Syphilis
Syphilis in STIsim progresses through exposed, primary, secondary, latent, and tertiary stages, with stage-dependent transmissibility and congenital syphilis outcomes. Treatment at any stage clears the infection but leaves the agent seropositive and susceptible to reinfection.
Class: sti.Syphilis | Alias: 'syphilis' / 'syph' | Base class: BaseSTI
States and transitions
┌─────────────┐
│ Susceptible │◄──────────────────────────────┐
└──────┬──────┘ │
│ infection │
▼ │
┌─────────────┐ │
│ Exposed │ Incubation │
│ (~50 d) │ Not yet infectious │
└──────┬──────┘ │
│ │
▼ │
┌─────────────┐ │
│ Primary │ Chancre stage treatment at
│ (~6 wk) │ Transmissibility: 1x any stage
└──────┬──────┘ clears infection
│ (sets susceptible
▼ + ever_exposed)
┌─────────────┐ │
│ Secondary │ Systemic symptoms │
│ (~3.6 mo) │ Transmissibility: 1x │
└──────┬──────┘ │
│ │
▼ │
┌─────────────────────────────┐ │
│ Latent │ Transmissibility: │
│ Early (~12-14 mo) │ decays with │
│ Late (indefinite) │ half-life of 1 yr │
└──────────┬──────────────────┘ │
│ │
┌───────────┼───────────┐ │
│ │ │ │
▼ ▼ ▼ │
┌──────────┐ ┌──────────┐ ┌───────────┐ │
│Reactivate│ │ Tertiary │ │ Remains │ │
│ (35%) │ │ (35%) │ │ latent │ │
│→Secondary│ │ (~20 yr) │ │ (30%) │ │
└──────────┘ └────┬─────┘ └───────────┘ │
│ │
▼
┌─────────────┐
│ Death (5%) │
└─────────────┘
Treatment and reinfection: Treatment (via SyphTx) clears all stage states and returns the agent to susceptible. The ever_exposed flag persists -- these agents are seropositive (detectable by treponemal tests) but can be reinfected. This reinfection cycle is a key driver of syphilis epidemics: symptomatic agents seek care, get treated, become susceptible again, and are reinfected by their partners.
Key properties:
active= primary | secondary (used by connectors for HIV susceptibility effects)infectious= active | latentnaive= never exposed (seronegative)sus_not_naive= susceptible & ever_exposed (seropositive but susceptible to reinfection)
Parameters
Natural history
| Parameter | Default | Description |
|---|---|---|
dur_exposed |
normal(50 d, 10 d) | Duration of incubation period |
dur_primary |
normal(6 wk, 1 wk) | Duration of primary stage |
dur_secondary |
lognorm(3.6 mo, 1.5 mo) | Duration of secondary stage |
dur_early |
uniform(12 mo, 14 mo) | Duration of early latent stage |
p_reactivate |
0.35 | Probability of reactivation from latent to secondary |
time_to_reactivate |
lognorm(1 yr, 1 yr) | Time to reactivation |
p_tertiary |
0.35 | Probability of progressing to tertiary |
time_to_tertiary |
normal(20 yr, 2 yr) | Time to tertiary stage |
p_death |
0.05 | Probability of death from tertiary syphilis |
time_to_death |
lognorm(5 yr, 5 yr) | Time to death after tertiary |
Transmission
| Parameter | Default | Description |
|---|---|---|
beta_m2f |
0.1 | Per-act male-to-female transmission probability |
eff_condom |
0.0 | Condom efficacy (syphilis transmits via skin contact) |
rel_trans_primary |
1 | Relative transmissibility during primary stage |
rel_trans_secondary |
1 | Relative transmissibility during secondary stage |
rel_trans_latent |
1 | Baseline latent transmissibility (decays exponentially) |
rel_trans_tertiary |
0.0 | Relative transmissibility during tertiary (non-infectious) |
rel_trans_latent_half_life |
1 yr | Half-life of latent-stage transmissibility decay |
Congenital syphilis
Birth outcomes depend on maternal stage of infection (probabilities sum to 1 across 5 outcomes):
| Maternal stage | Miscarriage | Neonatal death | Stillborn | Congenital syphilis | Normal birth |
|---|---|---|---|---|---|
| Active (primary/secondary) | 0.00 | 0.10 | 0.20 | 0.45 | 0.25 |
| Early latent | 0.00 | 0.05 | 0.10 | 0.40 | 0.45 |
| Late latent | 0.00 | 0.00 | 0.10 | 0.10 | 0.80 |
Initialization
| Parameter | Default | Description |
|---|---|---|
init_prev |
0.0 | Initial active syphilis prevalence |
init_latent_prev |
0.0 | Initial latent syphilis prevalence |