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