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Gastrointestinal strongyloses in sheep
Under the name gastrointestinal strongyloses, we bring together several roundworms which live in the abomasum and intestine of sheep and goats. Unlike the red abomasum worm, which causes spectacular anemia, these worms mainly damage the dig...
What is it?
Under the name gastrointestinal strongyloses, we bring together several roundworms which live in the abomasum and intestine of sheep and goats.
Unlike the red abomasum worm, which causes spectacular anemia, these worms mainly damage the digestive wall: the animal digests less well, eats less, and no longer properly processes what it receives.
The loss is therefore often invisible. The flock does not look sick, but the lambs are growing more slowly, the ewes are losing weight, the carcasses are lighter and the wool is of poorer quality.
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:
Soft to liquid diarrhea, often greenish, which soils the hindquarters and the wool of the tail.
Weight loss or simple stagnation of weight, even though the ration seems correct.
Lambs that are taking poorly, batches that are downgrading, increasingly visible differences in size between animals of the same age.
Dull, brittle wool, which comes loose in places.
Reduced appetite, animals grazing for less time and lying down.
Dehydration and weakness when diarrhea lasts.
In young lambs in spring, certain worms cause sudden and severe diarrhea which affects an entire flock within a few days.
Many infested animals show no visible signs while penalizing the herd's 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 adult worms lay eggs in the digestive tract, the eggs leave in the droppings and give rise to larvae on the plot.
The larvae survive in low vegetation and decaying droppings, then are swallowed along with the grass.
Once in the digestive tract, the larvae burrow into the wall of the abomasum or intestine. The organ heals poorly, absorbs less well and leaks proteins: it is this mechanism, more than the number of worms, which explains the poor performance.
Some species can remain dormant in the digestive wall during the bad season, then leave all together, often at the time of giving birth.
Animals acquire partial immunity with age and exposure: well-fed adults defend themselves better, but are never totally protected.
Lambs and lambs, whose immunity is still incomplete.
Ewes around birth and during lactation.
High stocking rate, low grazing, lack of rotation, wet plots.
Animals that are poorly nourished or have a protein and mineral deficiency: they tolerate the same parasitic load much less well.
Mixture of animals of different ages on the same plots.
Farms where dewormer is given systematically to the entire herd on a fixed date: resistance sets in more quickly.
Quarantine-free purchases, animal fairs and gatherings.
Transmission
Contamination occurs through grass and contaminated water, not through direct contact between animals, and these worms are not transmitted to humans.
Ewes at the end of gestation and at the beginning of lactation eliminate many more eggs than usual: they contaminate the plots at the precise moment when their lambs begin to graze.
Plots grazed continuously by the same animals gradually become loaded with larvae, season after season.
An animal introduced without quarantine can bring onto the farm worms that are already resistant to antiparasitic drugs.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
The diagnosis cannot be made by eye: diarrhea can have many other causes, and a heavily parasitized animal may not have any.
The veterinarian carries out a dropping analysis, coproscopy, with egg counting. The sample concerns several animals from the same batch, which gives an image of the situation of the group and not of an individual.
A culture of larvae allows, if necessary, to know which species of worms dominate, useful information for choosing the strategy.
Regular weighing of lambs, comparing the growth of flocks, is a simple and effective monitoring tool.
A dropping control carried out some time after a treatment makes it possible to verify that the product is still working on the farm. This is the most useful test when a dewormer no longer seems to be effective.
In diarrhea in young lambs, the veterinarian also rules out other causes, particularly infectious, before concluding.
Treatment
Several families of internal antiparasitics act on these worms. The choice is up to the veterinarian, who takes into account the species of worms present, the age of the animals, milk production and the state of resistance in the region.
Doses and rhythms cannot be improvised or copied from one neighbor to another: an under-dosed product cleans poorly and selects the most resistant worms.
Animals that have a lot of diarrhea need clean water, shelter, quality food and sometimes rehydration: deworming alone does not get a weakened animal back on its feet.
The systematic treatment of the entire herd on a fixed date, without analysis, is the practice which has done the most harm: it is this which has rendered entire families of products ineffective on many farms. The approach recommended today consists of having the droppings analyzed, treating only the batches or animals that really benefit from it, and leaving part of the herd untreated in order to preserve worms that are still sensitive.
Alternating product families only makes sense on veterinary advice and on the basis of an effectiveness check: changing bottles at random does not solve anything.
Waiting times for meat and milk must be strictly respected; they vary depending on the product and some are prohibited in females whose milk is consumed. Ask for them before any treatment.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Rotate the plots and prevent animals from razing the grass, where the larvae concentrate.
Reserve the less heavily loaded plots, for example after mowing or cultivation, for lambs and ewes at the end of gestation.
Alternate with cattle or horses when the farm allows it.
Quarantine and treat, on veterinary advice, any animal purchased before releasing it to pasture.
Monitor the growth of lambs by weighing: stalling growth is often the first signal.
Ensure sufficient nutrition, particularly in proteins, minerals and trace elements.
Avoid drinking from trampled puddles and stagnant water sources.
Keep for breeding animals that remain in good condition without treatment, and cull those that fail each season.
Possible complications
Stunted growth that never completely catches up in young people.
Weight loss in ewes, poor recovery after lactation, declining fertility.
Dehydration and severe weakness during prolonged diarrhea.
Permanently soiled hindquarters, which attracts flies and prepares the ground for myiasis during the hot season.
Increased susceptibility to other herd diseases.
Long-lasting loss of effectiveness of pest control on the farm.
When to see a veterinarian
Several animals in the same batch have diarrhea or soiled hindquarters.
The lambs no longer gain weight even though the diet is correct.
Ewes lose weight without explanation after giving birth.
Diarrhea suddenly affects a batch of lambs in spring.
An animal is dehydrated, depressed or refuses to get up: consult without delay.
The results do not improve after deworming: it is necessary to check its effectiveness rather than repeat it.
Prognosis
Caught in time, with appropriate treatment and good nutrition, animals recover well.
Severely affected lambs in full growth often remain permanently delayed, even when cured: this is the true cost of the disease.
At the herd level, the prognosis depends mainly on grazing management and the state of resistance to antiparasitics.
Frequently asked questions
My sheep don't look sick, should we still take care of the worms?
Yes. The most expensive form is precisely the one that is not visible: the animals eat, they do not really lose weight, but they do not use their ration well. A scat analysis costs much less than the lost growth on a flock of lambs.
Can I deworm my entire herd two or three times a year, as we have always done?
This is what has made the products ineffective in many farms. The current logic is different: we analyze the droppings, we treat the batches or animals that need it, and we voluntarily leave part of the herd untreated to preserve sensitive worms. This strategy is developed with the veterinarian.
How do I know if my dewormer is still working?
By comparing the number of eggs in the droppings before and some time after treatment. It is a simple examination that the veterinarian organizes on a few animals and which avoids continuing to pay for a product that has become useless.
Can sheep worms contaminate me or my children?
No, these sheep digestive worms are not transmitted to humans. Other herd parasites, on the other hand, present a human risk: this is the case of the dog tapeworm responsible for the hydatid cyst, which justifies deworming herding dogs and washing their hands.
Can we sell milk or slaughter an animal immediately after deworming?
No. Each product imposes a waiting period before the meat or milk is edible, and certain products are prohibited for dairy females. This deadline must be requested from the veterinarian and respected without exception.
References
FAO — Organisation des Nations unies pour l'alimentation et l'agriculture
ANSES — Agence nationale de sécurité sanitaire (France)
Content reviewed by the TunisieVet editorial team · Updated 09 August 2026
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