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Cattle gastrointestinal strongles
Gastrointestinal strongyles are small roundworms that live in the abomasum and intestine of cattle. They are almost always present at low noise in grazing animals. The problem is not their presence, but their number. Beyond a certain level...
What is it?
Gastrointestinal strongyles are small roundworms that live in the abomasum and intestine of cattle. They are almost always present at low noise in grazing animals.
The problem is not their presence, but their number. Beyond a certain level of infestation, they damage the digestive wall, the animal digests poorly and no longer benefits from its ration.
It is especially young cattle in the first grazing season that pay the high price. Adults, exposed for years, have developed partial immunity and compensate better.
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:
Diarrhea or persistent soft dung, especially in young cattle on pasture.
Slowed growth, animals which remain small in relation to their age, heterogeneity of the batch.
Weight loss or body condition that does not recover despite good weed.
Dull and pitted coat, animals that have no shine.
Decreased appetite, fatigue, animals staying behind in the pasture.
In infestations with bloodsucking worms: pale mucous membranes and pocket of edema under the jaw.
In adult dairy cows, the most common sign is production below potential, with no other symptoms.
Signs can appear at the end of winter in animals that have returned apparently healthy: certain larvae remain dormant in the digestive wall and then wake up all together.
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 species of roundworms are involved; some settle in the abomasum, others in the small intestine.
Adult worms lay eggs which are eliminated in the dung. The larvae develop in dung and then migrate to grass, where cattle swallow them.
The cycle is therefore entirely linked to grazing: an animal that does not go out does not become infested.
Certain larvae swallowed in the fall encyst in the digestive wall and resume their development at the end of winter, causing sudden problems in animals returned in good health.
Young cattle in the first grazing season: these are the most sensitive by far.
Continuous grazing of the same plot all season, without rotation or rest.
High loading: many animals on a small surface area.
Humid and mild seasons, which prolong the survival of larvae on grass.
An insufficient ration in energy and protein: a poorly nourished animal supports the same parasitic load much less well.
The introduction of animals without quarantine or reasonable grazing treatment.
Animals weakened by another illness or recent stress.
Transmission
Through grass: cattle swallow the larvae present on blades of grass, especially near dung and at the bottom of vegetation.
Mild, humid conditions favor the survival of larvae; drought and full sun destroy them.
A plot of land grazed continuously by the same animals gradually becomes denser over the course of the season.
There is no contamination by direct contact, neither through milk nor to humans.
Purchased animals bring their own worms, including treatment-resistant worms, into the herd and onto the plots.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
Don't judge the infestation by eye: talk to the veterinarian as soon as growth or production does not follow.
A dung analysis (egg counting) carried out on several animals in the batch gives an image of the parasitic load. An isolated sample doesn't mean much.
Blood tests exist to assess abomasum damage, and tests on tank milk to locate a dairy herd.
Weighing young cattle is a diagnostic tool in its own right: a decline in growth over several weeks is a reliable signal.
These analyzes are also used to verify that a treatment has worked well: it is the only way to identify resistance before it sets in.
Treatment
Treatment is based on internal antiparasitics, called anthelmintics. Several families exist; the choice is up to the veterinarian.
Resistance of worms to anthelmintics is a real problem, growing, and irreversible once established on a farm. It is directly favored by systematic and repeated treatments of the entire herd.
The rule is therefore to treat the right animals at the right time, on veterinary advice, and not out of habit or schedule.
Prioritize the animals that need it: young growing animals, late animals, batches whose analyzes justify it. Adults in good condition often have no interest in being treated.
A product poorly administered or poorly adapted to the animal's actual weight promotes resistance: the prescription and instructions for use must be followed exactly.
Alternate the families of anthelmintics on veterinary advice, and check the effectiveness by analyzing the dung after treatment.
It is imperative to respect the waiting times for milk and meat, which only the prescribing veterinarian can tell you.
However, systematically treat any animal purchased during its quarantine: this is the main entry point for resistant worms into a farm.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Pasture management is the first lever, before medications.
Rotation of the plots with sufficient rest time: the grass grown back after a vacuum is cleaned up.
Avoid putting young cattle on the heaviest plots; reserve the healthiest meadows for them, such as regrowth after mowing.
Alternation with another animal species or with a crop cleans up a plot, the worms being largely specific to the species.
Do not graze too low: the larvae concentrate in the first centimeters at ground level.
Take care of the ration: well-fed animals are much better able to withstand a given parasitic load.
Monitor the growth of the young by regular weighings rather than treating blindly.
Quarantine and treatment of introduced animals, so as not to import resistant worms.
Make an annual review with your veterinarian to build a plan adapted to your plots and your management.
Possible complications
Growth retardation which never completely recovers in young cattle.
Persistent diarrhea with dehydration in massive infestations.
Anemia with blood-feeding species.
Decreased milk production and fertility in adults.
Increased susceptibility to other diseases in weakened animals.
Installation of resistance to anthelmintics in breeding, which permanently deprives the breeder of available solutions.
When to see a veterinarian
Young cattle on pasture have soft dung or diarrhea that lasts.
The growth of the batch is late or very heterogeneous.
Animals lose weight or keep dull coats despite good grass.
You observe a soft pocket under the jaw or pale mucous membranes.
Animals returned in good condition develop digestive problems at the end of winter.
An antiparasitic treatment no longer seems to have any effect: this is a signal of possible resistance, to be explored with the veterinarian.
Prognosis
Animals treated in time recover, but significant growth retardation in young cattle never fully recovers.
The economic consequences are mainly due to accumulated silent losses, not to visibly sick animals.
Farming that combines plot management and targeted treatments maintains its parasites at a sustainable level.
Conversely, a farm that treats the entire herd systematically ends up selecting resistant worms and finds itself without an effective solution.
Frequently asked questions
Should I deworm my entire herd on a fixed date each year?
No, and it's even counterproductive. Systematically treating all animals selects resistant worms, and established resistance does not disappear. The good principle is to treat the animals that need it, at the right time, on veterinary advice and if possible after analyzing the dung.
Do my adult cows need to be dewormed like young ones?
Rarely so much. Adults have developed partial immunity over the grazing seasons. It is the first year youngsters who concentrate the risk and who warrant most of the attention.
How do I know if my dewormer is still working?
By comparing a dung analysis before and after treatment, with your veterinarian. This is the only way to spot emerging resistance, well before the animals show signs.
Can I sell milk from a dewormed cow?
Not immediately, and not with all products: some are prohibited in lactating cows, others impose a waiting period. This information appears on the prescription: ask the veterinarian for it and respect it strictly.
References
FAO — Organisation des Nations unies pour l'alimentation et l'agriculture
Content reviewed by the TunisieVet editorial team · Updated 09 August 2026
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