Trichomonads

Trichomonads are single-celled, flagellated protozoa, meaning they move using whip-like tails called flagella, and are found in humans and many animals. They belong to the order Trichomonadida, a group of anaerobic organisms that thrive in low-oxygen environments such as the intestines, oral cavity, and genital tract. Although most species coexist peacefully with their hosts, some are capable of causing infection when the host’s immune system is compromised or when local microbial balance is disturbed.

Human-Associated Species

Among humans, three species are most studied. Trichomonas vaginalis is a well-established pathogen that causes trichomoniasis, one of the most common sexually transmitted infections worldwide. It affects both men and women, increasing the risk of reproductive tract cancers, pregnancy complications, and HIV transmission. Trichomonas tenax is typically found in the mouth, especially in individuals with periodontal (gum) disease, and has occasionally been linked to respiratory infections such as empyema, a collection of pus in the chest cavity. And Pentatrichomonas hominis (P. hominis) resides mainly in the large intestine and is often considered non-pathogenic. However, emerging research suggests it may trigger diarrhea or colitis, particularly in people with weakened immune systems, such as those receiving anti-TNF therapy for autoimmune conditions.

Interactions with the Immune System

Beyond infection, recent studies suggest that trichomonads, especially P. hominis, may interact with the host immune system and microbiome in more subtle ways. Laboratory models indicate that P. hominis can promote the development of regulatory T cells (Tregs), which help suppress excessive immune responses. This raises the possibility that certain trichomonads may play a modulatory role in gut immune balance rather than purely causing disease.

Clinical Interpretation

That said, trichomonads are opportunists: they may persist harmlessly in one person but provoke inflammation in another, depending on immune status and microbial context. Their detection in stool or oral samples should therefore be interpreted cautiously. In healthy individuals, they often represent transient colonization rather than true infection. But in immunocompromised or critically ill patients, trichomonads can sometimes invade tissues or disseminate beyond their usual habitats, leading to severe outcomes like peritonitis or systemic infection.