Gastrointestinal parasitism in the Common Guillemot (roundworms)

What is it?

Gastrointestinal parasitism caused by roundworms (nematodes) is a common condition in the Common Guillemot, a diving seabird that feeds on small fish. These parasites live in the stomach, proventriculus (a specialized part of the stomach), or intestines of the bird. They are present in many wild seabirds without necessarily causing visible disease. The problem becomes significant when the parasite burden is heavy, or when the bird is already weakened by other factors such as an oil spill, food shortage, or severe stress. Guillemots admitted to rehabilitation centers after stranding often show this type of parasitism, sometimes alongside other health problems.

Commonly observed signs

These signs are neither exhaustive nor specific. Only a veterinarian can establish a diagnosis.

Causes and risk factors

Several families of nematodes (roundworms) can parasitize the digestive tract of the Common Guillemot, particularly those found in fish-eating seabirds. These parasites often have a complex life cycle involving an intermediate host, usually a fish or small marine invertebrate. The guillemot becomes infected by eating fish carrying infective larvae. The larvae then develop in the bird's stomach or intestines to the adult stage, where they lay eggs shed in the feces, which return to the marine environment to continue the cycle.

Young birds and individuals weakened by other causes, such as oil contamination, lack of food, or injury, are more sensitive to the clinical effects of parasitism.

A high availability of fish carrying larvae in the guillemot's fishing area increases the risk of heavy infestation.

Stress from captivity, for example during a stay at a care centre following stranding, can also bring out clinical signs of parasitism that was previously asymptomatic.

Transmission

Infection occurs through food intake, by eating fish or small marine invertebrates carrying nematode larvae. There is no direct transmission between guillemots.

Diagnosis and care

Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.

Treatment

Treatment is based on using broad-spectrum antiparasitic drugs from the anthelmintic family, adapted to the wild bird by a veterinarian skilled in wildlife medicine.

General supportive care is often provided alongside treatment: rehydration, nutritional supplementation, and stress management during convalescence at a care centre.

No treatment should be given without veterinary supervision, as wild seabirds are very sensitive to handling errors and incorrect dosing.

Prevention

Possible complications

Heavy untreated parasite burden can lead to chronic inflammation of the digestive wall, malabsorption of nutrients, and severe weight loss.

In rare cases, digestive obstruction from a mass of worms can occur.

Combined with other stressors, parasitism can contribute to exhaustion and death of the bird, particularly during mass strandings following marine pollution.

When to see a veterinarian

Prognosis

The outlook is generally good when parasitism is detected early and treated in a bird that is otherwise in fair general condition.

It becomes more guarded in birds already weakened, particularly following pollution or food shortage, where parasitism worsens an already fragile state.

Rapid care at a specialized wildlife rehabilitation centre significantly improves the chances of recovery before release.

Frequently asked questions

References

WOAH — Organisation mondiale de la santé animale (ex-OIE)

Cornell Feline Health Center

Content reviewed by the TunisieVet editorial team · Updated 08 September 2026