Animal Theileriosis Overview, Species, and Control

Definition

• • • Group of tickborne diseases

    • Caused by protozoan parasites of the Theileria genus

  • Hosts

    • Domestic and wild ungulates

    • Cattle

    • Sheep

    • Horses

    • Goats

    • African buffalo (carriers of T. parva)

  • Geographic Distribution

    • Tropical and subtropical regions

    • Occurrence linked to geographic distribution of tick vectors

  • Relation to Babesia

    • Both members of suborder Piroplasmorina

    • Babesia primarily parasites of RBCs

    • Theileria use successively WBCs and RBCs

    • Transovarial transmission occurs in Babesia but not Theileria

  • Life Cycle and Pathogenesis

    • Infective sporozoite stage transmitted in tick saliva

    • Sporozoites invade leukocytes (WBCs)

    • Develop to schizonts within a few days

    • Pathogenic species (T. parva, T. annulata) multiply predominantly in host WBCs

    • Less pathogenic species multiply mainly in RBCs

    • Schizonts cause host WBCs to divide, parasite also divides

    • Parasitized cell population expands and disseminates throughout lymphoid system

    • Later in infection, some schizonts undergo merogony

    • Merogony releases merozoites

    • Merozoites infect RBCs, giving rise to piroplasms

    • Uptake of piroplasm-infected RBCs by feeding ticks

    • Complex development cycle in tick

    • Transmission by ticks feeding in next instar (trans-stadial transmission)

    • Occult phase (5-10 days) before infected lymphocytes detected

    • Rapid increase of parasitized cells in lymphoid system

    • Merogony and piroplasms detected from ~day 14 onward

    • Coincides with progressively severe lymphocytolysis, lymphoid depletion, leukopenia

    • Disease severity varies (inapparent to fatal)

  • Specific Theilerioses

    • In Cattle

      • East Coast Fever

        • Caused by Theileria parva

        • Acute disease

        • Usually characterized by high fever, lymph node swelling, dyspnea, high mortality

        • Serious problem in east and southern Africa

        • Transmitted by Rhipicephalus appendiculatus ticks

        • Ticks acquire infection from infected cattle or African buffalo

        • Cattle- and buffalo-derived T. parva are highly pathogenic to cattle

        • Buffalo-derived T. parva (Corridor disease) differ by lower schizonts and no piroplasms

        • Clinical signs: Fever (7-10 days post-infection, >107°F), pronounced/generalized lymph node swelling, anorexia, rapid condition loss, lacrimation, nasal discharge, terminal dyspnea, sharp temperature decrease before death, pulmonary exudate from nostrils

        • Death usually occurs 18–24 days after infection

        • Postmortem lesions: Generalized lymph node enlargement, massive pulmonary edema and hyperemia, hemorrhages, necrosis in lymph nodes/thymus

        • Anemia is not a major diagnostic sign

        • Recovered animals are immune to same strains

        • Most recovered/immunized animals remain carriers

      • Tropical Theileriosis

        • Causal agent: T. annulata

        • Widely distributed in north Africa, Mediterranean coast, Middle East, India, southern former USSR, Asia

        • Transmitted by Hyalomma ticks

        • Can cause mortality up to 90%

        • Strains vary in pathogenicity

        • Kinetics and clinical features similar to T. parva

        • Anemia is often a feature (unlike East Coast fever)

        • Characteristic signs: Fever, swollen superficial lymph nodes, rapid condition loss

        • Recovered animals immune to subsequent challenge

      • Other Theilerioses of Cattle

        • Theileria orientalis group (T. orientalis, T. buffeli, T. sergenti)

          • Worldwide distribution

          • Transmitted by Haemaphysalis ticks

          • Piroplasms larger than T. parva and T. annulata

          • Multiply principally by intraerythrocytic division

          • Usually nonpathogenic

          • Can cause disease with anemia and occasional mortality (usually in dairy cattle)

          • Outbreaks of more severe disease caused by one subtype in Australia and New Zealand

        • T. mutans and T. velifera

          • Found in Africa

          • Transmitted by Amblyomma ticks

          • Multiplication mainly by intraerythrocytic division

          • Piroplasms morphologically indistinguishable from T. orientalis

          • Differentiated by serology (indirect fluorescent antibody) and DNA typing

          • Some strains of T. mutans are pathogenic

          • Concurrent infection may add to T. parva pathogenicity

    • In Sheep and Goats

      • Theileria lestoquardi

        • Causes disease in sheep and goats

        • Disease similar to T. annulata in cattle

        • Closely related to T. annulata

        • Transmitted by Hyalomma ticks

        • Similar geographical distribution to T. annulata

        • Mortality can approach 100%

        • Schizonts readily demonstrated in lymph node biopsies

      • T. luwenshuni and T. uilenbergi

        • Cause severe disease in sheep in China

        • Morphologically indistinguishable

        • Cause similar disease

        • Distinguished by DNA typing

        • Transmitted by Haemaphysalis ticks

        • Schizonts detected in range of tissues, later and in smaller numbers than other pathogenic species

        • Piroplasms consistently detected in RBCs

        • Morbidity and mortality up to 65% (T. luwenshuni) and 75% (T. uilenbergi) in susceptible animals

        • Affected animals show sustained fever and anemia

      • Other nonpathogenic species (T. ovis)

        • Widely distributed

        • Piroplasms are polymorphic

    • In Horses

      • Theileria equi

        • Formerly Babesia equi

        • Reclassified based on DNA analysis and development in leukocytes before erythrocytes

        • Infection often asymptomatic

        • Can result in outbreaks with fever, anemia, lethargy

        • Can lead to death

        • Schizont-infected leukocytes observed in lymphoid tissues

        • Disease largely attributable to destruction of erythrocytes

        • Thrombocytopenia and hemorrhage may occur

  • Diagnosis

    • Based on clinical signs

    • Detection of parasites

      • In lymph node aspirates

      • In stained blood smears (piroplasms)

    • Serology (value in detecting previous infection in recovered animals)

    • Microscopic examination of Giemsa-stained smears

      • Lymph node needle aspirates (for schizonts in leukocytes)

      • Stained blood smears (for piroplasms in RBCs)

      • Piroplasms: small, rod-shaped or oval

      • Schizonts and piroplasms of T. parva and T. annulata are morphologically similar

    • Definitive diagnosis:

      • Antigen-specific ELISAs

      • PCR on lymph node aspirates

  • Treatment and Control

    • Treatment

      • Only a single compound available: Buparvaquone

      • Often accompanied by anti-inflammatory drugs and antidiuretics (if pulmonary edema)

      • Effective in early stages

      • May require more than one dose

      • Less effective in advanced stages (extensive tissue destruction)

      • Development of resistance reported for T. annulata

    • Prevention

      • Prevention of tick infestation

        • Spraying or dipping animals with acaricides

        • Needs regular intervals

        • Pyrethroid compounds used where challenged with tickborne diseases and trypanosomes

      • Vaccination

        • T. parva: Infection-and-treatment procedure

          • Cryopreserved sporozoite stabilate of infected ticks (appropriate strain(s))

          • Single dose of long-acting oxytetracycline given simultaneously

          • Oxytetracycline inhibits parasite development at the outset of infection

        • T. annulata: Live vaccines using in vitro-cultivated parasitized bovine cells (schizont stage)

          • Cells held as cryopreserved stock

          • Approximately one million cells administered subcutaneously

          • Parasitized cells need prolonged passage in vitro for attenuation

        • Cattle should be immunized 3-4 weeks before being on infected pasture

      • Innate resistance in some indigenous cattle in endemic areas