Theileria orientalis

Theileriosis refers to the clinical disease syndrome caused by the genus theileria, a tick-borne protozoa that inhabits the blood (both red and white blood cells) and can infect a range of hosts. In Australia, Theileria orientalis can cause infections and potentially severe clinical disease in cattle with range of symptoms including but not limited to anaemia, lethargy, depression, jaundice, abortion, fever and death.

What causes Theileria?

Theileria is primarily spread by Haemaphysalis longicornis which is commonly known as the “bush tick”.  Other biting insects such as mosquitoes and lice could potentially result in infection but will not cause clinical disease.

Theileria impacts both beef and dairy cattle. It is common in coastal regions and is typically limited by the Great Dividing Range. The occurrence of Theileria can be impacted by seasonality of bush tick distribution, tick movements and alternative hosts that carry bush ticks.

Theileria orientalis has 11 genotypes (also known as ‘strains’ or ‘variants’). Of these 11 genotypes, Ikeda and Chitose are the most virulent and capable of causing severe disease in cattle. The genotype Buffeli is considered benign in cattle.

Our team recently held an information session on Theileria.

How do I know if my cattle have theileria?

Signs of theileriosis in cattle include any of the following:

  • anaemia (pale pink or white membranes of the conjunctiva, vulva and gums)
  • icterus or jaundice (yellowing of the conjunctiva, vulva and gums)
  • lethargy, difficulty walking, are slow to muster or are extremely depressed such that affected cattle prefer to lie down or unable to rise
  • abortion at any stage of pregnancy or calves are born dead
  • fever (rectal temperatures of 40°C and above)
  • respiratory distress (e.g. difficulty breathing, increased respiratory effort or coughing due to the profound anaemia Theileria causes)
  • profuse drooling, salivation or behavioural changes
  • weight loss and signs of dehydration (e.g. sunken eyes, gums that are dry, cold or sticky, increased skin tenting or changes in skin elasticity, decreased water content in stools). Affected calves may be seen straining or have rock hard faeces due to the effects of dehydration
  • death.

For more information regarding a field vet’s perspective on Theileria infections on the North Coast, refer to the information video presented in 2020.

How do you diagnose theileria in cattle?

A qPCR blood test of affected livestock provides a confirmatory diagnosis and a post mortem provides an ancillary diagnosis. On post mortem, changes in the internal organs of affected cattle may be seen due to the effects of Theileria acting on internal organs.

Theileria is best diagnosed using a qPCR blood test that can determine load levels of each strain type. This can be used to assess whether cattle herds or individual animals are at peak infection or have achieved a carrier state of immunity. Reach out to your Local Land Services District Veterinarian or a private veterinarian if you are interested in getting your herd checked for their Theileria status.

In endemic areas where Theileria is present, surviving calves generally develop immunity by 6 months of age. Unexposed or naïve surviving adult cattle from endemic and non-endemic areas generally develop immunity 3 months after initial exposure.  Not all infected animals will go on to develop clinical disease. Mortality rates are usually low (1-3%), but mortality rates of up to 30% are possible.

Once an animal becomes infected and reaches a carrier state, it is highly likely to have life-long immunity against clinical disease.

An animal that has previously reached a carrier state could potentially relapse with clinical disease when placed under severe stress.

When can Theileria infection occur?

High risk periods for Theileria infection are during or following peak bush tick activity which is typically during spring and summer months. Infections outside of these periods do still occur as the bush tick can survive year-round in endemic areas.

The bush tick is a three-host tick. This means that the tick can infest a variety of hosts other than cattle including deer and kangaroos. However, cattle are the preferred hosts. The various life stages of the bush tick—the larvae, nymphs and adults — can all feed on a host for a minimum of 3 days and up to 7 days before dropping off the host to moult or lay eggs in the paddock. If not fully engorged, the nymphs and adults (which can both transmit theilerial infection) can also be easily overlooked. This is why many people do not find ticks on diseased cattle or mistakenly think their property has no bush ticks present.

In areas where Theileria is present, bush ticks can begin biting and infecting calves from the day they are born. Similarly, any adult cattle introduced into an endemic area can also become infected or exposed on the day of arrival. Signs of clinical disease begin when Theileria levels in the blood reach their highest. Disease progression can depend on the environmental bush tick challenge levels present at the time and the corresponding levels of Theileria present in the blood.

Calves can show symptoms of Theileria as early as two months of age and up to four months of age. The first 2 to 3 months in general for susceptible cattle (calves and introduced naïve cattle) are when Theileria infections are at their peak. During this time period you may observe clinical signs in your cattle.

How do I treat Theileria infection?

There is no cure for Theileria and treatment of clinical disease in affected cattle consists of supportive nursing care focusing on hydration, nutrition and low-stress handling.

When cattle have significant burdens of Theileria present in their blood, this can lead to severe anaemia. Any mustering, stress or forced movement of anaemic cattle could potentially cause collapse or sudden death. If any movement of suspect Theileria affected cattle is required, do so at very slow pace. In some instances, cattle may be better off left to recover quietly in the paddock.

If broad-spectrum antibiotics such as oxytetracyclines can be administered without causing the animal undue stress, their use maybe justified in some cases to treat secondary infections. Otherwise, the best approach is ensuring cattle have access to good quality feed, water and sheltered paddocks that are easily accessible and in close proximity. Beware fatalities have been seen in anaemic cattle that have attempted to drink from steep dam banks or boggy dam edges.

Theileria risk when introducing stock from non-endemic areas

Livestock producers located in Theileria endemic areas should be aware of the risks associated with introducing cattle from outside their area. Theileria infection has been seen in livestock introduced from areas where the parasite is not present. In the case of introduced livestock, the disease is mostly seen about 6 - 12 weeks after introduction. This corresponds to the peak levels of Theileria present in the blood stream that rise sharply within the first 2 to 3 months of arrival. Heavily pregnant cattle are often most severely affected and can result in abortions, stillbirths and even death of the affected animal. Cattle in early stages of pregnancy can silently miscarry and return to oestrus.

Cattle that are lactating or have calves at foot should be avoided. Lactating cows and young calves at foot are particularly vulnerable to disease due to their lowered immunity. Bulls can also be affected. Due to the sheer weight and size of a breeding bull, a downer bull from the effects of anaemia can have severe secondary health impacts that could lead to possible death.

The introduction of any these suspectable stock to endemic areas should be avoided. Consider instead introducing non-pregnant adult breeders and adult steers in the colder months when the bush ticks are less active. Allow any introduced stock to acclimatise and develop Theileria immunity in paddocks away from bushland or paddocks infested with deer and kangaroos.

What are the risks of introducing stock from endemic areas to non-endemic areas?

Stock from endemic areas that have achieved a carrier state of immunity do not pose a threat to your herd or other animals. Although lice and mosquitoes could potentially transmit infection, it will not cause disease. The bush tick vector and animal hosts carrying the bush tick must be present in your area to maintain the life cycle of Theileria for it to continue causing disease in a designated area. This means that if a Theileria outbreak were to occur in an area where the bush tick vectors are not present, the disease will eventually die out.

Beware that cattle sourced from endemic areas that have achieved a carrier state of immunity can sometimes relapse and express clinical disease after being transported over very long distances or when they are heavily stressed. Sourcing young calves less than 12 weeks of age from Theileria endemic areas may still be fighting an active Theileria infection and may not have had a chance to become immune enough to achieve a carrier state.

What can I do to reduce the risk of Theileria infections in endemic areas?

Theileria infection can be reduced by limiting or slowing the tick challenge with persistent and effective tick treatments in affected areas. These ideally should be initiated from day zero or within the first 24 to 48 hours of the birth in calves. Naïve cattle should be treated before arrival and post arrival. In general, the first 2 to 3 months in these susceptible animals are your critical times where Theileria infections are at their highest. It is not necessary to prophylactically treat calves with tick products early in life unless you know there is an issue with Theileria on your farm.

There are very few acaricides (chemicals that destroy ticks) that have a label claim for bush tick. Most of the commercially available tick products were developed for cattle tick. Cattle tick is present in parts of Northern Australia and is rarely detected in NSW. In NSW, any detections of cattle tick must be reported to your local LLS office.

This means for bush tick control, acaricides are routinely used ‘off label’ to reduce bush tick infestations. Off label means that the tick product is not registered to treat bush tick and you are using at your own risk in terms of treatment and efficacy.

The bush tick can adapt to a variety of hosts other than cattle. Other common bush tick hosts include kangaroos and deer. Uncommon alternative hosts might include wombats, sheep, dogs and horses. The adaptability of the bush tick makes it more difficult to control than a one-host tick like the Queensland cattle tick.

In general, tick products that kill ticks within the first 24 to 48 hours and have a longer persistence period will require fewer treatments to reduce tick numbers. For a list of tick products that have a label claim for bush tick and products that could potentially be used off label.

Download our information sheets on treatments:

Other strategies to limit bush tick infestations and rates of Theileria infections include:

  • calving away from bushland or kangaroo infested paddocks
  • calving and introducing new cattle when ticks are less active (e.g. over winter/colder months)
  • using a ‘house paddock’ for at-risk or susceptible animals.

If you have specific questions or concerns regarding Theileria, we encourage you to reach out to your Local Land Services District Veterinarian or your local private veterinarian.

Theileria information session

An information session ‘Theileriosis As Much As We Know’ was presented by Professor David Emery DVSc BSc (Vet), Emeritus Professor, Veterinary Immunoparasitology, Sydney School of Veterinary Science, University of Sydney for cattle producers and cattle industry workers in Berry, NSW.

The presentation includes an overview of Theileria infections and the disease process in cattle, the bush tick vector, current research findings and use of integrated parasite management in Theileria affected areas.

This information session was funded through the Early Needs Recovery Program. The Early Needs Recovery Program is part of the $150 million Primary Industry Support Package which is co-funded by the Australian and NSW Governments.

Question and answer session

After the presentation, a Q&A session was held. Answers by Professor David Emery and LLS District Veterinarian Evelyn Walker.

Natural or artificial mating

Can Theileria transfer via natural mating and/or via artificial means (use of embryos, semen) from infected animals?  


Maternal foetal transfer

Can Theileria transfer from cow to calf via the placenta?  

Very rarely. This has been shown to occur in Japan, but no evidence of this transfer has been found in Australia.

Can Theileria transfer from cow to calf via colostrum or milk?  

Yes, but only a very small percentage. There may be some red blood cells from the cow that leak into the milk or colostrum. This means a calf can become infected this way, but this does not develop into clinical disease in the calf. A bush tick-borne infection is needed for this.

Can a calf be born with Theileria?  

Yes, but it is extremely rare as it would require transfer in the foetus. Similar to the above answer, but actual bush tick-borne infection only starts when they are in the environment from the day the calf is born.

Theileria in calves

We are seeing clinical disease and calf deaths even earlier than what you describe (e.g. 3-4 weeks of age) and other farms much later than what you describe (e.g. 13 weeks of age and even at 4-5 months of age). Why the variation in affected age groups of calves across different farms in an endemic area?  

The appearance of disease and deaths depends on how much theilerial sporozoites from the bush tick, or bush ticks, is injected to cause the infection. The higher the infection level, the faster clinical signs will appear. This means the more bush ticks that your calves are exposed to, the faster they will become infected. If infection levels rise too high, you will see deaths in your calves.

Do you think Theileria infections in calves can cause chronic issues (lasting several months) with calves failing to thrive or doing poorly?  

There is some anecdotal and observational evidence chronic issues can arise, but this can also be impacted by how well your calves are able to recover from possible underlying anaemia (due to the effects of Theileria) and the availability of good nutrition on offer to your calves.

Genetic susceptibility and resilience

Is there any evidence that calves born to certain types of cows are more susceptible to Theileria? 

Possibly in terms of their behaviour or genotype, but I am not aware of any research into resistant breeds, even in Japan.

Other bush tick hosts

Are wombats and deer possible hosts for bush ticks? What about dogs?  

Wombats could be possible hosts, but this is unlikely. Common hosts include species that are more mobile Dogs are possible hosts. Although the bush tick can adapt to a variety of hosts, cattle are the preferred host of bush ticks. Uncommon and possible hosts include wombats, sheep, dogs and horses. The adaptability of the bush tick thus makes it more difficult to control than a one-host tick, like the Queensland cattle tick.

Other insect vectors

Can Theileria be spread by lice or mosquitoes?  

Lice and mosquitoes can carry infected blood. While this could result in active transmission from very small amounts of blood, it will not cause any clinical disease and the animal will become a carrier. We have done this experimentally by injecting small amounts of blood. To get clinical disease and recovery in animals who will then become carriers, bush ticks are required for the initial infection.

Treatment options in acute or clinical cases of Theileria

What are the treatment options for treating clinical cases of Theileria?  

TLC and supportive nursing care are the best treatment options. Various people have tried oxytetracycline and Imidox® with mixed success. Research is limited and results have been variable. Prevention is best by limiting the bush tick challenge load before the first peak of parasitaemia by using chemicals with persistent tick activity early in life. Infections can start from the day calves are born and exposed to bush ticks in the environment and from the day naïve cattle are introduced to an area where bush ticks are present.

What is your opinion of use of Imidox® in treating calves as we find it very effective?

Imidox® is a veterinary drug with a label claim to treat cattle tick fever infections caused by Babesia and Anaplasma species. There is no label claim for the treatment of Theileria infections, which is transmitted by bush ticks. I haven’t tested Imidox® for treatment but if it works, it works - provided you don’t have any adverse effects from treatment.

What is your opinion of the use of tetracycline in clinical disease?  

In Kenya, the vaccine consists of an infection and treatment protocol, such that a dose of Theileria parva is injected and this is followed by a long-acting dose of tetracycline. This has been shown to slow the parasite right down so the animal has time to develop immunity.

In our studies, we found elimination of the carrier state could not be achieved with use of tetracycline.  We were not able to investigate whether tetracycline would have worked to either block or slow the rate of Theileria infection in naive animals. We did find, however, that when we fed bush ticks tetracycline, the Theileria uptake from an infected cow was compromised. This is why we possibly get a response to tetracycline treatment.

Please always be wary of ‘treatments that work/cure’ once clinical symptoms are seen as the animal may be recovering towards the carrier state.

What is your opinion on the use of oxytetracycline in calves with clinical disease?  

Oxytetracycline is a commonly used broad-spectrum antibiotic used to treat a wide range of bacterial infections in cattle. There is no-label claim for the treatment of Theileria infections in cattle.  If you are going to treat calves early in life with oxytetracycline, you could potentially destroy their delicate microbiome (gut flora).

The microbiome of calves develops very rapidly soon after birth and they continue to develop this microbiome over several weeks to months. Using a broad-spectrum antibiotic is not the first option I would recommend in treating calves. It is, however, still something that could potentially work and may possibly be worth sacrificing the microbiome to see if it works. I do not have any evidence to support the use of oxytetracycline in calves.

What is your opinion on the use of BPQ (Buparvaquone) as an effective treatment for Theileria? Can we obtain it in Australia?   

BPQ is a highly effective treatment and is currently available in New Zealand. Unfortunately, we cannot get it in Australia. At the moment, BPQ can only be obtained via a restricted minor use permit through veterinary application to the APVMA. Restrictions placed on the permit by APVMA may make it difficult as a practical option for cattle producers. There are also serious concerns regarding tissue and meat residues associated with the use of Buparvaquone that could potentially threaten our Australian meat export markets.

What is the mode of death in cattle with clinical disease of Theileriosis?  

Dehydration, cardiac failure and/or sludging of the red blood cells in the brain that are heavily parasitized with virulent Theileria strains.

Theileria immunity

Is there any cell mediated immunity for the two virulent strains of Theileria, Chitose and Ikeda?  

Cattle can develop immunity to different types of pathogens via different methods. Immunity in general can be via innate (natural) immunity and/or via acquired (specific) immunity. Acquired immunity can involve antibody and/or cell-mediated responses. We do know that for cattle in endemic areas that were previously infected develop a form of ‘premunity’. This offers protection from further Theileria infections from bush ticks.  We also know that there is no protective antibody response in calves born to carrier dams.

Cell-mediated immunity basically involves activation of various cell types that work together to destroy infected and foreign cells. We do not have a laboratory test to investigate whether there is cell-mediated immunity occurring in the Chitose and Ikeda strains present in Australia. We do know that there is cell-mediated immunity to East Coast Fever (Theileria parva) in Kenya. But bear in mind, similar to COVID-19, your immunity to one strain doesn’t necessarily transfer to the other.

The benefit, however, of being in an endemic zone is that once an animal reaches a carrier state, they will virtually be protected against all the Theileria strains. For some reason, even just carrying low levels of Ikeda, Chitose or Buffeli will still protect the animal against clinical disease. We still don’t quite know how this mechanism works.

Once an animal has immunity or reaches carrier state, can they still succumb to clinical disease due to Theileria and/or abort their calf? We still have issues with the odd heifer aborting her calf - is there a certain stage of pregnancy they will abort?

It takes a fair bit of stress and a very heavy parasite load to break through that carrier state and cause clinical disease. There may be a very high bush tick challenge on your property. And that’s similar with any type of immunity, the bigger dose you challenge them with, the greater the risk that you will break their immunity.

In terms of stage of pregnancy at which they abort, it depends on the parasite load and what her packed cell volume is at during the various stages of pregnancy. Packed cell volume (PCV) is a blood test that measures the percent of red blood cells present in the blood. It reflects the oxygen carrying capacity of the blood. Changes in PCV in livestock can be a useful tool in assessing dehydration or anaemia of an animal.

Bush tick treatments and prevention

We apply tick products to our calves at one day old using a pour-on tick treatment. By giving calves tick treatment so early on in life, are we potentially extending the length of time it takes for the calf to go into a carrier state?

We find that calves are well into the carrier state long before 13 weeks of age. It depends on the persistence period of the tick product you are using. What we do know based on our research is that a product that will kill ticks within 24 to 48 hours and has a persistence period of at least 28 days will reduce the bush tick challenge and reduce the Theileria load levels. There is a timing balance here between the dose of Theileria causing clinical signs and possible death, and the rate that you achieve the carrier state. At present, we do know that slowing the rate of infection by slowing or lowering tick infection, the greater the chance that calves will survive.

Is there a chance that we are actually slowing down rate of immunity to Theileria by over treating with tick treatments and maybe this is why we don’t see infections until much later in our calves? Theoretically, there will be some ticks still attaching since you are not in a completely sterile environment. You are also not able to physically remove every single tick that jumps on and off the calf. Even the best of tick acaricides will not achieve a 100% kill in time to prevent some infection, but the time to reach the carrier state will be longer if infection rates are reduced. You are simply trying to prevent deaths from too great a tick challenge.

Is there any cross-protection from using a tick fever vaccine?  

No. The tick fever vaccine is for cattle tick which transmits Babesia bovis, Babesia bigemina and Anaplasma marginale. The bush tick transmits Theileria for which there is no vaccine.

Is anyone doing any studies on immunising naïve cattle with Theileria infected cattle?

There is some very promising work we have done by pre-infecting naïve cattle with small injectable doses of infected blood obtained from carrier animals. Cattle that received pre-infected blood were protected against bush tick challenge and had decreased Theileria levels in their blood.   However, it is still early days as we need a guaranteed source of disease-free material. This used to be available through the Tick Fever Centre,but is no longer available. Negotiations are continuing with NSW Department of Primary Industries (DPI) at Elizabeth Macarthur Agricultural Institute (EMAI) Menangle.

Can you apply anything to the pasture to kill off the tick vectors?

No, please do NOT. Any chemical application for such a purpose would wreck the entire soil microbiome and all the other useful organisms that are vital for soil and plant health. Waterways and aquatic life could also be seriously impacted by any chemical application.

We run a small mob of cattle and have never had Theileria that we are aware of. We do, however, have plenty of kangaroos and bush ticks. We are able to handle our calves from day one. Do you recommend we inject our calves with tick treatments as preventative for Theileria? 

I would not be injecting anything without confirming the Theileria status of your herd. You can get a veterinarian to collect blood samples from your herd to check their Theileria status.

You mentioned the Python ear tags in your study and in your presentation, is that something we should be using in calves for prevention?  

If you can get it, yes, but availability is difficult and as I stressed, you will use them off-label’. The Python ear tags are used in the control of buffalo fly, biting lice and paralysis ticks. There are other brands similar to the Python tags as well. This could be used in conjunction with a persistent tick treatment like Dectomax® as an example used in our study. Remember, the majority of tick products do not have a label claim for bush ticks.

If you maintain a closed herd in an endemic area, will you have less disease on farm?   

All of the cattle born on your property over 6 months of age are likely to be carriers. You will still have issues with calves born in an endemic area. These calves will have no immunity even if their mothers are ‘immune’ or carriers. If you source cattle from other endemic or affected areas, they will have immunity as well. Where you will run into trouble is if you bring in cattle from other areas that do not have immunity to Theileria.

Geographic distribution of Theileria

Is there a Theileria geographic line?

No. It can be seasonal depending on bush tick distribution. In general, the disease is typically coastal, limited by the Great Dividing Range. It’s mainly a coastal parasite.

Production impacts and use of animal products from Theileria carriers

Does Theileria affect milk or meat products?  

No. Studies in both beef and dairy cattle in Australia and New Zealand have found that there were no productivity differences between Theileria carriers and non-carriers. You will not get Theileria by consumption of edible cattle by-products. Theileria is a disease of cattle only.

If you have specific questions or concerns regarding Theileria, we encourage you to reach out to your LLS District Veterinarian or your local private veterinarian.

For more information regarding current Theileria research in Australia, download a list of published open access journal articlesPDF, 81.22 KB.

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