the animal's stomach, where they feed and cause local lesions. The vast majority of wild elephant seals harbor these parasites without necessarily showing serious signs, but a high parasitic load can affect health, especially in young, weakened or stressed animals.
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 young elephant seals (weanlings), a significant infestation can lead to progressive weight loss despite a sometimes preserved appetite, a dull coat and a general loss of vitality.
The animal may present vomiting, loose stools or diarrhea, as well as tenderness or swelling in the abdomen.
In severe forms, there is marked growth retardation, significant fatigue and depression which make the animal more vulnerable to secondary infections and predation.
Many carrier animals, however, remain asymptomatic, the parasite being very common in wild populations without causing visible disease.
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 (roundworms) of the family Anisakidae, mainly the genera Contracaecum and Anisakis. These parasites have a complex life cycle involving several intermediate hosts, notably crustaceans and then fish, which are then consumed by the elephant seal, the definitive host.
Once ingested via infested prey, the larvae attach to the wall of the stomach where they develop into adult worms, causing local inflammation (parasitic gastritis) and sometimes ulcerations.
Young animals in the process of weaning or recently weaning are most exposed, because their immune system is still immature and their dietary transition exposes them to new parasitized prey.
Significant stress, undernutrition, concomitant disease or general weakening increase the risk of complications linked to a high parasitic load.
Population density and the availability of prey infested by larvae also influence the level of animal exposure.
Transmission
Transmission occurs through food: elephant seals become infected by eating fish or cephalopods carrying nematode larvae. There is no direct transmission between individuals.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
The diagnosis is based on clinical observation of the animal (body condition, behavior, digestive signs) associated if possible with a stool analysis to look for parasite eggs, as well as an endoscopy or a post-mortem examination allowing direct visualization of the worms in the stomach.
In animals stranded or taken care of in a care center, the veterinarian evaluates the general condition, hydration and weight gain to judge the severity of the infestation.
Treatment
Treatment, when implemented in a care center, is based on the administration of antiparasitics belonging to the anthelmintic family, under strict veterinary supervision.
Nutritional support and rehydration may be necessary in addition to weakened animals.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
In the wild, there are no preventive measures applicable directly to animals. Management is mainly based on the monitoring of populations and their general state of health.
In care or rehabilitation structures (rescue centers), good health management, controlled feeding and regular veterinary monitoring make it possible to limit the impact of parasitism in the animals taken care of.
Possible complications
A massive and untreated infestation can lead to severe gastritis, ulcerations of the stomach wall, or even perforations in extreme cases.
The resulting general weakening makes the animal more vulnerable to secondary infections, predation and reduces its chances of survival, particularly among young people in the critical phase of weaning.
When to see a veterinarian
Progressive or marked weight loss despite an apparently normal diet
Repeated vomiting
Persistent diarrhea or abnormal stools
Significant depression, lethargy or general weakness
Swollen belly or tender to the touch
Retarded growth visible in a young animal
Prognosis
The prognosis is generally good in animals in good general health carrying a low to moderate parasitic load, most of them tolerating the presence of these worms without major consequences.
In young people who are weakened or heavily infested, the prognosis may be more guarded if treatment is not put in place, particularly in the event of severe digestive complications.
Frequently asked questions
Do all elephant seals have these worms?
Yes, the vast majority of wild elephant seals harbor these types of nematodes in their stomachs. This is a very common situation that does not necessarily cause illness.
Can this parasite be transmitted to humans?
Worms of the Anisakidae family can occasionally infest humans through the consumption of raw or undercooked fish, but this risk is linked to human diet and not to contact with the elephant seal itself.
How do you know if a young elephant seal is too infested?
Warning signs are marked weight loss, dull coat, unusual fatigue and sometimes vomiting or diarrhea. Only a veterinary examination can confirm the severity of the infestation.
Can a wild elephant seal be treated for these worms?
In the natural environment, no treatment is applied directly. Antiparasitic treatment is only considered for animals cared for in a care or rehabilitation center, under strict veterinary supervision.
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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