Home › Animalia › Animal health › Animal diseases › Sheep theileriosis
Sheep theileriosis
Theileriosis is due to a microscopic parasite transmitted by ticks and which, unlike babesiosis, is not limited to red blood cells: it begins by invading the lymph nodes and white blood cells, where it multiplies, before reaching the blood....
What is it?
Theileriosis is due to a microscopic parasite transmitted by ticks and which, unlike babesiosis, is not limited to red blood cells: it begins by invading the lymph nodes and white blood cells, where it multiplies, before reaching the blood.
This explains the sign which most clearly distinguishes it from other blood diseases transmitted by ticks: a large lymph node, often that of the shoulder, which the breeder can feel under the hand on the side where the tick is attached.
It is transmitted by large ticks, well known in the hot and dry regions around the Mediterranean and the Maghreb, which readily lodge in the ears, under the tail and in the groin, and which also hide in the cracks of sheepfolds.
Key point
A febrile sheep with a large lymph node in front of the shoulder, in hot season and in a tick zone, must be seen quickly: the treatment of theileriosis is specific and it is only of value if it is given early.
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:
High fever that lasts for several days, often the first sign.
Swelling of one or more lymph nodes, especially the one in front of the shoulder, palpable under the skin. This is the most suggestive sign and it appears early: learn to look for it with the veterinarian.
Deep depression, refusal to eat, animal which separates from the herd and stops grazing.
Progressive pallor of the mucous membranes as red blood cells are destroyed.
Yellow discoloration of the mucous membranes and the white of the eye in advanced forms.
Discharge from eyes and nose, first clear then thick, swollen eyelids.
Rapid weight loss, dull wool.
Difficulty breathing, animal wheezing, sometimes accumulation of fluid in the lungs in severe forms.
In some cases, red spots visible on the mucous membranes of the mouth or eye, indicating bleeding under the skin.
Lying animal that never gets up again, then dies within a few days in malignant forms.
Urine generally does not change color, unlike babesiosis: this is a useful point for orientation.
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 parasite is inoculated by the saliva of the tick while it gorges.
It first reaches the lymph nodes, where it invades the white blood cells and forces them to multiply with it: this is what causes the lymph node closest to the puncture site to swell.
From the lymph nodes, it spreads throughout the body, then passes into the red blood cells, causing anemia and a yellow tint to the mucous membranes.
This double stage explains the seriousness of the disease and the difficulty of treatment: when anemia becomes visible, the parasite is already installed everywhere.
The ticks responsible have a particular lifestyle: part of their cycle takes place in buildings, in cracks in walls, under bird feeders and in old wood. Treating only the animals without taking care of the sheepfold leaves the problem unsolved.
A cured animal remains healthy for a long time and serves as a reservoir.
Hot and dry regions around the Mediterranean, where the responsible ticks live; hot season in particular.
Old stone or earth sheepfolds, with cracked walls and old wood feeders: these are reservoirs for ticks.
Animals purchased in a disease-free zone and introduced into a risk zone, or vice versa.
Young animals that have never been exposed, and adult animals from a herd that has been protected for a long time.
Stress, transport, high heat, giving birth, undernutrition.
Associated internal parasitism or other blood disease transmitted by the same tick.
Absence of treatment of buildings in the tick control program: this is the most common oversight.
Groupings of animals and periods of heavy commercial traffic.
Transmission
Only through the tick bite: there is no direct contagion between animals, nor to humans.
The ticks in question are active during the warm season and are found both on paths and in buildings, which makes exposure possible even for animals that rarely go out.
Animals introduced from a free region into an area where the disease is circulating are most exposed to fatal forms.
Cattle can harbor neighboring parasites transmitted by the same ticks: combating ticks benefits all species on the farm.
Animal movements, fairs, transhumance and gatherings preceding Eid, displace both the carrying animals and their ticks.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
The veterinarian systematically palpates the lymph nodes, particularly the one located in front of the shoulder: a large lymph node associated with fever in a sheep during tick season is very suggestive.
He takes the temperature, examines the color of the mucous membranes and checks the color of the urine, which helps distinguish theileriosis from babesiosis.
He looks for ticks on the animal, paying particular attention to the ears, groin, under the tail and udder, and asks about the condition of the sheepfold.
A colored blood smear and especially a puncture of the lymph node, examined under a microscope, show the parasite and allow rapid confirmation.
Specialized laboratory analyzes identify the species in question and distinguish the serious form from the mild form, which changes the course of action to take.
A blood test measures anemia and general condition.
Examining a dead animal is very informative: large lymph nodes, enlarged spleen, bleeding spots on the organs. It allows you to make the right decisions for the rest of the herd.
Treatment
There is a specific antiparasitic agent for theileriosis. It has nothing to do with dewormers or with products used against babesiosis: each blood disease has its own treatment, which makes diagnosis essential before treating.
This product is not always easy to obtain and it is expensive; its prescription and administration are the responsibility of the veterinarian. No dose or rhythm should be decided alone.
Here again, everything depends on precocity: treated as soon as fever and swelling of the lymph node occur, an animal has a good chance; treated once lying down and anemic, he has little, even with the right product.
An antibiotic may be combined by the veterinarian depending on the case, particularly when other agents transmitted by the same tick are suspected.
Supportive care is essential: shade, calm, clean water within reach, palatable food. Do not chase or transport a very anemic animal.
Removing ticks present on the animal is part of the treatment, and the entire lot must be examined at the same time: one case reports high tick pressure and other animals are often incubating.
The waiting periods for meat and milk 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
The fight against ticks is the decisive measure, and at the same time it protects against babesiosis, anaplasmosis and other agents inoculated by the same bite. The products exist in several families and in several forms; the calendar is built with the veterinarian according to the season and the region.
Treating the sheepfold is as important here as treating the animals, because these ticks hide in buildings. Filling cracks in walls, replacing or cleaning old wooden feeders, cleaning and disinfecting thoroughly between batches: without this, animals are constantly reinfested.
Inspect animals by hand during the hot season, paying particular attention to the ears, groin, underside of the tail and udder.
Remove ticks completely, without crushing or burning them, while wearing gloves: certain large ticks can transmit serious diseases to humans unrelated to those of sheep.
Quarantine any purchased animal, rid it of ticks and monitor its temperature for the first few weeks. This is especially important when shopping before Eid and after fairs.
Avoid introducing naive animals in the middle of the hot season onto a tick path.
Maintain a good diet and control internal parasites: a healthy animal tolerates infection much better.
Possible complications
Death of the animal, common in the malignant form when treatment arrives late.
Profound anemia and lasting weight loss, with a very slow recovery in survivors.
Bleeding and damage to several organs in severe forms.
Accumulation of fluid in the lungs, with respiratory distress at the end of the course.
Abortion in pregnant ewes, weak lambs.
Collapse of milk production.
Cured animals remaining carriers, which maintain the disease in breeding.
Association with other diseases transmitted by the same tick, which makes the picture confusing and the prognosis worse.
When to see a veterinarian
An animal has a fever and a swollen lymph node, palpable in front of the shoulder: call the veterinarian immediately.
An animal is slaughtered, refuses to eat and separates from the herd during the hot season.
The mucous membranes become pale or yellow.
An animal has discharge from the eyes and nose with fever.
A recently purchased pet becomes ill within weeks of its arrival.
Several animals are deteriorating at the same time while you find many ticks on the herd.
An animal dies suddenly during tick season: have it examined rather than making it disappear, this protects the rest of the flock.
Prognosis
The benign form heals on its own and most often goes unnoticed.
The malignant form is serious: without early and specific treatment, it kills a significant proportion of affected animals, particularly those that have never been exposed.
An animal treated at the onset of fever and swelling of the lymph node has a good chance of recovering, but it will take weeks to recover.
At the herd level, the prognosis depends on two things: the speed of calling the veterinarian, and the consideration of buildings in the fight against ticks. Treating animals by leaving ticks within the walls of the barn gives disappointing results year after year.
Frequently asked questions
How to distinguish theileriosis from other tick-borne diseases?
In the field, swelling of a lymph node, often that of the shoulder, associated with fever, is the most suggestive sign of theileriosis. Babesiosis tends to give red-brown urine, anaplasmosis causes pallor and jaundice with normal urine. But these diseases are similar and can coexist: only the veterinarian, with a blood or lymph node examination, can decide, and the treatment is not the same.
I treat my sheep against ticks but I still have cases, why?
Often because the ticks responsible also hide in the sheepfold: cracks in the walls, old wooden feeders, corners that have never been cleaned. The animals reinfest themselves when they return. Treating buildings at the same time as animals completely changes the results.
Do the sheep I buy before Eid present a particular risk?
Yes, on two counts: they can arrive with their own ticks and contaminate your route, and if they come from a free area they themselves are very vulnerable. A quarantine, an examination for ticks and monitoring the temperature during the first weeks avoid a lot of trouble.
Can a classic dewormer act on this parasite?
No. It is a parasite of the blood and lymph nodes, not an intestinal worm. You need a specific antiparasitic agent for theileriosis, prescribed by the veterinarian, and it is only effective if given early.
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 11 August 2026
AT A GLANCE
Need a veterinarian?
Find a practitioner near you on TunisieVet, with their specialities, hours and contact details.
Find a veterinarianCategories
This information is provided for educational purposes only and is not a substitute for a veterinary consultation. If you have any concerns about your animal's health, consult a veterinarian.
To discover
Related diseases and linked Animalia content.
Rabies
Viral · Vaccine available
Toxoplasmosis
Parasitic
Coccidiosis
Parasitic
Foot-and-mouth disease
Viral · Vaccine available