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Theileria Orientalis in Virginia Cattle


Theileria orientalis is an intracelluar protozoan parasite that has traditionally been classified by genotype (Buffeli, Chitose, and Ikeda). The Buffeli genotype has been endemic in the United States, and with rare exceptions is considered non-pathogenic. The genotype which has gained recent acclaim in Virginia is Ikeda, which can be a more considerable pathogen.


Erythrocytes are obligate hosts of Theileria piroplasms, and as such, are the target tissue for clinical signs associated with theileriosis. Erythrocyte destruction leads to anemia, thus clinically affected cattle have presentations similar to, and physical exam findings comparable with cattle affected by anaplasmosis. Signalment may be of some use in differentiating the two diseases as calves may be affected by theileriosis but are generally spared by anaplasmosis.


Mortality is possible, and though the literature shows wide ranges of mortality rates, field data in Virginia suggests mortality rates are likely to range from 0-5% in affected herds. It is important to note that while some animals experience acute disease events, surveillance data shows that a large proportion of cattle are carriers of the organism, yet exhibit no clinical manifestations of disease or production loss. Once parasitized, cattle are considered persistently infected. Periods of stress could lead to relapse of clinical disease.


The significant number of non-clinical, test positive cattle necessitates the reminder that cattle afflicted by other common maladies may test positive for Theileria, yet may not have clinical disease caused by theileriosis. A clinical diagnosis should be accompanied by compatible clinical presentation, positive test results, and exclusion of other common differential diagnoses.


There is no approved treatment in the United States, and treatments in other countries are generally cost-prohibitive, or have significant withdrawal times. Attempted interventions should focus on supportive measures for individual cattle, and on vector control and elimination of blood-borne transmission at the herd level.


Theileria sporozoites can be transmitted via the saliva of competent tick vectors, or via direct blood transmission. Thus, implementation of control programs akin to those used in managing other blood borne pathogens, such as Bovine Leukemia Virus or Anaplasma marginale, should be useful in limiting the spread of Theileira as well. Although absolute vector exclusion is likely not possible, all practical tick control practices should be supported in effort to slow the spread of disease. Virginia Cooperative Extension has developed a list of best management practices for tick control available at: https://www.pubs.ext.vt.edu/ENTO/ENTO-382/ENTO-382.html


The Asian Longhorned Tick (Haemaphysalis longicornis) is the known vector of T. orientalis in countries outside of the U.S. While transmission by H. longicornis is not proven in the U.S., it bears reminding that the tick now appears to be endemic in Virginia and many other states.


Since initial identification, the reported ranges of T. orientalis and H. longicornis are spreading in Virginia. As this is a rapidly changing situation, please see: https://www.vetmed.vt.edu/lab-services/docs/theileria-summary.pdf for an updated map of Virginia counties affected. Spread appears efficient, and given the current distribution of H. longicornis in the U.S. along with the suitable habitat afforded by much of North America, T. orientalis Ikeda genotype is likely to establish in numerous states.


Although the Ikeda genotype was not identified in the U.S. prior to 2017, clinical theileriosis caused by T. orientalis is not recognized as a reportable foreign animal disease because of the endemic Buffeli genotype. This should be contrasted with East Coast Fever, caused by Theileria parva, which is endemic in parts of Africa and is reportable as a foreign animal disease in the United States. There are currently no restrictions on interstate movement of cattle due to T. orientalis.


VDACS and VMCVM have been proactive in identifying the prevalence of T. orientalis and H. longicornis in Virginia. Passive, randomized surveillance at livestock markets, along with active investigation of suspect cases is underway. Accredited veterinarians are an important part of this and while the disease is not reportable, we are asking for your assistance in this matter. If you suspect theileriosis please contact either VDACS or VMCVM as noted below. Personal observations of useful tick control methods and supportive care for affected animals are accepted with interest.


For suspect cases, PCR is the diagnostic test of choice, and is available at VMCVM. EDTA anticoagulated whole blood is the preferred matrix, but in cases of post-mortem evaluation, splenic samples will suffice. Samples can be chilled on ice and submitted, along with an accession form, to: Virginia Tech Animal Laboratory Services (ViTALS) 245 Duckpond Dr. Blacksburg, VA 24061

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