# Situation: A public health nurse in a rice-farming municipality reviews a published prospective cohort study of farmers before planning leptospirosis prevention with the local farmers' association. In the study, 400 farmers who waded in flooded fields without boots and 600 farmers who did not were followed through one rainy season. Leptospirosis developed in 24 farmers in the first group and 6 in the second. What is the relative risk? Round off to one decimal place.

> source: MyMerci (mymerci.kr)  
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> language: ko  
> subject: Nursing Practice I — Community Health Nursing

## 문제

Situation: A public health nurse in a rice-farming municipality reviews a published prospective cohort study of farmers before planning leptospirosis prevention with the local farmers' association.

In the study, 400 farmers who waded in flooded fields without boots and 600 farmers who did not were followed through one rainy season. Leptospirosis developed in 24 farmers in the first group and 6 in the second. What is the relative risk? Round off to one decimal place.

## 보기

1. 0.2
2. 4.0
3. 5.0
4. 6.0 **✔ 정답**

**정답: 4**

## 해설

Relative risk = incidence in the exposed ÷ incidence in the unexposed. Incidence among those who waded without boots = 24 ÷ 400 = 0.06 (60 per 1,000); among the others = 6 ÷ 600 = 0.01 (10 per 1,000). RR = 0.06 ÷ 0.01 = 6.0: farmers who waded without boots had six times the risk.

## 심화 해설

Relative risk calculation

This question asks for the relative risk (RR), which compares the probability of developing leptospirosis between two groups: farmers who waded in flooded fields without boots (exposed) and farmers who did not (unexposed).

Step 1 — Incidence in the exposed group

Among 400 farmers who waded without boots, 24 developed leptospirosis.

Incidence = 24 ÷ 400 = 0.06 (or 60 per 1,000)

Step 2 — Incidence in the unexposed group

Among 600 farmers who did not wade without boots, 6 developed leptospirosis.

Incidence = 6 ÷ 600 = 0.01 (or 10 per 1,000)

Step 3 — Relative risk

RR = incidence in exposed ÷ incidence in unexposed = 0.06 ÷ 0.01 = 6.0

Farmers who waded in flooded fields without boots had six times the risk of developing leptospirosis compared with those who did not. The correct answer is 6.0.

Why this matters for leptospirosis prevention

Leptospirosis is a zoonotic bacterial infection transmitted through direct or indirect contact with water, soil, or mud contaminated with the urine of infected animal reservoirs, particularly rodents [1]. In rice-farming municipalities, flooded fields create an ideal environment for Leptospira survival and human exposure. The bacterium can penetrate intact or abraded skin, and prolonged immersion in contaminated water—especially when protective footwear is absent—substantially increases the risk of infection [1].

The epidemiological pattern in this cohort reflects a classic occupational exposure. Farmers who waded barefoot or without boots sustained prolonged skin contact with potentially contaminated floodwater, whereas those who did not wade had minimal or no such exposure. The sixfold elevation in risk is not merely a statistical artifact; it aligns with the known transmission dynamics of leptospirosis in agricultural settings where heavy rainfall and flooding drive disease occurrence [1].

Key point! Relative risk is the preferred measure in cohort studies because it directly compares the probability of disease between exposed and unexposed groups. An RR of 6.0 indicates a strong association, not a causal proof, but it provides compelling evidence for targeting prevention.

Watch out! Do not confuse relative risk with odds ratio. In a cohort study, RR uses incidence rates, whereas odds ratio is calculated differently and is typically used in case-control designs. Also, when rounding to one decimal place, 0.06 ÷ 0.01 yields exactly 6.0, not 6—the decimal place is required by the question stem.

Clinical and public health application

For the public health nurse planning prevention with the local farmers' association, this RR of 6.0 translates into a clear priority: reduce barefoot or unprotected wading in flooded fields. Interventions may include provision of waterproof boots, education on early recognition of leptospirosis symptoms, and coordination with agricultural authorities to minimize standing water exposure during the rainy season. The high RR also supports targeted surveillance among farmers who report wading without protective footwear, as early antibiotic treatment can reduce morbidity [1].

| Group | Developed leptospirosis | Total farmers | Incidence | Relative risk |
| --- | --- | --- | --- | --- |
| Waded without boots (exposed) | 24 | 400 | 0.06 | 6.0 |
| Did not wade (unexposed) | 6 | 600 | 0.01 |  |

The calculation demonstrates that exposure to floodwater without boots is a major modifiable risk factor for leptospirosis in rice-farming communities. A relative risk of 6.0 means the incidence in the exposed group is six times higher, reinforcing the need for protective measures during the rainy season when flooding and rodent activity converge to amplify transmission [1].References (research sources)

- [1]Leptospirosis in Southeast Asia: Investigating Seroprevalence, Transmission Patterns, and Diagnostic Challenges.Research articleAlmazar CA, Montala YB, Rivera WL. (2026) · DOI: 10.3390/tropicalmed11010018

## 임상 시나리오

Relative Risk in Field ExposureCalculating and applying RR for leptospirosis prevention
Relative risk compares incidence in exposed versus unexposed groups. Here, exposed incidence is 24/400 = 0.06, and unexposed incidence is 6/600 = 0.01, giving RR = 6.0.

In rice-farming areas, flooded fields contaminated with rodent urine are a major source of Leptospira. Wading without boots allows prolonged skin contact and increases infection risk.

CautionAlways verify the denominator for each group before dividing; mixing exposed and unexposed totals is a common error that produces incorrect RR values.

## 핵심 개념

- **Relative risk** — Ratio of incidence in exposed group to incidence in unexposed group; RR > 1 indicates increased risk with exposure.
- **Incidence** — Number of new cases during a specified period divided by the population at risk at the start of that period.
- **Cohort study** — Observational study that follows a group over time to compare outcomes between exposed and unexposed individuals.
- **Leptospirosis** — Zoonotic bacterial infection transmitted through contact with water or soil contaminated by infected animal urine.
- **Occupational exposure** — Contact with a hazard in the workplace; here, wading in flooded fields without boots increases infection risk.

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