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Trichomonas
Two trophozoites of "Trichomonas vaginalis" stained with Giemsa
Two trophozoites of Trichomonas vaginalis stained with Giemsa
Scientific classification Edit this classification
Domain: Eukaryota
Clade: Metamonada
Phylum: Parabasalia
Class: Trichomonadea
Order: Trichomonadida
Family: Trichomonadidae
Genus: Trichomonas
Species[1]

Trichomonas is a genus of anaerobic excavate parasites of vertebrates. It was first discovered by Alfred François Donné in 1836 when he found these parasites in the vagina of a patient suffering from vaginitis, an inflammation of the vagina. Donné named the genus from its morphological characteristics. The prefix tricho- originates from the Ancient Greek word θρίξ (thrix) meaning hair, describing Trichomonas's flagella. The suffix -monas (μονάς – single unit), describes its similarity to unicellular organisms from the genus Monas.[2]

Habitat and ecology

[edit]

Trichomonas is typically found in anaerobic environments. It is a known parasite of many different animals including humans, birds, dogs, and cats.[3][4][5][6] In humans, it can be found in the urogenital tract and in the oral cavity. It is estimated that 276 million new cases of urogenital infections occur each year.[3][5][6] Depending on the Trichomonas species, it can either be transmitted through direct sexual contact or through contaminated water sources.[3][5][6] In birds, it can be found in the upper digestive tract and is transmitted when adult birds regurgitate food to feed their young, when a bird of prey feeds on an infected bird, and through contaminated food or water.[7]

Morphology

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Trichomonas is around 10 μm in length and is normally pear-shaped. It has four flagella at its anterior end, distinguishing itself from closely related organisms that have different numbers of anterior flagella. At the base of these flagella are the parabasal bodies, kinetosomes accompanied by Golgi stacks. The pelta is a sheet of microtubules that curve around the flagellar bases. Posterior to the pelta is the axostyle, is a bundle of microtubules that extends from the anterior end of the organism all the way to the posterior end. The nucleus of Trichomonas is situated close to where the pelta and axostyle meet.[7][6][8][9]

Another distinguishing feature of Trichomonas is the presence of an undulating membrane. The undulating membrane is a fin-like extension of the plasma membrane located on the side of the organism. A flagellum that extends to the posterior end of the organism is attached to the outer edge of the undulating membrane. At the base of the undulating membrane is a striated fiber called the costa which is thought to exist for structural support.[7][10][6][8][9]

Trichomonas has a very interesting organelle: the hydrogenosome.[6] Hydrogenosomes are double-membraned organelles used by trichomonads, instead of mitochondria, to produce ATP. They do not require oxygen and instead use pyruvate:ferredoxin oxido-reductase and hydrogenase to produce ATP from pyruvate, generating hydrogen gas as a by-product.[11]

Genetics

[edit]

Trichomonas vaginalis, being the species that causes the most complications in humans, is the only fully sequenced Trichomonas species. Through whole-genome shotgun sequencing, the Trichomonas vaginalis genome is estimated to be around 160 Mb long, divided into six chromosomes. However, at least 65% of its genome was found to be redundant. The redundant genetic material is hypothesized to have emerged during Trichomonas's transition from aerobic to anaerobic environments.[12]

In addition to discovering the large proportion of repetitive DNA in Trichomonas vaginalis genome, the sequenced genes were also characterized. Approximately 60,000 protein-coding genes were found. Transfer RNAs for all 20 amino acids and approximately 250 ribosomal RNA were all found on the same chromosome.[12]

Life cycle

[edit]

Trichomonas has a trophozoite form, its pear-shaped form, which is most commonly observed, and an amoeboid form, which appears during host colonization.[12] It lacks a cyst form, but many studies have noted a unique form in which Trichomonas appears ovoidal rather than its typical pear-shaped form. In this ovoidal form, all its flagella are retracted in endocytic vacuoles, giving the impression of a cystic form. However, due to the lack of a cystic wall surrounding the organism, many studies describe this form as a pseudocystic form.[7][6]

In its trophozoite form, Trichomonas undergoes cell division through an interesting process called cryptopleuromitosis. There are three common forms of mitosis: open, closed, and semi-open. In open mitosis, the nuclear envelope disappears so that mitotic spindles can interact with the chromosomes. In closed mitosis, the nuclear envelope does not disappear but mitotic spindles appear within the nucleus to separate the chromosomes. In semi-open mitosis, the nuclear envelope remains intact but the mitotic spindles pierce through the nuclear envelope to divide the chromosomes. Cryptopleuromitosis is different from all the other more commonly known methods of cell division. In cryptopleuromitosis, the chromosomes divide without the breakdown of the nuclear envelope and without the entry of mitotic spindles into the nucleus.[13]

Diseases

[edit]

Trichomonas causes disease in humans and in birds. In humans, the causative species is Trichomonas vaginalis and Trichomonas tenax.[3][5][6] In birds, the causative species are Trichomonas gallinae, Trichomonas gypactinii, and Trichomonas stableri.[14][15][7]

In humans

[edit]

Trichomonas vaginalis is a sexually transmitted disease and causes trichomoniasis. It resides on squamous epithelium of the urogenital tract. Many carriers of Trichomonas vaginalis, especially men, are asymptomatic. Complications for symptomatic women include vaginitis, endometritis, infertility, and cervical cancer. Complications for symptomatic men include urethritis, prostatitis, epididymitis, and infertility. It is also associated with increased risk of transmission and acquisition of HIV.[5][6]

Trichomonas tenax is transmitted through exchange of saliva and contaminated water sources. It is an opportunistic pathogen and may cause pulmonary trichomoniasis.[3]

In birds

[edit]

Trichomonas in birds inhabit the upper digestive tract and also cause trichomoniasis. It creates lesions in the trachea and esophagus, occupying space and eventually causing emaciation and asphyxiation.[14][15][7]

Species

[edit]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Trichomonas

View on Grokipedia
from Grokipedia
Trichomonas is a of anaerobic, protozoans in the family Trichomonadidae and phylum Metamonada, consisting of pear-shaped, motile trophozoites measuring 5–23 μm in length, typically featuring four anterior free , one recurrent flagellum along an undulating supported by a costa, an axostyle, and a pelta, with no stage in their life cycle. These unicellular parasites reproduce asexually via binary fission and lack mitochondria, instead possessing hydrogenosomes for energy metabolism. The encompasses over a dozen that primarily infect the mucosal surfaces of vertebrates and , with being the most clinically significant, causing —a prevalent, curable estimated 156 million new cases in 2020 worldwide, particularly in the urogenital tract. Other human-associated include Trichomonas tenax, a commensal in the oral cavity often linked to poor dental and occasionally associated with respiratory infections, and formerly recognized Trichomonas hominis (now classified as Pentatrichomonas hominis), an intestinal commensal. The biology of Trichomonas species is adapted to anaerobic or microaerophilic environments, where they adhere to host epithelia using surface proteins and adhesins, potentially leading to inflammation, tissue damage, and increased susceptibility to other infections like . T. vaginalis, for instance, exhibits strain variability in virulence, influenced by symbiotic interactions with mycoplasmas and viruses such as Trichomonas vaginalis virus, which can enhance pathogenicity and metronidazole resistance. Transmission occurs directly through contact with infected secretions, with no intermediate hosts or environmental outside the body, emphasizing the of sexual in T. vaginalis spread and close contact or poor hygiene for oral species. Diagnosis typically involves microscopic examination of wet mounts for motile trophozoites, amplification tests for higher sensitivity, and in specific media. Treatment relies on nitroimidazoles like or , though emerging resistance poses challenges. Beyond humans, species like Trichomonas gallinarum parasitize birds' intestines, while related genera such as Tritrichomonas affect and cats, highlighting the zoonotic potential and economic impacts in .

Taxonomy and etymology

Classification

Trichomonas is a genus of eukaryotic microorganisms classified within the domain Eukarya, supergroup , phylum Metamonada, class Parabasalia, order Trichomonadida, and family Trichomonadidae. This placement reflects its position among early-diverging anaerobic protists, characterized by adaptations to low-oxygen environments and the absence of typical mitochondria. The genus was first described in 1836 by Alfred François Donné based on observations of motile cells in samples. Trichomonas belongs to Parabasalia within Metamonada, which also includes other groups such as Diplomonadida (exemplified by ); these metamonads share features as anaerobic flagellates adapted to parasitic or commensal lifestyles in vertebrates. This relationship underscores the evolutionary convergence in and energy metabolism among metamonads, distinguishing them from aerobic eukaryotes. The genus Trichomonas is distinguished taxonomically by the presence of four anterior free , an undulating membrane formed by a recurrent fifth along the cell's margin, and hydrogenosomes—double-membrane-bound organelles that generate ATP anaerobically via . These traits differentiate it from related genera like Tritrichomonas (three anterior ) and Tetratrichomonas (variations in flagellar arrangement), emphasizing the role of cytoskeletal and metabolic features in .

History

The genus Trichomonas was first described in 1836 by French physician and microscopist Alfred François Donné, who observed motile, flagellated protozoa in purulent vaginal discharge from patients, identifying the organism now known as Trichomonas vaginalis as a distinct entity through early microscopic examination. Donné's work marked the initial recognition of Trichomonas species as parasitic protozoa, building on the era's advancing microscopy techniques to distinguish them from other vaginal microbes. The name Trichomonas derives from the words thríx (meaning "hair") and monás (meaning "unit" or "single entity"), alluding to the hair-like flagella that enable the organism's characteristic . Throughout the , Trichomonas was increasingly acknowledged as a parasite associated with mucosal infections, though its precise pathogenic role remained debated, with some early observers viewing it as a commensal rather than a primary cause of . In the 20th century, key advancements solidified Trichomonas as a significant ; for instance, in 1916, German gynecologist Ottomar Höhne coined the term "" to describe the caused by T. vaginalis, establishing its etiological link to genital infections. By the mid-20th century, particularly the , T. vaginalis was widely recognized as a sexually transmitted , prompting improved diagnostic methods and treatment protocols amid rising awareness of its role in reproductive health complications. A major milestone occurred in , when the draft sequence of T. vaginalis was published, revealing its large, repetitive and providing foundational insights into its and as a eukaryotic parasite.

Morphology and ultrastructure

General morphology

Trichomonas species exist primarily in a stage, characterized by a distinctive pear-shaped (pyriform) morphology observable under light microscopy. The typically measures 7-23 μm in length and 5-15 μm in width, though dimensions can vary slightly depending on environmental conditions and . This compact, elongated form enables efficient navigation through host mucosal environments. Motility is a hallmark feature, achieved through the presence of four anterior flagella that project forward from the cell's apical region, propelling the organism with a characteristic jerky motion. Additionally, a fifth flagellum is incorporated into an undulating membrane that runs along the posterior margin, enhancing locomotion by creating wave-like ripples across the cell surface. A prominent cytoskeletal element, the axostyle, consists of a rod-like structure composed of microtubules that extends longitudinally from the anterior end near the nucleus to the posterior tip of the cell. This internal scaffold not only reinforces the trophozoite's elongated shape but also protrudes slightly at the rear, contributing to overall structural integrity during movement.

Cellular features

Trichomonas species, exemplified by T. vaginalis, harbor hydrogenosomes as their primary energy-producing organelles, which are double-membrane-bound structures analogous to mitochondria but adapted for anaerobic conditions. These spherical organelles, typically around 0.5 µm in diameter, generate ATP through by converting pyruvate to , while simultaneously producing hydrogen gas via [FeFe]-hydrogenase activity in the absence of external electron acceptors. Unlike canonical mitochondria, hydrogenosomes lack a , relying entirely on nuclear-encoded proteins imported post-translationally, and they do not support or a respiratory chain. The pelta represents a key cytoskeletal element in Trichomonas, forming a stable, sheet-like array of that reinforces the anterior flagellar canal. This structure is intimately associated with the basal bodies of the four anterior flagella, integrating into the broader pelta-axostyle complex to provide mechanical support and maintain the integrity of the mastigont system during locomotion. Electron microscopy reveals the pelta's role in anchoring flagellar elements, contributing to the parasite's characteristic pear-shaped morphology and motility. The nucleus in Trichomonas is a prominent, ovoid positioned anteriorly, containing six chromosomes with an unusual arrangement marked by a random, granular distribution surrounding a small central karyosome. This dispersed pattern, visible under electron microscopy, reflects the organism's early-diverging eukaryotic lineage and supports closed with an extranuclear spindle. Trichomonas possesses a prominent Golgi apparatus, consisting of stacked cisternae, which is closely associated with striated parabasal filaments to form the parabasal apparatus, supporting secretory functions.

Life cycle and reproduction

Developmental stages

The life cycle of Trichomonas species, such as Trichomonas vaginalis and Trichomonas tenax, is characterized by a single active stage known as the , which serves as the primary form for both transmission and . This pear-shaped or ovoid protozoan measures approximately 7–23 μm in length and exhibits characteristic motility through anterior flagella and undulating membrane, enabling rapid movement in liquid environments. Upon contact with host tissues, the undergoes a rapid morphological transformation into an amoeboid form, retracting its flagella and extending to facilitate and tissue invasion. This amoeboid state enhances the parasite's ability to penetrate epithelial layers, promoting during . In response to environmental stresses outside the host, such as low , cold temperatures, nutrient deprivation, or exposure to agents, can degenerate into a form. This stage features a rounded morphology with retracted , internalized flagella, and reduced metabolic activity, rendering it non-motile and non-proliferative yet viable. Unlike true cysts in other protozoans, the lacks a protective wall and can revert to the form upon restoration of favorable conditions, providing temporary resistance to , fluctuations, and disinfectants. Proteomic studies indicate that this transformation involves downregulation of glycolytic pathways and upregulation of stress-response proteins, supporting survival during transient exposure to harsh conditions. Trichomonas species notably lack a true dormant stage, distinguishing them from many other parasitic protozoans and necessitating direct host-to-host transmission via bodily fluids or fomites. Without a resilient cystic form for prolonged environmental persistence, the or must be transferred promptly to maintain , which influences the of infections like . This absence of encystation limits the parasite's ability to survive outside the host beyond short periods, typically hours to days under optimal conditions.

Reproduction mechanisms

Trichomonas species reproduce asexually through binary fission, a process characterized by longitudinal division of the form, which is the primary replicating stage in the life cycle. This division occurs via cryptopleuromitosis, a specialized closed where the remains intact throughout the process, and an internal spindle forms within the nucleus to facilitate chromosome segregation without typical condensation. The fission results in two genetically identical daughter cells, each inheriting one set of flagella and other organelles after precise duplication and partitioning. During binary fission, flagellar duplication begins in the S-phase of the , with new flagella migrating toward the nuclear poles and emerging from the spindle poles at , ensuring synchronized segregation of flagella and nuclei. This coordination maintains the and structural integrity of the daughter cells. The process is rapid, with division times as short as 6-8 hours in nutrient-rich conditions, allowing rapid within the host environment. No definitive evidence supports in Trichomonas, as no gametes, zygotes, or meiotic stages have been observed under laboratory or natural conditions. However, genetic studies indicate possible genetic exchange through parasexual mechanisms, such as or transient diploidy, which could contribute to diversity without a canonical sexual cycle.

Genetics

Genome characteristics

The of , serving as the primary model for the Trichomonas, measures approximately 177 Mb (ranging 173–181 Mb across recent assemblies) and is distributed across six chromosomes. The initial sequencing in 2007 produced a draft assembly of ~160 Mb into over 17,000 scaffolds, but as of 2025, chromosome-level assemblies provide higher-quality references with just six super-scaffolds covering over 90% of the . This structure endows T. vaginalis with one of the largest known eukaryotic s among protists. Recent annotations predict approximately 30,000–40,000 protein-coding genes (e.g., 24,691 in a 2024 assembly and 43,326 in a 2025 assembly), reflecting high coding capacity compared to other unicellular eukaryotes, though lower than the initial ~60,000 estimate from the draft. Approximately 65% of the consists of , dominated by repetitive elements that contribute to its overall size and complexity. This repetitiveness arises largely from transposable elements, which comprise about 60% of the sequence and include diverse families such as mariner-like elements with thousands of copies. Gene duplications further amplify this, leading to expanded gene families and pseudogenes that constitute a significant portion of the non-coding regions. Aneuploidy is common in T. vaginalis strains, often resulting in variable numbers that deviate from the typical diploid set of six. This variability underscores the genomic plasticity observed across isolates.

Molecular adaptations

Trichomonas species, exemplified by the human pathogen Trichomonas vaginalis, exhibit profound molecular adaptations to thrive in anaerobic or microaerophilic environments within host mucosal tissues. Central to this is the presence of hydrogenosomes, double-membrane-bound organelles that replace conventional mitochondria and support ATP generation via . These organelles harbor genes encoding key enzymes such as pyruvate: oxidoreductase (PFOR) and [FeFe]-hydrogenase, which catalyze the oxidation of pyruvate to , CO₂, and H₂, thereby disposing of reducing equivalents in the absence of oxygen. The expression of these hydrogenosomal enzymes is upregulated under anaerobic conditions, enhancing the parasite's metabolic efficiency. Unlike typical eukaryotes, T. vaginalis lacks and canonical mitochondrial genes, relying instead on extended cytosolic for the bulk of its energy needs, supplemented by hydrogenosomal . This is bolstered by extensive lateral transfer (LGT) from bacterial donors, which has introduced prokaryotic-origin genes for enzymes like PFOR and into the nuclear genome, enabling the evolution of hydrogenosomal functions from a mitochondrial . Phylogenetic analyses confirm that many hydrogenosomal proteins, including those involved in iron-sulfur cluster assembly, trace back to bacterial LGT events, underscoring the role of horizontal acquisition in metabolic diversification. For host interaction and immune evasion, T. vaginalis employs a vast repertoire of surface adhesins, notably the TvBspA protein family, which comprises over 900 genes encoding leucine-rich repeat-containing proteins that mediate adherence to host epithelial cells and components. These adhesins facilitate initial colonization and cytoadherence, critical for establishing infection in the urogenital tract. The TvBspA family exhibits antigenic variation through mechanisms such as , recombination, and differential expression, allowing the parasite to alter its surface and evade host adaptive immunity. This is supported by the large nuclear of approximately 177 Mb, which encodes an unusually high number of protein-coding genes.

Habitat and ecology

Environmental distribution

Trichomonas species thrive in anaerobic or microaerophilic environments, reflecting their adaptation to low-oxygen niches typically found within host tracts but also influencing their limited free-living persistence. These exhibit optimal growth at temperatures between 35°C and 37°C, aligning with mammalian body conditions, and prefer slightly acidic ranges of 5.5 to 6.5, where metabolic processes such as attachment and are most efficient. Deviations from these parameters, such as elevated or oxygen levels, inhibit proliferation and induce stress responses. Outside hosts, Trichomonas demonstrates short-term survival in aquatic and moist settings through the formation of pseudocysts, dormant structures that internalize flagella and resist mild or . These pseudocysts enable short-term persistence for hours, up to 24 hours, in moist environments such as , , or . However, prolonged exposure to drying or extreme conditions rapidly diminishes viability, limiting environmental reservoirs. The global distribution of Trichomonas mirrors host populations, with elevated in tropical and subtropical regions due to favorable climatic conditions facilitating transmission through increased host contact and density. This pattern underscores the parasite's reliance on warm, humid ecosystems for indirect dissemination, such as via contaminated water sources.

Host interactions

Trichomonas species primarily colonize hosts, including humans in the urogenital and oral tracts, birds in the upper digestive system, and various mammals such as dogs and cats in the intestinal tract. These parasites exhibit a for mucosal environments, where they establish through direct interaction with host epithelial cells and tissues. The demonstrates a broad host range across vertebrates, with adaptations that allow persistence in diverse anatomical niches without necessarily invoking severe . Adhesion to host mucosal surfaces is facilitated by specialized molecules, including that recognize carbohydrate moieties on epithelial cells and proteases that degrade components to promote attachment. Lectin-mediated binding enables initial contact, while proteases, such as CP30, contribute to stable adherence by cleaving host proteins and modulating the local environment. motility further aids in navigating layers to reach attachment sites. Immune evasion is achieved through mechanisms like antigenic variation, where the parasite sheds and renews surface antigens to avoid recognition by host antibodies, alongside phenotypic switching that alters surface protein expression. The role of Trichomonas in hosts can range from commensal to pathogenic, influenced by factors such as host immune status and parasite strain . In immunocompetent hosts, certain strains may persist asymptomatically as part of the mucosal , contributing to ecological balance without overt harm, whereas in immunocompromised individuals or with more aggressive strains, interactions can escalate toward tissue disruption. This variability underscores the parasite's opportunistic nature, where host immunity modulates the balance between mutualism and .

Diseases and pathogenicity

In humans

Trichomoniasis, the infection caused primarily by , is the most common non-viral (STI) worldwide, with an estimated 156 million new cases annually among individuals aged 15–49 years in 2020, including 73.7 million in females and 82.6 million in males. The highest prevalence occurs in women in this age group, at approximately 5.3%, compared to 0.6% in men, driven by factors such as multiple sexual partners and unprotected intercourse. , an estimated 2.6 million persons are affected, disproportionately impacting and those of lower . In women, T. vaginalis infection typically manifests as , characterized by a frothy, yellow-green with a fishy , genital itching, burning, soreness, and . Men often remain but may experience with urethral discharge, , or pruritus. Complications include increased risk of , , and premature in pregnant women, as well as heightened susceptibility to acquisition. Up to 70–85% of infections in both sexes are , complicating detection and control. Trichomonas tenax, primarily a commensal in the human oral cavity associated with poor dental hygiene, has been occasionally implicated in respiratory infections, particularly in immunocompromised individuals or those with underlying pulmonary conditions, though its pathogenicity remains debated and direct causation is not firmly established. Diagnosis relies on nucleic acid amplification tests (NAATs), such as the Aptima T. vaginalis assay, which offer high sensitivity (over 95%) for detecting T. vaginalis DNA in vaginal, endocervical, or urine samples from women and urethral swabs or urine from men, outperforming traditional methods. Wet-mount microscopy of fresh vaginal secretions remains a rapid, low-cost option but has lower sensitivity (50–70% in women, even less in men) due to the parasite's motility requirement and rapid die-off in specimens. Culture and point-of-care tests like OSOM Trichomonas Rapid Test are alternatives, though NAATs are recommended by the CDC for symptomatic and high-risk individuals. Treatment involves oral nitroimidazoles, with 500 mg twice daily for 7 days as the preferred regimen for women to reduce recurrence, or a 2 g single dose for both sexes; 2 g as a single dose is an effective alternative with potentially fewer side effects. Sexual partners should be treated concurrently to prevent reinfection, and or consistent use is advised until resolution. resistance has emerged since the 2010s, with prevalence up to 10% in some U.S. populations, necessitating susceptibility testing via CDC resources for persistent cases and alternative dosing or combination therapies.

In animals

Trichomonas infections in birds primarily involve Trichomonas gallinae, which causes avian trichomonosis, manifesting as frounce in raptors or in columbids such as pigeons and doves. The parasite induces caseous oral lesions and plaques in the upper digestive tract, leading to , regurgitation, and , with high mortality rates—often exceeding 90% in affected nestlings—particularly in young pigeons and doves. Transmission occurs through regurgitation of infected from parents to offspring or via direct beak-to-beak contact during courtship, as well as indirectly through contaminated water or feed shared among flocks. In , such as hawks and eagles, ingestion of infected columbids can result in secondary infections, exacerbating liver lesions and overall mortality. In mammals, trichomoniasis affects various species, with veterinary significance in companion animals and . In cats, Tritrichomonas foetus (now classified as T. blagburni) causes chronic large-bowel , characterized by soft, mucous-laden stools, often in young or purebred individuals housed in multi-cat environments like shelters; infections are frequently subclinical, persisting without resolution and contributing to or in symptomatic cases. In dogs, trichomonads such as Pentatrichomonas hominis are occasionally detected in feces, primarily causing mild or subclinical intestinal disturbances like intermittent , though their pathogenicity remains uncertain and respiratory involvement is rare. Bovine , caused by , is a venereal transmitted during breeding, leading to early embryonic death, mid-gestation s, , and reduced fertility in cows, while bulls remain asymptomatic carriers. Herds experience economic losses from lowered conception rates and culling of infected animals, with abortion rates varying but often below 5% in detected cases. Diagnosis of trichomoniasis in animals relies on molecular methods, particularly polymerase chain reaction (PCR) assays targeting fecal samples in cats and dogs or preputial washes in , which offer high sensitivity for detecting low parasite loads compared to traditional . Treatment varies by host: in cats, ronidazole at 30 mg/kg orally once daily for 14 days is the primary option, achieving clearance in approximately 60-90% of cases despite its narrow safety margin and potential . For birds, particularly pigeons, carnidazole administered as a single 10-20 mg/kg dose is effective against T. gallinae, often resolving infections rapidly with minimal recurrence when combined with hygiene measures. In , management focuses on test-and-cull strategies rather than , as no approved treatments exist for food animals. Ecologically, plays a role in wildlife , contributing to declines in avian species through epizootics; for instance, outbreaks have driven rapid reductions in greenfinch and turtle dove populations in , with mortality events linked to virulent strains spreading via shared feeders. In birds of prey, such as Bonelli's eagles, the parasite indirectly causes nestling losses—up to 14% in some Iberian populations—by accumulating in the from infected columbid prey, amplifying impacts on vulnerable raptors. These dynamics underscore as an emerging factor in , particularly in fragmented habitats where host density facilitates transmission.

Known species

Major species

Trichomonas vaginalis is the most clinically significant species in the genus, recognized as a major human urogenital pathogen that causes , the most common non-viral worldwide. The trophozoites are pyriform in shape, measuring 7-30 μm in length, and exhibit high motility due to four anterior and one posterior along the undulating membrane. This species infects the urogenital tract of both men and women, with an estimated 156 million new cases annually worldwide among people aged 15–49 (as of 2020). Trichomonas gallinae is a prominent avian parasite primarily affecting the upper digestive tract of birds, particularly pigeons, doves, and raptors, where it is the causative agent of trichomoniasis, commonly known as frounce in birds of prey. The trophozoites display variable morphology, typically ovoidal to pyriform and ranging from 5-20 μm in size, with four anterior flagella and a recurrent flagellum forming the undulating membrane that contributes to their motility. This species is widespread in columbiform and passeriform birds, leading to significant morbidity in wild and domestic populations. Trichomonas tenax serves as a common oral commensal in humans, often residing in the mouth and associated with conditions like , though its pathogenic role remains under investigation. The trophozoites are smaller than those of other major species, measuring 5-16 μm in length and exhibiting an ellipsoidal or pear-shaped form, propelled by five including four anterior ones and a recurrent flagellum. It is frequently detected in individuals with poor and is considered non-pathogenic in most cases but may contribute to oral infections in compromised hosts.

Minor or less-studied species

Among the less-studied species in the genus Trichomonas are T. gypaetinii and T. stableri, both of which are pathogens in raptors and other scavenging , inducing lesions in the upper that resemble those caused by T. gallinae. These species were formally described through morphological examination and genetic analyses, including sequencing of the 18S rRNA gene, which revealed distinct phylogenetic positions within the genus. T. gypaetinii was first isolated from the crops of vultures such as the Egyptian vulture (Neophron percnopterus) and (Gypaetus barbatus), highlighting its adaptation to scavenging hosts. Similarly, T. stableri has been detected in columbiform birds like band-tailed pigeons ( fasciata), where it contributes to outbreaks of trichomonosis, though its prevalence and transmission dynamics remain under-explored compared to major species. The genus Trichomonas has over 100 species described, though taxonomic revisions have reclassified some to other genera, and many remain poorly studied. Numerous species inhabit the intestines of reptiles, amphibians, and other vertebrates as commensals or opportunistic parasites; however, significant research gaps exist, including the absence of complete assemblies for most and incomplete documentation of their host specificities and ecological roles. These lesser-known taxa underscore the diversity of trichomonads beyond and avian pathogens, yet molecular and genomic studies lag, impeding broader understanding of their evolutionary relationships and potential zoonotic implications.

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