Yak gastrointestinal parasitism (roundworms)
Gastrointestinal parasitism caused by roundworms (nematodes) is a very common disease in yaks, especially in livestock farms in extensive high altitude pastures. These small worms live in the abomasum (stomach) and intestines of the animal....
What is it?
Gastrointestinal parasitism caused by roundworms (nematodes) is a very common disease in yaks, especially in livestock farms in extensive high altitude pastures.
These small worms live in the abomasum (stomach) and intestines of the animal.
They feed on blood or the digestive mucosa, which gradually weakens the animal.
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:
The signs are often subtle and gradual, making them easy to overlook at first.
There is diarrhea or loose stools, sometimes intermittent.
The coat becomes dull and shaggy, the animal loses weight despite sometimes retaining its appetite.
A swelling under the jaw (called “Morgagni bottle”) can appear in severe forms, linked to a loss of proteins in the blood.
The mucous membranes (gums, around the eyes) may turn pale in cases of blood-sucking worms like Haemonchus.
In young animals, there is a clear growth delay compared to the rest of the herd.
In massive infestations, marked depression and dehydration may occur.
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 several species of nematodes (roundworms) which parasitize the abomasum and intestines.
The genera most involved are Ostertagia, Haemonchus, Trichostrongylus, Cooperia and Nematodirus, often in mixed infestations.
Yaks become infested by grazing on grass contaminated by larvae from the excrement of carrying animals.
The parasitic cycle strongly depends on the climate: humidity and moderate temperature favor the development of larvae on the pasture.
At altitude, the cold can slow down the cycle but some larvae survive dormant in grass or in animals until more favorable conditions.
Young age is a major risk factor, because immunity against these parasites builds gradually with repeated exposure.
Females at the end of gestation or lactating are more sensitive due to a natural and temporary drop in their immune defenses.
Overgrazing and the high density of animals on the same plot increase contamination of the grass.
Moist pasture, abundant rainfall and mild temperatures favor the survival of the larvae.
An already weakened body condition (undernutrition, other illness) worsens susceptibility to parasites.
Transmission
Transmission occurs orally, by grazing on pastures contaminated by infective larvae from parasitized animal dung.
There is no direct transmission between animals without passing through 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 clinical signs (diarrhea, weight loss, dull coat, pallor) associated with grazing history and the season.
The definitive diagnosis is based on laboratory analysis of stools, which makes it possible to count parasite eggs (coproscopy) and estimate the level of infestation.
In some cases, a blood test may be useful to assess anemia or protein levels.
An autopsy can sometimes confirm the diagnosis in cases of mortality, by identifying worms directly in the stomach or intestines.
Treatment
Treatment is based on the use of families of antiparasitics (wormers) active against gastrointestinal nematodes, to be chosen and administered under veterinary supervision.
The choice of molecule and the timing of treatment depend on the season, the level of infestation and the risk of parasitic resistance.
Nutritional support (energy and protein intake) often accompanies treatment in weakened animals.
Rehydration may be necessary in severe cases with significant diarrhea.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Prevention is based on reasoned management of grazing: rotation of plots, avoidance of overgrazing, rest of certain areas.
A targeted antiparasitic treatment, decided with the veterinarian according to the time of year and the level of infestation, makes it possible to break the parasitic cycle without systematically treating the entire herd.
Regular monitoring of body condition and, if possible, stool analyzes make it possible to adapt interventions.
Avoiding grazing young animals on the most contaminated plots (used the previous year by already infested animals) reduces the risk.
A good diet, particularly in proteins and minerals, helps animals to better resist infestation.
Possible complications
Untreated infestation can lead to lasting growth retardation in young, reduced milk production in females, and general weakening leading to other diseases.
In severe cases, significant anemia or loss of blood proteins (with edema) can put the animal's life at risk.
Resistance to antiparasitics can develop in the event of repeated and unreasoned use, complicating the future management of the herd.
When to see a veterinarian
Persistent or recurrent diarrhea in one or more yaks in the herd
Progressive weight loss despite a preserved appetite
Dull, pitted coat associated with stunted growth in young
Swelling under the jaw
Paleness of the mucous membranes or marked weakness
Multiple animals in the herd showing similar signs at the same time
Prognosis
The prognosis is generally good if the disease is detected early and treated appropriately, with full recovery of body condition within a few weeks.
In heavily infested young animals or in the event of delay in treatment, growth effects may persist.
Severe forms with marked anemia or edema have a more guarded prognosis and require rapid treatment.
Frequently asked questions
Are yak roundworms dangerous for humans?
No, these nematodes are specific to ruminants like yak and are not transmitted to humans.
Should all yaks in the herd be dewormed systematically?
Not necessarily. Reasoned treatment, based on the condition of the animals and sometimes stool analyses, is preferable to systematic treatment to limit resistance.
At what age are yaks most at risk?
Young animals, especially after weaning, are the most vulnerable because their immunity against these parasites is not yet developed.
Can you see worms in stool?
Generally no, these worms are small and are not visible to the naked eye in the feces; only laboratory analysis can detect their eggs.
References
WOAH — Organisation mondiale de la santé animale (ex-OIE)
ESCCAP — European Scientific Counsel Companion Animal Parasites
FAO — Organisation des Nations unies pour l'alimentation et l'agriculture
Content reviewed by the TunisieVet editorial team · Updated 13 September 2026
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