Abstract: Equine theileriosis, an economically important disease that affects horses and other equids worldwide, is caused by a tick-borne intracellular apicomplexan protozoa . Genotyping of based on the 18S rRNA gene revealed the presence of two, three, four or five genotypes. In previous published reports, these genotypes have been labelled either alphabetically or numerically, and there is no uniformity in naming of these genotypes. The present study was aimed to revisit the phylogeny, genetic diversity and geographical distribution of based on the nucleotide sequences of the V4 hypervariable region of the 18S rRNA gene available in the nucleotide databases. Unassigned: Out of 14792 nucleotide sequences of available in the GenBank™, only 736 sequences of containing the complete V4 hypervariable region of the 18S rRNA gene (>207 bp) were used in multiple sequence alignment. Subsequently, a maximum likelihood phylogenetic tree was constructed based on the Kimura 2-parameter model (K2+I). Unassigned: The phylogenetic tree placed all the sequences into four distinct clades with high bootstrap values which were designated as clades/ genotypes A, B, C and D. Our results indicated that the genotype B of Nagore et al. and genotype E of Qablan et al. together formed the clade B with a high bootstrap value (95%). Furthermore, all the genotypes probably originated from clade B, which was the most dominant genotype (52.85%) followed by clades A (27.58%), and C (9.78%) and D (9.78%). Genotype C manifested a comparatively higher genetic diversity (91.0-100% identity) followed by genotypes A (93.2-99.5%), and B and D (95.7-100%). The alignment report of the consensus nucleotide sequences of the V4 hypervariable region of the 18S rRNA gene of four genotypes (A-D) revealed significant variations in one region, between nucleotide positions 113-183, and 41 molecular signatures were recognized. As far as geographical distribution is concerned, genotypes A and C exhibited far-extending geographical distribution involving 31 and 13 countries of the Asian, African, European, North American and South American continents, respectively. On the contrary, the genotypes B and D exemplified limited distribution with confinement to 21 and 12 countries of Asian, African and European continents, respectively. Interestingly, genotypes A and C have been reported from only two continents, viz., North and South America. It was observed that genotypes A and C, and B and D exhibit similar geographical distribution. Unassigned: The present study indicated the presence of only four previously described T. equi genotypes (A, B, C and D) after performing the molecular analyses of all available sequences of the complete V4 hypervariable region of the 18S rRNA gene of isolates in the GenBank™.
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Overview
This study re-examines the genetic classification (genotypes) of Theileria equi, a parasite that causes disease in horses, by analyzing a specific variable gene region (V4 region of the 18S rRNA gene).
The researchers aimed to clarify the genetic diversity, phylogenetic relationships, and geographical spread of these genotypes using available genetic sequences from public databases.
Background
Equine Theileriosis: A significant tick-borne disease impacting horses and related animals worldwide.
Cause: The intracellular protozoan parasite Theileria equi.
Genotyping Importance: Understanding genotypes based on genetic markers like the 18S rRNA gene helps in epidemiology, diagnosis, and control of the disease.
Previous studies identified multiple genotypes (ranging from two to five), but there has been inconsistency in how these genotypes are named.
Study Objective
To revisit and standardize the classification of T. equi genotypes based on the nucleotide sequence of the V4 hypervariable region of the 18S rRNA gene.
To analyze genetic diversity and phylogenetic relationships for all available sequences of this gene region in global databases.
To understand the geographical distribution of the identified genotypes.
Methodology
Data Collection:
From GenBank™, 14,792 T. equi nucleotide sequences were available.
Only 736 sequences contained the complete V4 hypervariable region of the 18S rRNA gene (>207 bases) and were selected for analysis.
Sequence Alignment:
Multiple sequence alignment was performed on these 736 sequences to identify similarities and differences.
Phylogenetic Analysis:
A maximum likelihood phylogenetic tree was constructed using the Kimura 2-parameter model (K2+I) to illustrate evolutionary relationships.
Key Findings
Genotype Clustering: All sequences grouped into four distinct clades/genotypes, designated as A, B, C, and D, each with strong statistical support.
Genotype Relationships: Previously reported genotypes by other researchers (like Nagore and Qablan) corresponded to these clades, for example, Nagore’s genotype B and Qablan’s genotype E grouped together as clade B.
Genotype Origin: Clade B is likely the ancestral or most dominant genotype, representing 52.85% of sequences.
Frequency of Genotypes:
Clade B: 52.85%
Clade A: 27.58%
Clade C: 9.78%
Clade D: 9.78%
Genetic Diversity:
Clade C showed the highest genetic diversity (sequence identity between 91.0-100%).
Clades A, B, and D showed slightly lesser diversity, with identities ranging from 93.2 to 100%.
Molecular Signatures: Significant nucleotide variations were found between positions 113-183 in the V4 region, with 41 unique molecular markers identified distinguishing the four genotypes.
Geographical Distribution
Genotypes A and C have wide geographic spread:
Reported in 31 and 13 countries, respectively.
These countries span Asia, Africa, Europe, North America, and South America.
Genotypes B and D show more limited distribution:
Clade B found in 21 countries across Asia, Africa, and Europe.
Clade D found in 12 countries, also in Asia, Africa, and Europe.
Note: Genotypes A and C are the only ones reported from the American continents (North and South America).
Genotype pairs A/C and B/D tend to have similar distribution patterns, suggesting possible epidemiological linkage.
Conclusions
Four distinct genotypes of Theileria equi (A, B, C, D) exist based on the complete V4 hypervariable region analysis.
The study clarifies previous discrepancies in genotype naming and classification, offering a uniform genotyping framework.
Recognizing molecular signatures aids in rapid identification and tracking of genotypes across global regions.
The wide geographic spread of certain genotypes highlights the global significance of T. equi and the need for targeted surveillance.
Cite This Article
APA
Nehra AK, Kumari A, Moudgil AD, Vohra S.
(2024).
Revisiting the genotypes of Theileria equi based on the V4 hypervariable region of the 18S rRNA gene.
Front Vet Sci, 11, 1303090.
https://doi.org/10.3389/fvets.2024.1303090
Department of Veterinary Parasitology, Lala Lajpat Rai University of Veterinary and Animal Sciences, Hisar, Haryana, India.
Kumari, Ansu
Department of Veterinary Medicine, Lala Lajpat Rai University of Veterinary and Animal Sciences, Hisar, Haryana, India.
Moudgil, Aman Dev
Department of Veterinary Parasitology, Lala Lajpat Rai University of Veterinary and Animal Sciences, Hisar, Haryana, India.
Vohra, Sukhdeep
Department of Veterinary Parasitology, Lala Lajpat Rai University of Veterinary and Animal Sciences, Hisar, Haryana, India.
Conflict of Interest Statement
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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