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Nematodiosis of lambs
Nematodiosis is a sudden diarrhea of lambs, caused by a roundworm of the small intestine of the genus Nematodirus. It is a separate disease among sheep parasitoses, for two reasons which completely change the way it is managed. First, it...
What is it?
Nematodiosis is a sudden diarrhea of lambs, caused by a roundworm of the small intestine of the genus Nematodirus.
It is a separate disease among sheep parasitoses, for two reasons which completely change the way it is managed.
First, it is not the adult worms that cause the disease, but the wave of larvae swallowed at once: thousands of larvae attack the wall of the small intestine at the same time, even before becoming adults.
Key point
Nematodiosis can kill lambs within a few days due to dehydration, and scat analysis can be of little use at the worst times. Sudden diarrhea in a batch of lambs justifies calling the veterinarian without waiting for a laboratory result.
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 with sudden onset, liquid, greenish to dark, which affects several lambs from the same batch at the same time.
Marked thirst: sick lambs crowd around water, which is a very characteristic sign.
Rapid dehydration, sunken eyes, skin that keeps wrinkles, dry muzzle.
Despondency, lambs which stop suckling and grazing, remain lying down, allow themselves to be caught.
Visible weight loss in just a few days in previously healthy animals.
Hindquarters and tail wool permanently soiled.
Possible death quickly among those most affected, particularly those who become dehydrated.
The ewes of the same flock generally remain normal: this is a point which strongly guides, the disease only affects the young.
Lambs that survive are often left behind, with dull coat and wool for weeks.
When to consult without delay
This list does not replace veterinary advice. It gathers situations that justify a prompt consultation.
Causes and risk factors
Adult worms live in the small intestine and lay eggs which come out with the droppings.
These eggs are exceptionally resistant: they survive on the plot throughout the bad season, under frost as well as under the sun, and wait.
The larva develops inside the egg and remains enclosed there. It needs a cold period, then a warming, for the hatching to start. All the larvae then hatch almost together, producing a massive wave on the grass within a few days.
If lambs are on the plot at this time and have started grazing, they will swallow a huge load of larvae in one go.
The larvae attack the wall of the small intestine, which loses its absorption capacity: water and nutrients are no longer retained, resulting in diarrhea and dehydration.
Adult sheep develop good immunity and practically no longer suffer from the disease, but they can harbor the parasite discreetly.
Lambs that have started to graze seriously, generally in their first months: these are the only ones really exposed.
Plots that had lambs the previous year, particularly those where diarrhea had already been observed.
Warming period following a cool season, when mass hatching begins.
Numerous lots, high loading, short grass.
Lambs whose milk intake decreases and which compensate by grazing more.
Poor availability of clean water: dehydration sets in much more quickly.
Lambs already weakened, malnourished or carrying other digestive parasites.
Transmission
Contamination occurs only through grass: there is no direct contagion between lambs nor any risk for humans.
Decisive feature: the danger does not come from the lambs of the current batch, but from the eggs laid by the lambs of the previous year. A batch of lambs placed on a plot which had sick lambs last season is in danger, even if no animal in the current flock is affected.
This explains why the disease repeats itself every year in the same places, on the same plots, often at the same time of the season.
Sheep contaminate little: it is not on them that we must act.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
The context is very orienting: sudden diarrhea, several lambs from the same batch, unharmed ewes, plot having borne lambs the year before.
Beware of a major pitfall: the dropping analysis can be negative or very weak at the very moment when the lambs die, because it is the larvae which cause the disease and they are not yet laying eggs. A reassuring result should never make you lower your guard.
The veterinarian may request a specific examination, the eggs of this worm being very large and recognizable under a microscope, but he will not rely solely on them.
He rules out other causes of diarrhea in lambs, notably infectious and coccidiosis, which are very similar and are not treated in the same way. This is precisely why we must not deal at random.
Examining a dead lamb often provides the answer: the wall of the small intestine is inflamed and the worms, fine and coiled in a spiral, are visible there.
Weighing the lambs then makes it possible to measure the delay taken by the batch.
Treatment
Treatment is based on internal antiparasitics active on both larvae and adults. The choice of family and shape is up to the veterinarian, who takes into account the age of the lambs and the local situation.
This parasite generally maintains a good sensitivity to the products available, but this does not exempt from checking the effectiveness in the event of failure.
No dose or rhythm is decided alone, and a treatment given to adults should never be copied.
Rehydration is as important as deworming: lambs die from dehydration, not from the worms themselves. Clean water permanently accessible, shelter, quality ration, rehydration solutions on veterinary advice for the most affected.
Removing the lot from the contaminated plot is part of the treatment.
Treating the entire batch at the same time, on the advice of the veterinarian, is more often justified here than in other parasitoses, because the contamination is simultaneous and massive. This remains a veterinary decision, not a reflex.
Meat waiting periods apply, including to lambs intended for slaughter in the near future: ask for them systematically.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
The most effective measure is a simple grazing rule: do not put this year's lambs on plots that bore lambs last year.
Reserve for young lots plots grazed by adults, by cattle, or plots mowed or cultivated the previous year.
Establish a rotation over several years between lamb plots, keeping a log of plots where diarrhea has been observed.
Ensure permanent access to clean water and shelter for young flocks.
Monitor the lambs during the warming season, a period when the wave of larvae can be triggered, and look at the animals' hindquarters every day.
Talk to the veterinarian about a possible prevention strategy for plots of known risk, rather than waiting for the first death.
React quickly to the first case: in this disease, a few days of delay cost lambs.
Possible complications
Death from dehydration within a few days, sometimes before treatment can even work.
Long-term growth retardation among survivors, with a flock downgrading and lambs never fully catching up.
Increased susceptibility to other illnesses during the following weeks.
Permanently soiled hindquarters, conducive to myiasis in the hot season.
Recurrence every year on the same plots if grazing management is not modified.
When to see a veterinarian
Several lambs from the same batch suddenly have diarrhea: call the same day, this illness spreads quickly.
A lamb is slaughtered, dehydrated, no longer gets up.
Lambs crowd around the water trough while having diarrhea.
Lambs die within a few days even though the ewes are fine.
A dropping analysis came back with little information but the lambs are not doing well: don't trust the result, call the veterinarian again.
The same diarrhea returns every year on the same plot: you need to review grazing management with the veterinarian.
Prognosis
Lambs treated and rehydrated early generally heal well.
Those who are already lying down and dehydrated have a guarded prognosis: death occurs quickly at this stage.
Even when cured, affected lambs in full growth often remain behind, which affects the results of the flock.
On a farm scale, the prognosis is excellent as long as the breeder applies the plot rule: it is a disease that is much better avoided than treated.
Frequently asked questions
My lambs have diarrhea but the dropping analysis is almost normal: can we rule out the worms?
No, and that is the great trap of this disease. It is the larvae that damage the intestine, and they do not yet lay eggs. Analysis can therefore wrongly reassure at the most dangerous moment. The veterinarian also thinks about the context: age of the lambs, season, history of the plot.
Why does the disease return every year on the same plot?
Because the eggs laid by last year's lambs survive all winter on site and hatch all together after a cold period followed by warming. It is the lambs from the previous year which contaminate those from this year, not the ewes. Changing the plot lambs is the most effective measure.
Should we treat the entire flock or just the sick lambs?
Here, contamination is simultaneous for the entire batch: the veterinarian often recommends treating all exposed lambs, unlike what is done for other worms. This decision is up to him, depending on the diagnosis.
Is water enough to save a very affected lamb?
Free, clean water is essential, but a lamb that is already lying down and dehydrated needs more: appropriate rehydration and treatment decided by the veterinarian. At this point, every hour counts.
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 11 August 2026
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