Gastrointestinal parasitism in swans (roundworms)
Gastrointestinal parasitism from roundworms (nematodes) is a common condition in swans, especially on water bodies shared with other waterfowl. Small threadlike worms establish themselves in the bird's intestine, stomach, or gizzard. They f...
What is it?
Gastrointestinal parasitism from roundworms (nematodes) is a common condition in swans, especially on water bodies shared with other waterfowl. Small threadlike worms establish themselves in the bird's intestine, stomach, or gizzard. They feed at the swan's expense and disrupt digestion. This disease affects young swans in particular and weakened birds or those living in overcrowded conditions.
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. Progressive weight loss may be seen despite maintained appetite, or conversely a decrease in appetite. Droppings may become soft or liquid, sometimes with worms visible to the naked eye. The bird becomes tired, less active, and stays apart from the group. In young swans, stunted growth and dull plumage are common. In heavy infestations, abdominal bloating or digestive discomfort may develop.
When to consult without delay
This list does not replace veterinary advice. It gathers situations that justify a prompt consultation.
Causes and risk factors
Several families of nematodes can affect swans, notably worms lodged in the gizzard, small intestine, or proventriculus (glandular stomach). Infection occurs through ingestion of eggs or larvae present in water, mud, or by eating intermediate hosts such as certain mollusks or aquatic insects depending on the parasite species.
Young swans are more vulnerable because their immune system is still immature. High bird density on the same body of water promotes the buildup of parasites in the environment. Stagnant water that is rarely refreshed, or water shared with many wild birds, increases the risk. Stress, inappropriate feeding by the public (particularly bread) and general weakness make the bird more susceptible.
Transmission
Contamination occurs through the mouth, by feeding in water, mud, or vegetation soiled by parasite eggs from the droppings of infested birds. Some parasites require passage through an intermediate host (mollusk, crustacean, or aquatic insect) before becoming infectious to swans again.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
The veterinarian relies on clinical examination of the bird and faecal analysis (coproscopy) to detect parasite eggs under the microscope. In some cases, endoscopy or post-mortem examination may reveal worms directly in the digestive tract. The bird's general condition, weight and history of its living site are also considered.
Treatment
Treatment is based on broad-spectrum antiparasitic medications suitable for waterfowl. Supportive care (rehydration, nutritional supplementation) may be needed if the bird is severely weakened. The choice of medication and treatment duration must be determined by a veterinarian.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Limiting the number of waterfowl on the same site helps reduce parasite pressure. Good water circulation and regular bank maintenance limit the accumulation of parasite eggs. Avoiding feeding bread or inappropriate foods preserves the bird's overall health. Regular parasite screening by a veterinarian, especially in captive or semi-captive populations, enables early detection.
Possible complications
Untreated heavy infestation can lead to partial digestive tract obstruction, anaemia, severe growth retardation in young birds, or death from general weakness, particularly in already compromised individuals.
When to see a veterinarian
The bird is losing weight despite appearing to eat normally. Worms are visible in the droppings or around the cloaca. The swan becomes unusually lethargic or stays apart from the group. A young swan shows clear growth retardation compared to siblings. Droppings become persistently soft or watery.
Prognosis
The outlook is generally good if infestation is detected and treated early. It becomes more guarded in young swans or birds already weakened by other causes, where infestation can worsen an already fragile condition.
Frequently asked questions
Can roundworms from swans be transmitted to humans?
No, gastrointestinal nematodes in swans are not considered transmissible to humans. The zoonotic risk is negligible for this parasitic infection.
How to tell if a swan is parasitized?
Signs are often subtle: weight loss, fatigue, loose stools. Only a stool analysis by a veterinarian can confirm the presence of parasites.
Should all swans on a body of water be systematically dewormed?
This decision is up to a veterinarian, who will assess the need for parasite control based on bird density, test results, and the health status of the site.
Does bread given by visitors promote parasitism?
Bread does not directly introduce parasites, but it weakens the nutritional status of swans, making them more susceptible to parasitic infestations.
References
ESCCAP — European Scientific Counsel Companion Animal Parasites
WOAH — Organisation mondiale de la santé animale (ex-OIE)
Content reviewed by the TunisieVet editorial team · Updated 21 August 2026
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