Current status and prospects for development of a vaccine against Trichomonas vaginalis infections

Highlights

• Most common non-viral and curable sexually transmitted infection globally.

• A non-reportable, often asymptomatic infection that increases HIV transmission.

• Underdiagnosis and under-recognition has lead to ongoing transmission and high incidence.

• Drug resistance in Trichomonas vaginalis is a concern for the future.

• An inexpensive vaccine for developing countries is needed and achievable.

Abstract

Trichomonas vaginalis is a sexually transmitted pathogen with an annual worldwide incidence of over 276 million infections, the highest of all curable and non-viral STI. A large proportion of cases are asymptomatic and under-diagnosed with conventional diagnostic tools. Infection has important maternal and fetal health consequences and can lead to a higher probability of HIV transmission and susceptibility. Lack of affordable accurate diagnostic tests globally and metronidazole resistance hinder T. vaginalis control efforts. Based on data from current vaccination studies in animal models, a human vaccine is achievable to intervene on the substantial incidence of infection.

Introduction

Recent World Health Organization estimates of the global incidence and prevalence of selected curable sexually transmitted infections reaffirms the need for public health intervention to control spread of Trichomonas vaginalis (Tv), a neglected parasite compared to other sexually transmitted infections (STI). Despite ranking as the most common curable and most common non-viral STI world-wide, relatively little research is conducted to understand its biology and pathogenesis. Furthermore, lack of education and screening programs allow the pathogen to go unreported and often undetected in millions of people across the globe. Incidence of Tv has increased by 11.5% since 2005 and is now estimated in 2008 surveys at 276.4 million new infections each year. The parasite's prevalence has increased by 22.2% since 2005 with recent reports of 187 million concurrent infections at any given time [1], [2]. To emphasize the severity of these numbers, Tv prevalence accounts for over half of curable STI; more than Chlamydia trachomatis (100.4 million), Neisseria gonorrhoeae (36.4 million) and syphilis (36.4 million) combined [1], [2]. Alternative control methods are clearly needed.

Men and women are infected in roughly the same proportion. However, women are considered to be impacted by the burden of disease more severely than men. Firstly, prevalence of Tv in women is roughly 10 times higher than men in any given region [2]. Women infected with Tv will often remain asymptomatic, with symptoms potentially developing within three months. Clinical manifestations of Tv infection, or trichomoniasis, include vaginal discharge of abnormal color and malodor, vulvar and vaginal irritation and/or erythema, colpitis macularis and a raised vaginal pH (>5) [3], [4], [5], [6]. Moreover, Tv infections are associated with cervical cancer (3.3 relative risk), pelvic inflammatory disease, birth complications including premature rupture of membranes and premature labor, infertility, and HIV acquisition and transmission [7], [8], [9], [10], [11].

T. vaginalis infection in men is considered a nuisance disease and men are most often transient or asymptomatic carriers. This lack of signs and symptoms helps facilitate the spread of Tv. Asymptomatic cases account for over 50% of Tv infections in men, though a range have been reported in literature (14–77.3%) [5], [12], [13], [14], [15]. Low sensitivity of laboratory testing used, discussed below, is the most probable explanation for the wide range of reported asymptomatic cases [14], [16]. An infection in the male urinary tract can remain asymptomatic until resolution [12], [17]. Following an asymptomatic incubation period male trichomoniasis presents itself as any of persistent urethritis, urethral discharge, dysuria, frequency of micturition, prostatitis, lower abdominal pain, pruritis, and epididymitis. Other complications have been ascribed including infertility and benign prostatic hyperplasia [7], [12], [17], [18]. Among a cohort of men with untreated Tv infection, the rate of recovered organisms dropped from 70% to 30% infected within 2 weeks of diagnosis suggesting spontaneous resolution [12]. However, this data has not been replicated using more sensitive molecular diagnostic techniques. Resolution in males has lead to the description of Tv as a nuisance disease, which undermines its impact on maternal/child health and has restricted interest in developing public policy in diagnosis, treatment and prevention strategies to understand the burden of Tv and reduce its impact as an STI pathogen.

T. vaginalis prevalence in men and women during the reproductive years is a major concern. Particularly, pregnancies coinciding with an active vaginal Tv infection may result in preterm birth, premature membrane rupture, and low birth weight [7], [19]. Investigation of the factors of premature rupture of membranes by Draper and colleagues [20], [21] revealed a possible connection between a decrease of protective vaginal proteases and the elastic strength of the amnion and chorion. Additionally, in the in vitro model of premature membrane rupture, weaker membranes was inoculum dependent, and was demonstrated by both presence of live Tv organisms but as well Tv free cell culture filtrates [20], [21]. This data coincides with the identification of Tv secreted cysteine proteases that have been shown to digest host-secreted protein soluble leukocyte protease inhibitor (SLPI) [22]. This host-derived serine protease is found on mucosal surfaces, interacts with innate inflammatory responses, and is protective of the vaginal milieu against HIV-1 [23]. Dysregulation of the inflammatory response during pregnancy related to SLPI could be responsible for the birth complications observed during pregnancy with concurrent Tv infection.

T. vaginalis infections during pregnancy are commonly given standard metronidazole treatment, the first line anti-Tv drug. Metronidazole was once considered to be teratogenic, however 50 years of usage has quelled that concern. However, treatment of Tv during pregnancy did not have the impact of reducing pregnancy complications as hoped. Metronidazole treatment during pregnancy was found to increase preterm labor (relative risk 3.0) compared to placebo (untreated) Tv infections [24], [25]. A potential conflicting factor of the results from the Klebanoff study is a nonstandard metronidazole dosage regime. Yet while no evidence of direct causality has been reported, it is speculated that dying Tv or the release of virus contained in some strains of Tv may result in stimulation of innate immune response or changes in bacterial flora that affect the pregnancy outcome, but studies are required to confirm this [25]. The overall data regarding Tv infection and pregnancy strongly suggests the value of screening and treatment of women seeking to become pregnant, or are at risk of unplanned pregnancies, and their male partners.

Reports regarding the increased transmission and acquisition of HIV in Tv infected study participants has stimulated recent interest in the parasite. The odds ratio of a female with Tv acquiring HIV has been measured between 1.52 and 2.74 [10], [26], [27]. A mathematical model of HIV infection based on a 1.8 odds ratio of acquiring HIV when infected with Tv estimates that 2% of all HIV acquired by females in the United States may be attributable to Tv [28]. In regions where Tv is more prevalent such as in Africa, the impact of Tv on HIV transmission could be higher.

Guenthner and colleagues [29] investigated the ability of HIV-1 to pass through a polarized monolayer of epithelial cells in conjunction with Tv. They demonstrated p24 gag could be detected in the basolateral supernatant in greater quantities compared to controls without Tv. Furthermore, differences in amount of epithelial damage based upon the Tv isolate was positively associated with HIV-1 passage through the monolayer. An additional experiment investigated the ability of Tv-stimulated peripheral blood mononuclear cells (PBMC) acutely infected with HIV-1 to induce replication of HIV-1. Activation of the acutely infected PBMC promoted HIV-1 replication. Thus two proposed mechanisms of synergy of Tv and HIV-1 were the pathogenesis of the Tv isolate's ability to induce damage to epithelial cells and the activation of acutely infected PBMC [29]. The relationship of Tv and HIV is reviewed in more detail elsewhere [30].

Co-infection of Tv and HIV in men and women is positively associated (odds ratio of 1.22 and 1.31, respectively) [31] with further reports identifying more Tv infections in HIV+ than HIV− patients and an odds ratio of 2.12 for HIV+ individuals to acquire Tv [26], [32]. Lower CD4 counts (40–140 and 150–250 cells/mL) and higher viral loads have been reported to be associated with likelihood of Tv diagnosis [33], [34]. The co-infection of HIV and Tv has also been associated with higher HIV viral loads in male genital ulcers and female vaginal discharge [8], [35], [36]. Additionally, a study of treatment of various vaginal infections in HIV+ participants revealed a significant reduction of HIV-1 RNA in vaginal secretions following treatment of Tv [37] and a decrease in frequency of viral shedding 3 months after treatment [8]. Overall, since Tv infections have a greater propensity to be present in HIV+ individuals and viral loads are increased in this scenario it is important to diagnose and treat Tv infections in HIV+ individuals to reduce the probability of HIV transmission.

Current treatment for cases of Tv is either a single 2 mg oral dose of metronidazole or a 2 mg oral dose of tinidazole [38]. Metronidazole and tinidazole are nitroimidazole compounds that are taken up by Tv as a prodrug by passive diffusion and activated by non-enzymatic reduction in the hydrogenosome, the Tv equivalent of a mitochondrion. Toxic nitro-radical molecules are produced that likely interfere with proteins and protein trafficking [39]. Unfortunately, metronidazole resistance has been detected as early as 1959 and is currently found in 2.5–10% of isolates tested [40], [41], [42], [43]. This value may be underreported given the number of untreated infections and the fact that in some infections the disease becomes subclinical despite treatment [44], [45]. Metronidazole resistance and high probability of asymptomatic reinfection up to one year following treatment are strong reasons for a prevention approach using vaccination [24].