To become infected by Theileria, a cow must be bitten by a tick carrying the disease. It is not spread by direct animal to animal contact. Cattle can be infected with the Theileria parasite without necessarily showing any signs of clinical disease.


  • Theileriosis only affects cattle and is transmitted by cattle ticks
  • Theileriosis causes anaemia in cattle and can sometimes be fatal
  • Cows during calving and young calves (2-3 months) are at most risk from infection

Once the animal is bitten, it takes about 6-8 weeks for the parasite to build up the significant levels in the blood. At this stage the body reacts by trying to destroy the parasite. Because the parasite is living inside the red blood cells, the body attacks its own infected red blood cells (haemolysis) to destroy the parasite. Unfortunately, this can lead to a huge loss in red blood cells which are responsible for carrying the oxygen around the body; this is known as anaemia and results in the following signs in affected animals:

  • Pale or yellow, rather than healthy pink, vulva (open up the vulva and look at the colouring inside).
  • Pale of yellow whites of eyes (a sign of jaundice)
  • Lethargy – exercise intolerance, cows lagging on the walk to the shed
  • Sick cows not responding as expected to treatment for conditions such as milk fever
  • Cows are off their food and appear hollow sided
  • A decrease in milk production
  • Sudden death especially in late pregnancy or early lactation.

The signs of anaemia associated with Theileriosis are more likely to be seen around calving time, in calves (2-3 months), and if cows are coping with other health challenges, or potentially at mating time.

If you notice any of the above signs or would like more information about Theileria, please contact us.

Assessment can be determined by:

Assessment of the herd

Use the vulva colour guide to screen the herd and see how many animals might be affected

Red Blood Cell Count (PCV)

Blood sample suspect animals. A low PCV is an indication of severe blood loss.