Hemoplasmas, short for hemotropic mycoplasmas, are wall-less bacteria belonging to the class Mollicutes. Unlike most bacteria, they cannot be grown easily in the laboratory, which has long hampered efforts to study them. Instead, scientists rely on molecular techniques, chiefly the polymerase chain reaction, or PCR, to detect their genetic material in blood samples. Two species are known to infect cattle worldwide: Mycoplasma wenyonii, often abbreviated Mw, and a bacterium currently designated “Candidatus Mycoplasma haematobovis,” formerly known as “Candidatus Mycoplasma haemobos.” The Candidatus designation is applied by microbiologists to organisms that can be characterized by molecular methods but have not yet been cultivated or fully formally described.

Infections with these organisms are often subtle. Some animals show no obvious signs of disease, while others may develop anemia, fever, weight loss, reduced milk yield, or swelling of the limbs and teats. Because the clinical picture can be vague and the bacteria cannot be cultured routinely, hemoplasma infections in cattle are frequently overlooked, and their economic impact on dairy production is difficult to quantify. In tropical countries such as Thailand, where dairy farming is an important agricultural sector and arthropod vectors such as flies and ticks are abundant year-round, the question of what these bacteria are doing to herd health takes on added urgency.

To address this, a team led by Kritsada Thongmeesee of Chulalongkorn University in Bangkok, working with colleagues at Prince of Songkla University, Vongchavalitkul University, and the University of Melbourne in Australia, collected blood samples from 522 dairy cattle across Thailand. The animals were screened using a PCR assay that targets the 16S ribosomal RNA gene, a workhorse of bacterial identification. This gene, which encodes part of the small subunit of the bacterial ribosome, contains regions that are conserved across all bacteria as well as variable stretches that allow researchers to distinguish between closely related organisms. The team also examined the 23S ribosomal RNA gene, which encodes part of the large ribosomal subunit and provides an independent set of variable positions for comparison.

The screening produced a striking result: 161 of the 522 cattle, or 30.8 percent, tested positive for hemoplasma DNA. The 95 percent confidence interval for this estimate ranged from 26.9 to 35.0 percent, meaning that roughly one in three animals in the sampled population was carrying at least one of these blood-dwelling bacteria. That prevalence figure alone underscores how common hemoplasmas are in Thai dairy herds, even if most infections remain clinically silent or undiagnosed.

From the pool of positive samples, the researchers selected 52 for sequencing of both the 16S and 23S rRNA genes. The selection was not random; the team prioritized samples that had produced strong, discrete PCR bands, which are more likely to yield clean sequence data. While this approach maximizes the quality of the genetic information obtained, it also means the 52 sequenced samples represent a curated subset rather than a statistical sample of all infections, a caveat the authors acknowledge in their interpretation.

When the sequences were subjected to phylogenetic analysis, a method that reconstructs evolutionary relationships by comparing genetic differences, the results revealed a picture more complicated than the standard two-species model. As expected, some sequences clustered with Mycoplasma wenyonii and others with “Candidatus Mycoplasma haematobovis.” But in addition to these two recognized taxa, the analyses identified two further lineages that were clearly divergent, sitting on separate branches within what the researchers describe as the M. wenyonii complex. These lineages are distinct enough in their ribosomal RNA sequences to suggest they may represent previously unrecognized organisms, yet the team has deliberately refrained from naming them as new species.

That restraint reflects a growing consensus in bacterial taxonomy. Ribosomal RNA genes are powerful tools for detection and preliminary classification, but they capture only a small fraction of a bacterium’s genome, and organisms with similar rRNA profiles can nonetheless differ substantially in their metabolism, virulence, and ecology. The authors therefore regard the two divergent lineages as candidate lineages, a provisional status that will stand until whole-genome sequencing, additional independent genetic markers, and biological characterization, including studies of pathogenicity and transmission, can establish whether they truly deserve species-level recognition. This cautious approach avoids inflating the taxonomy of a group that is already difficult to study while flagging the lineages for future investigation.

The discovery has practical implications for veterinary medicine and livestock management in the region. If multiple genetically distinct hemoplasmas circulate in Thai dairy cattle, then diagnostic tests designed to detect one species may miss others, and studies that lump all hemoplasma infections together may obscure differences in disease potential between the lineages. Understanding which lineages cause illness, how they are transmitted, whether arthropod vectors play a role, and how they respond to antimicrobial treatment will require the kind of genomic and biological work the authors call for. It also raises questions about whether the divergent lineages are confined to Thailand or circulate more widely in cattle and buffalo populations across Asia and beyond, having simply gone undetected because most surveys rely on short diagnostic fragments rather than comparative sequencing of multiple genes.

For now, the study stands as a reminder that even well-known pathogens can harbor hidden diversity when examined closely. A routine prevalence survey of dairy cattle, conducted with standard molecular tools and a careful two-gene approach, was enough to double the number of recognized lineages within the bovine M. wenyonii complex. As sequencing becomes cheaper and whole-genome approaches reach veterinary microbiology, researchers expect more such surprises, not only in cattle but across the many mammalian species that host these elusive red blood cell parasites. The Thai findings provide a foundation for that work and a caution against assuming that the hemoplasmas infecting a herd are always the ones in the textbooks.

Subject of Research: Genetic diversity of hemotropic Mycoplasma lineages in dairy cattle in Thailand

Article Title: Two divergent hemoplasma lineages within the Mycoplasma wenyonii complex in dairy cattle from Thailand revealed by comparative analyses of partial 16S and 23S rRNA gene sequences

Article References: Thongmeesee, K., Tiyamanee, W., Narapakdeesakul, D., Kamkong, P., Wechtaisong, W., Colella, V., & Tiawsirisup, S. (2026). Two divergent hemoplasma lineages within the Mycoplasma wenyonii complex in dairy cattle from Thailand revealed by comparative analyses of partial 16S and 23S rRNA gene sequences. Parasites & Vectors. https://doi.org/10.1186/s13071-026-07678-6

Image Credits: AI Generated

DOI: 10.1186/s13071-026-07678-6

Keywords: hemoplasmas, Mycoplasma wenyonii, Candidatus Mycoplasma haematobovis, dairy cattle, Thailand, 16S rRNA gene, 23S rRNA gene, molecular diagnostics, phylogenetics, bacterial taxonomy, livestock health, Parasites & Vectors