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Estrosis
Estrosis is an infestation of the nasal cavities and sinuses of sheep by the larvae of a fly, Oestrus ovis. The adult fly does not bite or feed. In mid-flight, it projects live larvae into the nostrils of animals, then dies shortly after. T...
What is it?
Estrosis is an infestation of the nasal cavities and sinuses of sheep by the larvae of a fly, Oestrus ovis.
The adult fly does not bite or feed. In mid-flight, it projects live larvae into the nostrils of animals, then dies shortly after.
The larvae ascend into the nasal cavities and sinuses, cling there, grow for weeks or months, then descend to be sneezed onto the ground, where they transform into flies.
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:
The behavior of the animals during the summer is often the first clue: when flies approach, the ewes lower their heads, stick their noses to the ground or against the legs of others, huddle in a circle, stamp their feet and seek dust and shade. They stop grazing at the most useful times.
Discharge from the nose, first clear then thick and yellowish, sometimes streaked with blood.
Sneezing in bursts, loud sniffing, animal rubbing its nose against the floor or against the walls.
Noisy and difficult breathing, crusted nostrils.
Repeated head shaking.
Reduced appetite, animals that graze less and lose weight.
In the most severe cases, nervous signs appear: uncertain gait, animal turning. We talk about false dizziness because it resembles coenurosis without actually being it.
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 female fly projects her larvae into the nostrils of sheep, often in the heat and in the sun.
The larvae progress into the nasal cavities and sinuses, where they cling to the mucous membrane and permanently irritate it.
They can spend the winter idling in the sinuses, then resume growth in the spring.
Once developed, they are expelled by sneezing and turn into flies in the ground, which restarts the cycle.
Permanent irritation of the mucous membrane opens the door to bacterial infections, which explain the thick and purulent discharge.
Outdoor breeding in hot and dry regions, which corresponds to the majority of herds in the Maghreb.
Hot season, absence of shade and shelter areas during fly flight times.
Large gatherings of animals, shared routes.
Lack of strategic treatment at the end of the flight season, which leaves the larvae to spend the winter in the sinuses.
Animals weakened or already suffering from other respiratory diseases.
Transmission
There is no direct contagion between animals: it is the fly which deposits the larvae in flight.
Flight periods correspond to warm seasons; the risk is greatest in summer and early fall.
Animals grouped together in full sun, without shade, are the most exposed.
Goats are also affected, generally less markedly than sheep.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
The veterinarian first relies on the signs and the season: shedding, sneezing and characteristic behavior in hot weather are very suggestive.
He examines the nostrils and rules out other causes of throwing in sheep, in particular infectious respiratory diseases, which require different management.
Examining the head of a slaughtered or dead animal, by opening the nasal cavities and sinuses, directly shows the larvae and makes it possible to assess the extent of the infestation in the herd. This is the most telling element for deciding on a strategy.
When faced with nervous signs, it is important to differentiate from coenurosis: the course of action is not the same.
Treatment
Certain families of antiparasitics act on the larvae lodged in the sinuses; the choice of product, route of administration and timing is up to the veterinarian.
The treatment is based on the scale of the herd and not of the isolated animal, because the exposure is collective. It is often positioned at the end of the flight season, to destroy the larvae before they spend the winter in the sinuses and restart the cycle in spring.
No dose or rhythm is improvised: the strategy is built with the veterinarian according to the region, the climate of the year and the type of production.
A very embarrassed animal, with a purulent discharge, may require additional treatment of the established bacterial infection, on prescription.
Waiting periods for meat and milk also apply here and differ depending on the product: always ask for them before any slaughter or milking.
Never pour any product into the nostrils of an animal on your own initiative: it is ineffective and dangerous.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
There is no vaccine or easy way to keep flies away.
Providing shade and shelter during hot hours reduces animal exposure and stress.
Shift grazing to cool hours of the day during flight periods.
Set up, with the veterinarian, a strategic end-of-season treatment at the herd level, which breaks the cycle much better than scattered individual treatments.
Monitor the behavior of the herd in summer: animals that keep their noses to the ground and stop grazing indicate strong pressure.
Eliminate other causes of respiratory irritation, dust and overcrowding in the sheepfold, which aggravate the signs.
Possible complications
Bacterial superinfection of the nasal cavities and sinuses, with persistent purulent discharge.
Extension of the inflammation to the deep sinuses, difficult to treat.
Sustainable decline in general condition and production, simply by reducing the time spent grazing during the summer.
In rare cases, damage to structures neighboring the brain with nervous signs.
Confusion with other respiratory diseases, which delays the correct diagnosis.
When to see a veterinarian
Your animals keep their noses to the ground, huddle in a circle and stop grazing during hot hours.
A thick or yellowish discharge develops in many animals.
An animal is breathing noisily or seems embarrassed to breathe.
Burst sneezing occurs regularly in the herd.
An animal staggers or turns: you have to differentiate from dizziness, consult.
Animals lose weight during the summer without any other explanation.
Transmission to humans
The responsible fly can, while flying, project its larvae into the eye of a person near the animals: shepherds, shearers, people working among the flock in summer are the most exposed.
This causes external ocular myiasis: sharp and sudden pain, foreign body sensation, red and watery eyes, discomfort in light. The larvae generally remain on the surface of the eye and do not complete their cycle there.
The procedure to follow is simple: do not rub your eye, rinse with plenty of clean water and quickly consult a doctor or ophthalmologist, who will remove the larvae. The outcome is usually favorable, but medical advice should not be delayed.
Wearing protective glasses when working among the herd in the middle of the hot season limits this risk.
Prognosis
Most animals tolerate the infestation without dramatic consequences, but at the cost of a reduction in growth and production throughout the hot season.
Severe forms, with superinfection or nervous signs, have a more guarded prognosis.
A well-positioned herd strategy with the veterinarian clearly improves general condition and performance from one summer to the next.
Frequently asked questions
Why do my sheep keep their noses pressed to the ground in summer?
This is a typical defensive behavior against the nose fly: the animals protect their nostrils. This is often the first sign of estrosis in a herd, even before shedding.
My sheep is running in circles, is it dizziness or a fly in the nose?
Both can give nervous signs, but the course of action is completely different. Only a veterinary examination can make a decision: do not treat randomly.
Can I catch something while working in the middle of the herd?
The fly can project its larvae into the eye of a person close to animals. It is brutally painful but generally benign, provided you do not rub the eye, rinse with clean water and seek medical attention promptly.
Should the entire herd be treated?
Since the exposure is collective, the strategy is effectively considered at the flock level, generally at the end of the flying season. The choice of product and timing is up to the veterinarian, taking into account waiting times for meat and milk.
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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