Gastrointestinal parasitism in bison (roundworms)
Gastrointestinal parasitism caused by roundworms (nematodes) is a common parasitic disease in bison, especially in young animals and those raised on pasture. Several nematode genera (such as Ostertagia, Trichostrongylus, Haemonchus, and Coo...
What is it?
Gastrointestinal parasitism caused by roundworms (nematodes) is a common parasitic disease in bison, especially in young animals and those raised on pasture. Several nematode genera (such as Ostertagia, Trichostrongylus, Haemonchus, and Cooperia) establish themselves in the abomasum (stomach) and intestines, where they feed and cause inflammation of the digestive tract wall. This disease is widespread wherever bison graze and represents a major cause of growth retardation and economic loss in herds.
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 depend on the number of parasites present and the animal's age. Young bison are most susceptible. Diarrhea or soft stools are often observed, along with progressive weight loss despite sometimes retained appetite, dull and rough hair coat, and growth retardation in young animals. In more severe cases, swelling under the jaw (edema called "bottle jaw" due to protein loss) may be seen, pale mucous membranes in cases of anemia linked to blood-feeding worms, and general depression. In light infestations, animals may show no visible signs while still losing growth performance.
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 gastrointestinal nematodes whose larvae are ingested with contaminated grass. Once swallowed, the larvae develop in the abomasum or intestines and become adult worms that lay eggs, shed in feces and recontaminating the pasture. The cycle repeats in a loop on pastures, especially in warm and humid conditions favorable to larval development.
Young weaned bison and weakened or stressed animals are most vulnerable, as their immunity to parasites is still immature or compromised. Overgrazing, high stocking density on the same pasture, moisture and warmth that favour larval survival, and lack of pasture rotation all increase the risk. Poor nutritional condition or concurrent disease worsens susceptibility to infestation.
Transmission
Contamination occurs through ingestion of grass or water soiled by infective larvae present in the environment, themselves originating from feces of parasitized animals. There is no direct transmission between animals away from contaminated pasture.
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 herd history, clinical examination (body condition, mucous membranes, presence of oedema) and especially on faecal analysis to count parasite eggs and identify the genera involved. In some cases, blood testing may be useful to assess anaemia or protein loss.
Treatment
Treatment is based on antiparasitic drugs (dewormers) from different chemical families, selected by the veterinarian according to the parasites identified and the herd's situation. Nutritional supplementation may be added to help weakened animals recover.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Pasture management is essential: rotate fields, avoid overgrazing, and allow grassland to rest to reduce larval burden. Regular monitoring of body condition and growth of young animals helps detect problems early. Periodic faecal testing (faecal egg count) helps assess the herd's level of infestation and target antiparasitic treatments rather than treating all animals routinely, which limits the development of resistance.
Possible complications
Untreated heavy infestation can cause significant growth delay, severe anaemia, dehydration and, in severe cases in young or weakened animals, death. Repeated and uncontrolled use of antiparasitic drugs also promotes the development of parasite resistance, making future treatments less effective.
When to see a veterinarian
A young bison shows growth delay or unexplained weight loss.
Diarrhoea persists despite normal diet.
Swelling under the jaw appears.
Several animals in the herd show similar signs at the same time.
An animal appears depressed, pale or dehydrated.
Prognosis
The outlook is good when the condition is detected and treated early, with full recovery of growth and general health expected. However, a severe and prolonged infestation, especially in a young animal, can result in lasting growth problems or progress to death if left untreated.
Frequently asked questions
Can gastrointestinal parasitism in bison be transmitted to humans?
No, these nematodes are specific to ruminants and do not pose a risk to humans.
Should all bison in the herd be dewormed systematically?
No, it is recommended to base treatments on faecal analyses to target animals that are actually infested and limit resistance to antiparasitic drugs.
Are adult bison naturally protected?
Adults often develop partial immunity after repeated exposures, which makes infestations generally less severe than in young people, though without rendering them completely resistant.
Is pasture rotation sufficient to eliminate the problem?
It significantly reduces the parasite burden but does not eliminate it completely; it must be combined with regular monitoring and, if necessary, targeted treatments.
References
WOAH — Organisation mondiale de la santé animale (ex-OIE)
FAO — Organisation des Nations unies pour l'alimentation et l'agriculture
ESCCAP — European Scientific Counsel Companion Animal Parasites
AVMA — American Veterinary Medical Association
Content reviewed by the TunisieVet editorial team · Updated 27 August 2026
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