Theileria Infection- the Hidden Killer of Australian Cattle

A deadly new cattle disease spread by bush ticks has swept through Australian herds since 2006.

The species of Theileria found in Australia is very different from the Theileria found in Africa and Southern Asia. However, the new deadly Australian strain of Theileria orientalis (known as ‘Ikeda’) is identical to the species and strain found in Japan.

It started to cause deaths and disease outbreaks in cattle about 2006 on the NSW north coast. The diagnosis was only confirmed however in 2011, by which time the disease had already spread to all parts of the NSW coast and Victoria and caused disease outbreaks on dozens of properties.

The new deadly Ikeda strain subsequently spread to inland areas of these states and to Queensland, Western Australia and South Australia as well as New Zealand. So far, it has not been detected in Tasmania or the Northern Territory.

Bush ticks

Bush ticks (Haemaphysalis longicornis) were introduced from Asia over one hundred years ago, bringing a strain of relatively harmless Theileria infection with them. These ticks like cattle best but are also happy to live on any type of mammal (including humans) and some birds.

Bush ticks love warm humid conditions. They are found along the east coast of Australia from Gympie to the Gippsland but will survive introduction to the inland or further afield. In southwest Western Australia they are found around the southern coastal areas, while in Queensland they are seen more often in tableland regions such as Maleny.

The estimated distribution can be seen in blue on the map below. Note that ticks will survive outside these areas in good seasons, like this year.

The main way that both ticks and the Theileria parasites are spread is live cattle transport. The tick larvae and nymphs are only tiny, so it is easy to miss them when bought-in cattle arrive on a property. Horses, dogs, feral deer and even rabbits could spread the ticks locally.

Theileria parasites require bush ticks to complete their life cycle, but small numbers of parasites can be spread within a herd by sucking lice, or even by mechanical means e.g. injection needles when vaccinating cattle.

Life cycle

The life cycle of the bush tick Haemaphysalis longicornis (see Diagram 2) is very different from that of cattle ticks (Rhipicephalus australis).

1. The bush tick is known as a ‘3-host’ tick, meaning that it drops off the host every time it moults. It therefore spends a lot of its life on the pasture. To complete its life cycle, it must find and attach to 3 different hosts.

2. The bush tick population is almost entirely made up of females (estimated ratio of female:male is 400 to 1), which are able to reproduce without males (parthenogenesis).

3. Nymph (juvenile) ticks over-winter on pasture, then look for hosts (cattle or other animals) in the spring. Adult ticks are seen on cattle in summer. They lay eggs which hatch and larval ticks are seen on cattle in the autumn.

4. Bush ticks are very small and although their life cycle may take 9-12 months, they may only be visible on cattle for about 14 days during this entire time. So, there could be many bush ticks present in the paddock and even on cattle (as tiny ‘seed’ ticks only about 1mm long) that are invisible to the producer.

Diagnosis and treatment

See your veterinarian for advice on diagnosis and treatment. Diagnosis of theileriosis (disease caused by Theileria) is done by matching the clinical signs of anaemia (pale colour, weakness), fever, abortion and loss of appetite, with blood tests sent to the veterinary diagnostic laboratory that can confirm the presence of the disease agent Theileria orientalis and also identify the strain.

There are no specific treatments available to cure infection with Theileria orientalis. Infected cattle and calves may show mild clinical signs but may also suffer severe illness or death. Treatment is based on intensive care including confinement, intravenous fluids and blood transfusions.

Prevention

Because Theileria infection is spread by bush ticks, the only way to prevent them is controlling the ticks. In places with widespread (endemic) infection, calves pick up the disease soon after birth and have peak disease signs at the age of 8-10 weeks. After this they develop immunity which only breaks down when they are placed under nutritional or metabolic stress. Careful handling of calves to prevent situations that exacerbate disease (e.g. mustering over long distances) coupled with extra attention to nutrition and control of other disease conditions helps get them through the critical period.

Most disease in growing or adult cattle is seen when naïve (previously unexposed) cattle are introduced to an area that has bush ticks and Theileria. Similarly, the infection can be spread to an unexposed herd by cattle from an endemic area, where Theileria infection is common.

There are a few products known to kill bush ticks. Synthetic pyrethroid, organophosphate and amitraz products are registered for that purpose. To minimise the introduction of infected ticks, treat cattle prior to introduction and keep in yards for clean inspection before allowing them onto your paddocks.

When purchasing cattle, ask the question of the vendor- have you ever had or suspected Theileria on the property? Even if all ticks are eliminated from incoming cattle, if they are infected with Theileria, then bush ticks already on your property can bite the infected cattle and pass on the disease to your herd. This information won’t be found on your standard Cattle Health Statement, so needs to be checked with vendor or using veterinary diagnostic tests.