Discussion
The average size of Trichomonas tenax is 7.1x4.7 µm. The parasite possesses four anterior flagella, which are often divided into two groups. The posterior flagellum is as long as the undulatory membrane. The undulatory membrane is shorter than the body, and the axial column is slender; most of the nuclei contain stained plasmids with a dark particle color (11). Parasites in the oral cavity, dental plaque and dental cavities, as well as trophozoites, spread through direct or indirect contact. Parasites often coexist with periodontal diseases such as gingivitis (12).
Trichomonas tenax parasitizes the oral cavity, dental plaque and cavities of the teeth. The parasites feed on bacteria, live in a slightly aerobic environment and are often accompanied by a large number of Clostridium and spirochetes. At present, the infection of lower respiratory tract caused by Trichomonas tenax is considered to be by inhalation via the oropharynx (13).
To the best of our knowledge, a report on oral diseases caused by oral Trichomonas infection exists (14), but few reports discuss infections affecting the lungs or other organs. According to the literature, Trichomonas tenax has been detected in the pleural effusion fluid in only 9 reported cases since 1966 (11,13,15-21). These patients included patients with various systemic diseases requiring immunosuppressive therapy, as well as patients with malignant tumors, as detailed in Table I. In the lungs, the proliferation of Trichomonas tenax appears to be facilitated by the presence of bacteria (13). As indicated in Table I, all 9 patients had concurrent infections of Trichomonas tenax with bacterial or fungal species, suggesting that Trichomonas tenax may utilize aerobic or anaerobic bacteria as a food source (11). Treatment with metronidazole was administered to all patients, resulting in mostly favorable outcomes; however, metronidazole is not without potential side effects (15-17) in humans.
The present study shares one case in which Trichomonas tenax was detected in a patient with pleural effusion. Pathogen metagenomics sequencing (PMseq) provides a direct and high-throughput method for sequencing infected samples, allowing comprehensive detection of microorganisms in clinical samples through detailed reports. Initially, PMseq (BGI Group) was conducted on pleural effusion samples, but Trichomonas tenax was not identified in the initial results. Some oral bacteria, including Prevotella multiformis, Parvimonas micra, Fusobacterium nucleatum, Porphyromonas endodontalis, Dialister pneumosintes, Olsenella uli and Mogibacterium timidum, were detected. At the same time, a large pneumothorax in the right pleural cavity was detected. Therefore, it is speculated that the Trichomonas in the patient's pleural effusion was likely inhaled into the lower respiratory tract from the oropharynx, which led to the aforementioned clinical symptoms. Clinically, there are also reports that most cases of pulmonary infection caused by Trichomonas are complicated with bacterial infection, which further supports the speculation that the Trichomonas tenax is most likely to come from the pharynx.
Upon consultation with technical support, it was determined that the sequencing database (Shenzhen Huada Medical Inspection Laboratory) did not include Trichomonas tenax. Subsequent reanalysis of the data revealed 306 reads corresponding to Trichomonas tenax in the non-human database. This underscores the importance of selecting an appropriate database, as the choice of database can significantly impact the accuracy of the final results.
The patients in the 9 literature cases were administered metronidazole (2 g intravenously per day) four times a day for 7 days. In some cases, metronidazole (1,000 mg) was administered orally twice a day for 5 days. In the present study, the presence of trichomonads was negative from day 3 of the institution's metronidazole therapy. However, this patient's mixed infection status was serious, with bacterial infection in the early stage and fungal infection in the later stage, and the prognosis was poor.
Quick and accurate identification is crucial for effective infection control. Trichomoniasis, which has long been neglected in scientific research, is now gaining renewed attention regarding its pathogenesis (22). Variations in the host-parasite interaction may involve human polymorphisms and environmental factors (23).
There is limited research available on Trichomonas tenax. In the present study, the parasite was identified via molecular methods. Among the diagnostic techniques available in laboratories, culture and molecular assays are considered the most effective for detecting parasites (24). Culture is typically the gold standard for identifying pathogenic microbes in infections (25), despite its drawbacks, such as low sensitivity and specificity, and time-consuming processes. By contrast, PCR has greatly advanced microbial identification due to its high sensitivity and specificity. PCR allows for quick determination of a microbial presence in small samples (12). It is recommended that future studies employ molecular methods, as relying solely on microscopy can lead to underestimation of the true prevalence of oral parasites. Further research is needed to explore the molecular epidemiology of these oral parasites.
Oral Trichomonas species predominantly inhabit anaerobic environments, such as gingival crevicular spaces (26). When the host's immunity decreases, it creates a environment that is conducive to the proliferation of pathogenic bacteria, facilitating the growth and reproduction of Trichomonas tenax. The protozoa metabolize host epithelial cell glycogen through glycolysis to obtain energy, further promoting the growth of pathogenic bacteria. Consequently, patients, especially those undergoing chemotherapy or radiotherapy for tumors, may experience concurrent oral Trichomonas infections alongside various pathogenic bacteria. Chemotherapy and radiotherapy can induce side effects such as neutropenia and compromised humoral and cellular immunity, which increase susceptibility to severe bacterial, viral and parasitic infections (12,27).
The potential pathways for Trichomonas tenax infection can be defined on the basis of published literature on pulmonary Trichomonas tenax (13,28) as follows: i) Primary or secondary immunocompromised conditions (29), including but not limited to solid tumors, hematological malignancies, rheumatic diseases, use of immunosuppressive agents, long-term systemic corticosteroid use, and transplantation of solid organs and hematopoietic stem cells. ii) Oral/periodontal diseases, risk factors for aspiration and any procedure of a therapeutic or diagnostic nature related to the upper thorax or esophagus. The third risk factor is with regard to Trichomonas tenax infection in the lungs, given the residence of Trichomonas tenax in the human oral cavity.
The results of the study by Kooshki et al revealed a high frequency of oral cavity parasites in children who were diagnosed with malignancies or who were receiving treatment with chemotherapy in Lorestan Province, Iran (3). Renal disorders increase the susceptibility of patients to infections, including those caused by oral cavity infections (28).
The present case involved a postoperative patient with lung cancer, and in association with the immune status of the patient, the patient had poor immunity, which provides an opportunity for pathogens, including oral Trichomonas and other potential co-infectious pathogens (such as bacteria, fungi and viruses), to escape immunity. In addition, it is worth considering whether lung cancer surgery provides opportunities for oral Trichomonas to enter the lower respiratory tract, and whether there is the possibility of open communication between the digestive system and chest cavity of patients with lung cancer. Finally, chest CT on admission revealed a large pneumothorax in the right pleural cavity, which provided an opportunity for the oral Trichomonas to enter the lungs.
As a result, oncologists and dental practitioners need to be vigilant in managing oral health issues in patients susceptible to various infections, with particular emphasis on those undergoing cancer treatment. Laboratory professionals should maintain vigilance and careful observation to avoid overlooking positive indicators of infections.