Syphilis is a sexually transmitted disease caused by the spirochetal bacterium Treponema pallidum subspecies pallidum. Globally, an estimated 12 million cases of syphilis occur annually. In the United States, 13,997 cases of primary and secondary (infectious) syphilis were reported to the Centers for Disease Control and Prevention (CDC) in 2009, a 3.7% increase from 2008 and a 134% increase from 2000, when a post-war low of 5,979 primary and secondary syphilis cases was reported. Men who have sex with men (MSM) — especially those who are HIV infected — and blacks are disproportionately affected by syphilis. Geographically, urban areas and the Southeastern region of the United States have the highest rates.
Syphilis is most commonly transmitted by skin-to-skin (or mucous membrane) contact. Following exposure, the infection passes through the following stages:
Primary syphilis, characterized by a painless ulcer, called a chancre, usually develops three weeks after exposure (range 10 days to 90 days) at the site of inoculation. The chancre heals spontaneously after several weeks.
Secondary syphilis is most often characterized by a generalized rash that also resolves without treatment. Rash on the palms and soles can also occur, as can systemic manifestations such as fever, malaise, and lymphadenopathy. Given the widely variable nature of the rash and other manifestations of the disease, syphilis has acquired the moniker “The Great Imitator.”
Early (one year) latent syphilis, defined by the absence of signs or symptoms of disease and diagnosed by serologic evidence of infection.
Tertiary syphilis, which affects about a third of untreated patients and manifests with cutaneous, cardiovascular, or neurologic disease.
Syphilis can also be acquired in utero at any stage of pregnancy and lead to congenital syphilis. Routine syphilis screening and treatment in pregnant women has made congenital syphilis rare in the United States.
Approaches to syphilis diagnosis
Because T pallidum is too fragile an organism to be cultured in the clinical setting, diagnostic testing relies on two approaches: direct detection of the organism and indirect evidence of infection.
Direct methods include darkfield microscopy, molecular assays to detect T pallidum DNA, and histopathologic examination of biopsies of skin or mucous membranes (which can also provide indirect evidence of infection, on the basis of patterns of inflammation in the tissue). Direct methods have the advantage, in some cases, of detecting infection before a patient has mounted a measurable antibody response that results in a reactive serologic test result.
Darkfield microscopy allows visualization of live treponemes obtained from a variety of cutaneous or mucous membrane lesions, as follows.
In primary syphilis, the chancre teems with treponemes that can be seen with darkfield microscopy. The sensitivity of darkfield microscopy for the diagnosis of primary syphilis is approximately 80%. Darkfield sensitivity declines over time and can also decrease if the patient has applied topical antibiotics to the lesion(s). Of note, the mouth harbors normal non-pathogenic treponemes that are indistinguishable microscopically from T pallidum. Therefore, oral specimens cannot be used for darkfield microscopy because of the possibility of false-positive test results.
In secondary syphilis, mucous patches (as long as not oral) and condyloma lata (found in moist areas between body folds) are appropriate specimens for darkfield microscopy. Dry skin lesions usually do not contain sufficient organisms for darkfield testing.
In congenital syphilis, moist discharge from the nose (snuffles) and vesiculobullous lesions of the skin are high-yield specimen sources for darkfield testing.
Indirect methods of diagnosis include serologic testing of blood or cerebrospinal fluid (CSF) and detection of CSF abnormalities (elevated white blood cell count or protein) consistent with neurosyphilis. Serologic testing of blood involves demonstration of host antibody to either endogenous antigens (non-treponemal tests) or to antigens of T pallidum (treponemal tests). Non-treponemal tests, including the rapid plasma reagin test and the venereal disease research laboratory test, have historically been used as the initial screening tests for the serologic diagnosis of syphilis. If a patient’s non-treponemal test is reactive, confirmatory testing with a treponemal test is performed, using either the T pallidum particle agglutination test, the fluorescent treponemal antibody-absorbed test, or another treponemal test. A reactive treponemal test confirms the diagnosis of a new or previously treated case of syphilis. If the treponemal test is non-reactive, the positive non-treponemal test result is considered a biologic false-positive that is not diagnostic of syphilis. A newer algorithm that is gaining in popularity begins with a treponemal enzyme immunoassay as the initial test, followed by a non-treponemal test, and if necessary, a “tie-breaker” third test, using a different treponemal test.
In 1830, J.J. Lister (the father of Joseph Lister) invented the darkfield microscope, in which the standard brightfield (Abbe) condenser is replaced with a single- or double-reflecting darkfield condenser. The use of indirect light allows visualization of organisms too small to be seen under direct-light microscopy. In 1906 in Vienna, Karl Landsteiner and Viktor Mucha were the first to use darkfield microscopy to visualize T pallidum from syphilis lesions. Since then, darkfield microscopy has served a vital role in the diagnosis of infectious syphilis.
Clinicians and laboratorians should use universal precautions in collecting, transporting, and handling specimens for darkfield examination. Acquisition of syphilis through occupational exposures, including contact with specimens collected for darkfield microscopy, has been reported.
Proper specimen collection and handling is critical for optimizing the sensitivity of darkfield testing. The clinician should gently cleanse and abrade the lesion with moist gauze, while trying not to cause bleeding. The goal is to obtain serous exudate, while minimizing contamination by blood or pus caused by secondary infection. The clinician might need to apply pressure at the margins of the lesion to express adequate serous fluid. The clinician transfers the serous fluid to a glass slide, either by direct application of the slide to the lesion, or by transferring the fluid with a bacteriologic loop or the edge of a cover slip. If necessary to prevent drying of the specimen, a drop of non-bacteriostatic normal saline may be placed on the slide; however, the saline might dilute the specimen and reduce test sensitivity. The clinician places a cover slip on top of the specimen. A trained microscopist then examines the specimen as soon as possible, no greater than 20 minutes after specimen collection. Placing the slide in a closed container such as a Petri dish during transport to the microscope might reduce evaporative drying.
Definitive identification of T pallidum depends on visualizing not only its typical morphology but also its typical motility. T pallidum is a delicate, tightly spiraled, corkscrew-shaped organism that rotates as it slowly moves backwards and forwards (translational movement); these movements are sometimes accompanied by a slight side-to-side oscillation. T pallidum will occasionally flex or bend sharply in the middle when obstructed by cellular elements or debris in the field but then spring back to its usual linear shape. In the genital region, Treponema refringens, which is part of the normal genital flora, can be distinguished from T pallidum by T refringens’ more coarsely wound spirals, greater flexibility, and rapid translational movement across the slide. In addition, the less experienced observer must guard against misidentifying Brownian movement of fibers or other linear debris as T pallidum.
After a methodical scanning of the entire specimen field of each slide, results are reported as one of the following:
Positive darkfield: Organisms with the characteristic morphology and motility of T pallidum observed
Negative darkfield: Either no treponemes found or spiral organisms seen but without the characteristics of T pallidum.
Unsatisfactory darkfield: The specimen could not be interpreted either due to drying or the presence of too many refractile elements, such as blood cells or fibers.
Syphilis is a legally reportable disease in all health jurisdictions in the United States. A positive darkfield examination should trigger a case report, regardless of clinical presentation or serologic results.
Because up to 25% of patients with primary syphilis have non-reactive serologic test results for syphilis, darkfield microscopy provides a critical complementary role in the identification of infectious syphilis. Darkfield microscopy requires, however, a special microscope and a trained microscopist in close proximity to where patients are examined, and few clinical facilities other than STD clinics and some hospitals have the capacity to perform darkfield microscopy. Given the resurgence of syphilis in the United States, the development and maintenance of facilities and skills to perform darkfield microscopy are essential to syphilis prevention and control.
Elaine F. Pierce, MD, MPH, and Kenneth A. Katz, MD, MSc, MSCE, work in the HIV, STD, and Hepatitis Branch of Public Health Services in the Health and Human Services Agency of the County of San Diego in San Diego, CA.
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