Gastrointestinal parasitism of nurse sharks (roundworms)
Gastrointestinal parasitism by roundworms (nematodes) is a condition frequently observed in nurse sharks (Ginglymostoma cirratum), whether kept in public aquariums or studied in the wild. These parasites live in the stomach or intestine of...
What is it?
Gastrointestinal parasitism by roundworms (nematodes) is a condition frequently observed in nurse sharks (Ginglymostoma cirratum), whether kept in public aquariums or studied in the wild.
These parasites live in the stomach or intestine of the shark and feed in part on its digestive contents or its tissues. In small quantities, they are often well tolerated. In large numbers, they can cause a progressive weakening of the animal.
This disease is particularly monitored in aquariums because confinement and stress can encourage the multiplication of parasites already present in the animal at the time of its capture or transfer.
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 are often discreet and progressive, which makes detection difficult without specialized examination.
We may observe a reduction in appetite, or even a refusal of food. The shark can lose weight visibly, particularly in the abdominal region which becomes more hollow.
General fatigue can result in less active swimming, a shark remaining on the bottom longer than usual.
Stool abnormalities are sometimes visible in the pelvis: soft stools, whitish filaments or worms directly expelled. In advanced cases, the abdomen may appear swollen or tender.
These signs are not specific to this disease alone and require veterinary advice specialized in aquatic wildlife.
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 the presence of nematodes (roundworms) in the shark's digestive tract. Several genera of parasitic nematodes are known among elasmobranchs (sharks and rays), some being quite specific to this group of animals.
The life cycle of these parasites often involves one or more intermediate hosts, such as small invertebrates or fish consumed by the shark. The infestation therefore occurs mainly through food.
Several factors increase the risk or severity of infestation:
- Diet based on uncontrolled whole fish or of wild origin, likely to carry parasite larvae;
- Prolonged captivity in an aquarium with an environment conducive to re-infestation if the tank is not properly managed;
- Stress linked to transport, capture or cohabitation with other species;
- Weakened immune system, particularly in young individuals or already sick animals ;
- Parasitic load already present when captured in the natural environment.
Transmission
Transmission occurs mainly through food: the shark becomes infested by consuming prey (fish, invertebrates) which already harbor larvae of the parasite. There is no known direct transmission between sharks through simple contact.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
Diagnosis is based on examination of the shark's stools (looking for eggs or worms) carried out by an aquatic wildlife veterinarian or a competent laboratory.
In some cases, endoscopy or other imaging techniques can be used to directly visualize parasites in the digestive tract, particularly when the animal can be handled safely.
The animal's dietary history and breeding conditions are also taken into account to guide the diagnosis.
Treatment
The treatment uses antiparasitics adapted to aquatic fauna, chosen and administered only by a specialized veterinarian, taking into account the particular sensitivity of sharks to certain molecules.
Nutritional support can be put in place to support the animal during the treatment phase, particularly if significant weight loss has been observed.
Environmental management (water quality, reduction of stress) is an integral part of the overall care.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Prevention is mainly based on the management of food and the environment in captivity.
It is recommended to offer a controlled diet, if possible frozen at a sufficiently low temperature and for a suitable period of time to kill the parasite larvae before being given to the animal.
Regular veterinary control, including stool examinations, allows early detection of an infestation before it becomes severe.
Quarantine of new arrivals in the aquarium, with examination parasitological, limits the introduction of new parasites into the pool.
Good water quality and management of the animal's stress also contribute to maintaining its natural immunity.
Possible complications
A severe and untreated infestation can lead to marked weight loss, partial obstruction of the digestive tract in extreme cases, and a general weakening which makes the animal more vulnerable to other diseases.
In an already weakened animal, parasitosis can contribute to a fatal outcome, although this remains rare if treatment is put in place in time.
When to see a veterinarian
The shark has refused to feed for several days.
Visible weight loss is noted, including a hollowing of the abdomen.
Abnormal worms or filaments are observed in the stools or pelvis.
The animal shows an unusual decrease in activity or remains immobile at the bottom of the pelvis.
The abdomen seems swollen, hard or tender to the touch.
Prognosis
The prognosis is generally favorable if the infestation is detected early and treated appropriately by a specialist veterinarian.
If left untreated, the animal's condition may gradually deteriorate, with an increased risk of complications, particularly in already weakened or young animals.
Frequently asked questions
Can a parasitized nurse shark continue to do well?
Yes, in cases of low parasite load, the animal may show no visible signs and remain in apparent good health. It is often a significant infestation that causes noticeable symptoms.
Can these worms be transmitted to humans?
Shark-specific nematodes are not known to infest humans. The zoonotic risk is considered negligible for this particular parasitosis.
How can we prevent other fish in the pond from being contaminated?
Good feeding management, quarantining new animals and regular parasitological monitoring help limit the spread in a shared aquarium.
Is antiparasitic treatment risky for the shark?
Sharks can be sensitive to certain molecules; this is why only a veterinarian specializing in aquatic wildlife should choose and adapt the treatment to the species and condition of the animal.
References
WOAH — Organisation mondiale de la santé animale (ex-OIE)
Content reviewed by the TunisieVet editorial team · Updated 30 October 2026
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