Trichomonads are single-celled, anaerobic protozoans with multiple flagella (typically 4–6). Trichomonas vaginalis (Tv) is a parasitic trichomonad which causes trichomoniasis (aka trich). Tv is the most common of the 3 trichomonads that infects humans.
Sexually transmitted, Tv resides in the urogenital system, with the vagina its preferred domicile: a warm, moist, inviting place with plentiful nutrition. Plus, men are a convenient carrier to the next vagina.
Once inside, the pear-shaped parasite paddles to an ideal spot on the lining of the vagina or cervix, where it sprawls out, amoeba-like. Tv then helps itself to a banquet of host cells. (Surprisingly, ~70% of women and men do not have symptoms when infected. Symptoms typically begin 5–28 days after exposure. Symptoms include itching, stinky vaginal discharge, burning when urinating, and pain during sex.)
Tv doesn’t work alone. Other microbes living in the vagina, and some inside the parasite itself, get into the fray.
Host immune response can be problematic. Tv kills T and B cells by poisoning them.
Neutrophils cope better. Neutrophils are immune system first-responders that attempt to engulf invaders. The problem in attacking Tv is that the protozoan is larger than a neutrophil. So, 3–6 neutrophils mob Tv and nibble it to death: a process called trogocytosis. Tv typically succumbs with 3–8 bites.
Tv’s preemptive countermove is to nestle into the vagina so that it cannot be surrounded. To do so, Tv peppers vaginal cells with tiny bubbles of proteins and RNA which prime host cells for parasite attachment by altering their surface membrane.
The vagina is home to a bacterial microbiome which is dominated by Lactobacillus in healthy women. The vagina feeds its lactobacilli. In return, the bacteria excrete an acidic substance that prevents many disease-causing microbes from taking hold of host cells.
Tv treats Lactobacillus as a threat: eating them and other protective bacteria while cultivating microbes amenable to mischief, which includes cooperating with Tv.
Tv also hosts its own helpful microbes: species of Mycoplasma bacteria and an assortment of viruses called TVVs (Tv-virus). TVVs assist Tv in sticking to host cells. TVVs and Mycoplasma also magnify inflammation and infection symptoms, thereby sowing confusion in the immune system as to the source of the infection.
Tv thrives when it has friends: showing more energy and growing faster. The mutualism improves nutrient collection, most notably arginine. Immune cells need arginine to make the nitric oxide which kills infectious microbes. By eating the arginine, Tv’s bacterial helpers thwart the immune system by removing the pesticide supply.