T. pallidum is a member of the order Spirochaetales and the family Treponemataceae (Fig. 18-1). The genus Treponema includes a number of species that reside in human gastrointestinal and genital tracts. T. pallidum, Treponema pertenue, and Treponema carateum are human pathogens responsible for significant worldwide morbidity (Table 18-1). Yaws, pinta, and bejel are diseases caused by bacteria closely related to T. pallidum. Yaws is common in the Caribbean, Latin America, Central Africa, and the Far East. Pinta is found only in Latin America and infection is limited to the skin. Bejel is found in eastern Mediterranean countries, the Balkans, and the cooler areas of North Africa.

Table 18-1

Treponema-Associated Diseases

Bacteria	Associated Disease
T. pallidum	Syphilis
T. pallidum (variant)	Bejel
T. pertenue	Yaws
T. carateum	Pinta

Figure 18-1 Treponema pallidum. (From Bauer JD: Clinical laboratory methods, ed 9, St Louis, 1982, Mosby.)

Direct examination of the treponemes is most often performed with darkfield microscopy. Pathogenic treponemes appear as fine, spiral (8 to 24 coils) organisms approximately 6 to 15 µm long. They have a trilaminar outer membrane similar to that of gram-negative bacteria.

Pathogenic treponemes are not cultivatable with any consistency in artificial laboratory media. Outside of the host, the pathogenic treponemes are extremely susceptible to a variety of physical and chemical agents. Treponemes may remain viable for up to 5 days in tissue specimens removed from diseased animals and from frozen cryoprotected specimens.

Epidemiology

Sexually transmitted diseases (STDs) remain a major public health challenge in the United States. The surveillance report by the Centers for Disease Control and Prevention (CDC) includes data on the three STDs that physicians are required to report to the agency—chlamydia, gonorrhea, and syphilis. Syphilis is considered to be primarily a venereal disease. It is the most common STD in the United States.

The three treponematoses—yaws, pinta, and bejel—are rarely seen in the United States but are prevalent in other countries. These diseases are associated with poverty, overcrowding, and poor hygiene.

In 2009, for the first time in 5 years, the CDC reported that syphilis cases did not increase overall among women. In addition, cases of congenital syphilis (transmitted from mother to infant) did not increase for the first time in 4 years. In 2008, 63% of the reported primary and secondary (P&S) syphilis cases were among men who have sex with men (MSM). In the surveillance period of 2004 to 2008, rates of P&S syphilis increased the most among 15- to 24-year-old men and women. The incidence of syphilis per capita is higher among blacks and Hispanics than among whites.

Syphilis remains a global problem, with an estimated 12 million people infected each year, despite the existence of effective prevention measures. The last decade has seen a pronounced resurgence of syphilis in countries of the Far East (e.g., China and Africa). Some fundamental social problems (e.g., poverty, inadequate access to health care, lack of education) are associated with disproportionately high levels of syphilis in certain populations.

Pathogenic treponemes are transmitted almost uniformly by direct contact. Treponemal infections of the skin or oral lesions contain many spirochetes that may be transmitted by personal, but not necessarily venereal, contact. These infections are generally acquired during childhood. In each of these diseases, infection elicits antibodies reactive in nontreponemal and treponemal methods.

Syphilis develops in 30% to 50% of the sexual partners of persons with syphilitic lesions. The risk of acquiring syphilis from a single sexual exposure to an infected partner is unknown. A high percentage of partners do seek medical treatment within 90 days of contact.

Syphilis can be acquired by kissing a person with active oral lesions. Very few cases of transfusion-acquired syphilis have been reported in recent years in the United States. During the first half of the twentieth century, however, syphilis was a major bloodborne infectious disease easily transmitted through the prevailing method of direct donor to patient blood transfusion. The danger of syphilis transmission still exists in tropical countries in which the organization of blood banks is deficient and the use of direct blood transfusion prevails in emergency situations. Refrigerated blood storage decreases accidental transmission of the microorganism because T. pallidum has a short survival period in stored blood. Spirochetes do not appear to survive in units of citrated blood at 4° C (39° F) for longer than 72 hours.

Cases have been reported of children who acquired syphilis by sharing a bed with an infected parent. In addition, syphilis may be transmitted transplacentally to the fetus. Spirochetes can be transmitted to the fetus during the last trimester of pregnancy, before the mother manifests postpartum evidence of infection.

Signs and Symptoms

Untreated syphilis is a chronic disease with subacute symptomatic periods separated by asymptomatic intervals, during which the diagnosis can be made serologically. The progression of untreated syphilis is generally divided into stages—primary, secondary, latent (hidden), and tertiary (late) (Table 18-2).

Table 18-2

Stages of Syphilis

Phase or Stage	Features and Comments	Test
Incubating phase	The incubation period usually lasts ≈3 wk but can range from 10-90 days.	Laboratory examination
Primary stage	• During the primary stage, a painless chancre develops at the site where the bacteria entered the body. • A person is highly contagious during the primary stage. • The chancre lasts 28-42 days and heals without treatment.

Darkfield examination Secondary stage

• This is characterized by a rash that appears from 2-8 wk after the chancre develops.

• A person is highly contagious during the secondary stage.

• A rash often develops all over the body, including palms of the hands and the soles of the feet. The rash usually heals without scarring in 2-12 wk.

• Open sores may be present on mucous membranes and may contain pus (condyloma lata).

• Symptoms can include nervous system abnormalities.

• RPR or VDRL

• TP-PA used to confirm a syphilis infection after another method tests positive for syphilis. It can be used to detect syphilis in all stages, except during the first 3-4 wk. This test is not done on spinal fluid.

• FTA-ABS test detects syphilis except during the first 3-4 wk after exposure to syphilis bacteria. It is more difficult to perform and may be used to confirm a syphilis infection after another method tests positive for the syphilis bacteria. It can be done on a sample of blood or cerebrospinal fluid. CSF

Latent (hidden) stage

• If untreated, an infected person will progress to the latent (hidden) stage of syphilis with no symptoms (latent period).

• The latent period may be as brief as 1 yr or range from 5-20 yr.

• A person is contagious during the early part of the latent stage and may be contagious during the latent period.

Relapses of secondary syphilis

• About 20%-30% of people with syphilis have a relapse of the secondary stage of syphilis during the latent stage.

• A relapse means that the person had passed through the second stage, was symptom-free, then began to reexperience secondary stage symptoms. Relapses can occur several times.

• When relapses no longer occur, a person is not contagious through contact.

• A woman in the latent stage of syphilis may still pass the disease to her unborn baby and may have a miscarriage, a stillbirth, or give birth to a baby infected with congenital syphilis.

• Nontreponemal tests measure IgM and IgG antibodies.

• It is best for testing for reinfection.

Tertiary (late) stage

• Most destructive stage of syphilis

• If untreated, the tertiary stage may begin as early as 1 yr after infection or at any time during a person’s lifetime. A person may never experience this stage of the illness.

• The symptoms of tertiary (late) syphilis depend on the complications that develop—gummata, large sores inside the body or on the skin, cardiovascular syphilis, or neurosyphilis.

• VDRL on cerebrospinal fluid (CSF) with concurrent RPR serum

• If RPR is negative and a high index of suspicion for neurosyphilis remains, perform FTA-Abs on serum.

• Some patients have nonreactive nontreponemal tests in late neurosyphilis.

FTA-ABS, Fluorescent treponemal antibody absorption; RPR, rapid plasma reagin; TP-PA, Treponema pallidum particle agglutination assay; VDRL, venereal disease research laboratory.

Initially, T. pallidum penetrates intact mucous membranes or enters the body through tiny defects in the epithelium. On entrance, the microorganism is carried by the circulatory system to every organ of the body. Spirochetemia occurs very early in infection, even before the first lesions have appeared or blood tests become reactive. Before clinical or serologic manifestations develop, patients are said to be incubating syphilis. The incubation period usually lasts about 3 weeks but can range from 10 to 90 days.

The late stage is seen in children older than 2 years who are untreated. Symptoms of the untreated late stage include eighth nerve deafness, keratitis, and Hutchinson’s teeth (Fig. 18-4) (hutchisonian triad), as well as arthropathy and neurosyphilis. Residual stigmata can develop. Other characteristics include fissuring around the mouth and anus, skeletal lesions, perforation of the palate, and collapse of nasal bones to produce a saddle-nose deformity.

Figure 18-4 Congenital syphilis (Hutchinson’s teeth). (From Kaye D, Rose LF: Fundamentals of internal medicine, St Louis, 1983, Mosby.)

Neurosyphilis

Although neurosyphilis may be asymptomatic, symptomatic forms include the following:

• Meningeal syphilis, usually less than 1 year after infection

• Meningovascular syphilis, usually 5 to 10 years after infection

• Parenchymatous syphilis

Meningeal neurosyphilis involves the brain or spinal cord. Patients can suffer from headaches and a stiff neck. Meningovascular syphilis involves inflammation of the pia mater and arachnoid space, with focal arteritis. A stroke syndrome involving middle cerebral artery is common in young adults. Parenchymatous neurosyphilis manifests as general paresis, joint degeneration, and tabes dorsalis (demyelination of posterior columns, dorsal roots, and dorsal root ganglia). Tabes dorsalis is characterized by a gait disturbance and bladder symptoms.

Immunologic Manifestations

In the treponemes, two classes of antigen have been recognized:

1. Antigens restricted to one or a few species

2. Antigens shared by many different spirochetes

Specific and nonspecific antibodies are produced in the immunocompetent host. Specific antibodies against T. pallidum (Treponema pallidum antibodies) and nonspecific antibodies against the protein antigen group common to pathogenic spirochetes are formed. Specific antitreponemal antibodies in early or untreated early latent syphilis are predominantly immunoglobulin M (IgM) antibodies. The early immune response to infection is rapidly followed by the appearance of IgG antibodies, which soon become predominant. The greatest elevation in IgG concentration is seen in secondary syphilis.

Nontreponemal antibodies, often called reagin antibodies, are produced by infected patients against components of their own or other mammalian cells. Although almost always produced by patients with syphilis, these antibodies are also produced by patients with other infectious diseases. Infectious diseases in which reagin can be demonstrated include measles, chickenpox, hepatitis, infectious mononucleosis, leprosy, tuberculosis, leptospirosis, malaria, rickettsial disease, trypanosomiasis, and lymphogranuloma venereum. Reagin can also be exhibited by patients with noninfectious conditions such as autoimmune disorders, drug addiction, old age, pregnancy, and recent immunization.

Delayed-hypersensitivity immune mechanisms (see Chapter 26) also contribute to the pathophysiology of syphilis. It has been suggested that the granulomatous reactions (gummas) result from delayed hypersensitivity in the immune host. In addition, the manifestations of congenital syphilis apparently result in part from an immune inflammatory reaction. Antigen-antibody complexes have been detected in the blood of patients with secondary syphilis and are responsible for the syphilis-associated glomerulonephritis. Suppression of the various aspects of cell-mediated immunity has been noted in syphilis and may contribute to the prolonged survival of T. pallidum.

Diagnostic Evaluation

The diagnosis of syphilis depends on clinical skills, demonstration of microorganisms in a lesion, and serologic testing. A variety of diagnostic procedures for syphilis are available (Tables 18-3 and 18-4). Classic serologic methods for syphilis measure the presence of two types of antibodies (Table 18-5), nontreponemal methods and treponemal methods.

Table 18-3

Comparison of Tests for Syphilis Diagnosis

Test, Methodology	Antibody	Antigen	Specimen and Clinical Notes	Technical Notes
Direct Microscopy Observation
Fluorescent	Antitreponemal antibody with fluorescent tag	T. pallidum	Patient specimen must be swab or discharge from active lesion.
Darkfield	None	T. pallidum	Patient specimen must be swab or discharge from active lesion.
Nontreponemal Assay
RPR	Reagin	Cardiolipin	Use a serum specimen; cannot be used for CSF.	More sensitive than VDRL in primary syphilis
VDRL	Reagin	Cardiolipin	Specimen can be serum or CSF.	Traditional method used less frequently than RPR
Treponemal Test
DNA probe	None	DNA from patient matched to treponemal DNA		Expensive form of testing
EIA	Anti-IgM or anti-IgG antitreponemal	Enzyme-labeled treponemal antigen	Antibody source is patient serum.	Less sensitive than other methods in later stages of syphilis
FTA-ABS	Antitreponemal	T. pallidum (Nichols strain)		Confirmatory assay; primary stage test results may be negative.
MHA-TP	Antitreponemal	Gel particles or sheep red blood cells; coated carrier particles of T. pallidum cell walls disrupted by high-frequency sound waves.		Less sensitive than FTA-ABS

RPR, Rapid plasma reagin; VDRL, venereal disease research laboratory; FTA-ABS, fluorescent treponemal antibody absorption; EIA, enzyme immunoassay; MHA-TP, microhemagglutination assay for antibodies direct against Treponemal pallidum; TP-PA, Treponemal pallidum particle agglutination assay.

Table 18-4

Select Tests for Syphilis Diagnosis

Test	Methodology	Comments
Darkfield examination	Darkfield microscopy
RPR	Charcoal agglutination
T. pallidum (VDRL), serum with reflex to titer	Flocculation
T. pallidum antibody, serum IgG by IFA	Indirect fluorescent antibody (IFA)	False-positive result in herpes, HIV, malaria, IV drug use, systemic lupus, rheumatoid arthritis, pregnancy, leprosy
T. pallidum antibody, IgM by ELISA	ELISA	If test results are questionable, repeat testing in 10-14 days.
T. pallidum antibody, IgG by ELISA	ELISA	If test results are questionable, repeat testing in 10-14 days.

IgG, Immunoglobulin G; HIV, human immunodeficiency virus; IV, intravenous; MHA, microhemagglutination; VDRL, Venereal Disease Research Laboratories.

Note: VDRL is the preferred test for CSF. Treponemal tests (TP-PA or FTA) are not recommended for CSF. FTAs on CSF may be tested, but TP-PA cannot be tested on CSF.

Adapted from Associated Regional and University Pathologists (ARUP) Laboratories: ARUP’s laboratory test directory, 2012 (http://www.aruplab.com/guides/ug/tests/ugs.jsp).

Table 18-5

Nontreponemal and Treponemal Assays

Nontreponemal Screening Assays	Treponemal Confirmatory^‡ Assays
T. pallidum (RPR)^∗	FTA-ABS
T. pallidum (VDRL)^†	T. pallidum particle agglutination (antibody to TP-PA) T. pallidum antibody IgM by ELISA T. pallidum antibody, IgG by Immunoblot T. pallidum antibody, IgG by indirect fluorescent antibody (IFA)^§

^∗Serum or CSF with reflex to titer.

^†Serum or CSF with reflex to titer.

^‡If nontreponemal screening assay is positive.

^§CSF.

Testing for syphilis can comply with a logical flow of observations and laboratory testing (Fig. 18-5). Seroconversion between acute and convalescent sera is considered strong evidence of recent infection. The best evidence for infection is a significant change in two appropriately timed specimens, in which both tests are performed in the same laboratory at the same time.

Figure 18-5 Syphilis algorithm. (© 2006-2012 ARUP Laboratories. All Rights Reserved. consult.com.)

Direct Observation of Spirochetes

Two methods of direct observation of spirochetes are available for the examination of a patient specimen from an active syphilitic lesion. These methods are darkfield microscopy and fluorescent antibody microscopy.

Darkfield Microscopy

For symptomatic patients with primary syphilis, darkfield microscopy is the test of choice. A darkfield examination is also suggested for immediate results in cases of secondary syphilis, with a titer follow-up test.

Direct and Indirect Fluorescent Antibody

This method of examination uses a fluorescent-labeled antibody conjugate to T. pallidum. An alternate indirect method uses antibody specific for T. pallidum and a second labeled anti-immunoglobulin antibody. This method offers the advantage of not requiring a live specimen. There is a risk of cross-reactivity with other subspecies of T. pallidum when monoclonal antibody is used for the procedure.

Nontreponemal Methods

Nontreponemal methods determine the presence of reagin, an antibody formed against cardiolipin. An antigen composed of cardiolipin, a lipid remnant of damaged cells, cholesterol, and lecithin is used to detect the nontreponemal reagin antibodies.

Rapid Plasma Reagin

The rapid plasma reagin (RPR) test is the most widely used nontreponemal serologic procedure, although Venereal Disease Research Laboratory (VDRL) methods may be used in some clinical and reference laboratories. Both these procedures are flocculation or agglutination tests in which soluble antigen particles coalesce to form larger particles that are visible as clumps when they are aggregated by antibody.

The RPR test, a charcoal agglutination test, can be performed on heated or unheated serum or plasma using a modified VDRL antigen suspension of choline chloride with ethylenediaminetetraacetic acid (EDTA). The RPR card test antigen also contains charcoal particles to which cardiolipin-containing antigen is bound for macroscopic reading. There are three versions of the RPR test. The original RPR method used unmeasured amounts of plasma and was used as a field procedure for screening large numbers of people. The modified RPR test uses the serum reagin test and is performed on measured volumes of unheated serum.

The RPR test measures IgM and IgG antibodies to lipoidal material released from damaged host cells and to lipoprotein-like material, and possibly cardiolipin released from the treponemes. If antibodies are present, they combine with the lipid particles of the antigen, causing them to agglutinate. The charcoal particles coagglutinate with the antibodies and show up as black clumps against the white card. If antibodies are not present, the test mixture is uniformly gray.

Antilipoidal antibodies are antibodies that are produced not only as a consequence of syphilis and other treponemal diseases, but also in response to nontreponemal diseases of an acute and chronic nature in which tissue damage occurs. Without some other evidence for the diagnosis of syphilis, a reactive nontreponemal test does not confirm T. pallidum infection.

The RPR test is more sensitive than the VDRL test for the detection of primary syphilis.

Venereal Disease Research Laboratory Test

The VDRL test, a flocculation test, is a qualitative and quantitative screening procedure. Flocculation is a specific type of precipitation reaction that takes place over a narrow range of antigen concentration.

Serum for testing must be heated to 56° C (133° F) for 30 minutes to inactivate complement. The test serum should be used promptly after inactivation. The antigen suspension is composed of cardiolipin, cholesterol, and lecithin. The VDRL test measures IgM and IgG antibodies to lipoidal material released from damaged host cells, to lipoprotein-like material, and possibly to cardiolipin released from the treponemes.

Without some other evidence for the diagnosis of syphilis, a reactive nontreponemal test does not confirm T. pallidum infection. Antilipoidal antibodies are antibodies that are not only produced as a consequence of syphilis and other treponemal diseases, but also may be produced in response to nontreponemal diseases of an acute and chronic nature in which tissue damage occurs. Without some other evidence for the diagnosis of syphilis, false-positive results may be caused by human immunodeficiency virus (HIV), herpes simplex virus (HSV), malaria, intravenous drug use (IVDU), systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), pregnancy, leprosy, or endemic treponemal conditions.

VDRL with reflex testing to titer is the preferred test for CSF. A positive VDRL test result on spinal fluid is diagnostic of neurosyphilis.

Treponemal Methods

Treponemal assays can confirm reactive (positive) reagin tests but should not be used as primary screening methods. The most common assays in this category are the following:

• Fluorescent treponemal antibody absorption (FTA-ABS)

• T. pallidum particle agglutination (TP-PA)

• T. pallidum antibody by enzyme-linked immunosorbent assay (ELISA)

• T. pallidum antibody by immunoblot (Western blot) test

Fluorescent Treponemal Antibody Absorption

FTA-ABS can be used to confirm that a positive nontreponemal test result has been caused by syphilis rather than by other biological conditions that can produce a positive serologic result. This test also can determine quantitative titers of antibody, which is useful for following response to therapy.

The FTA-ABS uses a killed suspension of T. pallidum spirochetes as the antigen. Most systems use nonviable T. pallidum (Nichols strain), extracted from rabbit testicular tissue, as a substrate (antigen). Sorbent, another reagent, is prepared from cultures of nonpathogenic Reiter treponemes. The sorbent that contains an antigen to the Reiter treponeme may or may not specifically absorb the reactivity that occurs in normal sera. Treponema pallidum antigen.

This procedure is performed by overlaying whole treponemes fixed to a slide with serum from patients suspected of having syphilis because of a previously positive syphilis serology. The patient’s serum is first absorbed with non–T. pallidum treponemal antigens to reduce nonspecific cross-reactivity. Fluorescein-conjugated antihuman antibody reagent is then applied as a marker for specific antitreponemal antibodies in the patient’s serum.

FTA-ABS may be helpful in late neurosyphilis when the RPR is negative but there is a high clinical suspicion of syphilis. FTA tests may produce false-positive result in a variety of disorders, such as autoimmune disease, leprosy, febrile illnesses, advanced age, or Lyme disease.

Treponema pallidum Particle Agglutination

This is a semiquantitative particle agglutination assay. It cannot be used to test CSF. This assay cannot differentiate between IgM and IgG antibodies. TP-PA is useful to diagnose infection in patient whose reactive screening test is positive with atypical signs of primary, secondary, or late syphilis. TP-PA compares favorably to the FTA test but is slightly less sensitive in untreated early primary syphilis. This assay is excellent for resolving inconclusive FTA-ABS results.

Treponema pallidum Antibody, Immunoglobulin G, by Enzyme-Linked Immunosorbent Assay

The ELISA assay can discriminate maternally derived IgG antibodies that cross the placenta from IgM antibodies that indicate active infection in a newborn. Congenital syphilis sensitivity is approximately 80%. Hence, congenital syphilis can be confirmed but a negative IgM level does not rule out congenital syphilis. The assay is highly specific (100%) and sensitive (91%).

Treponema pallidum Antibody, Immunoglobulin G, by Immunoblot (Western Blot) Test

A negative result is seen when no specific IgG antibodies against T. pallidum are detected. This test should not be used to determine relapse or reinfection of syphilis because of the persistence of reactivity, likely for a lifetime. Repeat testing in 2 to 4 weeks is recommended if results are equivocal. The presence of IgG antibody to T. pallidum is suggestive of current or past infection.

Sensitivity of Representative Procedures for Syphilis

Detection of syphilis by serologic methods is related to the stage of the disease and test method (Table 18-6).

Table 18-6

Percentage of Positive Tests for Syphilis

Test^∗	Stage
Test^∗	Primary	Secondary	Latent Late
Nontreponemal Assay
Rapid plasma reagin (RPR)	80-86	99-100	98^†
Treponemal Assays
FTA-ABS TP-PA, MHA-TP	84-85 85-100	100 98-100	95-100 98-100

MHA-TP, Microhemagglutination assay for antibodies directed against T. pallidum.

^∗Percentage of patients with positive serologic tests in treated or untreated primary or secondary syphilis.

^†Treated late syphilis.

Adapted from Tramont E: Treponema pallidum. In Mandell GI, Douglas RG Jr, Bennett Jr, editors: Principles and practice of infectious diseases, ed 2, New York, 1985, Wiley & Sons, and LaSalsa L et al: Spirochete infections. In Henry JB, editor: Clinical diagnosis and management by laboratory methods, ed 21, Philadelphia, 2007, WB Saunders, Table 58-1.

In the primary stage, about 30% of cases become serologically active after 1 week and 90% of patients demonstrate reactivity after 3 weeks. Reagin titers increase rapidly during the first 4 weeks of infection and then remain stable for about 6 months. Patients in the secondary stage of syphilis are serologically positive.

During latent syphilis, there is a gradual return of nonreactive serologic manifestations, as seen with nontreponemal methods. About one third of patients in the latent stage will remain seroreactive and presumably infectious. In late syphilis, treponemal tests are generally reactive and nontreponemal methods are nonreactive.