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Theileria (pronounced "tie-lear-ia") is a parasite transmitted by ticks which can cause a sudden drop in milk production, abortion, stillbirth and, in many cases, death.

Theileria first occurred in Australia in the early 1900s and only affected cattle in north-eastern Australia. However, there were reports in 2011 of theileria outbreaks in Victorian cattle herds that had introduced cattle from parts of Australia where the disease was known to occur.

It is thought that seasonal conditions at the time supported survival of bush ticks, responsible for transmission of theileria, and allowed the disease to spread into neighbouring herds. Since then, there have been over 500 outbreaks of theileria in cattle in New South Wales and Victoria.

What to look for

  • Sudden drop in milk production, lethargy and weakness
  • Very pale or yellow colour inside the vulva or of the third eyelid
  • Inability to walk very far (exercise intolerance)
  • May have a high fever (41°C)
  • Abortion and stillbirth
  • High death rate (25%)


Theileria is caused by a parasite called Theileria orientalis which infects red blood cells, causing anaemia, and is transmitted by ticks.

Animals likely to be affected

Cattle affected by theileria often show symptoms following a stressful event, such as calving. Affected animals may have been introduced from a non-theileria area to a theileria area, or mixed with animals from different areas.

Confirming the diagnosis

Theileria should be suspected when animals have a sudden drop in milk production, lethargy and weakness accompanied by a very pale or yellow colour inside the vulva. The diagnosis can be confirmed by a veterinarian following laboratory testing of a blood sample. Ticks are rarely observed on affected cattle.

Spread of the disease

The disease is spread by bush ticks. Only small numbers of ticks (less than 10) are required to spread infection. Often ticks are not observed on the animal as they spend a large part of their lifecycle on vegetation.


There is currently no registered treatment for theileria in Australia. Symptomatic treatment including intravenous fluids, good nursing, minimising stress and movement may be beneficial. Some veterinarians may also recommend the use of a specific antibiotic. Blood transfusions can be performed in high-value animals.

Risk factors

Recent introductions of cattle from a non-theileria area entering a theileria area and exposure to stress such as calving are the most important risk factors.


There is no specific prevention for theileria. Bush ticks are almost impossible to eradicate from a property as they spend most of their life cycle on vegetation rather than on animals. Recommendations to reduce risk include:

  • Sourcing cattle locally wherever possible
  • Avoid moving adult animals in late pregnancy
  • Avoid mixing cattle from areas where theileria is common with those from areas where it is uncommon
  • If possible, assess the risk by conducting blood testing for previous exposure

More information on theileria is available on the Agriculture Victoria website.

More information about dairy biosecurity can be on the biosecurity page.


An overview of the importance of a biosecurity plan and links to resources to help dairy farmers develop their own biosecurity plan.

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