Open Veterinary Journal, (2025), Vol. 15(9): 3931-3942

Review Article

10.5455/OVJ.2025.v15.i9.2

Hidden menace: Understanding the devastating consequences of dourine disease in horses

Rimayanti Rimayanti¹, Aswin Rafif Khairullah², Imam Mustofa¹, Budi Utomo¹, Tita Damayanti Lestari¹, Suzanita Utama¹, Adeyinka Oye Akintunde³, Sri Mulyati¹, Tatik Hernawati¹, Ahmed Qasim Dawood^4,5, Ginta Riady⁶, Imdad Ullah Khan⁷, Siti Darodjah Rasad⁸and Ikechukwu Benjamin Moses⁹

¹Division of Veterinary Reproduction, Faculty of Veterinary Medicine, Universitas Airlangga, Surabaya, Indonesia

²Research Center for Veterinary Science, National Research and Innovation Agency (BRIN), Bogor, Indonesia

³Department of Agriculture and Industrial Technology, Babcock University, Ilishan Remo, Nigeria

⁴Department of Veterinary Preclinical Sciences, Faculty of Veterinary Medicine, Universiti Putra Malaysia, Serdang, Malaysia

⁵Department of Physiology, Pharmacology, and Chemistry, Faculty of Veterinary Medicine, Al Shatra University, Al Shatra, Iraq

⁶Reproduction Laboratory, Faculty of Veterinary Medicine, Universitas Syiah Kuala, Banda Aceh, Indonesia

⁷Faculty of Veterinary and Animal Sciences, Gomal University, DI Khan, Pakistan

⁸Faculty of Animal Husbandry, Universitas Padjadjaran, Sumedang, Indonesia

⁹Department of Applied Microbiology, Faculty of Science, Ebonyi State University, Abakaliki, Nigeria

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ABSTRACT

Trypanosoma equiperdum is a protozoan parasite that causes the sexually transmitted disease known as “dourine” in horses. This chronic illness is directly spread from one animal to another during mating. Doflein proposed the name T. equiperdum in 1901. Despite being distributed worldwide, the broad use of artificial insemination technology over the past three decades has resulted in only a few cases being documented. The condition is typically fatal and is characterized by gradual emaciation, nervous system involvement, and edematous lesions of the genitals. The incubation period between exposure and the onset of clinical symptoms varies widely, ranging from a few weeks to several years. The diagnosis of dourine can be challenging because of factors such as a lack of knowledge about the parasite and host–parasite interactions following infection. However, in reality, the diagnosis is based on clinical evidence backed by molecular and serological testing. Coital exanthema, surra, infectious anemia in horses, viral arthritis in horses, and purulent endometritis cause such infectious equine metritis are examples of differential diagnosis. Coital exanthema, surra, equine infectious anemia, viral arthritis, and purulent endometritis are examples of conditions that should be considered in the differential diagnosis of infectious equine metritis. Dourine is the only trypanosomiasis that is not spread by an insect vector. Treatment is generally not advised in dourine-free areas due to the possibility of asymptomatic carrier animals and the concern that treated animals may continue to spread the disease. Dourine vaccination does not yet exist. Thus, preventing dourine is dependent on achieving an infection-free status, which is accomplished by checking the blood for T. equiperdum antibodies.

Keywords: Dourine, Equine, Neglected disease, Sexually transmitted disease, T. equiperdum.

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Introduction

Humans have greatly benefited from horses in many facets of life, including transportation, agriculture, sport, and leisure (Lönker et al., 2020). Although horses are sometimes portrayed as powerful animals, they are prone to some illnesses and other undesirable situations, the most common of which is parasite infections. Trypanosoma equiperdum is a protozoan parasite that causes the sexually transmitted disease known as “dourine” in horses (Gizaw et al., 2017). This chronic illness is directly transmitted from one animal to another during mating (Ahmed et al., 2018). The disease is more frequently spread from stallions to mares, but it can also spread from mares to stallions. This is because the seminal fluid and mucous exudate from the penis and sheath of infected males and the vaginal mucus of infected females contain parasites (Pal et al., 2024).

Trypanosoma equiperdum is a tissue parasite that is rarely found in the blood, which distinguishes it from other trypanosomes (Yasine et al., 2019a). The only known natural reservoir for the parasite is horses. The parasite occasionally leaves the genital system, and the animal stops being contagious for a few weeks or months (Claes et al., 2005a). The symptoms of dourine in horses include fever, vaginal swelling, skin eruptions, and plaques, as well as paralysis, incoordination, conjunctivitis, keratitis, anemia, and progressive emaciation (Pal et al., 2024). Although the clinical symptoms of dourine are frequently evident, serological and molecular testing are necessary to confirm the presence of the parasite for a definitive diagnosis (Hébert et al., 2023). The amount of time that passes between exposure and the onset of clinical symptoms can vary greatly; it might be as brief as 1 or 2 weeks or as long as many years (Gizaw et al., 2017).

Latin America, Asia, the Middle East, and Africa have all suffered significant economic losses due to dourine sickness (Yasine et al., 2019a). Despite this, the scientific community, veterinary authorities, and regulatory bodies view this disease as a mainly neglected animal disease. The unwillingness of many endemic nations to notify the World Organization for Animal Health about dourine worsens this dilemma (Büscher et al., 2019). The primary barriers to the local and worldwide management of equine trypanosomosis are the absence of a vaccine, the incapacity of medications to treat the neurological stage of illness, varying case definitions, and existing diagnostic restrictions. Recent European surra and dourine epidemics highlight the dangers and repercussions of bringing sick horses with equine trypanosomosis into non-endemic nations (Ungogo and de Koning, 2024). Proper testing and diagnostics for dourine management, in addition to dependable curative and preventative medications, are needed, given the growing number of horses traveling throughout the globe.

Dourine disease has a direct impact on livestock productivity, livestock management, and human settlements; it also has an indirect impact on food crop production. In most regions, it is a major agricultural output limitation that frequently results in death (Desquesnes et al., 2022a). Dourine presents a serious problem for the production of horses because it can spread anywhere due to climate influences and does not require an insect vector. This review addresses the devastating impact of dourine disease in horses, positioning it as a “hidden menace” within equine health and the broader veterinary landscape. Unlike previous studies, this article provides a comprehensive and integrative synthesis of the latest developments in dourine research, including its pathogenesis, epidemiology, clinical progression, and socioeconomic ramifications. This review critically evaluates emerging diagnostic techniques, particularly molecular methods, and examines vaccine development prospects, addressing long-standing challenges in disease detection and control. Furthermore, it introduces a One Health perspective, considering the potential ecological and cross-species implications of dourine transmission—an angle rarely explored in prior studies.

Importantly, the article highlights policy and surveillance deficiencies and offers targeted recommendations for improving disease monitoring and international control strategies. By consolidating these insights, the review provides a novel, multidimensional understanding of dourine, emphasizing its significance in animal health, global trade, and rural economies. The aim of this review article is to provide a comprehensive understanding of dourine disease, including its causative agent, transmission mechanism, clinical symptoms, diagnosis, and control methods. This article is intended to serve as a resource for researchers, veterinarians, and legislators working to improve dourine control.

Etiology

Trypanosoma are long, spindle-shaped, flagellated protozoa that are typically 20–30 μm long and 1.5–3.5 μm wide. Trypanosoma cells feature a free flagellum at the anterior end and a blunt posterior end (Olego-Fernandez et al., 2009). Trypanosoma equiperdum belongs to the category of trypanosomes that are not spread by tsetse (Cuypers et al., 2017). Trypanosomes are flagellated, fully parasitic protozoa. They belong to the phylum Sarcomastigophora, order Kinetoplastidae, family Trypanosomatidae, and genus Trypanosoma (Radwanska et al., 2018). The pathogenic species T. evansi, T. brucei, and T. equiperdum belong to the Trypanozoon subgenus (Amer et al., 2024). The three subspecies of T. brucei are the animal pathogen T. brucei brucei (found in horses and ruminants) and the two human infections that cause sleeping sickness in humans, T. brucei gambiense and T. brucei rhodesiense (Boundenga et al., 2022). Molecular studies have recently led to the classification of T. equiperdum as a subspecies of T. brucei (Bassarak et al., 2016).

Kinetoplast DNA (kDNA), the name given to flagellate kinetoplastid, comprises two kinds of circular DNA molecules that are entangled with one another: thousands of minicircle and hundreds of maxicircle (Lukes et al., 2002). Most of the unique mitochondrial genes found in maxcircle can only be translated following RNA editing (Chrzanowska-Lightowlers and Lightowlers, 2024). Decrypting the maxicircle transcript requires a guide RNA, which is encoded by the minicircle (Thomas et al., 2007). Trypanosoma equiperdum and T. evansi are dyskinetoplasty trypanosomes, whereas T. brucei is a kinetoplastid trypanosome (Yasine et al., 2019a). The maxicircle gene is absent in T. evansi, but it is present in T. equiperdum, albeit with a significant number of deleted genes (Li et al., 2007). A kinetoplastic T. equiperdum that lacks complete kDNA has not yet been found. T. evansi and T. equiperdum differ from T. brucei in two biological ways. First, since T. equiperdum is spread by sexual contact and T. evansi is spread by biting flies, they did not use flies as a vector of transmission. Second, T. evansi and T. equiperdum are dyskinetoplasty, meaning that they lack some or all of their kDNA (Brun et al., 1998). Trypanosomes in the bloodstream are locked from trypamastigotes in the host by partial dyskinetoplasty or total kinetoplast, and vertebrate-to-vertebrate transmission becomes solely mechanical, with no additional vector formation in dyskinetoplastic or akinetoplastic trypanosomes, the loss of kinetoplast DNA prevents further development in the insect vector. As a result, the parasite circulates in the vertebrate host only as bloodstream trypomastigotes, and transmission becomes limited to direct vertebrate-to-vertebrate routes, mainly through mechanical transfer or venereal contact (Pereira et al., 2024).

Trypanosomes of the subspecies T. equiperdum are typically located in the capillaries of the mucous membrane of the urogenital tract; thus, they are rarely found in the bloodstream of the host (Ahmed et al., 2018). Nonetheless, some trypanosomes can occasionally be found in the peripheral blood of animals. The discovery of T. equiperdum infections in foals indicates that the parasite can also directly spread through the milk, udder lesions, or the birth canal after parturition (Gizaw et al., 2017). The Mongolian isolate (IVM-t1), BoTat 1.1, Dodola 940 and 943, and OVI are the authentic T. equiperdum strains that have been successfully isolated and made available for use in a variety of investigations (Suganuma et al., 2016).

History

Dourine illness is thought to have existed since people domesticated horses between 4,000 and 3,000 years before Christ, yet there are no explicit written records of it until much later. Throughout the 19th century, dourine began to be identified as a disease apart from other trypanosomiases, such as surra (Giordani et al., 2016). Dourine is distinct from most trypanosomiasis because sexual interaction spreads the disease rather than insect vectors (Oldrieve et al., 2024). In 1796, dourine was first discovered and discussed in the European literature. In 1894, T. equiperdum, the causal agent, was isolated from horse blood and subsequently multiplied in other animals (Claes et al., 2005a). In 1901, Doflein proposed the scientific name T. equiperdum (Molinari and Moreno, 2018). Ancient Arabic literature also describes dourine-like ailments.

Host range

Dourine primarily targets mules, donkeys, and horses (Pascucci et al., 2013). Although it has not been established that zebras can carry or spread T. equiperdum, a positive complement fixation test (CFT) has been obtained from zebras (Claes et al., 2005b). Several laboratory animals have adapted to the organism (Lai et al., 2008). The condition is less common in native ponies, donkeys, and mules, but it seems to be more common in higher horse breeds (Büscher et al., 2019).

Horses are more susceptible to T. equiperdum than other animals (Gizaw et al., 2017). This disease affects the neurological system and rapidly advances in animals (Ungogo and de Koning, 2024). They typically die after a chronic illness that can last for 1–2 years. Although donkeys and mules are prone to illness, they either acquire a modest condition or continue to exhibit symptoms (Gizaw et al., 2017). Infected male donkeys pose a special threat to disease epidemiology because they may go undetected as carriers, given their potential for asymptomaticism. Horses and donkeys are the only known natural reservoir of T. equiperdum (Getahun, 2019).

A variety of laboratory animals, such as mice, rats, and rabbits, can be experimentally infected with the disease (Perrone et al., 2018). Ruminants do not seem to be vulnerable to T. equiperdum isolates, although sheep and goats infected with murine-adapted strains have shown signs of dourine infection (Gizaw et al., 2017).

Epidemiology

Although dourine is found all over the world, the extensive use of artificial insemination technologies over the past three decades has resulted in very few cases being reported (Timoney, 2007). Dourine was previously common in an era when horses were valued in agriculture, the military, and the economy (Sazmand et al., 2020). Dourine became a significant issue in both the US and Canada in the early 20th century (Nielsen, 2003). Currently, the US, Australia, and Western Europe are regarded as dourine-free (Gizaw et al., 2017). This disease is endemic to numerous regions of Asia (Tanaka et al., 2023), Africa (Barrowman and van Vuuren, 1976), Russia (Zablotskij et al., 2003), the Middle East (Luciani et al., 2024), and Eastern Europe (Podaliri Vulpiani et al., 2013). The countries with the most recent official reports of dourine (i.e., positive CFT cases) include Venezuela (Perrone et al., 2018), Italy (Pascucci et al., 2013), Germany (Bassarak et al., 2016), Ethiopia (Getahun, 2019), Botswana (Masupu and Majok, 1998), Namibia (Kumba et al., 2002), Mongolia (Davaasuren et al., 2017), and Turkey (Marenzoni et al., 2013). However, it is challenging to determine whether animals that test positive for T. equiperdum are indeed cases because of the potential for cross-reactions in CFT.

Pathogenesis

Trypanosoma equiperdum is found in the seminal fluid and mucosal membranes of the genitals of infected donor animals during sexual contact (Ahmed et al., 2018). After entering the bloodstream, the parasites can travel to different body parts. This metastatic invasion usually results in the development of distinctive skin plaques (Getahun, 2019). Dourine frequently results in death; however, it can also recover on its own. The length, severity, and incubation period of the disease vary greatly (Gizaw et al., 2017). The illness is typically mild and persistent and can persist for several years.

Pathology

The condition is typically fatal and is marked by gradual emaciation, nervous system involvement, and edematous lesions of the genitals (Yasine et al., 2019b). The condition is known as “dourine” because of its distinctive skin lesions, which are elevated circular plaques of thicker skin that resemble money or “douros” and range in size from 1 to 10 cm in diameter (Meseret et al., 2016). Numerous active biological products are released, and immune complexes are formed as a result of the parasite’s constant antigenic diversity. These processes are undoubtedly the primary causes of various clinical and pathological alterations (Calistri et al., 2013).

Gross pathological lesion

The symptoms of dourine include ptosis of the lower lip, vaginal lesions, edematous skin plaques, peripheral edema, anemia, muscle hypotrophy, ataxia, and lack of hindlimb coordination (Pascucci et al., 2013). Unlike other trypanosomes, T. equiperdum appears to be more suited to the peripheral nervous system than the central nervous system, as evidenced by the occurrence of neurological symptoms without sensory alterations (Gizaw et al., 2017).

A postmortem examination revealed gelatinous exudate beneath the skin. Stallions cause thickening and infiltration of the testis, scrotum, and sheaths of the tunica (Edwards, 2008). The testicle is sometimes not visible because it is lodged in a hard mass of sclerotic tissue (Yasine et al., 2019a). In mares, gelatinous infiltration may thicken the vulva, vaginal mucosa, uterus, bladder, and mammary glands. In the abdominal cavity, the lymph nodes enlarge, soften, and even bleed. Paraplegic animals frequently have soft, mushy, and discolored spinal cords, particularly in the lumbar and sacral areas (Aliyi et al., 2018).

Infection with T. equiperdum in stallions does not seem to affect libido or erection capacity, even when the glans sheath and scrotum are significantly enlarged (Büscher et al., 2019). Similarly, neither mares nor stallions seem to be negatively impacted by illness in terms of fertility. Additionally, the study documented three cases of infected mares being conceived by clean stallions and five cases of clean mares being conceived by infected stallions (Gizaw et al., 2018). Two healthy foals reared to adulthood were born to an afflicted mother.

Microscopic lesions

Hemostasis deposition in the spleen, iliac, supramammary, and popliteal lymph nodes exhibiting nonspecific reactivity with plasma cell hyperplasia, indications of elevated hemolymphatic activity, are characteristics of the disease as determined by histological analysis of tissue samples (Yasine et al., 2019a). The characteristic symptoms of pustular dermatitis are evident in the edematous plaques, which are particularly severe around the lesion. The skin adnexa is involved, and vacuolar degeneration and severe inflammation that extend into the deepest layers of the skin are observed. Perivascular plasma cell inflammation is also present (Yasine et al., 2019a). In the same region, there is an exudate of cellular debris known as “trypanosomal sand” that primarily consists of eosinophils and free parasitic protozoan bodies.

Inflammatory vasculitis and neurodegenerative lesions have been reported in the central nervous system of sick horses, along with edematous infiltration of the lingual and facial nerves (Mungun-Ochir et al., 2019). Russell bodies are present in the udder, together with high supramammary lymph node reactivity and histological lesions caused by extensive interstitial inflammation (Pascucci et al., 2013). Multifocal hepatitis is observed in the liver, and renal pelvic inflammation caused by plasma cells affects the kidneys. Infected horses also have periglandular inflammation of the vulva, vagina, uterus, and clitoris (Gizaw et al., 2021). The parasite appears to travel primarily through the lymphatic system, as evidenced by the consistent detection of positive iliac and supramammary lymph nodes and lymphatic activity on macroscopic observation and histological testing (Tanaka et al., 2020).

Although depigmentation surrounding the perineum is frequently mentioned as a feature of clinical instances of dourine, no microscopic description of these lesions has been referenced in earlier research. Severe dermatitis with hydropic degeneration, necrosis of stratum spinosum keratinocytes, and necrosis of basal cells, including melanocytes, with an excess of free melanin pigment in the epidermis have been reported recently (Scacchia et al., 2011). Since the pigmented areas are microscopically characterized by severe cell necrosis, abundant free melanin, and cystic formations within the epidermis, significant melanocyte necrosis may be the origin of the depigmentation around the vulvar skin of infected mares (Singh et al., 2021).

Clinical signs

The incubation period between exposure and the onset of clinical symptoms varies widely, ranging from a few weeks to several years (Ungogo and de Koning, 2024). The degree and presentation of clinical symptoms of dourine vary greatly. The main characteristics of the disease are genital swelling, skin plaques, and neurologic symptoms such as paralysis, incoordination, conjunctivitis, keratitis, anemia, and progressive emaciation, although the severity varies based on the strain’s virulence, the horse’s nutritional condition, and stressors (Tanaka et al., 2021). Clinical symptoms usually come and disappear with relapses, which may be brought on by stress, and usually take weeks or months to develop (Gizaw et al., 2017). This could occur multiple times before the animal passes away or significantly recovers. The death rate is estimated to be >50% (Luciani et al., 2024).

The disease’s progression has been divided into three phases: stage 1 (genital lesions), stage 2 (skin signs), and stage 3 (nervous signs). Stage 1 manifests 1–2 weeks after infection and is characterized by vaginal edema and swelling. Stage 2 is characterized by the appearance of distinctive skin plaques, often known as “silver dollar” plaques, along with skin thickening, which some authors deem pathognomonic. Neurological problems, increasing anemia, and spinal paresis mark stage 3, which frequently results in death (Podaliri Vulpiani et al., 2013).

The edematous plaque, which comprises elevated skin lesions up to 5–8 cm in diameter and 1 cm thick, is the pathognomonic symptom (Calistri et al., 2013). Although it can develop anywhere on the body, the plaque typically covers the ribs and lasts for 3–7 days. This plaque is not always present (Argaw, 2023). Intermittent pyrexia is accompanied by nervous symptoms, such as inability to coordinate, particularly with the hind limbs, lips, nose, ears, and throat (Hébert et al., 2023). The udder, perineum, and vaginal area may become pigmented. The initial clinical symptom of stallions is fluctuating enlargement of the prepuce and glans penis (Gizaw et al., 2017). The edema extends anteriorly throughout the inferior abdomen and posteriorly to the scrotum, inguinal lymph nodes, and perineum. Edema can spread over the entire abdominal floor in heavy-breed stallions (Argaw, 2023).

According to Podaliri Vulpiani et al. (2013), infected stallions exhibited less severe symptoms than infected mares. The stallion virtually exhibited no symptoms 6 months after infection. However, due to the small number of cases included in this study, gender-related differences could not be statistically evaluated. Pereira et al. (2024) demonstrated that, aside from the increase in virulence brought on by ongoing infection, the disease often progressed more in stallions than in mares.

Diagnosis

The diagnosis of dourine can be challenging because of factors such as a lack of knowledge about the parasite and host–parasite interactions following infection, but in reality, the diagnosis is based on molecular and serological testing (Büscher et al., 2019). Dourine can be diagnosed by identifying the parasite, although it is very difficult to locate and differentiate T. equiperdum from T. evansi under a microscope (Desquesnes et al., 2022b). Trypanosomes may be found in trace amounts in breast gland exudate, edematous fluid from the external genitalia, lymph, vaginal or preputial fluid, or scrapings (taken shortly after infection) (Smirlis et al., 2010). Organisms are only present in plaque for a few days and are more likely to be found shortly after edema or the first occurrence of plaque.

Wet and thick blood films

A coverslip was used to evaluate 5–10 μl of blood on a slide under a microscope at X400 magnification in thin and thick blood films. Infected animals exhibit the movement of parasites, known as trypanosomes, among their erythrocytes (Giordani et al., 2016). Despite being used today, its sensitivity is extremely low, and its detection limit can reach 10,000 trypanosomes/ml. The sensitivity of thin Giemsa or field-stained blood films is similarly low. It takes 10–20 minutes per slide and requires skill to identify parasites (Dóró et al., 2019).

Serology technique

It is extremely difficult to find the parasite in the bodily fluids of infected horses; thus, serological evidence is used to diagnose T. equiperdum infection (Gizaw et al., 2017). The CFT, which has been effectively employed in eradication operations, is still the sole globally recommended test for T. equiperdum, despite the development of antibody and antigen enzyme-linked immunosorbent tests (Bassarak et al., 2016).

A number of additional alternative serological tests are also employed, including the competitive immunoassay employing the cELISA, the immunodiffusion array method, and the Hagebock agar gel immunodiffusion test. The cELISA technique provides a number of benefits over CFT, including the ability to be automated, reproducible, and to provide objectively measured and computed results, and a lower turnaround time than the analogous CFT procedure (Khan et al., 2012). Although serological testing may be the preferred technique for mass population screening, it is insensitive to parasites, particularly T. equiperdum, which is regarded as a tissue parasite rather than a blood parasite (Miller et al., 2018).

Molecular technique

Fluid and tissue samples, as well as fluids from naturally dourine-infected horses, have been subjected to highly sensitive real-time polymerase chain reaction (PCR) for the subgenus Trypanozoon, which enables the detection of parasites in trace amounts (Sereno et al., 2022). Given its failure in blood samples following the early stage of infection, PCR and other comparable DNA amplification techniques have been used to analyze exudates or tissue samples (Ahmed et al., 2013). It is possible to detect parasites in bodily fluids, such as blood, with high sensitivity using direct diagnosis based on molecular methods (Ndao, 2009).

Differential diagnosis

Surra, coital exanthema, equine viral arthritis, equine infectious anemia, and purulent endometritis cause such infectious equine metritis (Pal et al., 2024). Differentiating T. equiperdum microscopically (morphology, motility) from other members of the subgenus Trypanozoon (T. evansi or T. brucei) in nations where Nagana or Surra are present is challenging (Büscher et al., 2019). Specifically, morphological characteristics cannot be used to discriminate between T. equiperdum and T. evansi. Both species are slender, monomorphic trypomastigote with free flagella, albeit with short and pleomorphic varieties. The typical strains of this parasite are between 15.6 and 31.3 μm (Ventura et al., 2002).

Although the pathogenicity of different strains of the parasite T. equiperdum has been documented, no strain of the parasite has been isolated in any nation since 1982, and the majority of strains currently found in national veterinary diagnostic laboratories are related to T. evansi. According to one theory, the “dourine” illness condition is actually a host-specific immunological response to T. brucei or T. evansi infection, rather than the existence of T. equiperdum as a distinct species (Choi et al., 2024). According to more recent kDNA research, T. equiperdum and T. evansi are both subspecies of T. brucei (Cuypers et al., 2017). The final classification of Dourine is still pending.

Transmission

In contrast to other trypanosoma infections, dourine is primarily transmitted through sexual contact (Pascucci et al., 2013). Dourine is the only trypanosomiasis that is not transmitted by insect vectors (Pal et al., 2024). Furthermore, unlike other trypanosomes, T. equiperdum is mostly a tissue parasite that is rarely found in the blood (Zablotskij et al., 2003).

The seminal fluid and vaginal mucous membranes of infected donor animals contain trypanosomes, which are transferred to the recipient during sexual contact (Yasine et al., 2019a). Since trypanosomes are typically found in the capillaries of the mucous membrane of the urogenital tract, they are rarely found in the circulation of the host (Getahun, 2019). Nonetheless, animals with persistent infections can occasionally have some trypanosomes in their peripheral blood. This condition may allow hematophagous insects to transmit the parasite mechanically, although such occurrences are considered to be extremely rare (Venturelli et al., 2022).

The presence of the parasite in the seminal fluid and mucous exudates of the penis and its sheath makes it easier for stallions to spread this infection to mares. Stallions contract the virus from infected mares because the parasite is present in their vaginal mucous (Tyrnenopoulou et al., 2021). According to a study that used laboratory and epidemiological analysis of the outbreak in Italy, as well as clinical findings, the infection is directly spread from animal to animal during coitus. This analysis was based on characteristics such as prevalence, age, reproductive activity, and relationships between affected animals (Calistri et al., 2013). Trypanosomes occasionally vanish from the vagina or urethra as the illness worsens; the critters are not contagious during these times. The non-infectious phase is more likely to occur later during the illness and can last for weeks or months (Pal et al., 2024). Therefore, transmission is most likely to occur early in the disease course. The positive PCR test result from a preputial swab taken from a stallion free of dorin just after mounting an infected mare is an intriguing discovery in the literature (Musinguzi et al., 2022). The horse tested negative for every subsequent test, confirming the hypothesis that the parasite is found in genital tissue but that sexual transmission is not always the case (Podaliri Vulpiani et al., 2013).

Trypanosoma equiperdum can infect foals by contaminating the nasal membranes or conjunctiva with vaginal fluids and can even penetrate intact mucous membranes (Constable et al., 2017). These infected foals can spread the organism as they mature (Tyrnenopoulou et al., 2021). Although there is no proof that arthropod vectors contribute to infection, other transmission routes are potentially feasible. Mechanical transmission by blood-sucking insects cannot be ruled out, as demonstrated by experimental intravenous or intraperitoneal infections (Desquesnes et al., 2022b). Foals born to mares infected with T. equiperdum may contract the infection during parturition or in utero (Constable et al., 2017). It is considered uncommon for foals to contract the infection by consuming contaminated milk or colostrum. Trypanosomes found in breast secretions may provide evidence that the infection can occasionally be passed from the mother to the foal during lactation (Alfituri et al., 2020). Passive transmission of antibodies will make foals who consume colostrum from an infected mare seropositive; these foals are often seronegative by the time they are 4–7 months old.

Treatment

Currently, the World Organization for Animal Health advises the killing of horses that test positive for CFT as a successful control measure (Hébert et al., 2023). Treatment is generally not advised in dourine-free areas due to the possibility of asymptomatic carrier animals and the concern that treated animals may continue to spread the disease (Constable et al., 2017).

Suramin, diminazene, quinapyramine, and Cymelarsan can effectively inhibit trypanosome species, according to in vitro drug sensitivity testing of T. equiperdum. Hagos et al. (2010) tested the in vivo efficacy of diminazene diaceturate (Diminasan^®) and bis(aminoethylthio)4-melaminophenylarsine dihydrochloride (Cymelarsan^®) in rats. Diminazene diaceturate failed to cure rats infected with the Ethiopian Dodola strain, even at high doses of up to 28 mg/kg body weight (four times the recommended dose in cattle).

Horses treated with Cymelarsan^® at doses of 0.25 and 0.5 mg/kg body weight did not exhibit any observable parasitemia 24 hours after treatment. The mean PCV levels also increased following therapy, and seroreversion in the trypanosome card agglutination test was noted beginning 150 and 170 days later (Hagos et al., 2010). This could be because the host system’s lack of an antigen source inhibits the generation of antibodies, which causes the antibodies to become diluted in the serum. According to Hagos et al. (2010), there was no recurrence, and the body state improved following therapy for a persistent infection. Furthermore, the clinical manifestations of weakness, ventral edema, and hindlimb incoordination vanished within 10 days, and the PCV gradually increased. However, a recent study revealed that parasites were still present in the CSF of horses treated with Cymelarsan^®, and that both Cymelarsan^® and Diminasan^® caused relapse in treated mice (Habte et al., 2014; Cauchard et al., 2014).

Recently, the ex vivo trypanocidal efficacy of 1-(2-hydroxybenzylidene) thiosemicarbazide against a Venezuelan strain of T. equiperdum was described (Parra et al., 2017). The ability of the compound to suppress parasites was more pronounced in culture media. Another recent study found that eCATH1 exhibited encouraging outcomes in in vitro laboratory experiments concerning its trypanocidal activity on Trypanozoon spp. through mitochondrial alterations and plasma membrane permeabilization (Schlusselhuber et al., 2014). Mortality decreased in mice infected with T. equiperdum when eCATH1 was administered at a dose of 10 mg/kg. According to these results, eCATH1 might be a viable option for the development of novel therapeutic molecules to treat trypanosomosis (Cauchard et al., 2014).

Prognosis

The removal of parasites from the bloodstream of cymelarsan-treated horses was quickly demonstrated (Hagos et al., 2010; Cauchard et al., 2014). However, since the parasite is a tissue parasite, it can hide in places that are difficult for medications to reach, leading to relapse (Cauchard et al., 2014).

If treatment is not received, most horses will either die or acquire a chronic form of dourine with the above-mentioned clinical symptoms (Podaliri Vulpiani et al., 2013). Although the overall state of the body gradually worsens, parasitemia may disappear after 80 days of illness (Hagos et al., 2010). Infected animals developed parasitemia 80 days after infection; nevertheless, when immunosuppressive medications were administered, they resumed exhibiting parasitemia (Hagos et al., 2010). Thus, the parasite is believed to be able to conceal itself from the immune system.

Control

Dourine vaccination does not yet exist. Preventing natural mating or artificial insemination with diseased horses (male or female) or contaminated stallion semen is the most crucial control measure because dourine is largely a sexually transmitted disease (Tyrnenopoulou et al., 2021). As a result, the goal of dourine prevention is to create an infection-free state, which is accomplished by checking the blood for T. equiperdum antibodies, which are more accurate than checking for the protozoan parasite itself (Büscher et al., 2019). A blood test for CFT antibodies should be performed on any horse coming in from an endemic or invaded area (Hébert et al., 2023).

Disease control depends on the killing of sick animals, required notification, and mobility restrictions enforced by legislation in most nations (Pal et al., 2024). In non-endemic regions, the cause of the infection must be determined, all individuals in contact must be tracked down and tested, and infected and seropositive horses must be euthanized (Gizaw et al., 2017). The World Organization for Animal Health (OIE) currently enforces an eradication approach that involves killing horses with seropositivity when treatment is forbidden (OIE, 2018). However, enforcing stringent testing and slaughter regulations to control dourine in developing nations is not economically viable. Based on the findings of in vivo drug sensitivity research, an improved pharmacological treatment plan, rather than eradication, can be recommended to the OIE in dourine-endemic areas (OIE, 2018).

Castration of an adult stallion should be handled carefully while trying to eradicate the species because it does not always change the animal’s copulation capacity (Waqas et al., 2024). Serum samples should be obtained following the isolation (quarantine) time to ensure that the animal is not in the incubation stage and to avoid the introduction of dourine. The inability to accurately diagnose T. equiperdum has made it more difficult to gather accurate data on the disease’s prevalence and distribution and to implement monitoring, treatment, and control programs. Furthermore, the lack of a vaccine to prevent trypanosomosis and the scarcity of trypanocide medications have made it difficult to control and prevent the disease in endemic areas (La Greca and Magez, 2011).

Prospects for the development of vaccines for dourine disease in horses

The absence of an effective vaccine is a major challenge in controlling this disease. Despite advances in molecular parasitology, T. equiperdum vaccine development has been hindered by the parasite’s antigenic variation mechanisms, particularly its variant surface glycoproteins, which enable immune evasion (La Greca and Magez, 2011; Horn, 2014; Pereira et al., 2022). This antigenic variability complicates the formulation of a stable, long-lasting vaccine, as seen in related trypanosome species such as T. brucei (Müller et al., 2018).

Moreover, the lack of reliable diagnostic tools further intensifies control efforts. Traditional diagnostic methods, including microscopic examination and serological tests such as the CFT, often suffer from low sensitivity and specificity (Vainionpää and Leinikki, 2008). Molecular diagnostics, particularly PCR-based assays, have shown promise due to their higher sensitivity and species-specific detection (Yang and Rothman, 2004; Oliveira et al., 2010). However, infrastructure requirements limit the use of PCR, especially in low-resource settings where dourine is endemic.

Despite these barriers, recent research has highlighted potential vaccine targets, including invariant antigens and conserved proteins such as flagellar pocket proteins or enzymes involved in parasite metabolism (Autheman et al., 2021). Advances in omics technologies and reverse vaccinology offer new avenues for identifying such candidates. Overall, addressing dourine requires an integrated approach that combines innovative vaccine research, improved molecular diagnostics, and enhanced international surveillance and policy coordination.

One Health perspective on dourine disease

The One Health approach, which recognizes the interconnectedness of human beings, animals, and environmental health, provides a comprehensive lens for understanding and managing dourine disease. Although dourine (T. equiperdum) is primarily an equine disease transmitted venereally, its implications extend beyond equine health, affecting socioeconomic stability, biodiversity, and potentially broader ecosystems.

Although T. equiperdum is not recognized as a direct zoonotic agent, its close phylogenetic relationship with T. evansi and T. brucei—both of which have zoonotic potential—raises concerns about possible cross-species transmission under certain ecological conditions (Claes et al., 2005a; Carnes et al., 2015; Suganuma et al., 2016). This underscores the need for vigilance in disease surveillance across species boundaries, particularly in regions with high interactions between livestock, wildlife, and human populations.

The environmental dimension is significant in the context of horse breeding and transboundary animal movements. Poor biosecurity measures, lack of diagnostic capacity, and unregulated breeding practices facilitate the silent spread of dourine across regions and borders, potentially impacting wildlife equids and disrupting local ecosystems (OIE, 2018; Taylor et al., 2020; White and Pelzel-McCluskey, 2023).

Furthermore, the persistence of dourine in endemic areas reflects the broader challenges of veterinary infrastructure, policy gaps, and surveillance weaknesses, especially in low- and middle-income countries. A One Health approach calls for integrated policy frameworks that strengthen veterinary services, promote safe breeding practices, and enhance molecular diagnostic capacity, coupled with public awareness campaigns to mitigate risks associated with equine trade and movement (Zhang et al., 2024; Lane et al., 2025).

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Conclusion

In summary, the chronic character of dourine and the difficulties in diagnosing and controlling it make it a danger to both equine health and agricultural productivity. The scientific community, veterinary authorities, and policymakers must immediately focus on this neglected disease to create efficient treatments and diagnostic instruments that will guarantee the health of horses and the stability of agricultural systems around the world. This integrative effort is crucial not only to control dourine but also to pre-empt the broader consequences of neglected vector-borne and parasitic diseases in an increasingly interconnected world.

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Acknowledgments

The authors thank the Faculty of Veterinary Medicine, Universitas Airlangga.

Conflict of interest

The authors declare no conflict of interest.

Funding

The authors would like to thank Universitas Airlangga for their managerial support.

Author’s contributions

RR, ARK, IM, and SU drafted the manuscript. AOA, SM, TDL, and IUK revised and edited the manuscript. BU, IBM, and SDR prepared and critically checked the manuscript. AQD, TH, and GR edited the references. All authors have read and approved the final version of the manuscript.

Data availability

All references are open access, so data can be obtained from the internet.

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