United States Department of Agriculture

Animal and Plant Health Inspection Service

Key Points

• Theileria orientalis is a tickborne protozoon 

that infects red and white blood cells and 

causes bovine infectious anemia. Clinical 

signs of theileriosis are similar to 

anaplasmosis in cattle and include anemia, 

jaundice, and weakness. Native genotypes 

of T. orientalis in the United States are 

usually nonpathogenic; however, the 

virulent Theileria orientalis Ikeda genotype 

was identified in the United States.

3

• The last documented theileriosis case in 

the United States was a beef herd in 

Missouri infected by T. buffeli in 2000.

7,13

• In August 2017, seven cattle from a herd in

Albemarle County, Virginia died after showing

signs of illness including severe lethargy and

weakness. The cattle ranged in age from 3

months to 13 years and included bulls, cows,

and steers.3

• In September 2017, an additional cow from 

the same herd was examined for weakness, 

icterus, and anemia. Anaplasmosis was 

suspected, however, testing revealed an 

infection with T. orientalis, which prompted 

quarantine of the affected farm and further 

investigation including a foreign animal 

disease investigation initiated in December 

2017.

3

• During the investigation, Virginia-Maryland 

College of Veterinary Medicine

(CMCVM) identified the blood-borne parasite 

as the virulent T. orientalis Ikeda genotype.15

• A recently published study reported that 

Haemaphysalis longicornis ticks, also known 

as the Asian longhorned tick (ALHT), were 

found in all sampled habitat types and were 

the most abundant of ticks collected from the

environment at the T. orientalis index farm in 

Virginia. A relatively high percentage (∼13%) 

of questing H. longicornis nymphs from this 

site were positive for T. orientalis Ikeda, further 

implicating this tick in the pathogen 

transmission on the index farm.11

• A recent preliminary report of an experimental

transmission trial performed by USDA’s

Agricultural Research Service (ARS) in

conjunction with the Virginia Tech Animal

Laboratory Services (ViTALS) laboratory has

confirmed vector competence of

H. longicornis for T. orientalis Ikeda in the United

States.

16

• T. orientalis Ikeda has been documented in 

cattle since September 2017 in at least 28 

counties in Virginia and three counties in 

West Virginia.14, 15 The National Veterinary 

Services Laboratories (NVSL) confirmed 

the initial two cases in Virginia. The 

remaining cases have been confirmed by 

blood smears and polymerase chain 

reaction (PCR) along with sequence and 

phylogenetic analyses at Kansas State 

Veterinary Diagnostic laboratory, VirginiaMaryland College of Veterinary Medicine, 

and the ViTALS laboratory.

3

• Some species of Theileria (T.parva and T. 

annulate) are reportable to the World 

Organisation for Animal Health (OIE); however, 

T. orientalis is currently not reportable to the 

OIE.17

• There are no known risks to human health.

Potential Economic Concerns

• T. orientalis Ikeda has caused major economic 

losses in Asia, New Zealand, and Australia 

primarily as a result of deaths or illness in beef 

and dairy cattle and ongoing milk losses.3

• An analysis of one dairy affected by T. 

orientalis in New Zealand in 2014 estimated the 

loss at more than $400 per cow.19 

Epidemiology

• Theileriae are obligate intracellular protozoan 

parasites that infect wild and domestic animals 

in the Bovidae family worldwide. They are 

transmitted by ixodid ticks.1

Emerging Risk Notice

January 2021

Theileria orientalis Ikeda
United States Department of Agriculture

Animal and Plant Health Inspection Service

• T. orientalis has a global distribution, but 

countries impacted by clinical oriental

theileriosis (OT) are Australia, New Zealand, 

Japan, Korea, China, and Vietnam.2

• T. orientalis has 11 genotypes identified 

(named: Chitose or type 1, Ikeda or type 2, 

buffeli or type 3, types 4 to 8, and N1 to N3). 

Genotypes Chitose and Ikeda are associated 

with severe disease.2

• T. orientalis Ikeda infections have been 

reported to cause mortality in up to 5% of 

infected cattle. Pregnant heifers and calves are 

particularly susceptible to the infection.

7

• Additional factors, such as breed or age, may 

increase disease susceptibility in cattle. In 

Japan, beef cattle of the Wagyu breed have 

been reported to be less susceptible to clinical 

infections. However, the effect of age has not 

been well studied.7 

• Clinical findings include weakness, reluctance

to walk, and abortion. Physical examination 

may reveal mucosal pallor, pyrexia, and 

elevated heart and respiratory rates.1, 2,8

• Cattle that recover from Theileria infections 

usually become carriers.2

• In Asia, Australia, and New Zealand, the 

primary tick vector for T. orientalis Ikeda is 

H. longicornis.

3

 The ALHT is native to 

eastern Asia and has spread to Australia, 

New Zealand, and several Pacific islands. 

4,5, 9

• ALHT are three-host ticks, with larvae, 

nymphs, and adults feeding on a wide 

range of wild and domestic species, 

including birds, white-tailed deer, 

companion animals, livestock, equines, 

and humans. Once introduced into suitable 

habitats, ALHT populations can increase 

rapidly, partly due to exhibition of multiple 

genetic types including being 

parthenogenetic and bisexual.4,12, 9

• As of August 2020, ALHT has been 

identified in 15 States (Arkansas, 

Connecticut, Delaware, Kentucky, 

Maryland, New Jersey, New York, North 

Carolina, Ohio, Pennsylvania, Rhode Island, 

South Carolina, Tennessee, Virginia, and 

West Virginia). USDA keeps a publicly 

accessible website that maintains 

information on ALHT and maps of the 

current tick distribution in the U.S.12

Transmission

• Cattle are thought to become infected within 

3 weeks of being placed on pasture 

containing infected tick vectors. Disease is 

seen more frequently when naïve animals 

are introduced into an endemic area or 

when infected animals are introduced to a 

herd where a competent vector is present.2

• Theileria sporozoites (the infective stage) 

are transmitted to susceptible animals 

during tick feeding through the tick’s saliva.2

• When a T. orientalis parasite infects a 

nymphal tick feeding on a naïve cow, the

cow can become infective to ticks in 10 days.2

• Theileria mature within the tick after it has been 

attached to the host for 48-72 hours. If 

environmental temperatures are high, infective 

sporozoites can develop in ticks on the ground 

(off host) and may enter the host within hours of 

tick attachment.2

• Ticks can remain infected on a pasture for up to 

2 years under favorable conditions.2

Diagnostic Testing

• T. orientalis can be diagnosed by various

methods such as microscopy, serology,

molecular techniques, and xenodiagnosis. PCR,

the gold standard, can detect infection in cattle

up to 2 weeks before the infected erythrocytes

can be observed under a light microscope. To

identify the Ikeda genotype, more advanced

molecular diagnostics like real time semiquantitative and quantitative PCR assays

designed to detect that genotype need to be

used.7

• Serological tests (i.e., immunofluorescence 

assay [IFA] and enzyme-linked immunosorbent 
United States Department of Agriculture

Animal and Plant Health Inspection Service

assay [ELISA]) are available for detection of T. 

orientalis; however, these assays are currently 

not genotype-specific and may also cross-react 

with other Theileria species.1,7

Treatment

• Currently, there is no approved treatment for T. 

orientalis infection.

Prevention

• Virginia Cooperative Extension has produced a 

document titled “Managing the Asian Longhorned 

Tick: Checklist for Best Management Practices for 

Cattle Producers.” The document can be accessed 

at: https://www.pubs.ext.vt.edu/ENTO/ENTO382/ENTO-382.html 18

• Vector control and herd management is important 

to reduce the rapid spread of T. 

orientalis outbreaks. Management practices such 

as keeping grasses short, regularly inspecting 

cattle for ticks, using pesticide-impregnated ear 

tags, and keeping cattle out of wooded areas can 

reduce disease spread.9, 18, 19

• Control of this tick species with acaricides 

alone is challenging due to a limited host 

attachment period, as the ticks spend most 

of their time in the environment and not on 

their biological host.7

• Restriction of grazing cattle movements may 

assist in reducing exposure to infected H. 

longicornis.

7 Amidines, organophosphates, and 

synthetic pyrethroids may be effective in 

controlling this tick species in the field over a 

limited time period.6

• No available acaricides in the U.S. contain a 

label claim against H. longicornis.

10 Isoxazolines

and macrocyclic lactones ectoparasiticides that 

are approved in the U.S. for similar 

ectoparasites are also approved in other H. 

longicornis endemic countries with an indication 

for that tick species.10

• Any FDA-approved drug products not labeled 

for H. longicornis are considered extra-label 

use.10

Sources

1. OIE. Manual of Diagnostic Tests and Vaccines for 

Terrestrial Animals 2019: Chapter 3.4.14: 

Theilieriosis2020;

https://www.oie.int/fileadmin/Home/eng/Health_sta

ndards/tahm/3.04.14_THEILIERIOSIS.pdf. 

Accessed March 30, 2020.

2. OIE. Technical disease cards: Theileriosis. 2020; 

https://www.oie.int/fileadmin/Home/eng/Animal_He

alth_in_the_World/docs/pdf/Disease_cards/THEIL

ERIOSIS.pdf. 

3. Oakes V, Yabsley M, Schwartz D, et al. Theileria 

orientalis Ikeda Genotype in Cattle, Virginia, USA. 

Emerging infectious diseases 2019:1653-1659.

4. Hutcheson HJ, Dergousoff SJ, Lindsay LR. 

Haemaphysalis longicornis: A tick of considerable 

veterinary importance, now established in North 

America. Can Vet J 2019: 60:27-28.

5. USDA. Factsheet: Longhorned Tick: Information 

for Livestock and Pet Owners 2020;

https://www.aphis.usda.gov/publications/animal_h

ealth/fs-longhorned-tick.pdf. Accessed March 30, 

2020.

6. Center UAVCH. Monitoring Haemaphysalis 

longicornis, the Asian longhorned tick, populations 

in the United States, 2019.

7. Yam J, Bogema D, Jenkins C. Oriental 

Theileriosis, 2018.

https://www.intechopen.com/books/ticks-and-tickborne-pathogens/oriental-theileriosis

8. Watts J, Playford M, Hickey K. Theileria orientalis: 

A review. New Zealand veterinary journal 2015;

64:1-21.

9. Heath ACG. Biology, ecology and distribution of 

the tick, Haemaphysalis longicornis Neumann 

(Acari: Ixodidae) in New Zealand. New Zealand 

Veterinary Journal 2016; 64:10-20.

10. AVMA. Options available to protect against 

longhorned tick. 2018; 

https://www.avma.org/javma-news/2018-09-

15/options-available-protect-against-longhornedtick. Accessed March 30, 2020.

11. Thompson A, White S, Shaw D et al. Theileria 

orientalis Ikeda in host-seeking Haemaphysalis 

longicornis in Virginia, U.S.A. 2020 

Sep;11(5):101450. doi: 

10.1016/j.ttbdis.2020.101450 

12. USDA. National Haemaphysalis longicornis (Asian 

longhorned tick) Situation Report. 2020; 

https://www.aphis.usda.gov/animal_health/animal_

diseases/tick/downloads/longhorned-ticksitrep.pdf. Accessed September 14, 2020.

13. Stockham SL, Kjemtrup AM, Conrad PA, et al. 

Theileriosis in a Missouri Beef Herd Caused by 

Theileria buffeli: Case Report, Herd Investigation, 

Ultrastructure, Phylogenetic Analysis, and 
United States Department of Agriculture

Animal and Plant Health Inspection Service

Experimental Transmission. Veterinary Pathology

2000;37:11-21.

14. Leonhardt K. WVDA Confirms Theileria in West Virginia. 

2020; https://agriculture.wv.gov/2020/01/24/wvdaconfirms-theileria-in-west-virginia/ last accessed October 

28, 2020.

15. Lahmers K. Theileria orientalis Ikeda genotype in cattle. 

2020; https://www.vetmed.vt.edu/labservices/docs/theileria-summary.pdf. last accessed 

October 28, 2020.

16. Dinkel, K., Herndon, D., Noh, S., et. al. 2020. A U.S. 

isolate of Theileria Orientalis, Ikeda Genoytype, is 

Transmitted to Cattle by the Invasive Asian Longhorned 

Tick, Haemaphysalis Longicornis. Parasites and Vectors. 

Preprint. 10.21203/rs.3.rs-90827/v1

17. World Organization for Animal Health (OIE) (2019). 

Terrestrial Animal Health Code, 28th Ed. OIE, Paris. 

Available at: www.oie.int/en/international-standardsetting/terrestrial-code/access-online/ (accessed on 18 

September 2020).

18. Dellinger, T. and Day, E. Virginia Cooperative Extension: 

Managing the Asian Longhorned Tick: Checklist for Best 

Management Practices for Cattle Producers.

https://www.pubs.ext.vt.edu/content/dam/pubs_ext_vt_ed

u/ENTO/ento-382/ENTO-382.pdf last accessed October 

28, 2020.

19. Stafford KC, Williams SC, and Molaei G. 2017. Integrated 

Pest Management in Controlling Ticks and Tickassociated diseases. Journal of Integrated Pest 

Management 8:1-7. 

For more information, contact:

The Center for Epidemiology and Animal Health

vs.ceah@usda.gov

(970) 494-7200

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