Investigate the transmission of a disease during an epidemic, including: mode of transmission (direct, indirect including airborne, vector-borne and waterborne or food-borne) of an infectious disease

What this dot point is asking

NESA wants you to classify the main modes of transmission, give a named example of a disease for each mode, and explain how the mode determines public health responses. Transmission mode comes up in multiple choice every year and is central to extended-response questions on epidemics.

The answer

Transmission is the process by which a pathogen moves from one host to another. The four main modes are direct, airborne, waterborne or food-borne, and vector-borne. The first is "direct"; the rest are forms of indirect transmission.

Direct transmission

The pathogen passes from infected host to new host through physical contact, with no intermediate.

Routes. Touch (skin, mucous membranes), sexual contact, mother-to-child during birth or breastfeeding, droplet spread over short distances (less than 1 metre).

Examples. HIV (sexual contact, blood-to-blood), glandular fever caused by Epstein-Barr virus (saliva), and tinea (skin-to-skin or shared towels).

Indirect transmission: airborne

The pathogen travels through the air on aerosol droplets or dust particles, sometimes over long distances.

Mechanism. Coughing, sneezing or talking produces aerosolised droplets. Smaller droplets (less than 5 micrometres) can remain suspended for hours and travel many metres.

Examples. Mycobacterium tuberculosis (tuberculosis), influenza A, SARS-CoV-2 (COVID-19), measles morbillivirus. Measles is one of the most contagious airborne pathogens, with an R0 of 12 to 18.

Indirect transmission: waterborne and food-borne

The pathogen is carried in contaminated water or food.

Mechanism. Faecal-oral cycle is the most common pattern. An infected host sheds the pathogen in faeces, which contaminates water supplies or food. A new host ingests the pathogen.

Examples. Vibrio cholerae (cholera, contaminated water), Salmonella enterica (food poisoning, undercooked poultry and eggs), hepatitis A virus (contaminated shellfish), Giardia lamblia (contaminated water).

Indirect transmission: vector-borne

A living organism, the vector, carries the pathogen between hosts. The vector is usually an arthropod (mosquito, tick, flea).

Mechanism. The vector picks up the pathogen from one host's blood, the pathogen may undergo development inside the vector, and the vector then transfers the pathogen to a new host through bites or faeces.

Examples. Plasmodium falciparum (malaria, Anopheles mosquito), Yersinia pestis (plague, fleas on rodents), dengue virus (Aedes aegypti mosquito), Trypanosoma brucei (African sleeping sickness, tsetse fly).

Plant pathogens

The same modes apply in plants, with some plant-specific routes such as transmission via grafting and by aphid vectors (e.g. tobacco mosaic virus).

Examples in context

Example 1. Cryptosporidium in Sydney's water supply 1998. In July 1998, WaterNSW (then Sydney Water) detected Cryptosporidium and Giardia oocysts in the water leaving Prospect Reservoir. Over the following three weeks, three "boil water" alerts affected 3 million Sydney residents. The pathogen is transmitted by the faecal-oral route via contaminated drinking water; oocysts are chlorine-resistant and pass through standard filtration unless additional ultrafiltration or UV treatment is applied. The crisis prompted a complete overhaul of Sydney's catchment monitoring, including the upgrade of the Prospect Water Filtration Plant and the establishment of the Sydney Catchment Authority. The event remains the textbook Australian example of waterborne indirect transmission and the public health response it demands.

Example 2. Ross River virus and Aedes mosquito vectors on the NSW north coast. Ross River virus is transmitted indirectly via mosquito vectors (particularly Aedes vigilax salt-marsh mosquitoes) that pick up the virus from infected marsupials, especially eastern grey kangaroos, then bite humans. NSW Health records about 4500 cases per year nationally, with peak transmission in the Tweed and Clarence catchments after summer floods that produce ideal breeding habitat. The virus replicates in mosquito salivary glands and is injected during feeding. Control strategies target the vector: aerial larvicide application to salt marshes, source reduction (removing standing water), and personal repellent use. Because the reservoir is marsupial rather than human, the disease cannot be eliminated by vaccination of people.

Try this

Q1. Identify the mode of transmission for each disease and one control strategy for each: (a) measles, (b) cholera, (c) malaria, (d) tetanus. [4 marks]

• Cue. (a) Airborne droplet, vaccination. (b) Waterborne, clean water supply. (c) Vector-borne (mosquito), bed nets and vector control. (d) Wound contamination (soil), vaccination and wound hygiene.

Q2. During a NSW gastroenteritis outbreak, 320 of 500 attendees at a Sydney wedding develop vomiting within 24 hours. Calculate the attack rate and suggest the most likely mode of transmission given the short incubation. [3 marks]

• Cue. Attack rate is 320/500 = 64 percent. Short incubation and common-source pattern suggests food-borne (e.g. Staphylococcus aureus toxin or norovirus) via contaminated catering.

Q3. Compare the public health strategies needed to control a respiratory droplet-transmitted disease and a vector-borne disease. (a) Identify one example of each. (b) Identify the key control measure for each. (c) Justify why these strategies differ. [2+2+3 marks]

• Cue. (a) Influenza vs Ross River virus. (b) Mask use and vaccination vs vector control and personal repellent. (c) Strategies target the actual transmission route - blocking droplet spread vs eliminating mosquito breeding.