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Hypodermosis
Hypodermosis is caused by the larvae of a fly that parasitizes cattle. It is commonly called the varron, from the name of the large lumps that we feel under the skin of the back. In summer, the adult fly lays its eggs on the hairs of the le...
What is it?
Hypodermosis is caused by the larvae of a fly that parasitizes cattle. It is commonly called the varron, from the name of the large lumps that we feel under the skin of the back.
In summer, the adult fly lays its eggs on the hairs of the legs. The larvae penetrate under the skin and travel for several months inside the body before lodging under the skin of the back at the end of winter and spring.
The fly does not bite or feed, but its buzzing panics the animals: the cows become agitated, run, stop grazing. This panic alone costs milk and growth.
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:
In summer: animals panicking in the meadow, tails in the air, running suddenly and looking for shade or water, with interrupted grazing.
At the end of winter and spring: firm and painful lumps appear under the skin of the back, on either side of the spine. Each has a small orifice through which the larva breathes.
The number of these balls varies from a few to several dozen.
Animals react when their backs are touched.
Decrease in milk production and slowed growth among young people.
At the slaughterhouse, the leather is punctured and the meat beneath the lesions must be trimmed.
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 disease is caused by the larvae of parasitic flies specific to cattle.
The adult fly lives a short time, in summer, and only lays eggs on the hairs of the legs and lower body.
The hatched larvae pass through the skin and then slowly migrate through the body over several months. Depending on the species, they stop along the spinal canal or the wall of the esophagus.
They then join the tissue under the skin of the back, where they complete their development, before falling to the ground to transform into flies.
Cattle grazing in summer, especially young ones.
Regions and farms where the fight is not organized collectively.
Purchasing animals from infested areas, without quarantine treatment.
Flocks put out to grass in sheltered, warm areas, where flies are active for longer.
Transmission
Contamination only occurs in pasture, in summer, when adult flies are active.
There is no direct contagion from one animal to another.
Animals introduced from an infested area can carry migrating larvae without any visible signs for months: they reintroduce the parasite into a cleaned region.
Human beings are not affected in current breeding conditions.
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 simple in spring: the veterinarian feels the animal's back and feels the characteristic lumps.
Outside of this period, the larvae are invisible: blood tests are available to detect carrier animals before the appearance of nodules.
Report lumps to the vet as soon as you feel them, and don't touch them yourself.
In areas or countries where organized control exists, cases must be reported: contact the veterinary services.
Treatment
Treatment is based on antiparasitics acting on the larvae, prescribed by the veterinarian.
The timing of the treatment is essential: it is carried out after the end of the fly season and before the larvae reach the sensitive areas of their route. Poorly timed treatment within a year can cause a serious reaction.
Never pierce, squeeze or crush a varron: the crushed larva releases substances that can trigger a violent reaction in the animal.
Treatment is based on the scale of the herd, and ideally the region: treating a single herd does not provide long-term protection if the neighbors do not do so.
Withdrawal times for milk and meat apply: ask the prescribing veterinarian and respect them.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Treatment of the herd at the right time of year, decided with the veterinarian and coordinated locally when possible.
Quarantine and treatment of any animal purchased, particularly if it comes from an infested area: this is how the parasite returns to sanitized regions.
Monitoring of the animals' backs in spring, on the entire herd and not just on suspicious animals.
Participation in collective campaigns when they exist: this parasite is effectively combated on the scale of a region, much less on the scale of an isolated farm.
No vaccine exists against this parasite.
Possible complications
Violent reaction, sometimes fatal, if larvae are crushed during migration or in back nodules.
Damage to the spinal canal which can lead to paralysis of the hindquarters when larvae are destroyed there.
Secondary infection of pierced nodules.
Perforated and depreciated leather, trimming of meat at the slaughterhouse.
Production losses due to summer unrest and the inconvenience caused by nodules.
When to see a veterinarian
You feel firm lumps under the skin on your cattle's back in spring.
An animal reacts strongly when its back is touched.
Your animals panic in the meadow in summer and interrupt their grazing.
An animal shows weakness or paralysis of the hindquarters after antiparasitic treatment: call immediately.
You have purchased animals from a region where the varron is present.
Prognosis
Treated at the right time, the parasite is controlled very well and the animals have no after-effects.
The nodules already formed heal after the emergence or destruction of the larvae, but the leather remains marked.
The main risk of treatment is linked to poor choice of period, hence the importance of veterinary prescription.
Where collective campaigns are carried out, the parasite can be greatly reduced or even eliminated.
Frequently asked questions
Can I pierce the balls to get the larva out?
No, never. Crushing or piercing a varron can trigger a violent, sometimes fatal reaction in the animal. The course of action to be taken is decided with the veterinarian.
Why do my cows run in the meadow in summer?
This is often the panic reaction caused by the buzzing of the flies causing the varron. They do not sting, but panic interrupts grazing and costs milk and growth.
Can we treat any time during the year?
No. The period is decisive: too late in the season, the destruction of the larvae in the wrong place in the body can cause a serious reaction. The veterinarian indicates the window to respect.
Is varron dangerous for me or for consumers?
No. It is a parasite of cattle, with no consequences for humans under current breeding conditions. The losses are economic: perforated leather, trimmed meat, reduced production.
References
FAO — Organisation des Nations unies pour l'alimentation et l'agriculture
Content reviewed by the TunisieVet editorial team · Updated 09 August 2026
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