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Haemonchosis
Haemonchosis is caused by a roundworm, Haemonchus contortus, which attaches itself to the abomasum, the true stomach of ruminants, and feeds on blood. It is the most feared internal parasite on sheep farms around the Mediterranean, includin...
What is it?
Haemonchosis is caused by a roundworm, Haemonchus contortus, which attaches itself to the abomasum, the true stomach of ruminants, and feeds on blood.
It is the most feared internal parasite on sheep farms around the Mediterranean, including in Tunisia and the Maghreb, because it likes heat and humidity.
Each worm only takes a drop of blood, but there are often thousands of them: the animal bleeds itself from the inside, day after day.
Key point
Haemonchosis can kill, often the most beautiful lambs, in a few days, and without ever causing diarrhea. Pale mucous membranes in a grazing sheep should trigger a call to the veterinarian the same day.
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:
Paleness of the mucous membranes: the inside of the lower eyelid, the gums and the vulva change from clear pink to pale pink, then to pearly white. This is the most reliable and earliest sign.
Soft, cold swelling under the jaw, between the two branches of the bone, which often appears in the evening and deepens under the finger: it indicates already advanced anemia.
Animals which stay behind the herd, which often stop, which huff after a few meters of walking or uphill.
Progressive weight loss, dull wool, which becomes loose or breaks.
Lambs that stop growing even though the grass is good.
In the most brutal form, animals are found dead in pasture without having appeared ill the day before, and they are often the ones in the best health.
Diarrhea is generally absent: its absence therefore never rules out the diagnosis.
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 worm lives in the abomasum and lays eggs in very large numbers, which go into the droppings.
On the ground, the eggs hatch into larvae which climb onto the blades of grass as soon as there is humidity, morning dew, rain or irrigation. Animals swallow them while grazing.
The heat associated with humidity causes larvae populations to explode in a few weeks: this explains the crises after the rains, on irrigated plots and around water points.
The swallowed larvae can also pause in the wall of the abomasum during the unfavorable season, then leave all together, often at the time of giving birth. The breeder then sees a crisis without the animals having been placed in a new plot.
Lambs and young animals: their immunity is not yet in place, they are the ones who pay the heaviest price.
Ewes around birth and at the start of lactation: their defenses naturally drop and they begin to eliminate a lot of eggs again.
Hot and humid season, periods following the rains, irrigated plots, lowlands, around watering troughs and shaded areas where animals park.
High stocking rate, low grazing, too frequent return to the same plots.
Introduction of animals without quarantine.
Insufficient nutrition, particularly in protein: a poorly nourished animal resists much less well.
Herds have been systematically dewormed on a fixed date for years, where resistant worms have taken over.
Transmission
Contamination occurs only through grass: there is no direct contagion from one animal to another, nor through contact with humans.
An infested animal sows eggs wherever it grazes; the plot remains dangerous as long as conditions allow the larvae to survive.
The larvae concentrate in the first centimeters above the ground: the shorter the grass, the more the animals swallow.
An animal purchased and released directly to the herd can introduce, in one go, worms resistant to antiparasitic drugs onto a previously untouched farm.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
The veterinarian begins by examining the color of the mucous membranes of several animals in the group, not just the one that alerted. A color grid, compared to the inside of the eyelid, makes it possible to classify the animals and identify those which must be treated first; its use is learned from the veterinarian, because a bad reading causes serious cases to be missed.
A scat analysis, coproscopy, counts parasite eggs. It is carried out on a representative sample of the batch, never on a single animal, and it distinguishes haemonchosis from other worms.
A blood test assesses the depth of the anemia.
The autopsy of a dead animal is very informative: the red worms, thin and coiled, are visible to the naked eye in the open abomasum. Do not throw away a dead animal without talking to the veterinarian.
Repeated coproscopy after a treatment makes it possible to verify that the product used is still working on the farm.
Treatment
Treatment is based on internal antiparasitics, which exist in several distinct chemical families. The choice of family, form and timing is up to the veterinarian, depending on what remains effective in the region and on the farm.
No dose or rhythm should be decided on its own initiative: underdosing is the first cause of failure and it directly selects resistant worms. The heaviest animals in the group serve as a reference for adjustment, which remains the responsibility of the veterinarian.
Very anemic animals need more than dewormer: rest, shade, clean water, rich food, freedom from transport and rough handling. A transfusion may be considered in valuable animals in the clinic.
Treating the entire herd on a fixed date, without analysis, is currently not recommended: this practice is the main driver of resistance to antiparasitics. The recommended approach consists of relying on scat analyzes and mucous membrane examinations to treat only the animals that need it, deliberately leaving part of the herd untreated in order to preserve worms that are still sensitive to the products.
The waiting times for meat and milk, indicated on the instructions for each product, must be respected to the letter: they differ from one molecule to another and certain products are prohibited for females producing drinking milk. Have this point clarified by the veterinarian before any slaughter or milking.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Manage grazing: rotate the plots, do not let the animals shave the grass, delay entry into wet areas, avoid lowlands in the early morning when the dew causes the larvae to rise.
Alternate, when possible, with cattle or horses on the same plot: they reduce larval pressure for sheep.
Reserve the healthiest plots for lambs and ewes at the end of gestation, which are the most fragile.
Quarantine any animal purchased, treat it according to the advice of the veterinarian and only release it to pasture afterwards: this is the action that avoids importing resistant worms.
Regularly monitor the color of the mucous membranes of your herd, and not just animals that appear sick.
Carry out poop analyzes before treating, and check from time to time that the product used is still working.
Feed correctly, particularly with protein: a well-fed herd tolerates parasitism much better.
In the longer term, cull animals that have repeated seizures and keep for breeding those that remain in good condition without treatment.
Possible complications
Death of the animal, sometimes within a few days and without warning signs.
Abortions, lambs that are small and weak at birth, ewes that do not produce milk.
Long-term growth delay in lambs, which does not completely recover.
Increased susceptibility to all other diseases, because an anemic animal defends itself poorly.
Devalued fleece and skinny carcasses.
Installation of antiparasitic resistance on the farm, which deprives the breeder of entire product families for the following years.
When to see a veterinarian
The inside of the eyelid or the gums of an animal are pale or whitish: it is an emergency, do not postpone until tomorrow.
A soft swelling appears under the jaw.
An animal remains lying down, no longer follows the herd or blows at the slightest effort.
One or more animals die suddenly on pasture without appearing ill.
The lambs are no longer growing even though the grass and feed are correct.
Animals feel bad shortly after deworming: the product may no longer work, this must be checked with the veterinarian.
A crisis occurs in the weeks following giving birth.
Prognosis
An animal spotted early, with mucous membranes still pink, recovers well after treatment and good nutrition.
An animal that is already very anemic, lying down, with edema under the jaw, has a guarded prognosis even when treated: blood reconstitution takes time and rough handling can be enough to kill it.
At the herd level, everything depends on the state of resistance: where several families of antiparasitics no longer work, control relies mainly on grazing, feeding and sorting of the animals.
Frequently asked questions
My sheep don't have diarrhea, could it still be worms?
Yes, and this is precisely the trap of haemonchosis. This worm feeds on blood, it does not cause diarrhea in most cases. The sign to watch for is not the poop but the color of the inside of the eyelid.
Why am I being told to stop deworming the entire herd on a fixed date?
Because this practice only allows worms capable of resisting the product to survive, which then repopulate the plot. After a few years, the product no longer does anything. Analyzing the droppings and treating only the animals that need it keeps the products effective much longer.
The swelling under the jaw, is it water or an abscess?
It is most often edema, an accumulation of fluid linked to the lack of proteins in the blood. It is soft, cold, painless and expands when pressed, unlike an abscess which is hot and painful. This is a sign of advanced anemia: call the veterinarian.
I recently dewormed and the animals are feeling bad, is this normal?
No. This suggests either an underdosage, or worms that have become resistant to the product, or immediate recontamination on a heavily loaded plot. The veterinarian can check this by carrying out a poop analysis after treatment.
Can I slaughter or milk an animal immediately after treatment?
No. Each product imposes a waiting period before meat or milk can be consumed or sold, and this period is not the same from one product to another. Some are even prohibited for dairy females. Always ask the veterinarian for this deadline or read the instructions.
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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