Theileria annulata
Junlong Liu ,1
Guiquan Guan ,1
and Hong Yin 1,2,
*
1
State Key Laboratory of Veterinary Etiological Biology, Key Laboratory of Veterinary Parasitology of Gansu Province, Lanzhou Veterinary Research Institute, Chinese Academy
of Agricultural Sciences, Lanzhou, PR China
2
Jiangsu Co-Innovation Center for the Prevention and Control of Important Animal Infectious Disease and Zoonosis, Yangzhou University, Yangzhou, PR China
Mammalian hostTick stages
Sporozoites
Infection of
leukocyte
Macroshizonts
Piroplasms in
erythrocyte
Microgamete
Zygote
Sporozoites in
salivary gland
Merozoites
released from
leukocyte
Proliferation of
parasitized cells
Life cycle of Theileria annulata
Transmissionbybloodfeeding
Mammalian hostTick vector
TrendsTrendsininParasitologyParasitology
Theileria annulata is a tick-transmitted parasite that causes tropical theileriosis. It is distributed widely across
southern Europe, north Africa, and Asia, where approximately 250 million cattle are at risk of T. annulata infection.
The parasite's life cycle contains a sexual phase in tick vectors and asexual developmental stages in mammalian
hosts. After invading host leukocytes, the parasites reprogram many host signaling pathways and induce replication
via poorly understood molecular mechanisms. Within red blood cells, multiplication of T. annulata causes anemia that
could aggravate the pathology of the disease. Buparvaquone is an effective drug for treating tropical theileriosis at the
early infection stage; however, the relatively high cost limits its application worldwide. Currently, low-pathogenicity
parasites derived from infected cells in vitro are used as vaccines in many countries.
KEY FACTS:
T. annulata reproduction includes
asexual and sexual patterns. The stages
of schizogony, merogony, and sporogony
occur via an asexual model for
reproduction. Sexual reproduction occurs
in the tick gut lumen.
Infected host lymphocytes can be easily
cultured in vitro; but the replication of
cells is reversible upon drug-induced
parasite clearance.
There are three genomes: a nuclear
genome (8.35 Mb encoding 3792 genes);
a mitochondrial genome (6.6 kb); and an
apicoplast genome.
T. annulata is only transmitted by ticks of
the genus Hyalomma via transstadial
transmission.
DISEASE FACTS:
T. annulata infection occurs in large
ruminants, including cattle, yaks, water
buffalos, and camels.
Local low-producing Bos indicus cattle
are resistant to clinical disease, while
exotic high-producing European
Bos taurus breeds are susceptible.
Clinical signs of tropical theileriosis in the
early stage are fever, dyspnea, and
swelling of the superficial lymph nodes.
Anemia and icterus are more prominent
during the later stages of infection.
Detection of parasites in Giemsa-stained
red blood cells and lymph node smears
by microscopy is necessary for diagnosis,
especially for acute infection.
Animals with subclinical infections could
act as reservoirs for tick infection and
disease spread.
TAXONOMY AND CLASSIFICATION:
PHYLUM: Apicomplexa
CLASS: Aconoidasida
ORDER: Piroplasmida
FAMILY: Theileriidae
GENUS: Theileria
SPECIES: T. annulata
*Correspondence:
yinhong@caas.cn (H. Yin).
H. lusitanicum
H. dromedarii
H. anatolicum
H. excavatum
H. scupense
H. impeltatum
H. rufipes
H. marginatum
H. brevipunctata
H. isaaci
Species not identified
Vector ticks
Distribution of Theileria annulata
TrendsTrendsininParasitologyParasitology
Trends in Parasitology, March 2022, Vol. 38, No. 3 © 2021 Published by Elsevier Ltd. https://doi.org/10.1016/j.pt.2021.11.001 265
Trends in Parasitology | Parasite of the Month
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (No. 31972706), ASTIP (CAAS-ASTIP-2016-LVRI), NBCITS (CARS-37); Jiangsu
Co-innovation Center programme for Prevention and Control of Important Animal Infectious Disease and Zoonose.
Declaration of interests
The authors declare no competing interests.
Resources
www.oie.int/en/disease/theileriosis/
https://piroplasmadb.org/piro/app
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266 Trends in Parasitology, March 2022, Vol. 38, No. 3 © 2021 Published by Elsevier Ltd. https://doi.org/10.1016/j.pt.2021.11.001