Theileria are transmitted predominantly by Haemaphysalis bush ticks, but also possibly other biting insects and injection needles.
Ticks become vectors when they feed on the blood of an infected cow, ingesting the juvenile form of the Theileria parasite that resides within the cow’s red blood cells.
Once inside the tick, the parasite develops and moves to invade the tick’s salivary glands. The Theileria are then transmitted via tick saliva and inoculated into new cattle hosts when the tick feeds.
Adult ticks can remain infective and transmit the parasite for as long as 18 months. The parasite does not transmit between tick generations (an infected female tick does not lay infected eggs).
Although bush ticks are mainly a cattle parasite (preferring to attach to softer tissues around the tail, udder, brisket, inside legs, face, ears and neck), they can also attach to many other species (including sheep, dogs and cats). However, Theileria can only be transmitted to cattle.
Infected animals may present as having a significant drop in milk production, loss of body condition, reluctance to walk, weakness, abortion, panting or having difficulty breathing, downers or sudden death.
Stressful events such as calving or transport can lead to onset of symptoms in animals carrying the disease.
Clinical signs of theileriosis may include pale mucous membranes, fever (41-42°C), marked reduction in rumination, salivation, swollen lymph nodes, cloudy eyes, and discharge from the eyes and nose. Diarrhoea is common and may be blood-tinged. Anaemia can be followed by severe jaundice in the latter stages.
After the onset of clinical signs, death usually follows within five to 15 days. Morbidity (number affected) can be around 50 per cent while mortalities have been reported of up to 25 per cent.
Diagnosis of theileriosis can be confirmed by your vet by performing laboratory testing on a blood sample.
Farms experiencing theileriosis may have a history of recent introductions of new stock (infected cattle entering a naive herd, or naive cattle entering an infected herd). Tick activity and numbers are also likely to increase when environmental conditions are favourable — warmth, moisture and good vegetation are preferred by ticks.
Currently, there is no registered treatment for bovine theileriosis in Australia.
Supportive care includes fluids, nursing, low-stress handling, providing a calm and comfortable environment, and good nutrition. Sometimes, antibiotics may be prescribed but efficacy against the parasite is unreliable.
Blood transfusions from healthy unaffected cattle may be considered in individual cases or for high-value animals, but are not usually a viable option in an outbreak situation.
There is also no specific prevention for Theileria, as bush ticks spend most of their life cycle on plants rather than attached to cattle.
Recommendations include sourcing local cattle wherever possible, avoiding stress or transport of late pregnant cattle, avoiding mixing of stock from high/low risk areas, rotational grazing, and strategic blood testing to assess animals for exposure levels if necessary.
Using registered tick treatments may help control spread between cattle, although will do nothing to help those already infected.
As always, having a good biosecurity plan in place is essential if you’re in a high-risk area or plan on introducing new animals to your herd from an area known to have Theileria or tick populations.
For more information, talk to your vet or go to: https://biosecurity.dairyaustralia.com.au
Lucy Collins is completing her Dairy Residency with The University of Melbourne. She works as an on-farm veterinarian for Apiam Animal Health, and alongside her partner on his family’s dairy farm in south-west Victoria. She is a 2021 Nuffield Scholar supported by Gardiner Dairy Foundation.
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