Benign Theileriosis in Victoria
Note Number: AG1434
Published: November 2011
Reviewed:
Benign theileriosis is a disease of cattle caused by a blood parasite, known as Theileria, transmitted by ticks. The disease seen in Australia is usually mild or ‘benign’ – deaths are uncommon in areas where the disease is endemic. The disease can however be more severe when it moves into new areas, where animals have not been previously exposed. The heavy summer rains in Victoria in early 2011 created an environment where the tick vector was able to flourish, leading to spread of this disease.
Where does the disease occur?
Theileriosis has a virtually worldwide distribution in cattle. Many different species of Theileria are involved, some more deadly than others. One of the most fatal forms of the disease is East Coast Fever, caused by Theileria parva. This disease does not occur in Australia and is found only in Africa.
Theileria infection was first detected in Australia in the early 1900s, and has become endemic in parts of north eastern Australia. Historically the parasite has generally not caused disease in endemic areas, in recent years there has been an increase in clinical disease, particularly in New South Wales and more recently, Victoria.
Cause
Benign theileriosis is caused by microscopic blood parasites from the Theileria orientalis group. The parasite enters the bloodstream where it first infects the white blood cells and then the red blood cells. These blood cells are destroyed and so the infected animal can become anaemic.
How is benign theileriosis transmitted?
The Theileria parasite is transmitted to cattle in the saliva of ticks. When the ticks attach to cattle and feed, they ‘inject’ the parasites into the bloodstream.
Benign theileriosis is suspected to be transmitted by the bush tick (Haemaphysalis longicornis), though other ticks could be involved. Bush ticks are characterised by their red legs and occur throughout Victoria, particularly in the wetter areas. Bush ticks thrive in a moist, temperate environment where there is plenty of plant cover. These conditions occurred in eastern Victoria in the summer and autumn of 2011. The ticks die off in dry conditions and when vegetation is lacking.
The time that ticks actually spend on cattle is very short – from one to a few weeks at the most – but this is sufficient time for them to transmit the parasite if they are carrying it. Ticks can become infected when feeding on infected cattle.u jst
Although heavy tick infestations are usually easy to see on cattle, very few ticks are required to transmit benign theileriosis, and they may not be easy to see when inspecting cattle. Ticks are smaller than a match-head, but swell up to pea-size when engorged with blood. They are usually found at the base of the tail, in the escutcheon (between the tail and the top of the udder or scrotum), on the dewlap and in the ears. Where infestations are heavy, ticks may be easily visible on the neck.
Maintenance of the transmission cycle requires cattle with a high number of circulating theileria in the blood sufficient to infect ticks that may attach to infected cattle; it also requires a sufficient population of ticks. If there are not enough theileria or ticks transmission will not occur and benign theileriosis will disappear.
After the initial infection in cattle, many blood cells become infected. Some cattle die of anaemia but the vast majority survive. As immunity builds up, less blood cells become infected, but most cattle remain infected for life. The number of blood cells infected may however be too low to pass the infection on to new ticks that have survived the winter.
The Theileria parasite may also be transmitted directly from animal to animal through the use of ‘multi use’ hypodermic needles and other procedures which can spread blood such as ear-notching. It has also been shown experimentally that some species of lice and biting flies may transmit infection, but it is not known whether this occurs in the field.
While bush ticks may infest a variety of animal species including horses, birds, sheep, goats and native wildlife, they are only able to transmit benign theileriosis to cattle.
It should also be remembered that not all bush ticks are infected with benign theileriosis – the presence of bush ticks on cattle does not necessarily indicate the presence of disease.
Heavy bush tick infestations do cause “tick worry” in all animal species that they infest, and this needs to be remembered. Effective tick management promotes animal wellbeing.
Haemaphysalis ticks are not the only ticks active in Victoria. Ixodes holocyclus, the paralysis tick, is also present in the south-eastern parts of the State. It may cause allergic reactions through its bite and is known paralyse dogs and cats. Farmers need to be aware of the various dangers posed by a variety of ticks in wet seasons.
What are the signs of benign theileriosis?
The signs of benign theileriosis are typical of diseases where anaemia is involved. Animals lose weight, are listless and depressed, their mucous membranes are pale and they run out of breath when moved. As the disease advances, liver damage occurs and the mucous membranes show signs of jaundice (i.e. they become yellow in colour). In the early stages of disease, a fever is obvious and pregnant cows may abort.
Death rates vary; in areas where animals are regularly exposed (a situation of endemic disease), clinical signs may not be seen at all or only be seen in each new crop of calves or in introduced cattle. In areas where the disease has recently been introduced, many animals may become ill and some will die. In Victoria, up to a quarter of visibly sick cattle have died in herds affected by benign theileriosis, however most infected cattle did not become visibly sick.
In order to confirm a diagnosis of benign theileriosis, a veterinarian will need to take blood samples from affected animals and have these examined at a laboratory.
Collection of ticks may also be useful to confirm their identification. Researchers are developing a test to detect Theileria parasites in ticks.
The situation in Victoria
Benign theileriosis has been present in east Gippsland for many years, but disease is rare. A recent study indicated that a relatively high percentage of cattle herds in east Gippsland may have cattle infected but not showing signs of disease.
During 2011, benign theileriosis began to appear in other parts of Victoria in cattle populations not previously exposed to the disease. In these populations, clinical disease and deaths were seen. In some cases, the disease appeared in herds after the introduction of animals from parts of Australia where the disease is known to occur. It is believed that local environmental conditions favouring the multiplication of the bush tick, particularly the warm and moist conditions in the summer and autumn, enabled the introduced disease to spread into previously unexposed local animals. It is likely that imported cattle were carriers and did not show clinical disease, and the local bush ticks fed on them and became infected. Over the next couple of months the ticks attached to local cattle and spread the parasite to them, causing disease and deaths in some instances.
As there is still much to be learnt about the disease, and the role that bush ticks and other possible vectors play, it is unknown if benign theileriosis will persist or spread within Victoria. It is likely to disappear if summers are dry, and persist in wet summers.
Treatment
There is currently no medication registered in Australia for the treatment of benign theileriosis. Only symptomatic treatment can be provided, including good nursing of affected stock and minimising stress and movement. There is no vaccine for the disease.
Managing the risk of benign theileriosis
As there is no specific treatment or vaccine, it is important for cattle producers to manage the risk of benign theileriosis.
If you are concerned about managing theileriosis risk the following tips may assist:
- When buying in new stock, ascertain their health status. Avoid importing animals from known affected properties or localities.
- Where the health status of bought-in stock is unknown, treatment with a registered tick treatment may be advisable prior to introduction. When using insecticides, always consult with your veterinarian and remember to observe the prescribed withholding periods before marketing products of treated animals.
- Rotational grazing practices may also help control ticks; the use of non-bovine species may act as ’vacuum cleaners’ to remove ticks from pasture before the introduction of cattle.
- Cattle showing clinical benign theileriois must not be stressed. They should be rested, nursed and given high quality feed.
- Seek veterinary advice if cattle are showing signs of fever, anaemia, multiple abortions or any other unusual signs.
Note that bush ticks are almost impossible to eradicate from a property as they are on and off the host in a week or so, and live in pasture for many months, as well as infesting other animals including wildlife.
Further information
Contact your veterinarian or local Department of Primary Industries (DPI) Animal Health staff for further information.
Benign theileriosis is not a notifiable disease.
Information on general biosecurity for cattle enterprises is available from www.farmbiosecurity.com.au
