Gastrointestinal roundworm parasitism in Northern Gannets
Gastrointestinal roundworm parasitism (nematode infection) is a common condition in Northern Gannets, large diving seabirds. These parasites live in the stomach, proventriculus, or intestine of the bird. They feed at the bird's expense and...
What is it?
Gastrointestinal roundworm parasitism (nematode infection) is a common condition in Northern Gannets, large diving seabirds. These parasites live in the stomach, proventriculus, or intestine of the bird. They feed at the bird's expense and can cause progressive deterioration of the bird's overall condition when present in large numbers. This disease mainly affects wild birds but also concerns Northern Gannets brought to wildlife care centres or kept in captivity (zoos, wildlife sanctuaries). In most cases, a healthy Northern Gannet carries a small parasite load without suffering ill effects; signs only appear with heavy infestation or when the bird becomes weakened.
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:
Signs are often subtle at first and may go unnoticed in wild birds. Gradual weight loss may be seen despite maintained appetite, or conversely a decrease in appetite may occur. Digestive disturbances are possible: regurgitation, abnormally soft droppings or change in faeces colour. The bird becomes more tired and less active, stays away from the group, and dives less to feed. The plumage may lose its shine and become dull. In severe cases, dehydration and pale mucous membranes (visible inside the beak) may appear, indicating severely compromised health. In young chicks, growth retardation is possible with heavy infestation.
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) that parasitize the digestive tract, belonging to different genera found in fish-eating seabirds, particularly worms of the genus Contracaecum and related species that lodge in the proventriculus or stomach. These parasites often have an indirect life cycle: larvae develop in intermediate hosts (small fish, crustaceans) before infecting the bird when it consumes these contaminated prey. The bird becomes infested mainly through its natural diet of fish.
Young birds, particularly chicks and juveniles, are generally more susceptible as their immune system is less mature. Pre-existing weakness (injury, oiling, exhaustion after a storm, food shortage) promotes parasite establishment and worsening. High population density in breeding colonies increases parasite circulation. Areas where water or prey are heavily contaminated with nematode larvae also represent a risk factor. In captivity, inadequate basin or enclosure hygiene and feeding with poorly controlled fish may favour infestation.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
Diagnosis starts with a thorough clinical examination of the bird by a vet, ideally one specialising in wildlife or birds.
Faeces analysis (coproscopic examination) allows checking for worm eggs in the droppings under a microscope.
In some cases, endoscopy or a post-mortem examination (in a bird found dead) may reveal adult worms in the stomach or intestines.
A blood test can help assess the disease's overall impact (anaemia, nutritional status).
Treatment
Treatment involves giving an antiparasitic medication prescribed and dosed solely by a vet experienced in wildlife care.
Supporting care is often provided alongside treatment: rehydration, nutritional supplementation, and rest in a calm, secure environment.
Treatment effectiveness is usually monitored by repeating the faeces examination after treatment.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
For birds in care facilities or captivity, regular parasite screening by faeces examination helps detect infestation early.
Checking the quality and source of fish used for feeding helps reduce the risk.
Keeping the environment clean—basin water and aviary floors—reduces the presence of infective larvae.
In wild birds, prevention focuses mainly on maintaining their general health and habitat, as direct parasite monitoring is not feasible.
Prophylactic antiparasitic treatment can be considered by a vet specialising in wildlife in certain captive or rehabilitation settings.
Possible complications
Heavy untreated infestation can lead to digestive blockage or perforation, severe anaemia, and extreme weight loss that may prove fatal, especially in already weakened birds.
In young birds, lasting growth delay is possible.
Intestinal parasitism also weakens the bird against other diseases and environmental stress.
When to see a veterinarian
A gannet found has visible weight loss or persistent loss of appetite.
The bird has abnormal droppings, soft or unusual in colour.
The bird is unusually dull, weak, or remains motionless.
Worms are visible in the droppings or vomit.
Any wild bird that is weakened or beached should be reported to a wildlife rescue centre rather than handled alone.
Prognosis
Prognosis is generally good when infestation is light to moderate and treated promptly with appropriate medication and supportive care.
It becomes guarded in cases of heavy infestation, digestive complications, or when the bird is already severely weakened by other factors (injury, malnutrition, oil contamination).
Young birds and already compromised individuals have a more cautious prognosis.
Frequently asked questions
Can the parasitism of the northern gannet be transmitted to humans or other domestic animals?
The nematodes in question are specific to seabirds and their aquatic prey. The risk of transmission to humans or domestic animals is considered negligible under normal conditions.
Are all Northern Gannets carrying parasitic worms sick?
No. A light parasitic load is common in healthy wild birds and generally does not cause visible signs. Symptoms only appear in cases of heavy infestation or when the bird is weakened.
What to do if you find a weakened gannet on a beach? If you come across a weakened gannet on a beach, here are the recommended steps: 1. **Observe from a distance** - Don't touch it immediately. Watch for signs of life and behavior. 2. **Contact wildlife authorities** - Call your local wildlife rescue center, animal protection service, or coastal wildlife organization right away. 3. **Provide temporary shelter** - If the bird is in immediate danger (from predators, tide, or weather), gently place a box or cloth over it to reduce stress and keep it warm. 4. **Keep it away from people and pets** - Prevent disturbance and potential injury from curious onlookers or domestic animals. 5. **Don't try to feed or give water** - Leave this to professionals, as improper care can cause harm. 6. **Document the location** - Note the exact spot so you can guide rescuers there. 7. **Minimize handling** - If you must move the bird to safety, wear gloves and support its body properly to avoid further injury. Gannets are seabirds that may strand due to illness, injury, exhaustion, or adverse weather. Professional wildlife rehabilitators have the expertise and facilities to provide appropriate care and increase the bird's chances of survival.
You should not attempt to treat it yourself. It is recommended that you contact a wildlife rescue center or a veterinarian specializing in wildlife, who will be able to assess its condition and provide appropriate care.
How do gannets become infested with roundworms?
They become infested mainly by consuming fish or small crustaceans carrying parasitic larvae, which serve as intermediate hosts in the parasite's life cycle.
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 21 August 2026
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