Molecular Investigation of the p28 Gene of <i>Ehrlichia canis</i> in Infected Dogs From Ho Chi Minh City, Vietnam

<i>Ehrlichia canis</i> (<i>E. canis</i>) is a tick-borne, obligate intracellular bacterium that causes canine monocytic ehrlichiosis (CME), a widely distributed infectious disease in dogs with variable clinical severity. While the 16S rRNA gene has been commonly used for detection, limited data are available on the genetic diversity of the immunogenic p28 outer membrane protein gene, particularly in Southeast Asia. Eighty-three canine blood samples were collected from various districts in Ho Chi Minh City, Vietnam, and screened using polymerase chain reaction (PCR) assays targeting the 16S rRNA and p28 genes. Ten representative p28-positive samples were selected for Sanger sequencing. Phylogenetic relationships were inferred using the maximum likelihood method. Genetic variability was assessed through nucleotide and amino acid entropy analysis. Of the 83 samples, 40 (48.19%) were positive for both 16S rRNA and p28 genes. Phylogenetic analysis of p28 sequences revealed three major clades, with all Vietnamese strains clustering within clade 3 and showing a 97%-100% nucleotide identity with global references. Entropy analysis identified 50 high-variability nucleotide sites <i>H</i>(<i>x</i>): 0.206-1.013 and 20 amino acid positions <i>H</i>(<i>x</i>): 0.206-1.264, including a prominent substitution at position 145 (S ⟶ D/T), suggesting a potential immunogenic variability. This study provides the first detailed molecular characterization of the <i>p28</i> gene in <i>E. canis</i> from Vietnam, demonstrating both high genetic conservation and localized variation within clade 3. These findings enhance the understanding of regional strain diversity and may inform future diagnostic or vaccine development efforts targeting the <i>p28</i> protein.