Predatory Protist DNA Found in Human Stool and Ticks in Northern China

A microscopic predator that usually spends its life hunting other microbes in ponds and soil has turned up, in the form of DNA, inside human fecal samples and ticks collected in northern China. In a study published in Acta Parasitologica, researchers screened thousands of samples from Hebei Province and found molecular traces of Colpodella, a genus of free-living protists that sits remarkably close to the evolutionary branch containing some of medicine’s most notorious parasites, including the agents of malaria and toxoplasmosis. The findings do not demonstrate that these organisms were infecting or harming anyone, but they add a growing body of evidence that Colpodella species are more entangled with vertebrate animals, and possibly with people, than their reputation as obscure environmental microbes suggests.

The research team, led by Huan Li and Lei Ma of Hebei Normal University together with Jing Huang of Hebei Children’s Hospital, assembled an unusually large sample set for this kind of survey: 2,448 human fecal specimens and 215 ticks of the species Haemaphysalis longicornis, an aggressive, wide-ranging biting tick common across East Asia. Using polymerase chain reaction assays designed to amplify the 18S rRNA gene, a standard molecular marker for identifying eukaryotic microorganisms, the researchers screened every sample for Colpodella DNA. The 18S rRNA gene is favored in studies of protist diversity because it is present in all eukaryotes, contains both conserved and variable regions, and allows newly recovered sequences to be compared against a deep archive of reference sequences from known organisms.

The results were striking in their consistency across the two very different sample types. Colpodella DNA was detected in 136 of the human fecal samples, a detection rate of 5.56 percent, and in 11 of the tick samples, a rate of 5.12 percent. The near-identical prevalence in feces and ticks is one of the more intriguing aspects of the study, hinting at some shared route of exposure or a common environmental reservoir linking the human and tick samples, though the design of the study cannot distinguish among the many possible explanations, including ingestion of contaminated food or water, environmental pass-through, or a genuine biological association with the tick vector.

Every PCR-positive sample yielded a high-quality 18S rRNA sequence when the amplified DNA was read by Sanger sequencing, a validation step that strengthens confidence in the specificity of the detection. Because some of the tick-derived sequences were identical to one another, the researchers collapsed the data into representative sequence types, ultimately selecting one tick-derived sequence and 21 distinct human-derived sequence types for deeper analysis. These 22 representative sequences were deposited in GenBank under accession numbers QB017305 through QB017326, making them available to other researchers for comparison and future surveillance work.

Phylogenetic analysis, the technique biologists use to place newly recovered gene sequences on the tree of life, showed that all 22 representative sequences from the study clustered within previously reported Colpodella lineages. In other words, the DNA found in human stool and ticks belongs to the same evolutionary neighborhood as Colpodella species that have been described from aquatic and terrestrial habitats around the world. The sequences did not branch off into entirely novel territory, which suggests that the organisms detected in Hebei are related to known forms rather than representing a radically divergent new lineage, although species-level identification in this group remains difficult because morphology and molecular data do not always align neatly.

To probe the genetic variation within their dataset, the team performed a haplotype analysis, a method that groups sequences by their shared sets of variable sites. Among the analyzed sequences, they identified three haplotypes, indicating a modest but real degree of genetic diversity among the Colpodella organisms whose DNA was recovered from the samples. Haplotype networks and diversity statistics are standard tools in population genetics, and applying them to environmental protist DNA can reveal whether a detection reflects a single widespread genotype or a mixture of related strains circulating in the environment. In this case, the limited number of haplotypes suggests relatively constrained diversity in the sampled region, though the authors caution that broader sampling would be needed to draw firmer conclusions.

Understanding why this matters requires a short detour into evolutionary biology. Colpodella species are predatory flagellates that feed by a process called myzocytosis, in which they attach to another microbe, puncture its membrane, and suck out its cytoplasmic contents. This dramatic feeding strategy has fascinated cell biologists in part because it may resemble the evolutionary steps that gave rise to intracellular parasitism. Colpodellids are considered the free-living sister group to the Apicomplexa, the phylum that includes Plasmodium, the malaria parasite, and Toxoplasma gondii. Studying Colpodella therefore offers a window into how some of humanity’s most burdensome parasites may have evolved from free-living ancestors that once hunted their prey from the outside.

The new study also fits into a rapidly expanding literature on where Colpodella DNA shows up outside of ponds and soil. Recent surveys have reported the organism’s genetic traces in goats, foxes, and birds; in Hyalomma ticks infesting camels in Egypt; in ticks feeding on ruminants in Italy; in horses; in zoo animals; and even in the urine of a human patient. Clinically, isolated case reports have described Colpodella-like parasites in a woman in China and in a patient with neurological symptoms whose infection was linked to a potential novel tick-borne Colpodella species. A 2025 scoping review of the genus’s biological characteristics and zoonotic potential underscored how much remains unknown about whether these detections represent transient environmental exposure, colonization, or true infection.

That uncertainty is central to how the Hebei findings should be interpreted, and the authors are explicit about the limits of their method. PCR-based detection confirms only that Colpodella DNA was present in the sample; it cannot determine whether live organisms were present, whether they were actively replicating, whether they colonized the human gut or the tick, or whether they caused any disease. DNA can persist after an organism has died, and environmental contamination or passive passage through the body can both produce positive signals. Distinguishing among these possibilities will require complementary approaches, including attempts to culture the organisms, sequencing of additional genetic markers beyond the 18S rRNA gene, and biological studies of how Colpodella interacts with animal hosts.

Nevertheless, the study marks a meaningful addition to surveillance data on an understudied group of protists. The large number of human samples screened, the near-matching detection rates in feces and ticks, the successful sequencing of every positive sample, and the public deposition of 22 representative sequences collectively provide a foundation for future work. As molecular surveys continue to sweep ever more environments and host species, Colpodella keeps appearing in unexpected places, and each new detection sharpens the questions that make this genus scientifically compelling: how these predatory relatives of apicomplexan parasites move between environmental reservoirs, animals, and occasionally people, and whether any of them pose a genuine health risk. For now, the answer in Hebei Province is a well-documented molecular presence and a clear agenda for the research that must follow.

Subject of Research: Molecular detection and genetic diversity of Colpodella protist DNA in human fecal and tick samples in China

Article Title: Molecular Detection and Genetic Characterization of Colpodella spp. DNA in Human Fecal and Tick Samples from Hebei Province, China

Article References: Li, H., Huang, J., Zhang, M., Zhang, Y., Chen, Y., & Ma, L. (2026). Molecular Detection and Genetic Characterization of Colpodella spp. DNA in Human Fecal and Tick Samples from Hebei Province, China. Acta Parasitologica, 71(5), Article 234. https://doi.org/10.1007/s11686-026-01419-6

Image Credits: AI Generated

DOI: 10.1007/s11686-026-01419-6

Keywords: Colpodella, protists, Apicomplexa, Haemaphysalis longicornis, ticks, 18S rRNA, PCR detection, genetic diversity, haplotypes, Hebei Province, zoonotic potential, myzocytosis