Disease progression

The EMOD typhoid model represents the progression of typhoid through a series of state changes, from susceptible through prepatent, acute, subclinical, and chronic. The following diagram illustrates the progression process used by EMOD, and the sections below describe each state.

The disease stages

Individuals infected with typhoid may experience a variety of symptoms over a period of days to weeks after infection. In order to capture those dynamics, the progression of a typhoid infection has been broken down into the following stages in the EMOD TYPHOID_SIM.

Susceptible

All individuals enter the model in the susceptible state. After exposure to the pathogens through contaminated water or food, infection is acquired and individuals progress to the prepatent state. Note that in EMOD, there is an “environmental contagion pool” which serves as the means for exposure. Some individuals may have immunity, and will remain in the susceptible category as long as they retain immunity. See Disease immunity for more information.

Prepatent

When susceptible individuals are exposed and infected, they enter the prepatent stage. During this stage, individuals shed contagion into the environment. Duration in the prepatent stage is inversely related to the level of exposure (high exposures have lower prepatent durations). At the end of the prepatent stage, individuals move to either the acute stage or the subclinical stage.

Acute

Infected individuals entering the acute stage continue to shed contagion into the environment, but are now symptomatic and may seek treatment. The duration of the acute stage is dependent upon age: the model uses different distributions of time for over versus under 30, with those over 30 having longer durations of acute infection. Individuals may leave the acute stage either through treatment, death, or by progressing into the chronic stage.

Subclinical

As with the acute stage, all individuals entering the subclinical stage entered via the prepatent stage. There are no symptoms of typhoid, so individuals will not seek treatment. The duration of the subclinical stage is also age-dependent, again with different distributions for over- and under-30. Despite lack of symptoms, subclinical individuals are still infectious and will shed contagion. These individuals may recover (and move back to the susceptible stage), or progress to the chronic stage. There is no disease-associated death for subclinical infections.

Chronic

Infected individuals become chronic carriers after the acute or subclinical stage. The probability of becoming a chronic carrier is both age- and gender-dependent, with probabilities calculated in 10-year age-bins separately for males and females. Individuals in this stage will still shed contagion, but there is no exit from the chronic stage: it is lifelong.