Syphilis

Syphilis in STIsim progresses through primary, secondary, latent, and tertiary stages, with stage-dependent transmissibility and congenital syphilis outcomes.

Class: sti.Syphilis | Alias: 'syphilis' / 'syph' | Base class: BaseSTI

States and transitions

                         ┌─────────────┐
                         │ Susceptible │
                         └──────┬──────┘
                                │ infection
                                ▼
                         ┌─────────────┐
                         │   Primary   │  Chancre stage
                         │  (~6 wk)   │  Transmissibility: 1x
                         └──────┬──────┘
                                │
                                ▼
                         ┌─────────────┐
                         │  Secondary  │  Systemic symptoms
                         │ (~3.6 mo)  │  Transmissibility: 1x
                         └──────┬──────┘
                                │
                                ▼
                  ┌─────────────────────────────┐
                  │        Latent               │
                  │   Early (~12-14 mo)         │  Transmissibility: decays
                  │   Late (indefinite)         │  with half-life of 1 yr
                  └──────────┬──────────────────┘
                             │
                 ┌───────────┼───────────┐
                 │           │           │
                 ▼           ▼           ▼
          ┌──────────┐ ┌──────────┐ ┌───────────┐
          │Reactivate│ │ Tertiary │ │ Remains   │
          │   (35%)  │ │  (35%)   │ │  latent   │
          │→Secondary│ │ (~20 yr) │ │           │
          └──────────┘ └────┬─────┘ └───────────┘
                            │
                            ▼
                     ┌─────────────┐
                     │ Death (5%)  │
                     └─────────────┘

Parameters

Natural history

Parameter	Default	Description
`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	Neonatal death	Stillborn	Congenital syphilis	Normal birth
Active (primary/secondary)	0.10	0.20	0.45	0.25
Early latent	0.05	0.10	0.40	0.45
Late latent	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