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Phtiriosis (sheep lice)
Sheep lice are small insects that live permanently in the fleece and on the skin of sheep. They never leave the animal, except to pass on a neighbor in contact. There are two kinds, and the distinction changes everything. Chewing lice eat s...
What is it?
Sheep lice are small insects that live permanently in the fleece and on the skin of sheep. They never leave the animal, except to pass on a neighbor in contact.
There are two kinds, and the distinction changes everything. Chewing lice eat skin debris and wool fibers: they irritate, scratch and destroy the fleece. Biting lice pierce the skin and feed on blood: they can, in large numbers, make animals anemic.
Biting lice live mainly where the wool is short or absent: the head, around the eyes, the neck, the bottom of the legs. Chewing lice prefer the back, shoulders and sides, under long wool.
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:
Continuous itching: animals rub against walls, stakes, feeders and feeders, chew their sides, scratch with their hind legs.
Disheveled fleece, torn strands, tufts of wool clinging to the fences and corners of the sheepfold.
Dirty, dull, felted wool, sometimes dotted with small light debris at the base of the strands: these are the nits, the eggs stuck to the fibers, impossible to slide off with the nail.
By removing the wool in broad daylight, we see small insects moving on the surface of the skin. They are tiny but visible to the naked eye when you take the time to look.
Red, crusty skin with scratching abrasions on back, shoulders and sides.
With biting lice, heavy infestation: inside of the eyelid and pale gums, tired animals, stagnant lambs.
Restless animals in sheepfolds, which get up at night, eat in spurts and lose condition.
The signs are greatest at the end of winter and diminish significantly in the weeks following mowing.
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 louse spends its entire life on the sheep: it lays its eggs stuck to the wool fibers, the young hatch on the spot and reproduce without ever leaving the animal.
Outside the host, the louse only survives for a very short time. The sheepfold is therefore not a lasting reservoir: it is the carrying animals that maintain the infestation.
The long fleece creates a warm and humid microclimate close to the skin, exactly what lice need to multiply. This is why populations swell in winter and collapse after mowing and under the summer sun.
A small number of discreet carriers, often old animals or animals in poor condition, is enough to restart a widespread infestation the following fall.
The itching doesn't just come from the bite: the skin reacts to the droppings and debris left behind by the lice, and this reaction continues the scratching long after the number of parasites has diminished.
Accommodation in buildings during winter, high density of animals, insufficient ventilation.
Long fleece, late shearing or no shearing.
Animals that are malnourished, deficient, old, or already weakened by internal parasites: these are the ones that carry the largest populations and contaminate the rest of the flock.
Purchases without quarantine or examination of the skin under the wool.
Mixture of batches of different origins, loans of rams, returns from transhumance.
Treatment limited to only scratching animals, leaving carriers in the herd.
Mowing equipment shared between farms without disinfection.
Transmission
By direct contact between animals, this is the main mode: winter sheepfold, animals crowded at the trough, mother and lamb, loaned rams.
By fresh wool left on feeders, barriers and rubbing areas, only for a short time.
By shearing equipment, cattle trucks and sorting yards used from one herd to another without cleaning.
By the introduction of an animal purchased at a fair or returned from a gathering, often without visible signs under long wool.
These lice are specific to sheep: they do not settle on humans or other species on the farm.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
The diagnosis is made on the living animal, by removing the wool at the base of the strands, in broad daylight, on several areas of the body: back, shoulders, sides, neck, head and bottom of the legs. Lice and nits can be seen with the naked eye, but you have to look in the right place and take the time.
The veterinarian examines several animals in the group, not just the one that alerted: the real importance of the infestation can never be judged on a single sheep.
Its main role is to differentiate from other causes of itching in sheep, which look very similar from a distance: scabies, false sheep louse, ringworm, dermatophilosis. The treatments and health obligations are not at all the same, and treating haphazardly means losing a season.
If in doubt, scraping the skin examined under a microscope can rule out scabies, which is the most common and costly mistake.
If animals have pale mucous membranes, the veterinarian looks for internal parasites in parallel: the two problems often affect the same animals, and anemia rarely comes from lice alone.
Treatment
External antiparasitics suitable for sheep, applied to the skin along the back, bathed or sprayed, as well as certain injectable antiparasitics, eliminate lice. The choice of product and route is up to the veterinarian: injectable products work well on biting lice, which feed on blood, but much less on chewing lice, which eat skin and wool. This is a key difference and a classic source of failure.
The entire herd is treated at the same time, never just the animals that scratch themselves: the discreet carriers recontaminate the lot in the days that follow.
The best time is right after mowing. The short wool lets the product reach the skin, where the parasites live; on a long fleece, an external treatment slides over the fibers without ever reaching its goal.
The eggs stuck to the fibers are not affected by most products: a second pass is often necessary to destroy the lice hatched after the first. The veterinarian sets the course and the interval.
No dose or rhythm can be improvised. Products intended for other species, homemade preparations, diesel, waste oils and household products are ineffective, burn the skin and are dangerous for the person handling them.
Thin animals need better nutrition as well as treatment: a sheep in good condition tolerates lice much better than a thin sheep.
Waiting times for meat and milk differ from one product to another and must be requested from 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
Quarantine any animal purchased or returned from a gathering, examine its skin under the wool and decide on treatment with the veterinarian before integrating it into the herd.
Mow regularly: Mowing is both the best time to spot infestations and the best time to treat.
Take advantage of the restraint during shearing to systematically inspect the skin of each animal.
Avoid crowding animals in sheepfolds in winter and ensure good ventilation.
Clean shearing equipment between herds and mow healthy flocks before suspect flocks, never the other way around.
Maintain good nutritional status: well-fed animals harbor fewer lice and recover more quickly.
Identify and cull the few animals that scratch themselves every winter despite treatments: they are often the ones who restart the infestation from one year to the next.
Possible complications
Fleece torn, felted and soiled, which loses all value on sale.
Scratch wounds which become infected and attract flies as soon as the heat returns, with a real risk of myiasis.
Anemia in animals heavily infested with biting lice, particularly lambs.
Stunted growth and lighter carcasses, simply because the animals spend their time rubbing each other instead of eating.
Clear worsening when internal parasites are added to the external infestation: the two accumulate on the same fragile animals.
Infestation which returns each winter when the treatment was partial or poorly positioned during the season.
When to see a veterinarian
Many animals constantly rub against walls, fences or feeders.
Tufts of wool remain clinging to the fences and the fleece becomes ruffled.
You see small insects moving at the base of the strands, spreading the wool.
Small light grains are stuck to the wool fibers and do not come off with the nail.
An animal has pale mucous membranes or loses weight even though it is properly fed.
Lambs do not gain weight in the barn during the winter.
You are hesitant between lice and scabies: have the veterinarian decide, because the treatments and health obligations differ.
Prognosis
The prognosis is good: correctly treated at the herd level and at the right time, the infestation is controlled without any after-effects.
The wool takes several months to grow back normally and the year's fleece remains devalued.
Anemic or stunted animals recover with good nutrition, but the most affected lambs often remain permanently lagging.
Recurrences almost always come from the same causes: partial treatment, poorly chosen product for the type of lice, application on wool that is too long, or animal introduced without quarantine.
Frequently asked questions
My sheep is scratching, how do I know if it's lice or scabies?
You can't tell by eye alone, and this is the most common confusion. Lice can be seen by removing wool in broad daylight; The scabies mite is invisible and requires skin scraping examined under a microscope. As treatments and health requirements differ, have your veterinarian confirm before purchasing a product.
Am I at risk of catching lice from my sheep?
No. These lice are specific to sheep and do not infect humans or other farm animals. Washing your hands after handling remains a good habit, but there is no risk of contamination.
I did an injectable and they still scratch, why?
Probably because they are chewing lice, which eat skin and wool without drawing blood: injectable products do not reach them well. This is exactly the kind of situation where the veterinarian must identify the parasite before choosing the treatment route.
Should I treat before or after mowing?
After. On long wool, an external product remains in the fibers and never reaches the skin, where the parasites live. Just after shearing, the treatment goes to the right place and the most infested animals are identified along the way.
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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