Trichomonas vaginalis. Trichomonas tenax. Giardia lamblia (Lamblia intestinalis)

Trichomonas vaginalis. Trichomonas tenax. Giardia lamblia (Lamblia intestinalis)

Three flagellates that live on our surfaces and never invade them, Trichomonas vaginalis in the urogenital tract, Trichomonas tenax in the mouth and Giardia lamblia in the small intestine, how each is built and passed on, and the diseases two of them cause.

Trichomonas: the trophozoite
On the left, a vaginal epithelial cell, a flat many-sided cell with a small nucleus, with three trophozoites flattened on it, amoeboid, their flagella at their front, and a fourth, pear-shaped, swimming above it. In the middle, Trichomonas vaginalis drawn large, its broad anterior end up: a large oval nucleus in the anterior end, a row of basal bodies just in front of it, four flagella free from the anterior end, a fifth running back along the edge of the undulating membrane on one side, which reaches about halfway down the body, with the costa, a rod, along its base, the axostyle, a glassy rod, down the middle and out through the posterior end as a spike, and small round hydrogenosomes along the axostyle and the costa. On the right, Trichomonas tenax, smaller, its undulating membrane two-thirds of the body. The flagella beat and the membranes ripple.

Point at a name or a part to light it.

Trichomonas lives only as a trophozoite, with no cyst. T. vaginalis is pear-shaped, 7 to 30 µm long, on average 10 by 7 µm: four flagella free from its anterior end, a fifth along the edge of an undulating membrane that reaches about halfway down the body, and an axostyle that runs through it and out of its posterior end. Attached to the vaginal epithelium, it flattens and turns amoeboid. T. tenax, in the mouth, is smaller, 5 to 16 µm, its membrane two-thirds of the body.

Simplified: not to scale: T. vaginalis drawn large, T. tenax smaller beside it, and the trophozoites on the epithelial cell far larger than they would be; most organelles left out: the Golgi, the pelta and the endoplasmic reticulum, and the hydrogenosomes far fewer and larger than they are; the attached trophozoites drawn simply, without their undulating membranes; T. tenax drawn as a smaller T. vaginalis with a longer membrane; its other small differences left out; the beat played far slower than life, as simple waves drawn flat on the page.

4 sources
  1. Centers for Disease Control and Prevention. DPDx: Trichomoniasis. Last reviewed 7 June 2024.
  2. Petrin D, Delgaty K, Bhatt R, Garber G. Clinical and microbiological aspects of Trichomonas vaginalis. Clin Microbiol Rev. 1998;11(2):300–317.
  3. Cheon SH, Kim SR, Song HO, Ahn MH, Ryu JS. The dimension of Trichomonas vaginalis as measured by scanning electron microscopy. Korean J Parasitol. 2013;51(2):243–246.
  4. Matthew MA, Yang N, Ketzis J, Mukaratirwa S, Yao C. Trichomonas tenax: a neglected protozoan infection in the oral cavities of humans and dogs, a scoping review. Trop Med Infect Dis. 2023;8(1):60.

At a glance

  1. Trichomonas and Giardia are flagellates that live on the surface of a tract, the urogenital tract, the mouth or the small intestine, and do not invade the tissue.
  2. Trichomonas has only a trophozoite, with no cyst, so it passes straight from person to person: T. vaginalis by sexual intercourse, T. tenax in saliva.
  3. Trichomoniasis is the most common non-viral sexually transmitted infection. Most infected people have few or no symptoms; women may have a frothy, yellow-green discharge and a strawberry cervix.
  4. Giardia has a trophozoite, with two nuclei, eight flagella and a ventral disc, and a cyst, with four nuclei, which is swallowed in water or food or from hands.
  5. Giardiasis brings diarrhoea, pale, greasy stools, bloating and malabsorption. Both infections are treated with a nitroimidazole, metronidazole or tinidazole, and with trichomoniasis the partner too.

Trichomonas, flagellates without a cyst

Two species of Trichomonas live in people: T. vaginalis in the urogenital tract and T. tenax in the mouth. They look much alike: a pear-shaped cell with a large nucleus in its broad anterior end, flagella in front, and an undulating membrane along one side. Neither has a cyst. Each lives only as the trophozoite, which multiplies by longitudinal binary fission where it lives and soon dies outside the body. A third trichomonad, Pentatrichomonas hominis, lives harmlessly in the large intestine.

T. vaginalisT. tenax
Wherethe vagina and urethra in women, the urethra and prostate in menthe mouth: gum pockets, plaque and tartar
Size7 to 30 µm long, on average 10 by 7 µmsmaller, 5 to 16 µm long
Undulating membraneabout half the length of the bodyabout two-thirds
Passed onby sexual intercoursein saliva: kissing, droplets, shared cups and dishes
Diseasetrichomoniasisnone proven

The parts to know, in T. vaginalis:

  • Flagella: five, all growing from basal bodies in the anterior end. Four are free and beat ahead of the cell; the fifth, the recurrent flagellum, runs back along the edge of the undulating membrane.
  • Undulating membrane: a fin of the cell's surface, held out by the recurrent flagellum and supported at its base by the costa, a slender rod that does not contract. Its ripple helps drive the cell in a jerky, wobbling swim.
  • Axostyle: a glassy rod of microtubules that starts at the nucleus, runs down the middle of the cell and comes out at the posterior end as a spike.
  • Hydrogenosomes: Trichomonas is an anaerobe: it makes its ATP without using oxygen, and has no mitochondria. In their place, small round organelles with a double membrane make ATP and give off hydrogen. They also activate metronidazole, the drug that kills it.

Swimming free, the trophozoite is pear-shaped. Once it holds on to a vaginal epithelial cell it flattens and turns amoeboid, spreading over the cell's surface.

Trichomonas: the life cycles
A map of the two trichomonads' life cycles, each between two people drawn as pale panels with a figure at the outer edge. Across the top, Trichomonas vaginalis: trophozoites multiply by binary fission in a woman's vagina and urethra, and sexual intercourse passes them to a man, in whose urethra and prostate they live, often without symptoms, and back again. Across the foot, Trichomonas tenax: trophozoites in the gum pockets and on the tartar of one mouth pass in saliva, by kissing, droplets or shared dishes, to another mouth, and back. Neither has a cyst: each passes straight from person to person.

Point at a name to light its pathway.

The trichomonads have one stage, the trophozoite, and no cyst, so they die soon outside the body and pass straight from one person to the next: T. vaginalis by sexual intercourse, T. tenax in saliva. People are the only host of T. vaginalis; T. tenax is also found in dogs, cats and other animals.

Simplified: a map, not anatomy: the people drawn as icons and the trophozoites drawn small, nothing to scale; only the main routes drawn: T. vaginalis also passes between women, and rarely from mother to baby at birth; T. tenax also on shared dishes and in droplets.

3 sources
  1. Centers for Disease Control and Prevention. DPDx: Trichomoniasis. Last reviewed 7 June 2024.
  2. Petrin D, Delgaty K, Bhatt R, Garber G. Clinical and microbiological aspects of Trichomonas vaginalis. Clin Microbiol Rev. 1998;11(2):300–317.
  3. Matthew MA, Yang N, Ketzis J, Mukaratirwa S, Yao C. Trichomonas tenax: a neglected protozoan infection in the oral cavities of humans and dogs, a scoping review. Trop Med Infect Dis. 2023;8(1):60.

The cycle is a single step. Trophozoites multiply in one person and pass to the next, directly: with no cyst to survive outside the body, T. vaginalis depends on sexual contact, and T. tenax on saliva. People are the only host of T. vaginalis; T. tenax is also found in dogs, cats and other animals.

Trichomoniasis

Trichomoniasis is the most common non-viral sexually transmitted infection in the world, with an estimated 156 million new cases in 2020 among people aged 15 to 49. In women the parasite lives in the vagina and urethra; in men, in the urethra and the prostate.

Most infected people, 70 to 85 per cent, have few or no symptoms, nearly all infected men among them, so they pass the parasite on without knowing.

  • In women: a vaginal discharge, yellow-green or grey, often frothy, profuse and foul-smelling, with itching of the vulva, pain on passing urine and pain during sex. The vagina is red and inflamed, and the cervix may be covered in small haemorrhagic spots, the strawberry cervix.
  • In men: usually nothing; at times a urethritis, with discharge and pain on passing urine, and more rarely epididymitis or prostatitis.

Trichomoniasis matters beyond its symptoms:

  • In pregnancy it makes preterm birth about 1.4 times as likely, and is linked to low birth weight and early rupture of the membranes.
  • It makes HIV about 1.5 times as likely to be caught, and in women with HIV it raises the risk of pelvic inflammatory disease.

Diagnosis

  • Nucleic acid amplification tests (NAATs), on a vaginal swab in women and on urine in men, are the tests of choice: they find more than 90 per cent of infections.
  • A wet mount, a drop of discharge seen at once under the microscope, shows the jerking, swimming trophozoites, but it misses about a third to a half of infections in women and more in men. A negative wet mount does not rule it out.
  • Culture finds more than the wet mount but takes days. Rapid antigen tests can be done at the bedside.

Treatment and prevention

The treatment is a nitroimidazole by mouth:

  • Metronidazole. US guidance gives women 500 mg twice a day for 7 days and men 2 g as a single dose; UK practice gives everyone 400 mg twice a day for 7 days, the single 2 g dose being less effective. A gel put in the vagina does not reach the urethra and is not enough.
  • Tinidazole, 2 g as a single dose, is the alternative. It is avoided in pregnancy, while the 7-day course of metronidazole can be given at any stage of it.

Every sexual partner is treated at the same time, whatever their symptoms, and sex is avoided until both have finished treatment, for at least a week; otherwise the infection simply passes back. Reinfection is common: US guidance tests women again about 3 months later, while UK practice tests again only if symptoms persist or come back. In 4 to 10 per cent of cases the parasite is less sensitive to metronidazole, and higher doses or tinidazole are used. Condoms, used every time, prevent it.

Trichomonas tenax

Trichomonas tenax lives in the mouth, in the pockets between the gums and the teeth and on plaque and tartar. It is found far more often where the gums are diseased and the mouth is poorly cared for. Whether it helps cause gum disease or simply thrives in it is not settled. It passes from mouth to mouth in saliva, by kissing, in droplets and on shared cups and dishes, and is also found in dogs.

Giardia lamblia

Giardia lamblia is the same organism as Giardia intestinalis and Giardia duodenalis, and the older Lamblia intestinalis: the names are synonyms. It lives in the small intestine, is found worldwide, and is the commonest parasitic infection in travellers returning to the UK. Unlike Trichomonas it has two forms:

TrophozoiteCyst
What it doeslives and multiplies in the small intestinesurvives outside the body and infects
Shape and sizepear-shaped and two-sided, 10 to 20 µm longoval, 8 to 19 µm long, with a tough, two-layered wall
Nucleitwo, side by sidefour when mature
Other partseight flagella in four pairs, a ventral disc, median bodies, axonemesthe axonemes and median bodies of the trophozoite it came from
Giardia: the trophozoite and the cyst
On the left, the Giardia trophozoite seen from its ventral side, drawn large, its broad round anterior end up and tapering to a narrow tail: a pale oval ventral disc fills the front half, its spiral drawn as rings, with two oval nuclei side by side in it like a pair of eyes, the basal bodies between them, two axonemes running down the middle to the tail, two dark curved median bodies across them behind the disc, and four pairs of flagella leaving the body, a pair at the front of the sides, a pair at the lower third of the sides, a pair that comes out of the ventral face behind the disc and runs back over it past the tail, and a pair from the tail. On the right, the cyst, an oval with a pale wall, the cell inside drawn back a little from it, four nuclei at one end and the axonemes and median bodies down its middle. The flagella beat.

Point at a name or a part to light it.

Giardia has two forms. The trophozoite, which lives in the small intestine, is pear-shaped, 10 to 20 µm long, with 2 nuclei, 8 flagella in four pairs and a ventral disc it holds on with. The cyst, the form that passes from person to person, is oval, 8 to 19 µm long, and has 4 nuclei when mature.

Simplified: not to scale: the trophozoite drawn far larger than the cyst beside it; the trophozoite drawn flat from its ventral side, the disc as an oval with its spiral as rings, and the flagella's paths inside the cell simplified; most organelles left out: the endoplasmic reticulum, the small vesicles under the surface, and the disc's ridges and lateral crest; the cyst drawn with all four nuclei together at one end and its wall as one layer; the beat played far slower than life, as simple waves drawn flat on the page.

2 sources
  1. Centers for Disease Control and Prevention. DPDx: Giardiasis. Last reviewed 5 June 2024.
  2. Adam RD. Giardia duodenalis: biology and pathogenesis. Clin Microbiol Rev. 2021;34(4):e00024-19.

The trophozoite is the most recognisable cell in parasitology. Seen from its flat ventral side, its two nuclei, each with a large central karyosome, and the curved median bodies below them make a face, with two eyes and a crooked smile. The ventral disc, a shallow sucker built from a spiral of microtubules, fills its front half and holds it on to the brush border of the gut lining. Its eight flagella leave the cell in four pairs, anterior, posterolateral, ventral and caudal, each from a basal body between the nuclei.

The life cycle

Giardia: the life cycle
A map of Giardia's life cycle, with one host. Across the top, outside the body on the white page: cysts passed in faeces, infective at once, are carried into water and food or onto hands, and are swallowed. Down into the patient, drawn as a pale panel with a figure at its left: in the duodenum trophozoites are freed from the cysts, hold on to the lining of the small intestine by their ventral disc and multiply by binary fission, and form cysts again as they pass on toward the colon. The cysts are passed in the faeces, and the cycle begins again.

Point at a name to light its pathway.

Giardia passes from person to person as the cyst, which is infective as soon as it is passed and survives for months in cold water. Swallowed, it releases trophozoites in the duodenum, which multiply on the lining of the small intestine and form cysts again on their way to the colon. People are infected from water, food and hands, and as few as 10 cysts can be enough.

Simplified: a map, not anatomy: the patient drawn as an icon and the parasites drawn small, nothing to scale; the steps within each stage left out: the cyst's nuclei dividing, and the divisions that turn one cyst into its trophozoites.

3 sources
  1. Centers for Disease Control and Prevention. DPDx: Giardiasis. Last reviewed 5 June 2024.
  2. Adam RD. Giardia duodenalis: biology and pathogenesis. Clin Microbiol Rev. 2021;34(4):e00024-19.
  3. Public Health Agency of Canada. Pathogen safety data sheets: infectious substances, Giardia lamblia.

The cyst is swallowed, in water or food contaminated with faeces, or from hand to mouth; as few as ten cysts can be enough. It passes the stomach and opens in the duodenum, where trophozoites are freed. These stay in the lumen of the duodenum and jejunum, free or held to the lining by the disc, and multiply by longitudinal binary fission. As they are carried on toward the colon they form cysts again, which leave in the faeces. Trophozoites appear in the faeces too, in diarrhoea, but die outside the body.

The cysts are infective as soon as they are passed, so Giardia spreads directly from person to person in day nurseries and institutions, within households and in sex that involves contact with faeces. They survive for months in cold water, so large outbreaks follow contaminated drinking water, and hikers drinking from clear streams are infected too. Animals carry Giardia, but their part in human infection is unclear.

Giardiasis

Many people who pass cysts have no symptoms at all, especially where Giardia is common. In others, giardiasis starts one to two weeks after the cysts are swallowed:

  • diarrhoea, watery at first, with nausea, loss of appetite, cramps, bloating and wind;
  • if it goes on, pale, greasy, foul-smelling stools that float and are hard to flush, the steatorrhoea of fat malabsorption;
  • weight loss, which can be marked; in children who are infected for long, poor growth.

There is no blood and usually no fever, because the parasite does not invade. Most illness settles over two to six weeks, but in some it lasts for months or years. The lining of the small intestine shows at most a mild partial villous atrophy; how the parasite damages it without invading is not fully known, though the host's immune response and an overgrowth of bacteria both seem to play a part. Afterwards some people are left with an irritable bowel or with chronic fatigue. It is worse and longer in people who make too little antibody.

Diagnosis

  • Stool samples, three of them over several days, since the parasite is passed irregularly: examined under the microscope for cysts and trophozoites, or tested for Giardia antigen.
  • PCR, now part of many stool panels, is more sensitive: a single sample tested by PCR finds more than three looked at under the microscope.
  • Rarely, the trophozoites are found in fluid aspirated from the duodenum or in a biopsy of the small intestine.

Treatment and prevention

  • Tinidazole as a single dose, or metronidazole for 5 to 10 days; nitazoxanide for 3 days, or albendazole, are alternatives.
  • Fluids by mouth for the diarrhoea: dehydration is the danger in infants and in pregnancy. In pregnancy paromomycin, which is barely absorbed, is preferred.
  • If symptoms persist, reinfection from the household or a nursery and too short a course are considered first; then two drugs of different classes are given together.

Chlorine at the doses used to treat water does not reliably kill the cysts. Water from streams, lakes and springs is boiled for at least a minute or passed through a fine filter, and hands are washed with soap and water after the toilet, after changing nappies and before handling food.

Check yourselfAnswer, then say how sure you were: a lucky guess still counts as a weak point.

1How does Trichomonas vaginalis pass from one person to the next?

2What does Trichomonas have in place of mitochondria?

3A woman has a frothy, yellow-green, foul-smelling vaginal discharge, and the cervix shows many small red spots. Her partner has no symptoms. What is the right plan?

4How many nuclei does a mature Giardia cyst have?

5Which picture fits giardiasis?

6Why can a town's chlorinated water supply still spread Giardia?