CDC Laboratory Recommendations for Syphilis Testing, United States, 2024

Introduction

Background

Treponema pallidum subsp. pallidum, primarily transmitted through sexual contact, is among four pathogenic species in the genus Treponema, which is in the family Treponemataceae (1). The other three pathogenic Treponema species cause skin diseases mostly transmitted by direct skin-to-skin contact. Yaws is caused by T. pallidum subsp. pertenue and is found in tropical areas in Africa, Asia, and Latin America (2). Treponema carateum infection results in pinta which, although rare, is found in tropical areas of Latin America (3). Endemic syphilis or bejel, caused by T. pallidum subsp. endemicum, occurs mostly in children and is mainly found in the eastern Mediterranean, West Africa, and Cuba (4,5). However, phylogenic analysis of lesion specimens from certain patients outside of areas where bejel is endemic who had received a diagnosis of syphilis revealed that T. pallidum subsp. endemicum might be sexually transmitted. These patients have a clinical course similar to syphilis (5–8). For this report, T. pallidum subsp. pallidum will be abbreviated to T. pallidum unless further distinction between the subspecies is necessary.

T. pallidum causes a systemic infection and might lead to serious sequalae in multiple organ systems, including the central nervous system (CNS) and the ocular and otic systems. Vertical transmission can cause congenital syphilis, which might result in spontaneous abortions, miscarriages, or stillbirths; infants with congenital syphilis can have clinical signs of infection at birth or months to years after birth. Clinical features in adults progress through different stages beginning with primary syphilis, which often appears about 3 weeks after exposure, with an incubation period of 10-90 days (9). Primary syphilis is characterized by single or multiple ulcerative-like lesions (chancres) that often are painless and therefore might be unnoticed when they occur inside the mouth, vagina, or rectum. Chancres can persist for 2-6 weeks before healing spontaneously. Secondary syphilis typically begins 2-24 weeks after most primary lesions heal and is commonly characterized by a mucocutaneous rash appearing on the trunk, palms, and soles; mucous patches in the mouth or condylomata lata on the genitals or rectum occur in approximately one fourth of patients. Primary and secondary syphilis symptoms can occur concurrently, which is more likely in persons with HIV infection. Moist primary and secondary syphilis lesions contain infectious T. pallidum that can be transmitted through sexual contact to susceptible persons. Secondary clinical manifestations also can consist of lymphadenopathy, alopecia, and occasionally neurologic and ocular manifestations. Signs and symptoms of secondary syphilis typically resolve in approximately 3 months, with a range of 1-12 months (10,11) but can periodically recur for the first several years of infection in ≤25% of untreated persons (12).

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The interval between primary to secondary and secondary to tertiary syphilis is known as latency when no symptoms or signs of syphilis are present. The interval from secondary to tertiary syphilis can last for years or decades before symptoms appear. In up to two thirds of patients, the disease can remain latent for life and never progress to tertiary syphilis (13–15). Latent asymptomatic syphilis is divided into three categories: early latent infections thought to have been acquired within the past year; late latent infections thought to be longer than 1 year duration; and latent syphilis of unknown duration where the timing of acquisition cannot be determined based on available clinical, historical, or laboratory data. Clinical signs of tertiary syphilis, a rare condition, include cardiovascular syphilis, with aneurysms or stenosis resulting from multiplication of treponemal spirochetes in the thoracic aorta or coronary arteries; syphilitic gummas, with soft granulomatous growths that can cause tissue destruction in any organ system including bones and cartilage; and neurosyphilis, with late neurologic manifestations including tabes dorsalis and general paresis. Neurosyphilis can occur during any stage of syphilis and can be asymptomatic or symptomatic during any stage of infection.

Rationale for New CDC Recommendations

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Syphilis, a nationally notifiable disease with approximately 176,000 cases in the United States reported to the CDC in 2021 (16) and approximately 6 million new cases occurring worldwide (17), is caused by T. pallidum. A syphilis epidemic is occurring in the United States, with sustained increases in primary and secondary syphilis from 5,979 cases reported in 2000 to 133,945 cases reported in 2020, a 2,140% increase (16,18). The epidemic is characterized by health disparities, particularly among sexual and gender minority populations, intersections with the HIV and substance use epidemics, and increased morbidity and mortality attributable to congenital syphilis infections (16).

Laboratories have a critical role in the public health response to the syphilis epidemic. The responsibility of the laboratory is to test specimens and report results in a timely manner, allowing clinicians to efficiently make clinical diagnoses for patient management. Public health reporting by laboratories also allows local health departments and CDC to conduct surveillance and monitor disease trends. This report details CDC’s new recommendations for syphilis testing, including laboratory-based tests, point-of-care (POC) tests, processing of samples, and reporting of test results to aid laboratorians and clinicians in the diagnosis of syphilis. Future revisions to these recommendations will be based on new research or technologic advancements for syphilis clinical laboratory science.

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