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(Theileria orientalis)

  • Theileria are protozoan parasites that can cause anaemia in cattle.
  • The parasite infection is transmitted by bush ticks (Haemaphysalis longicornis).
  • The disease is most common in eastern, coastal Australia but also occurs in the southwest of Western Australia.
  • Young calves, heavily pregnant and recently calved cows are most susceptible.
  • Immunity usually develops by 6 months of age and adults in endemic regions are rarely sick.
  • In adults, the most susceptible are those with no prior exposure (naïve) that are moved to endemic areas (signs 6-8 weeks after introduction).
  • Currently no vaccine or specific treatment is available.
  • Introduce & maintain tick control on the property and manage ticks where present to minimise risk.

The protozoan parasite Theileria orientalis can cause theileriosis (bovine anaemia) in cattle and is transmitted by bush ticks, which feed on cattle and inject the parasite into their bloodstream. Parasites enter red blood cells, multiply and destroy them, eventually resulting in anaemia.

The parasite has been present in Australia for more than 100 years and although it causes lifelong infection, it was rarely associated with disease. Since 2005, reports of theileriosis outbreaks have greatly increased with up to 30% mortalities in some herds attributed to Theileria. This change is associated with a new parasite strain and is sometimes referred to as BATOG or ‘bovine anaemia due to Theileria orientalis group’.

Figure 1. Theileria stains dark pink inside the round, red blood cells of an infected cow. Image courtesy Tick Fever Centre

It is important to distinguish BATOG from highly pathogenic East Coast Fever, which is caused by exotic strains of Theileria and is confined to eastern Africa.

Theileria is commonly detected in wet coastal areas where bush ticks are prevalent and is considered endemic (repeatedly found in a region) in much of coastal Queensland, New South Wales and Victoria. Although Theileria has been detected in all states and territories except Tasmania, theileriosis cases have only been reported in South Australia and Western Australia within the last decade.

Figure 2. Geographic distribution of the bush tick in Australia. Image adapted from Virbac.

The Theileria parasite is mainly spread by the bush tick and signs can occur with less than 10 ticks, which can easily be missed when inspecting cattle. Bush ticks cannot spread this disease to other animals that they might live on (i.e. other livestock, birds, dogs and cats) and not all bush ticks carry Theileria.


  • Fever, lethargy (reluctance to walk), weakness.
  • Laboured or heavy breathing.
  • Pale or yellow mucous membranes (gums, eyes/conjunctiva, vulva).
  • Brown urine.
  • Reduced appetite and weight loss.
  • Decreased milk production in dairy cows.
  • Abortion, premature births, stillbirths.
  • Sudden death (highest rates seen in heavily pregnant cows).


The signs of theileriosis can also be linked to other conditions, so it is important to confirm a diagnosis with your veterinarian. This involves examination of blood under a microscope for the presence of Theileria parasites. It can also be confirmed using other laboratory based genetic tests too if necessary.

No registered drugs or vaccines are currently available for the specific treatment or control of theileriosis. The effect theileriosis may have can be minimised through appropriate management of ticks where present to reduce cattle exposure.

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