Gastrointestinal roundworm parasitism in cormorants
Gastrointestinal parasitism caused by roundworms (nematodes) is a common condition in cormorants, fish-eating birds that live in colonies near water. These parasites settle in the stomach, proventriculus (digestive pouch), or intestines of...
What is it?
Gastrointestinal parasitism caused by roundworms (nematodes) is a common condition in cormorants, fish-eating birds that live in colonies near water. These parasites settle in the stomach, proventriculus (digestive pouch), or intestines of the bird. They feed on ingested food or the intestinal wall, which disrupts digestion. The condition mainly affects young birds and weakened individuals, but often remains mild in healthy 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:
Signs vary depending on the level of infestation. A moderately infested cormorant may show no visible symptoms. As infestation increases, progressive weight loss can occur despite maintained or reduced appetite. Plumage may become dull and less waterproof. Digestive problems are possible, including diarrhea, regurgitation, or abdominal bloating. In young birds, stunted growth and general weakness are common. In severe cases, worms may be visible in droppings or regurgitated material.
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 condition is caused by gastrointestinal nematodes, particularly species from the genera Contracaecum and Cosmocephalus, well adapted to fish-eating birds. Cormorants become infected by eating fish or aquatic invertebrates (intermediate hosts) that carry parasitic larvae. Once ingested, the larvae develop within the bird's digestive tract to the adult stage, where they lay eggs that are shed in the droppings.
Young cormorants are more susceptible due to immature immune systems.
Dense colony living promotes parasite buildup in the environment.
An abundant diet of small infested fish increases risk.
Stress, concurrent illness, or water pollution can worsen infestation.
Feeding areas shared with many fish-eating birds raise parasite pressure.
Transmission
Transmission occurs through the food chain, via ingestion of fish or aquatic invertebrates carrying infective larvae. There is no direct contagion between birds; the cycle requires passage through the aquatic environment and intermediate hosts.
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 a full veterinary examination including assessment of overall condition and plumage.
Fecal analysis (searching for parasite eggs) is the basic test.
In some cases, endoscopy or necropsy allows direct identification of worms in the digestive tract.
The veterinarian tailors tests to the context: a weakened wild bird found, or a bird kept in captivity.
Treatment
Treatment relies on administering antiparasitic drugs specific to roundworms, prescribed and dosed only by a veterinarian experienced in wildlife or aquatic bird medicine. Supportive care (rehydration, nutritional supplementation) often accompanies treatment in weakened birds.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Complete prevention is not possible in the wild, as the parasite cycle is tied to the aquatic ecosystem.
In captivity or at wildlife care facilities, risk can be reduced by controlling the quality of fish provided and maintaining good basin hygiene.
Regular monitoring of body condition and droppings allows early detection.
Routine parasitology testing is useful in collections of aquatic birds (parks, care centers).
Possible complications
Untreated heavy infestation can lead to digestive blockage or perforation, anemia, and severe weakness that may result in death, especially in young birds or those already compromised by other factors (pollution, injury, food scarcity).
When to see a veterinarian
The bird appears thin despite eating normally.
Worms are visible in droppings or regurgitate.
The bird shows persistent diarrhea or marked lethargy.
A young cormorant displays obvious growth retardation.
The bird is found weak and unable to fly or feed normally.
Prognosis
Prognosis is generally good in healthy adult birds with moderate infestation, especially after antiparasitic treatment. It becomes guarded in young birds or already weakened individuals, where heavy infestation can worsen an already precarious health status.
Frequently asked questions
Is gastrointestinal parasitism in cormorants dangerous for humans?
No, these nematodes are specific to piscivorous birds and are not transmitted to humans under normal conditions.
How does a cormorant catch these worms?
By eating fish or aquatic invertebrates carrying parasitic larvae, which then develop in its digestive tract.
Should a wild cormorant infested with parasites necessarily be treated?
Not necessarily. Many wild birds harbor parasites without showing any signs of disease. Treatment is considered mainly if the bird displays clinical signs or is taken in by a care center.
Can this infestation be prevented in a captive cormorant?
You can reduce the risk by controlling the quality and origin of the fish given to the bird, and by regularly monitoring the bird's health status.
References
WOAH — Organisation mondiale de la santé animale (ex-OIE)
ESCCAP — European Scientific Counsel Companion Animal Parasites
Content reviewed by the TunisieVet editorial team · Updated 21 August 2026
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