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