We follow GBD 2017 and "defined diarrhoeal disease episodes as three or more loose stools in a 24-hour period." (p. 88 of [GBD-2017-YLD-Capstone-Appendix-1]).
Diarrhea has various etiologies, with infectious diarrhea accounting for the vast majority of global diarrheal disease burden. The top pathogens responsible for diarrhea include norovirus, rotavirus, E. Coli, Camplyobacter, and Salmonella. Bacterial infections, and specifically species of Shigella, account for the majority of bloody diarrhea.
Infection most commonly occurs via feces-contamined water, and can also spread via contamined food and person-to-person contact. ([WHO])
The global prevalence of diarrhea thus varies considerably accoring to resource access. In particular, resource-limited countries have a "baseline frequency... superimposed with epidemic cases of diarrhea" ([UpToDate_1]). The top risk factors for diarrheal diseases thus include crowding (such as living in refugee camps) and poor sanitation, in addition to immune system-compromising conditions, such as living with HIV.
The most significant outcomes of a nonfatal diarrhea episode are dehydration and the loss of nutrition. In particular, in low-income countries, the high prevalence of diarrhea is a major cause of child malnutrition ([WHO]), which in turn makes such children more susceptible to future diarrheal episodes and other negative sequelae.
- The WHO-recommended measures for diarrhea prevention include:
- Access to safe drinking water;
- Use of improved sanitation;
- Hand washing with soap;
- Exclusive breastfeeding for the first six months of life;
- Good personal and food hygiene;
- Health education about how infections spread; and
- Rotavirus vaccination.
Noninfectious diarrhea etiologies are far less common, but are more likely among chronic cases of diarrhea. Causes of noninfectious diarrhea include ischemic colitis, inflammatory bowl disease, among others ([UpToDate_2]).
The GBD diarrheal diseases model follows a standard GBD framework, including a cause of death (CoD) model and a nonfatal model.
The CoD model estimates the cause-specific mortality rate (CSMR) within the total population, and a cause fraction. These estimates are based on vital registration and verabal autopsy data.
The nonfatal model is run in DisMod. The primary inputs are prevalence epi data, for which self-reported prevalence is the reference definition, and the CSMR estimates from CodCorrect. Separately, the ratio of mild/moderate/severe diarrhea is estimated, based on data from a systematic review. These severity ratio estimates do not vary by age/sex/location/year, and are applied to the prevalence and incidence estimates produced by DisMod to produce the three sequela of diarrheal diseases: mild diarrheal diseases, moderate diarrheal diseases, and severe diarrheal diseases. In our model, every individual will have the average severity for their age/sex/location/year.
The GBD 2017 adjusted for seasonal variation in diarrheal disease, but we have not attempted to include this variation in Vivarium yet. (p. 89)
There is substantial additional effort in GBD to divide diarrhea burden into the aetiologies of diarrhea, but we have not included aetiologies in this simple model.
S: Susceptible to diarrheal diseases
I: Infected and currently experiencing a diarrheal disease bout
| State | State name | Definition |
|---|---|---|
| S | Susceptible | Simulant currently has diarrheal disease |
| I | Infected | Simulant does not currently have diarrheal disease |
| State | Measure | Value | Notes |
|---|---|---|---|
| I | prevalence | prevalence_c302 | |
| I | birth prevalence | 0 | |
| I | excess mortality rate | \frac{\text{deaths\_c302}}{\text{population} \,\times\, \text{prevalence\_c302}} | |
| I | disability weight | \displaystyle{\sum_{s\in \text{sequelae\_c302}}} \scriptstyle{\text{disability\_weight}_s \,\times\, \text{prevalence}_s} | |
| S | prevalence | 1-prevalence_c302 | |
| S | birth prevalence | 1 | |
| S | emr | 0 | |
| S | disability weight | 0 | |
| All | cause-specific mortality rate | \frac{\text{deaths\_c302}}{\text{population}} |
| Transition | Source State | Sink State | Value | Notes |
|---|---|---|---|---|
| i | S | I | \frac{\text{incidence\_rate\_c302}}{1-\text{prevalence\_c302}} | We transform incidence to be a rate within the susceptible population. |
| r | I | S | remission_rate_m1181 | Already a rate within with-condition population |
| Value | Source | Description | Notes |
|---|---|---|---|
| prevalence_c302 | como | Prevalence of diarrheal diseases | |
| deaths_c302 | codcorrect | Deaths from diarrheal diseases | |
| incidence_rate_c302 | como | Incidence of diarrheal disease within the entire population | |
| remission_rate_m1181 | dismod | Remission of diarrheal disease within the infected population | |
| population | demography | Mid-year population for given age/sex/year/location | |
| sequelae_c302 | gbd_mapping | List of 4 sequelae for diarrheal diseases | Note Guillain-Barre due to diarrheal diseases is included in sequelae. |
| prevalence_s{sid} | como | Prevalence of sequela with id sid | |
| disability_weight_s{sid} | YLD appendix | Disability weight of sequela with id sid |
| Restriction type | Value | Notes |
|---|---|---|
| Male only | False | |
| Female only | False | |
| YLL only | False | |
| YLD only | False | |
| YLL age group start | Early neonatal | age_group_id = 2; [0-7 days) |
| YLL age group end | 95 plus | age_group_id = 235; 95 years + |
| YLD age group start | Early neonatal | age_group_id = 2; [0-7 days) |
| YLD age group end | 95 plus | age_group_id = 235; 95 years + |
.. todo:: Describe tests for model validation.
| [WHO] | (1, 2) Diarrheal disease Fact Sheet. World Health Organization, 2 May 2019. Retrieved 14 Nov 2019. https://www.who.int/news-room/fact-sheets/detail/diarrhoeal-disease |
| [UpToDate_1] | Approach to the adult with acute diarrhea in resource-limited countries Retrieved 26 Dec 2019. https://www.uptodate.com/contents/approach-to-the-adult-with-acute-diarrhea-in-resource-limited-countries |
| [UpToDate_2] | Approach to the adult with acute diarrhea in resource-rich countries Retrieved 26 Dec 2019. https://www.uptodate.com/contents/approach-to-the-adult-with-acute-diarrhea-in-resource-rich-settings |
| [CDC] | Diarrhea: Common Illness, Global Killer. https://www.cdc.gov/healthywater/global/diarrhea-burden.html |
| [Wikipedia] | Diarrhea. From Wikipedia, the Free Encyclopedia. Retrieved 14 Nov 2019. https://en.wikipedia.org/wiki/Diarrhea |
| [GBD-2017-YLD-Capstone-Appendix-1] | Supplement to: GBD 2017 Disease and Injury Incidence and Prevalence Collaborators. Global, regional, and national incidence, prevalence, and years lived with disability for 354 diseases and injuries for 195 countries and territories, Disease Study 2017. Lancet 2018; 392: 178 (pp. 88-94) (Direct links to the YLD Appendix hosted on Lancet.com and ScienceDirect) |