Theileria orientalis: A newly emerging disease in India

Theileria orientalis: A newly emerging disease in India

Dr. Warsha Chaudhary^1*, Dr. Raunak Chaudhary²

¹PhD Scholar, Department of Veterinary Medicine, Post Graduate Institute of Veterinary Education and Research (PGIVER), JAIPUR, Rajasthan

²PhD Scholar, Department of Veterinary Parasitology, Indian Veterinary Research Institute (IVRI), Izatnagar, Bareilly, UP

^*Corresponding Author E. mail- warshachaudhary1234@gmail.com

Abstract

Theileriosis, a significant tick-borne disease affecting livestock in tropical and subtropical regions, is caused by protozoan parasites of the genus Theileria. Among these, Theileria orientalis, once considered benign, has emerged as a pathogenic species, particularly impacting cattle and buffalo in Asia, Australia, and the United States. Oriental theileriosis is primarily transmitted by ticks such as Haemaphysalis bispinosa, Rhipicephalus microplus, and R. annulatus in India, and it causes clinical signs including anaemia, jaundice, fever, abortion, and mortality. Pathogenic genotypes, notably Ikeda and Chitose, have been linked to recent outbreaks and economic losses.The taxonomy of T. orientalis is defined by genetic diversity in the major piroplasm surface protein (MPSP) gene, with eight recognized genotypes (types 1–8) and additional novel variants (N1–N3). Molecular studies have confirmed the distribution of T. orientalis in multiple Indian states, revealing genotype-specific prevalence and the involvement of diverse tick vectors. The parasite follows a complex life cycle involving schizogony in vertebrates and gametogony and sporogony in ticks, without host leukocyte transformation—unlike T. parva and T. annulata. The pathogenesis involves erythrocyte destruction and immune-mediated anaemia, with stress and environmental factors exacerbating disease severity. Diagnosis relies on microscopy, serology, and advanced molecular methods like PCR, which enable early and genotype-specific detection. Treatment options remain limited, with buparvaquone and oxytetracycline offering partial efficacy. Tick control through acaricides and supportive care are essential, though vaccine development is hampered by genetic diversity and the lack of robust culture systems.

Key Words- Theileriosis, T. orientalis, MPSP gene

INTRODUCTION

Theileriosis is a significant tick-borne disease that poses a major challenge to livestock development globally, especially in tropical and subtropical regions (Ica et al., 2007; Jenkins, 2018). It is caused by protozoan parasites of the genus Theileria (phylum Apicomplexa, order Piroplasmida, family Theileridae), which infect both domestic and wild ruminants (Uilenberg, 1995).

The first documented outbreak occurred in Africa in 1902 (Robertson, 1904). The disease leads to high morbidity and mortality in cattle and buffalo, reducing productivity and causing substantial economic losses. Globally, tick and tick-borne diseases (TTBDs) are estimated to cause annual losses between USD 22–30 billion (Lew-Tabor & Valle, 2016). In India alone, TTBDs result in an estimated annual control cost of USD 498.7 million (Minjauw & McLeod, 2003), with the highest impact observed in crossbred cattle (Singh et al., 2022).

Among the Theileria species, T. annulata and T. parva are highly pathogenic, while species like T. mutans and T. taurotragi typically cause subclinical infections (Jabbar et al., 2015). Theileria orientalis, previously considered benign, has emerged as a pathogenic species affecting cattle and buffalo in Asia, Australia, and other regions (Aktas et al., 2006; Aparna et al., 2011; Patial et al., 2021).

Oriental theileriosis, caused by T. orientalis, primarily affects red blood cells and is transmitted transtadially by ticks such as Haemaphysalis bispinosa and Rhipicephalus microplus in India. Recent outbreaks have been associated with anaemia, jaundice, fever, abortion, and even death (Watts et al., 2016; Yam et al., 2018). Clinical signs and hematological changes, like lymphocytosis or neutropenia, help assess disease severity and guide treatment.

Molecular diagnostic tools such as PCR can detect infections before microscopic evidence appears. The major piroplasm surface protein (MPSP) of T. orientalis is an important diagnostic and immunological marker. Variants of the MPSP gene are used to study the parasite’s genetic diversity (McFadden et al., 2011; Gebrekidan et al., 2017).

Taxonomy of T. orientalis

The taxonomy of Theileria orientalis has been studied using various molecular markers, with the major piroplasm surface protein (MPSP) gene being the most widely used (Yokoyama et al., 2011; Perera et al., 2015). Based on MPSP gene sequences, eight primary genotypes (types 1–8) have been identified globally, including Chitose (type 1), Ikeda (type 2), and Buffeli (type 3) (Sivakumar et al., 2014). Additionally, three more genotypes (N-1 to N-3) have been reported in sheep, buffalo, and cattle (Khukhuu et al., 2011).

Among these, the Ikeda and Chitose genotypes are considered pathogenic, contributing to clinical disease and economic losses in regions such as Australia, New Zealand, and India (McFadden et al., 2011). Genomic analyses have shown significant genetic divergence of the Ikeda genotype from others, suggesting its classification as a distinct species (Bogema et al., 2018).

Theileria orientalis infects a wide range of domestic and wild animals, with its prevalence highest in tropical and subtropical regions, where competent tick vectors are present. Its distribution is strongly linked to vector availability and climate conditions. The parasite has been reported in cattle, buffaloes, and yaks across Australia, New Zealand, Southeast and East Asia, South Asia, the Middle East, Africa, Europe, and the Americas, including countries like India, Pakistan, and Bangladesh (Dodd, 1910).

In Australia, T. orientalis was first identified in cattle in 1910 and initially referred to as T. mutans (Dodd, 1910). Global reviews have documented the spread and genotype distribution of this parasite across multiple regions.

Distribution of Theileria orientalis in India

Theileria orientalis has been increasingly reported across various Indian states, with molecular studies confirming its presence in both cattle and buffalo populations.

• Uttar Pradesh (2023): A novel genotype of orientaliswas identified in buffalo calves, exhibiting 84.8% similarity to the N2 genotype previously reported in Kerala. This marks the first clinical outbreak in Indian buffaloes attributed to this new genotype.
• Kerala (2015 & 2024): Fatal infections in water buffaloes were linked to the N2 genotype. Subsequent studies detected pathogenic Ikeda and Chitose genotypes in Rhipicephalus annulatusticks, underscoring the role of tick vectors in disease transmission.
• Karnataka (2024): Outbreaks in crossbred and indigenous cattle revealed the presence of multiple genotypes, including the pathogenic genotype 7 and Chitose B. High genetic diversity was noted among the isolates.
• Assam (2023): Molecular characterization confirmed the presence of the pathogenic Chitose strain in crossbred cattle, with phylogenetic analysis showing close relation to South Indian and Myanmar strains.
• Himachal Pradesh (2017): An outbreak in a Holstein-Friesian breeding farm reported a high prevalence of orientalis, with 93.3% of blood samples testing positive. The disease was associated with anemia and reproductive issues.
• Mizoram (2024): Molecular studies in slaughtered bovines detected orientalis, indicating its spread to northeastern regions of India.

Vectors of Theileria orientalis

Biological Vectors

T.orientalisis primarily transmitted through transstadial passage in ticks; transovarial transmission has not been confirmed. In India, key tick vectors include Haemaphysalis bispinosa, Rhipicephalus microplus, and R. annulatus. Although R. microplushas been considered a potential vector, recent studies suggest it may not effectively transmit T. orientalis . In contrast, Haemaphysalis longicornis is recognized as the primary vector in Australia, New Zealand, and parts of the United States.

Mechanical Vectors

Mechanical transmission occurs when blood-feeding insects transfer the parasite between hosts without involving tick vectors. Insects such as stable flies (Stomoxys calcitrans), mosquitoes, lice (Linognathus vituli), and tabanid flies have been implicated in this mode of transmission . These insects can carry infected blood in their mouthparts or digestive tracts and transmit the parasite during subsequent feedings. While mechanical transmission is less efficient than biological transmission, it may contribute to the spread of T. orientalis in areas with low tick populations.

Other Transmission Routes

• Vertical Transmission: There is limited evidence for transplacental transmission of orientalis. Studies have detected the parasite in a small percentage of newborn calves from infected dams, suggesting that vertical transmission is possible but occurs at a low rate.
• Colostral Transmission: The transfer of orientalisthrough colostrum has been considered, as maternal antibodies and possibly infected cells can be present in colostrum. However, the significance of this route in the epidemiology of the disease remains unclear.
• Iatrogenic Transmission: The reuse of contaminated needles, surgical instruments, or other equipment can facilitate the spread of orientalisbetween animals. Implementing strict hygiene practices during veterinary procedures is essential to prevent iatrogenic transmission.

Life Cycle of Theileria orientalis

Theileria orientalis exhibits a complex life cycle involving both vertebrate hosts (primarily cattle) and tick vectors. Unlike transforming Theileria species (e.g., T. parva, T. annulata), T. orientalis does not induce host leukocyte transformation.

. orientalis has an indirect life cycle involving both tick vectors and vertebrate hosts. In the tick, the parasite undergoes sexual reproduction (gametogony) in the gut, forming zygotes that develop into kinetes. These migrate to the salivary glands, where they become sporozoites. During tick feeding, sporozoites are transmitted to the vertebrate host .

In the vertebrate host, sporozoites invade leukocytes, forming schizonts, which then produce merozoites that infect erythrocytes, leading to the formation of piroplasms. Notably, T. orientalis does not transform host leukocytes, distinguishing it from other Theileria species like T. parva and T. annulata .

Key Stages:

1. Schizogony (Asexual Reproduction in Vertebrates):Sporozoites introduced by tick bites invade host lymphocytes, developing into schizonts. These schizonts undergo merogony to produce merozoites, which then infect erythrocytes, forming piroplasms .
2. Gametogony (Sexual Reproduction in Ticks):Ticks ingest piroplasm-infected erythrocytes during feeding. Within the tick’s gut, piroplasms differentiate into gametocytes, fuse to form zygotes, and develop into kinetes .
3. Sporogony (Asexual Reproduction in Ticks):Kinetes migrate to the salivary glands, where they develop into sporoblasts and subsequently into infective sporozoites. These are transmitted to the vertebrate host during the tick’s next blood meal.
4. Pathogenesis

• The pathogenicity of orientalisis genotype-dependent. The Ikeda and Chitose genotypes are associated with clinical disease, causing anemia, jaundice, and reduced productivity in cattle.
• Anemia results from both the destruction of infected erythrocytes and immune-mediated removal of uninfected ones. Oxidative damage, including elevated methemoglobin levels, contributes to erythrocyte destruction.
• Stress factors like calving or environmental changes can exacerbate the disease. Infected cattle may experience abortions, decreased milk production, and, in severe cases, death.

Clinical and Postmortem Findings of T. orientalisInfection

Clinical diagnosis of oriental theileriosis is supported by signs such as anorexia, lethargy, fever, anaemia, jaundice, pale mucosa, haemoglobinuria, increased heart/respiratory rate, nasal discharge, lymphadenopathy, depression, abortions, reduced productivity, stillbirths, and death in severe cases (McFadden et al., 2011; Eamens et al., 2013; Perera et al., 2014). Occasionally, aggression may occur due to cerebral hypoxia. These symptoms primarily result from anaemia, with packed cell volume (PCV) dropping to 8% in severe cases.

Mixed-genotype infections are common, especially with Ikeda and Chitose genotypes, while Buffeli is more often linked with subclinical cases (Eamens et al., 2013). Ikeda is the most virulent and dominant in clinical infections (Bogema et al., 2015). Disease outbreaks are more frequent when naïve cattle are introduced into endemic areas, with parasite loads peaking around 40 days post-exposure and anaemia developing 8–10 days later.

Susceptibility is influenced by age and breed. Wagyu cattle show lower clinical incidence (Higuchi et al., 1997), but calves aged 6–14 weeks may suffer high morbidity and mortality. Although maternal antibodies are present in colostrum, they wane within 4 weeks (Swilks et al., 2017). Seroconversion to the major piroplasm surface protein (MPSP) occurs ~14 days post-patency and is more common in clinical cases (89%) than subclinical (45%).

Postmortem findings include jaundice, pale and enlarged liver, kidney and spleen, haemorrhagic duodenitis, abomasal ulcers, pulmonary oedema, and enteritis (Izzo et al., 2010; Aparna et al., 2011; McFadden et al., 2011; Gebrekidan, 2017).

Endocardial haemorrhage (Aparna et al., 2011)      Haemorrhagic duodenitis. (Aparna et al., 2011)

Immune Response to T. orientalis

Cell-mediated immunity plays a primary role in defense against intracellular pathogens, while humoral immunity may prevent re-infection by blocking cell entry (Brown et al., 2006). Cattle recovering from clinical theileriosis typically develop protection against future disease episodes. However, cross-protection between different T. orientalis genotypes remains uncertain. In transforming Theileria species (T. parva, T. annulata), immunity is genotype-specific and mediated by cytotoxic T lymphocytes (CTLs), which target infected leukocytes but not erythrocytes (Radley et al., 1975). In non-transforming species like T. orientalis, similar CTL-driven responses are believed to be involved (Jenkins et al., 2015).

Diagnosis

Theileria orientalis can be diagnosed using microscopy, serology, and molecular methods:

Microscopy: Giemsa-stained blood smears reveal piroplasms within erythrocytes and help estimate parasitaemia, especially in acute infections (Aktas et al., 2006; Izzo et al., 2010). However, its sensitivity is low in subclinical cases and it cannot differentiate between genotypes or other Theileriaspecies (Perera et al., 2013; Chauhan et al., 2015).

Giemsa-stained blood smear showing piroplasms of Theileria orientalis in red blood cells of cattle

Giemsa stained blood smear showing paired pear-shaped red blood cell form of Theileria orientalis   and  intraerythrocytic rod shape of t.orintalis (1000x )  

Serology: Tests like IFAT, ELISA, and latex agglutination detect antibodies against surfaceproteins of the parasite. ELISA is commonly used for herd-level prevalence studies but cannot distinguish genotypes or differentiate between past and current infections (Mans et al., 2015).

Molecular Techniques: PCR-based methods (conventional PCR, nested-PCR, qPCR, MT-PCR), LAMP, and RLB assays provide high sensitivity and specificity. These approaches can detect and quantify parasite DNA and differentiate between pathogenic and non-pathogenic genotypes (Perera et al.,2013).

Treatment and Control of T. orientalis

• Chemotherapy
  Effective treatment of orientalisremains a challenge, with several drugs tested including primaquine, buparvaquone, imidocarb, oxytetracycline, and halofuginone, showing variable success (Watts et al., 2016). Buparvaquone is effective when used early, especially in Japan, but has concerns about drug residues and is not approved in Australia (Stewart et al., 1990; Mhadhbi et al., 2010). Imidocarb shows limited efficacy (Bailey et al., 2013). In India, combined buparvaquone and oxytetracycline reduced symptoms but did not fully clear infection (Goud et al., 2020). Drug development is costly and slow, highlighting the need for alternative control methods.
• Vector Control and Animal Management
  Controlling the tick vector Haemaphysalis longicornisis crucial. Strategies include restricting cattle movement and applying acaricides such as flumethrin and synthetic pyrethroids (Shimizu et al., 2000; Watts et al., 2016). However, acaricide resistance and the tick’s three-host lifecycle complicate control efforts. Vaccines targeting tick antigens show promise, inspired by successful Boophilus microplus vaccines and tick saliva protein trials (Willadsen et al., 1995; Mulenga et al., 1999).
• Supportive care—such as blood transfusions, iron dextran injections, intravenous fluids, and nutritional supplements—can aid recovery but are costly and mostly reserved for valuable animals (Watts et al., 2016; Nakamura et al., 2010). Stress reduction and careful animal management help prevent relapse.

Vaccine Development

• Vaccination is the preferred method to control oriental theileriosis, but currently, no vaccine exists for orientalis. Live vaccines have been effective for related species like T. parvaand T. annulata, which cause East Coast fever and tropical theileriosis, respectively (De Vos and A. J., 2011). However, these approaches are not directly applicable to T. orientalis due to its non-transforming nature and lack of cultivation methods.
• The “infect and treat” strategy used for parvashows potential but is limited by the absence of suitable drugs to manage infection in T. orientalis. Live vaccination with benign T. orientalis genotypes might offer cross-protection, but outbreaks caused by the pathogenic Ikeda genotype suggest limited cross-immunity (Onuma et al., 1997).
• Subunit vaccines targeting the major piroplasm surface protein (MPSP) showed promising results in early studies, reducing parasitaemia and clinical signs after immunisation (Onuma et al., 1997). However, genetic diversity among orientalisstrains poses challenges for subunit vaccine efficacy, with protection often strain-specific. Despite these challenges, vaccine development remains an important goal due to the significant impact of the disease in Asia and Australasia.

Conclusion

Recent outbreaks of oriental theileriosis in cattle across the Asia-Pacific and the USA have revealed that it is not always a benign infection, causing significant economic losses in dairy and beef industries. The clinical disease is mainly associated with the T. orientalis Ikeda and Chitose genotypes. Key areas such as transmission, pathogenesis, epidemiology, and control still require further study.Traditional diagnostic methods for T. orientalis have limitations in sensitivity and specificity, but these are largely addressed by molecular techniques. Conventional PCR targeting the MPSP gene is widely used for genotype identification, while other markers like 18S rRNA and ITS are also employed.

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