Theileriosis

• Theileria parasites can cause anaemia in cattle.
• The parasite infection is transmitted by bush ticks, wallaby ticks and bandicoot ticks (all in the genus Haemaphysalis).
• The disease is most common in eastern, coastal 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 typically appear 6–8 weeks after introduction).
• Currently, no vaccine or specific treatment is available.
• Follow best practice herd management and biosecurity protocols to minimise risk.

The protozoan parasite Theileria orientalis can cause theileriosis (bovine anaemia) in cattle and is transmitted by bush ticks, wallaby ticks and bandicoot 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’.

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

The Theileria parasite is mainly spread by Haemaphysalis ticks and signs can occur with less than 10 ticks, which can easily be missed when inspecting cattle. Cattle that have recovered from the disease continue to be carriers remaining as Theileria parasite sources in endemic zones. A recent study from New Zealand has shown that sheep can carry detectable levels of Theileria infection, although the levels of infection were much lower than those found in cattle. Research is needed in Australia to determine if species other than cattle could be involved in transmission.  Not all Haemaphysalis ticks carry Theileria.

Signs

• 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).

Diagnosis

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 molecular tests too, if necessary.

Treatment and control

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 practices to maintain a healthy herd.

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

Figure 2. Theileria has the same coastal distribution as its Haemaphysalis tick host (bush tick distribution shown). Image adapted from Virbac.