Trichomonas vaginalis: reviewing the available diagnostic testing options

Trichomonas vaginalis is the most common curable sexually transmitted infection (STI), with increasing prevalence worldwide. The World Health Organization estimates that 276.4 million cases occurred in 2008, which represents an 11.2% increase over the previous estimate from 2005.¹ Despite the high prevalence of this infection, we have had little call for laboratory testing for this organism in the past. However, T. vaginalis is receiving more attention of late, which may result in increased demand for both diagnosis of trichomoniasis and screening for asymptomatic infection. I will discuss the cause for the increased interest in this pathogen and the currently available diagnostic options for centralized laboratories.

The increasing need: why now?

T. vaginalis has long been recognized as a common cause of vaginitis, known as trichomoniasis, but it has only been acknowledged as being exclusively transmitted through sexual contact within the last thirty years. However, assumptions regarding infection with T. vaginalis have affected the management of this infection. As laboratorians, it is important to understand these assumptions in order to be able to provide accurate feedback to clinicians as the demand for T. vaginalis testing increases.

The first of these assumptions is that all women with T. vaginalis infection have symptoms and thus can be diagnosed based on clinical observations. This dogma has been reinforced by the practice of testing only women with symptoms; thus, all cases come from symptomatic women. However, as more studies have been performed using non-microscopy methods, including culture, antigen detection, and nucleic acid amplification tests (NAATs), data suggest that as many as 50% to 60% of women with T. vaginalis do not exhibit symptomatic discharge or dysuria.² Given that this organism is not commensal, we may miss infection with a known pathogen if we rely solely on clinical presentation. There has been an increasing amount  of literature calling for targeted screening as well as improved assays for diagnosis of symptomatic individuals.

The need for improved diagnostics is not universally supported, in part due to the second assumption: that wet preparation microscopy is a sensitive diagnostic method. It is often hard for people who have seen a trichomonad wiggling through the field of vision on a microscope slide to imagine that they could actually miss detecting this organism! Unfortunately, that perception is responsible for the continued reliance on what is in fact a highly insensitive diagnostic method.

Why insensitive? The swab sample first has to capture sufficient numbers of organisms; then the saline prep has to have been taken from a portion of the saline that contained the organisms; finally the sample needs to be read microscopically within 10 minutes of collection. Trichomonads lose motility quickly when at ambient temperature³ and appear, in size and shape, similar to white blood cells, which are plentiful in women with discharge. Estimates of the sensitivity of wet prep range from 51.3% to 61.5% when compared to highly sensitive diagnostic assays.^4,5 The specificity of microscopy is good due to the characteristic asynchronous movement of trichomonads, and thus wet prep may be an excellent tool for identifying infections as a first stage in a clinical process that would also involve more sensitive, but also more expensive, technologies for women with wet prep-negative results.

The other rationale for not supporting targeted screening is based on the third assumption: that infection with T. vaginalis is self-limiting and has no, or few, negative consequences if left untreated. As a direct result of our lack of strong epidemiologic data regarding the true prevalence of this disease and its consequences, T. vaginalis does not receive a great deal of federal funding for epidemiologic or health services research. Despite the limited number of studies, however, the data are highly suggestive that untreated infection, even in asymptomatic women, may in fact have serious health consequences. These consequences include premature rupture of membranes and low birth weight babies in infected pregnant women,⁶ pelvic inflammatory disease,⁷ and, most significantly, increased risk of both acquisition^5,8 and transmission of HIV.^9,10

While no large-scale randomized control treatment trials have demonstrated improved pregnancy outcomes, the limited number of these trials suggests that additional studies are needed to determine the true impact of diagnosis and treatment early in the course of pregnancy. The association with HIV remains of highest concern given what we know about the epidemiology of T. vaginalis infection. The prevalence rates are often higher than those estimated for chlamydia and gonorrhea combined. Thus, even a small increase in HIV risk is exacerbated due to the large number of people with infections. Strong data from prospective studies have shown that risk of HIV infection in women with T. vaginalis is at least twofold higher than in women without T. vaginalis regardless of the presence or absence of symptoms.^5,8

Additionally, the risk of acquiring T. vaginalis is higher for women living with HIV,⁹ and this leads to increased genital compartment HIV viral load,¹⁰ which is the single most important risk factor for transmission of HIV to a sexual partner. Treatment has been shown to reduce the genital compartment viral load, and thus transmission risk, in both men and women.^10,11 Given the populations who are at shared risk for T. vaginalis and HIV, screening and provision of single dose treatment should be evaluated as an HIV prevention method. Studies of cost-effectiveness and treatment impact are clearly needed. However, such studies are expensive and time-consuming. Public health action with even the potential to reduce HIV rates in vulnerable populations should be considered while waiting for the results of these types of studies.

In summary, we now know more about the distribution and potential consequences of T. vaginalis infection than we have known in the past. It is often asymptomatic, but it may lead to serious consequences if left untreated. There is currently rigorous debate in our field regarding the applicability of targeted screening and how this affects management of patients.^12-15 Such debate is healthy and will continue to raise the profile of this often ignored infection.

For the laboratory: what’s on the way?

As a direct result of this increase in attention paid to T. vaginalis, laboratories should expect to see ever-increasing numbers of requests for trichomoniasis diagnostics. Currently my laboratory supports bundled molecular testing for any patients being tested for chlamydia/gonorrhea infections. Populations screened include women in the first trimester of pregnancy attending the local antenatal clinic, adolescents seeking contraceptives, women attending the local STD clinic, women seeking screening at non-clinical venues (e.g., health fairs, high schools, etc.), and women in correctional settings.

The local STD clinic uses a triaged approach to trichomoniasis screening in order to minimize costs while maximizing case finding. Women with symptoms are screened first with microscopy, and if positive they are treated. Women with negative wet prep results are tested using the sample collected for chlamydia/gonorrhea screening (no second sample collection is necessary). Women without symptoms collect their own vaginal samples prior to interacting with a clinician. These samples are tested for chlamydia/gonorrhea and trichomoniasis with no microscopy. As a result of this inclusion with routine screening, our overall testing volume for STIs has increased by ~25%. Currently, men are not routinely screened for trichomonas in any settings. However, at the STD clinic men who do not respond to therapy for non-gonococcal urethritis are subsequently tested for T. vaginalis.

The laboratory-based testing options for T. vaginalis include an antigen-based immunochromatographic assay that has good sensitivity, detecting approximately 80% of infections¹⁶ (this assay is intended as a point-of-care test); an instrument-based direct specimen DNA-probe-based diagnostic test (intended for use in clinical settings to identify the causative agent of vaginitis—only to be used on samples from symptomatic patients); a nucleic acid amplification test; polymerase chain reaction; and culture. The DNA probe assay has been estimated to be approximately 62% sensitive when compared to NAATs.¹⁷ Generally, the cost compared to wet prep, which is considered to have acceptable sensitivity, has been the factor preventing uptake of these assays.

Culture is collected in a closed puch system which is then read microscopically in its entirety.¹⁸ However,  despite the logistical improvements offered by this system, culture remains fairly insensitive, less than 75%,^19,20 and requires a trained microscopist and three to seven days of continued reading. Thus, the advantages offered by this laboratory method are modest at best.

The true reason for the increased attention to T. vaginalis is the availability of molecular diagnostics (see review by Hobbs, et al¹²). We now have the capacity to offer testing using  samples provided for chlamydia/gonorrhea screening or samples received specifically for  trichomonas diagnostics. High-throughput platforms are now available that use RNA-based amplification and have sensitivity estimates at or above 95%. A polymerase chain reaction-based assay currently under evaluation also has sensitivity reported to be at or above 90%.

Those molecular assays provide distinct advantages over other types of diagnostics. First, they can be performed using samples collected for chlamydia/gonorrhea screening on fully automated systems. This allows clinicians to order a more complete diagnostic or screening panel from a single sample. Additionally, these assays are ideally suited for vaginal samples, the recommended sample type for chlamydia/gonorrhea screening, and can be self-obtained. Given the high estimates for prevalence among women that are appropriate for chlamydia/gonorrhea screening, addition of screening for trichomonas has high potential for identifying previously missed infections. Beyond bundled screening of women being tested for chlamydia/gonorrhea, the distribution of trichomonas is somewhat different from these other STIs with prevalence rates peaking in older women (40-55 years of age).¹⁴ Thus, for women at elevated behavioral risk, or for women with symptoms, this technology provides highly accurate results for populations that are not being routinely screened for chlamydia/gonorrhea.

A limitation to some of the currently available assays is that the trichomonas tests are distinct from the chlamydia/gonorrhea assays and therefore require processing a single sample through an analyzer twice, once for chlamydia/gonorrhea and once for trichomonas results. This has impact on the throughput of samples, and if the volume of testing becomes substantial, may require careful scheduling of the testing to ensure that all three test results are available for reporting on the same day. This is less of an issue with systems that have random access capability.

The availability of molecular diagnostics, especially the ability to utilize samples not restricted to collection by a clinician, has improved our ability to begin to collect epidemiologic data regarding the distribution and associated risks of T. vaginalis infection. As more data are generated, laboratories should expect to see continued increases in requests for T. vaginalis testing. This may lead to improved studies of the negative outcomes of untreated infections as well as new treatment trials to evaluate the impact of treatment on prevention of the consequences of infection. However, despite the expected increase in test requests, we should not expect this STI to be added to the notifiable diseases list anytime in the near future.²¹ Until stronger epidemiologic and prevention data are available, using public health resources to track this infection in the population is not warranted.

This is an exciting time in the development of control efforts for T. vaginalis. Centralized laboratories will play a significant role in generating the data necessary to make sound patient management decisions and to set public health policies going forward. Now is the time to consider adding T. vaginalis diagnostics to your menu in preparation for the future.