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Bovine theileriosis
Theileriosis is a blood disease transmitted by ticks. A microscopic parasite, theileria, is inoculated into the cow during the tick's meal. It first multiplies in the lymph nodes, then invades red blood cells. It is one of the most serious...
What is it?
Theileriosis is a blood disease transmitted by ticks. A microscopic parasite, theileria, is inoculated into the cow during the tick's meal. It first multiplies in the lymph nodes, then invades red blood cells.
It is one of the most serious parasitic diseases of cattle throughout the Mediterranean, including the Maghreb. It strikes especially during the hot season, when ticks are active.
Local breed cattle, exposed from a young age, often resist better. Imported animals of susceptible dairy breeds can die within a few days due to lack of immunity.
Key point
Severe and rapidly fatal disease in non-immune cattle, particularly imported animals. A delay of a few days changes the prognosis.
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 first sign is often a large lymph node, palpable under the skin, on the side where the tick bit: in front of the shoulder, at the level of the neck or stifle.
High fever that lasts, depression, the animal stays away from the herd.
Loss of appetite, cessation of rumination, rapid drop in milk production.
Mucous membranes of the eyes and gums become increasingly pale, then sometimes yellow: the parasite destroys the red blood cells.
Tearing, running eyes, swollen eyelids.
Rapid weight loss, short and painful breathing in advanced forms.
Ticks are frequently visible on the animal: ears, udder, under the tail, armpits, between the thighs.
In some cases, nervous signs appear at the end of the course.
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 a microscopic parasite of the genus Theileria, transmitted by tick bites.
After inoculation, the parasite multiplies in the lymph nodes, which explains their early swelling, then spreads to red blood cells and causes anemia.
Several species of theileria exist. The one which dominates around the Mediterranean is responsible for the so-called tropical form, which is particularly severe in non-immune cattle.
Imported cattle, or those from selected dairy breeds, are the most exposed: they have never encountered the parasite and develop serious illnesses.
Calves and young livestock previously unharmed.
Periods of high tick activity: heat, late spring and summer.
Old stables, with cracked walls and uncleaned corners, which harbor ticks all year round.
Animals weakened by another illness, lack of water or insufficient nutrition.
The introduction of purchased animals without quarantine or tick examination.
Transmission
Transmission occurs only by ticks: there is no direct contagion from one cow to another, neither through milk nor through contact.
Tick vectors are active from late spring to fall, peaking in summer.
Some vector ticks do not stay permanently on the animal: they hide during the day in the cracks of the walls, under the feeders and in the corners of the stable. So the building is part of the problem.
A cured animal can remain a carrier of the parasite without being sick and serve as a reservoir for ticks in the herd.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
Call the veterinarian as soon as you have a fever and a large lymph node: in this disease, every day counts.
The clinical examination looks for enlarged lymph nodes, pale mucous membranes and the presence of ticks.
The diagnosis is confirmed in the laboratory: blood smear and, above all, puncture of a lymph node observed under a microscope, where the parasite is visible. More in-depth analyzes are available in specialized laboratories.
A blood test assesses the extent of anemia and general condition.
Inform the veterinarian of the origin of the animals and the date of their introduction: a recently purchased batch strongly influences the diagnosis.
Treatment
Treatment is the responsibility of the veterinarian. A specific antiparasitic treatment for theileriosis exists; it is all the more effective if it is started early, before anemia sets in.
Supportive care accompanies the treatment: rehydration, anti-inflammatories if the veterinarian deems them useful, palatable food, clean water at will, shelter in the shade.
In very anemic cases, a transfusion may be considered.
Ticks present on the animal must be eliminated using a suitable acaricide, without crushing them with bare hands.
None of these products should be used without a prescription: the withdrawal periods for milk and meat must be respected, and the veterinarian is the only one who can indicate them.
An animal treated late can recover but maintain a drop in production for a long time.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Everything is based on the fight against ticks, carried out at the herd level and not animal by animal.
Inspect cattle regularly during the hot season: ears, udder, under the tail, armpits, between the thighs. Remove ticks with a hook or tweezers, without crushing them between your fingers.
Acaricide treatments for the herd according to a schedule established with the veterinarian, based on the season of tick activity.
Also treat buildings: cleaning, liming and filling cracks in walls, feeders and water troughs, where ticks take refuge.
Quarantine any purchased animal, examine it and remove ticks before introducing it into the herd.
Protect imported animals or sensitive breeds as a priority during their first warm season on site.
Alternate the families of acaricides on veterinary advice: repetitive use of the same product ends up selecting resistant ticks.
Vaccination
A live attenuated vaccine against tropical theileriosis exists and is used in several countries around the Mediterranean.
It is administered before the tick season and its benefit is particularly clear for imported animals or sensitive breeds.
Its availability varies depending on the country and countryside: check with your veterinarian or the veterinary services in your region.
Vaccination does not replace the fight against ticks: the two complement each other.
Possible complications
Severe anemia, with an animal that no longer gets up.
Lung damage and difficulty breathing in advanced forms.
High mortality in susceptible cattle not treated in time.
In animals that recover: lasting weight loss, growth retardation in young animals, prolonged drop in milk production.
Secondary infections favored by general weakening.
When to see a veterinarian
A cow has a fever and a swollen lymph node under the skin during tick season: call the veterinarian immediately.
A recently purchased or imported animal is slaughtered, eats less and loses its milk.
The mucous membranes of the eyes or gums become pale or yellow.
You find many ticks on several animals in the herd.
A bovine has difficulty breathing or remains lying down without getting up: it is an emergency.
Prognosis
Taken early, in an animal that is still healthy, the treatment gives good results.
Discovered late, in very anemic cattle, the disease often remains fatal despite treatment.
Local animals exposed young develop immunity and generally develop mild forms.
In a farm where ticks are truly controlled, cases become rare.
Frequently asked questions
Why do my imported cows get sick while the local ones are fine?
Because local cattle encountered the parasite at a very young age and became immune to it, often without the breeder realizing it. An imported animal has no defenses: it has a severe form. It must therefore be protected in a reinforced manner during its first hot season.
Can the disease be transmitted to my other cows through contact?
No. It is only transmitted by tick bites. But if a cow is sick, this means that infected ticks are circulating on the farm: the rest of the herd is exposed, hence the importance of acting on ticks and the building.
Can I consume or sell milk from a treated cow?
Not freely. Any treatment imposes a waiting period for milk and meat, which depends on the product used. Only the veterinarian who prescribed can tell you this, and this deadline must be respected to the letter.
Is treating only animals enough against ticks?
Often no. Some vector ticks spend part of their lives in cracks in walls and under bird feeders. Without cleaning and treating the building, the tick population rebuilds every year.
References
WOAH — Organisation mondiale de la santé animale (ex-OIE)
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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