Syphilis and chancroid are two distinct sexually transmitted infections (STIs) characterized by the initial appearance of genital sores. While both present with a lesion at the site of infection, their causative agents, clinical courses, and ultimate health impacts are profoundly different. Understanding these differences is necessary for accurate diagnosis and effective treatment, which prevents severe, long-term health complications.
Understanding the Causes and Primary Lesions
The difference between these two infections begins at the microbial level, involving two entirely different types of bacteria. Syphilis is caused by the spirochete bacterium Treponema pallidum, a fragile, spiral-shaped organism difficult to culture outside of a host. Chancroid is caused by Haemophilus ducreyi, a small, rod-shaped bacterium that requires specialized media to grow in a laboratory setting.
The primary physical symptom for each disease exhibits contrasting characteristics. Syphilis begins with a lesion known as a chancre, which typically appears between 10 to 90 days after exposure, averaging about three weeks. This chancre is described as a firm, round, and painless ulcer with a clean base and a raised, indurated (hardened) edge.
The initial lesion of chancroid, often called a “soft chancre,” is the opposite of the syphilitic chancre. Chancroid lesions emerge much faster, usually within 4 to 10 days of infection, starting as a small, tender bump that quickly breaks down into an ulcer. This ulcer is notably painful, soft to the touch, and features irregular, undermined edges with a grayish or yellowish purulent base that bleeds easily.
Chancroid is also characterized by the development of painful, tender, and often unilateral swelling of the lymph nodes in the groin, known as buboes. This lymphadenopathy occurs in about half of infected individuals. In approximately 25% of these cases, the buboes progress to become suppurative (filled with pus) and may rupture spontaneously. The syphilitic chancre may cause non-tender regional lymphadenopathy, but it rarely progresses to the destructive bubo formation typical of chancroid.
Disease Progression and Systemic Impact
A major distinction between the two infections is their patterns of spread throughout the body. Chancroid remains largely a localized infection, with its primary complications tied to the local destruction of tissue and the lymph system in the groin. If left untreated, the genital lesions will eventually resolve on their own, typically within one to three months, though this process may leave significant scarring.
The progression of chancroid is primarily confined to severe complications arising from ruptured lymph nodes, which can lead to chronic draining sinuses and deep tissue damage. While the infection can cause serious local disfigurement and increase the risk of HIV transmission due to the open sores, it does not disseminate throughout the body in the multi-stage manner seen in syphilis.
Syphilis is a systemic infection from the moment the spirochete enters the body, capable of affecting nearly any organ system if left untreated. The disease progresses through four defined stages: primary, secondary, latent, and tertiary. The secondary stage follows the healing of the primary chancre, typically appearing two to eight weeks later, and is marked by the multiplication and widespread dissemination of the bacteria.
This systemic spread manifests as a variety of symptoms, including a non-itchy rash often found on the palms of the hands and soles of the feet, along with flu-like symptoms such as fever, fatigue, and headache. Following the secondary stage, the infection enters the latent stage, which is asymptomatic but still requires treatment to prevent later complications.
The most severe outcome is tertiary syphilis, which can emerge years or even decades after the initial infection in untreated cases. This late-stage disease involves destructive manifestations like gummas (soft, tumor-like growths), cardiovascular syphilis (affecting the heart and blood vessels), and neurosyphilis, which causes severe neurological and mental disorders.
Key Differences in Diagnosis and Treatment
The systemic nature of syphilis and the localized character of chancroid necessitate distinct approaches to diagnosis and treatment. Diagnosis of syphilis relies primarily on blood tests because the bacteria cannot be easily cultured in a laboratory setting. The diagnostic process uses a combination of non-treponemal tests (such as RPR or VDRL) to measure antibody activity, and treponemal tests, which specifically detect antibodies to Treponema pallidum.
In contrast, a definitive diagnosis of chancroid traditionally requires the identification of Haemophilus ducreyi from a swab of the ulcer base using special culture media. This method is not widely available and has limited sensitivity. Consequently, chancroid is often diagnosed clinically, based on the presence of painful genital ulcers, tender inguinal lymphadenopathy, and the exclusion of other common causes like syphilis and herpes simplex virus.
The treatment protocols for the two infections also differ significantly in terms of the preferred medication. Penicillin G, administered parenterally, is the preferred drug for treating all stages of syphilis due to the bacterium’s unique sensitivity to the antibiotic. For early-stage syphilis, a single intramuscular dose of Benzathine penicillin G is typically curative.
Chancroid is treated with different classes of antibiotics, with single-dose regimens often preferred to ensure patient adherence. Recommended options include a single oral dose of azithromycin or a single intramuscular injection of ceftriaxone. For both conditions, it is necessary to treat all sexual partners to halt transmission and prevent reinfection.
Prevention and Screening
Preventive measures for both syphilis and chancroid focus on reducing the risk of transmission through sexual contact. Consistent use of barrier methods, such as condoms, significantly lowers the risk of acquiring either infection by minimizing skin-to-skin contact with infectious lesions. Limiting the number of sexual partners is another strategy for reducing overall exposure risk.
Screening practices differ based on the public health threat posed by each disease. Routine screening for syphilis is a standard public health measure due to its potential for long-term complications and the existence of the asymptomatic latent stage. Screening is strongly recommended for pregnant individuals during their first prenatal visit to prevent congenital syphilis, which can cause severe outcomes for the baby.
Screening for chancroid is not typically performed routinely in the general population, as the disease is rare in many developed countries. Testing for chancroid is usually initiated when a person presents with a characteristic painful genital ulcer or when contact tracing identifies an exposure to a confirmed case. Retesting for both syphilis and HIV is also recommended three months after a chancroid diagnosis, as the open ulcers increase the likelihood of acquiring other STIs.