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Week 2 Case Study

The Gastro Cluster

The Scenario

Paramedic Amir is working a Friday night shift at the Springfield station in Ipswich, Queensland. Over the past three shifts (Wednesday to Friday), he has personally attended four cases of acute gastroenteritis in the suburb of Redbank Plains—all presenting with severe vomiting, watery diarrhoea, abdominal cramping, and low-grade fever. During a handover conversation, a colleague mentions attending two similar cases in the same suburb on Wednesday, and another crew reports a further three on Thursday. In total, at least nine cases have been transported from a four-block radius over 72 hours.

Amir's most recent patient is Devi, a 34-year-old woman who reports that her symptoms started approximately 18 hours ago after eating a chicken salad from "Fresh Bites Café," a small local takeaway shop. She is visibly dehydrated: dry mucous membranes, reduced skin turgor, HR 108, BP 96/62, temp 37.9°C. She tells Amir that her husband and two children are also unwell but "not as bad." While establishing IV access for fluid replacement, Amir asks Devi if she knows anyone else who is sick. She says, "My neighbour went to hospital yesterday with the same thing. We both ate at the same café."

Amir recalls from his PMC1101 studies that paramedics can play a role in disease surveillance—they are often the first healthcare contact for acutely unwell patients and can identify patterns before the formal public health system detects them. He decides to document the suspected cluster in his patient care record, noting the common food exposure, geographic clustering, and temporal pattern. At the end of his shift, he raises the concern with his station officer, who contacts Queensland Health's communicable disease unit.

Discussion Questions

1 Using the epidemiological data in this scenario (9 cases in 72 hours from a defined geographic area), calculate or estimate the attack rate if the suspected source (Fresh Bites Café) served approximately 120 customers over those three days. What additional information would you need to calculate a precise attack rate?
2 Describe the difference between incidence and prevalence. Which measure is more useful in investigating this acute outbreak, and why?
3 What type of epidemiological study design would be most appropriate to investigate this cluster? Explain why a case-control study or a retrospective cohort study might be used, and outline the basic methodology for each.
4 Amir identified this cluster through observation during routine clinical work. Discuss the role of paramedics in disease surveillance. How does syndromic surveillance differ from traditional laboratory-based surveillance, and what advantages do paramedics offer as "sentinel observers"?
5 What are notifiable diseases in Queensland? Is gastroenteritis notifiable? Under what circumstances would this cluster trigger formal notification to Queensland Health? What pathogen might be suspected given the food exposure and clinical presentation?
6 Construct a simple epidemic curve (epi curve) for this outbreak using the information provided. What does the shape of the curve suggest about the likely source (point source vs propagated)?

Key Concepts Applied

Attack Rate

The proportion of exposed individuals who develop illness—critical for quantifying outbreak severity and identifying the source.

Incidence vs Prevalence

Incidence measures new cases over time, making it the key metric for tracking an evolving outbreak rather than prevalence.

Study Designs

Case-control and retrospective cohort studies are the workhorses of outbreak investigation, each with distinct strengths.

Disease Surveillance

Paramedics act as frontline sensors in the surveillance system, detecting patterns before laboratory confirmation is available.

Epidemic Curves

Visual tools for plotting case onset over time. The curve shape reveals whether the source is a single point exposure or ongoing transmission.

Notifiable Diseases

Certain infections require mandatory reporting to public health authorities to enable rapid response and containment.

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