prevalence and risk factors of cholera

Pathogenic Diseases – Cholera

1. Overview (AO1 – Knowledge)

• Causative agent: Vibrio cholerae (serogroups O1 and O139).
• Transmission: Ingestion of water or food contaminated with faeces containing the bacterium.
• Clinical picture: Profuse watery diarrhoea → rapid fluid loss → dehydration and, if untreated, death.
• Geographic relevance: Classic example of how human‑environment interactions shape the distribution of infectious diseases.

2. Global Prevalence (2023‑2024) – Scale & Spatial Variation (AO1)

The WHO Global Health Observatory (2024) estimates 1.3–4.0 million cholera cases and 21 000–143 000 deaths each year.

Region (2023)	Incidence (cases per 100 000)	Case‑Fatality Ratio (CFR %)
Sub‑Saharan Africa	12.4	2.1
South Asia	8.7	1.8
Latin America & Caribbean	1.2	0.9
Middle East & North Africa	0.9	1.2
East Asia & Pacific	0.5	0.7

Source: WHO Global Health Observatory, 2024 (averages for calendar year 2023).

3. Key Geographical Concepts (explicitly linked to the syllabus)

• Scale: Global (WHO data), regional (tables), national (case study Haiti), local (community water points).
• Change over time: Trends in incidence 2020‑2024; impact of climate change projections to 2050.
• Place: Physical (river basins, coastal deltas) and human (urban slums, refugee camps) characteristics that affect risk.
• Spatial variation: Differences between regions shown in the prevalence table and map.
• Cause‑and‑effect / Systems: Interaction of water supply, sanitation, poverty, and climate in a feedback loop that sustains transmission.
• Challenges & opportunities: Short‑term (ORS, oral cholera vaccine) vs. long‑term (piped water, sanitation) interventions.
• Diversity & equality: Gender (women water‑collectors) and age (children under 5) differentials in exposure and mortality.

4. Socio‑environmental Risk Factors (AO2 – Data interpretation)

Relative risk (RR) values are drawn from Ali et al. (2022). Values > 1 indicate increased risk compared with a baseline population.

Risk Factor	Transmission Mechanism	Relative Risk (RR)
Contaminated drinking water	Direct ingestion of V. cholerae from unsafe sources	≈ 3.5
Open defecation / inadequate sewage	Environmental contamination of water bodies	≈ 2.8
High population density (urban slums)	Shared water points & rapid spread of faecal contamination	≈ 1.9
Flooding & cyclones	Disruption of water treatment, overflow of sewage	≈ 2.2
Poverty (low household income)	Limited access to piped water, health services & education	≈ 2.5
Gender (women) & age (children)	Women collect water; children play in contaminated water	1.6 (women), 1.8 (children)
Land‑use change (urban expansion, agricultural runoff)	Increased surface‑water contamination, reduced natural filtration	≈ 1.4
Climate‑change trends (↑temp, ↑precipitation)	Warmer, wetter conditions promote bacterial growth & extend transmission season	Projected RR ≈ 1.3 by 2050 (RCP 4.5)

Source: Ali et al., “Risk factors for cholera: a systematic review”, Int. J. Infect. Dis., 2022.

5. Case Study – 2010 Haiti Cholera Outbreak (Place & Change over time)

Aspect	Details
Trigger	Contamination of the Artibonite River by untreated sewage from a UN peace‑keeping camp.
Spatial spread	River‑basin diffusion, later to Port‑au‑Prince via market routes.
Most affected groups	Children < 5 yr (CFR ≈ 4 %); women water‑collectors.
Immediate response	ORS kits, cholera treatment centres, emergency chlorination.
Long‑term interventions	250 km piped water, community‑led total sanitation (CLTS), oral cholera vaccine (OCV) 2018.
Effectiveness evaluation	ORS reduced CFR from 4 % to <1 % within 12 months. OCV (2‑dose) achieved 78 % coverage; modelling predicts 30 % incidence reduction. Only 45 % of households have reliable piped water after 5 years – infrastructure remains under‑funded.
Governance & policy	WHO‑UNICEF JMP coordinated with Haiti’s Ministry of Public Health; highlighted need for stronger international accountability.

Sources: WHO Cholera Situation Report, Haiti 2021; UNICEF WASH Annual Review, 2022.

6. Transmission Dynamics – The R₀ Concept (Systems)

For cholera, the basic reproduction number can be expressed as:

\[ R_0 = \frac{\beta}{\gamma} \]

• β (transmission rate) = contact rate (people accessing a contaminated water source per day) × probability of infection per contact (depends on bacterial concentration, water temperature, hygiene).
• γ (recovery rate) = 1 / average duration of infectiousness. Prompt ORS shortens the infectious period from ~5 days to ≈2 days, raising γ and lowering R₀.

Typical endemic values: R₀ = 1.2–2.5. Values > 1 indicate potential for epidemic expansion. Modelling (Hartley et al., 2020) shows that a 30 % reduction in β (water treatment) combined with a 50 % increase in γ (rapid ORS) can push R₀ < 1, halting transmission.

7. Management, Mitigation & Evaluation (AO3 – Evaluation)

Intervention	Effectiveness (incidence reduction)	Cost‑Effectiveness (US$ per DALY averted)	Sustainability / Governance
Oral Rehydration Solution (ORS) & zinc	≈ 85 % reduction in case‑fatality	≈ $30	Highly sustainable; minimal infrastructure; WHO Essential Medicines List.
Improved water supply (chlorination, piped water)	30‑60 % reduction in incidence (coverage‑dependent)	≈ $150‑$300	Capital‑intensive; requires strong water authority and maintenance funding.
Sanitation upgrades (latrines, sewerage)	≈ 40 % reduction in incidence	≈ $250‑$500	Needs community mobilisation; yields co‑benefits for other diseases.
Oral Cholera Vaccine (OCV) – 2‑dose regimen	≈ 60 % short‑term protection; 50 % after 3 years	≈ $4‑$5 per dose (Gavi‑supported)	Effective for rapid outbreak control; requires cold‑chain and periodic boosters.

Cost data: Gavi OCV Investment Strategy 2023; effectiveness: WHO “Cholera Toolkit”, 2022.

Evaluation Task (AO3)

Using the table above, write a short paragraph (≈ 80 words) comparing the long‑term sustainability of improved water supply with the oral cholera vaccine. Consider factors such as cost, required infrastructure, community acceptance, and potential for reducing future outbreaks.

8. Wider Geographic Impacts (Diversity & Equality, Economic Change)

• Population‑health interactions: Outbreaks can trigger temporary rural‑to‑urban migration, straining city health services and housing.
• Economic consequences: Lost labour, reduced tourism, and increased health‑care spending can depress GDP, especially in low‑income economies.
• Gender & age differentials: Women’s water‑collection duties increase exposure; children under five suffer the highest CFR, underscoring the need for targeted education.
• Climate change projection: IPCC AR6 predicts a 0.5‑1.0 °C rise in average temperature across many cholera‑endemic regions by 2050, potentially extending the transmission season by 2‑3 months (Kalkstein et al., 2023).

9. Transmission Cycle Diagram (Systems)

10. Key Takeaways for Students (AO1)

• Cholera is most prevalent where water, sanitation and health services are inadequate – a clear indicator of global health inequality.
• Risk factors operate at multiple scales (global climate, regional water infrastructure, local hygiene practices) and are intensified by gender, age and poverty.
• Understanding R₀ and relative risk helps predict outbreak potential and evaluate control strategies.
• Effective control combines short‑term measures (ORS, OCV) with long‑term investments (piped water, sanitation) and strong governance.
• The 2010 Haiti outbreak demonstrates the importance of rapid response, community engagement, and sustainable infrastructure for reducing mortality and preventing future epidemics.

11. Practice Questions (AO2 & AO3)

1. Data interpretation (AO2): Using the prevalence table, calculate the overall global case‑fatality ratio (weighted by regional incidence) and comment on what this reveals about health‑service gaps.
2. Cause‑and‑effect analysis (AO2): Explain how a severe flood could increase cholera incidence in a coastal city, linking at least three risk factors from the table.
3. Evaluation (AO3): Assess the advantages and limitations of using oral cholera vaccine campaigns in emergency settings compared with improving water supply.
4. Conceptual mapping (AO1): Match each of the syllabus key concepts (scale, change over time, place, spatial variation, systems, challenges & opportunities, diversity & equality) to the relevant section of these notes.