Biology • Year 12 • Module 7 • Lesson 4
Modes of Transmission
Apply the three transmission modes, epidemic-curve interpretation and Snow's epidemiological method to real Australian outbreak data and novel scenarios.
1. Interpret notified dengue fever cases in Queensland, 2008–2019
The graph below shows the annual number of locally acquired (vector-borne) dengue fever cases notified in Queensland, compared with dengue cases classified as overseas-acquired for the same period. Dengue is transmitted exclusively by the Aedes aegypti mosquito, which in Australia is restricted to parts of north Queensland. 7 marks
Figure 1.1. Notified dengue fever cases in Queensland, 2008–2019, by acquisition. Adapted from Queensland Health Notifiable Conditions Annual Reports (Queensland Government, 2020). Values approximate.
1.1 Describe the trend in locally acquired dengue cases from 2008 to 2019. Identify the year of peak transmission and estimate the number of cases. 2 marks
1.2 Overseas-acquired cases remain relatively stable across the period while locally acquired cases fluctuate dramatically. Using your knowledge of vector transmission, explain what this pattern suggests about the Aedes aegypti mosquito population in Queensland. 3 marks
1.3 A public health officer proposes quarantining all dengue patients arriving from overseas as the primary control strategy. Evaluate this proposal, referring to the data and the appropriate control measure for vector-borne diseases. 2 marks
2. Cause-and-effect chain — why airborne TB is hard to control
The boxes below form a cause-and-effect chain. The first and last boxes are provided. Fill in the three empty effect boxes to show how airborne transmission of TB leads to the control strategy of negative-pressure isolation rooms. 5 marks
3. Compare the transmission modes of COVID-19 and cholera
Complete the comparison table below. Where a cell is marked “N/A”, write ‘N/A’ and explain briefly why that mode does not apply. 8 marks
| Feature | COVID-19 (SARS-CoV-2) | Cholera (Vibrio cholerae) |
|---|---|---|
| Primary transmission mode | ||
| Specific route within that mode | ||
| Does a vector play a role? | ||
| Key intermediate (if indirect): fomite / water / food / airborne? | ||
| Most effective control measure targeting the transmission route | ||
| Why person-to-person isolation is/is not the primary control | ||
| Australian-context example | ||
| One shared feature of both transmission routes | Write your answer here (applies to both diseases): | |
4. Case study — norovirus outbreak aboard Pacific Explorer, 2018
In March 2018, 192 of 2 200 passengers aboard the Australian cruise vessel Pacific Explorer developed acute vomiting and diarrhoea over a five-day voyage from Sydney. Symptoms appeared between 12 and 48 hours after embarkation. The ship's medical log recorded: all symptomatic passengers initially used the main buffet restaurant; a secondary cluster appeared among cabin-mates of early cases on day 3; stool samples confirmed norovirus GII.4. Norovirus is stable on surfaces for days and infectious at fewer than 20 viral particles. 6 marks
4.1 Identify the most likely initial transmission route using the case data. Classify it by mode. 2 marks
4.2 The secondary cluster among cabin-mates of early cases suggests a second transmission route became active on day 3. Identify this route and explain the evidence supporting it. 2 marks
4.3 Predict and justify which of the following control measures would be most effective for controlling norovirus transmission on a cruise ship: (i) surface disinfection of high-touch areas, (ii) quarantining all ill passengers, (iii) providing antibiotic treatment. 2 marks
Q1.1 — Dengue trend (2 marks)
Locally acquired cases peaked sharply in 2008 (approximately 925 cases) [1], then fell dramatically in most subsequent years, with a secondary smaller peak in 2012 (~300 cases) before remaining near zero to 2019 [1]. Accept any description that notes the sharp peak in 2008 and general decline with values within ±15% of the stated estimates.
Q1.2 — What the pattern suggests (3 marks)
Dengue is vector-borne — it requires a living Aedes aegypti mosquito to transmit the virus; an infected overseas traveller arriving in Australia does not cause local spread unless a mosquito bites that person and subsequently bites another [1]. The large spikes in locally acquired cases in 2008 and 2012 indicate conditions that year supported a larger mosquito population (warm, wet season), sufficient for local transmission chains [1]. The stability of overseas-acquired numbers across all years confirms that travellers consistently return with dengue from endemic regions — but local outbreaks only occur when the vector population is dense enough to sustain transmission, confirming the critical role of the Aedes mosquito in local disease ecology [1].
Q1.3 — Evaluate quarantining overseas arrivals (2 marks)
The proposal is insufficient as a primary strategy [1]. The data show that overseas-acquired cases remain roughly constant while locally acquired cases vary enormously — implying that the determinant of local outbreaks is not the number of infected travellers but the size and distribution of the local Aedes mosquito vector population. The primary control for vector-borne disease is vector control (draining breeding sites, insecticides, mosquito surveillance), not isolating infected people [1].
Q2 — Cause-and-effect chain (sample answer)
Box 2: Droplet nuclei <5 µm are too small to fall quickly under gravity, so they remain suspended in the air of an enclosed space for minutes to hours. Box 3: Suspended droplet nuclei can travel distances greater than 1–2 m, meaning people who are not in close contact with the index case can still inhale infectious particles. Box 4: Because transmission occurs via airborne particles (indirect contact) rather than large respiratory droplets, physical distancing alone does not prevent transmission inside an enclosed space. Award 1 mark per valid, causally linked step, + 1 for the final link to negative-pressure rooms being necessary.
Q3 — COVID-19 vs cholera comparison (8 marks, 1 per row)
Primary mode: COVID-19 → direct contact (respiratory droplets); cholera → indirect contact (waterborne). Specific route: COVID-19 → large droplets >5 µm from infected person at close range (<1–2 m); cholera → water contaminated with V. cholerae. Vector role: both → No. Key intermediate: COVID-19 → airborne aerosol (limited) / fomite (minor); cholera → contaminated water (primary). Control measure: COVID-19 → masks (surgical/N95), ventilation, physical distancing; cholera → water chlorination, sewage treatment, access to safe water. Isolation rationale: COVID-19 → isolation is central (direct person-to-person spread); cholera → isolation alone is insufficient (disease spreads via water, not primarily person-to-person). Australian context: COVID-19 → 2020–2022 pandemic; cholera → not endemic in Australia (risk for travellers/aid workers in Pacific/South-East Asia). Shared feature: both pathogens enter a new host through mucosal surfaces (GI or respiratory tract).
Q4.1 — Initial route (2 marks)
Most likely initial route: indirect contact via fomites [1] — contaminated surfaces in the buffet restaurant (serving equipment, trays, handrails). Norovirus persists on surfaces for days and has an extremely low infectious dose (<20 particles), making fomite transmission highly efficient in a crowded buffet setting [1].
Q4.2 — Secondary route (2 marks)
The secondary cluster among cabin-mates suggests direct contact (or fomite spread within the cabin) — the shared confined cabin space allows vomit-contaminated surfaces and aerosolised particles from vomiting to reach close contacts [1]. Evidence: the cluster appeared on day 3 (after an incubation period of 12–48 h from cabin contact with an infected person), and affected people who shared a cabin with early cases, not random passengers [1].
Q4.3 — Most effective control (2 marks)
(i) Surface disinfection is the most effective measure [1], because the initial and sustained transmission is fomite-mediated — norovirus is stable on surfaces, highly infectious at low dose, and the buffet setting provides many shared high-touch points. Quarantine of ill passengers (ii) also helps reduce direct-contact secondary spread but does not address contaminated surfaces already in the environment. Antibiotics (iii) are ineffective against norovirus, which is a virus [1].