Topic 1.3 The Chain of Infection and Modes of Transmission

Every infectious disease follows a predictable sequence of events that allows microorganisms to spread from one host to another. This sequence, known as the chain of infection, is one of the most important concepts in infection prevention and control. By understanding each step in the chain, Occupational Therapists (OTs) can identify opportunities to interrupt disease transmission and implement practical measures that protect clients, assistance animals, themselves and the broader community.

Infection does not occur simply because a microorganism is present. For disease transmission to occur, several conditions must exist simultaneously. A source of the microorganism must be present, the organism must be capable of leaving that source, survive long enough to reach another susceptible host, and successfully enter the new host through an appropriate route. If any one of these links is broken, transmission is interrupted and infection cannot occur.

For Occupational Therapists working with assistance animals, understanding the chain of infection is particularly important because practice often involves close interaction between people, animals, equipment and diverse environments. Therapy sessions may occur in hospitals, schools, workplaces, aged care facilities, community settings, public transport and clients’ homes, each presenting unique biological hazards. Understanding how infectious diseases spread enables practitioners to undertake appropriate risk assessments and implement evidence-informed infection prevention strategies without unnecessarily restricting access to assistance animals.

This topic explores each component of the chain of infection, the various modes of disease transmission, and the practical strategies Occupational Therapists can use to minimise infection risks while maintaining inclusive, person-centred practice.

The Chain of Infection

The chain of infection is a model used to explain how infectious diseases spread. It consists of six interconnected links:

1. Infectious agent
2. Reservoir
3. Portal of exit
4. Mode of transmission
5. Portal of entry
6. Susceptible host

Disease transmission only occurs when all six links remain intact. Infection prevention strategies aim to interrupt one or more of these links, preventing the microorganism from successfully reaching a new host.

Link 1: Infectious Agent

The infectious agent is the microorganism capable of causing disease.

These include:

• bacteria
• viruses
• fungi
• parasites
• prions.

Not all microorganisms are infectious agents. Many bacteria and fungi are beneficial or harmless. Only microorganisms capable of causing disease under particular circumstances are considered pathogens.

The characteristics of an infectious agent influence its ability to spread, including:

• virulence (ability to cause disease)
• infectious dose (number of organisms required to cause infection)
• ability to survive outside the host
• resistance to disinfectants
• susceptibility to medications.

Some pathogens survive for only minutes outside the body, whereas others remain viable on surfaces for days, weeks or even months.

Link 2: Reservoir

The reservoir is the place where the infectious agent normally lives, grows or multiplies.

Reservoirs may include:

• humans
• assistance animals
• wildlife
• livestock
• insects
• contaminated food
• water
• soil
• medical equipment
• environmental surfaces.

Importantly, individuals or animals acting as reservoirs may appear completely healthy.

For example:

• A healthy person may carry Staphylococcus aureus on their skin.
• An assistance dog may carry naturally occurring bacteria within its gastrointestinal tract without showing any signs of illness.
• Environmental reservoirs may include contaminated water systems or poorly cleaned equipment.

Understanding reservoirs allows Occupational Therapists to identify potential sources of infection and implement appropriate cleaning, hygiene and biosecurity measures.

Link 3: Portal of Exit

The portal of exit is the pathway through which a microorganism leaves its reservoir.

Common portals of exit include:

• respiratory secretions
• saliva
• blood
• urine
• faeces
• vomit
• wound drainage
• skin lesions
• reproductive fluids.

For assistance animals, portals of exit may also include:

• nasal secretions
• saliva
• faeces
• urine
• infected skin lesions
• respiratory secretions.

Routine hygiene, wound management and prompt veterinary care reduce opportunities for microorganisms to leave the reservoir.

Link 4: Mode of Transmission

Once microorganisms leave the reservoir, they require a method of reaching another host.

Modes of transmission describe how this movement occurs.

The major transmission pathways include:

• direct contact
• indirect contact
• droplet transmission
• airborne transmission
• common vehicle transmission
• vector-borne transmission.

Understanding these pathways enables Occupational Therapists to select appropriate infection prevention measures based on actual risk.

Direct Contact Transmission

Direct contact occurs through physical contact between an infected or colonised individual and another person or animal.

Examples include:

• touching infected skin
• contact with body fluids
• bites or scratches
• direct handling of infected animals.

Within occupational therapy practice, direct contact may occur during:

• physical assistance
• transfers
• manual handling
• wound care
• grooming activities
• fitting assistance animal equipment.

Good hand hygiene and appropriate personal protective equipment substantially reduce direct contact transmission.

Indirect Contact Transmission

Indirect contact occurs when microorganisms are transferred via contaminated objects or surfaces.

These objects, known as fomites, may include:

• harnesses
• leads
• mobility aids
• therapy equipment
• door handles
• keyboards
• examination tables
• feeding bowls
• toys.

Regular cleaning and appropriate disinfection are essential for reducing indirect transmission.

Droplet Transmission

Droplet transmission occurs when respiratory droplets are expelled during:

• coughing
• sneezing
• talking
• shouting
• singing.

These droplets generally travel short distances before settling onto nearby people or surfaces.

Respiratory illnesses such as influenza and COVID-19 may spread through droplets.

Protective measures include:

• respiratory hygiene
• masks when indicated
• maintaining appropriate distance
• adequate ventilation
• hand hygiene.

Airborne Transmission

Some infectious particles remain suspended in the air for prolonged periods.

These airborne particles may travel greater distances and be inhaled by susceptible individuals.

Examples include:

• measles
• tuberculosis
• chickenpox.

Although airborne diseases require specialised infection prevention measures, Occupational Therapists should understand when airborne precautions are necessary and follow organisational policies.

Common Vehicle Transmission

Common vehicle transmission occurs when multiple individuals are exposed through a shared contaminated source.

Examples include:

• contaminated food
• drinking water
• medications
• medical products.

Although less common in occupational therapy practice, practitioners should remain aware of food safety, water quality and medication handling where relevant.

Vector-Borne Transmission

Vectors are living organisms that transmit infectious diseases between hosts.

Examples include:

• mosquitoes
• ticks
• fleas.

Vector-borne diseases are generally influenced by environmental conditions and geographic location.

Routine parasite prevention for assistance animals helps minimise vector-associated disease risks.

Link 5: Portal of Entry

The portal of entry is the route through which microorganisms enter a new host.

Common portals include:

• respiratory tract
• gastrointestinal tract
• urinary tract
• eyes
• nose
• mouth
• broken skin
• mucous membranes
• bloodstream.

Protective barriers such as intact skin provide important defence against infection.

Practitioners should consider how everyday clinical activities may create opportunities for microorganisms to enter the body and implement appropriate preventive measures.

Link 6: Susceptible Host

The final link is a susceptible host.

Not everyone exposed to a pathogen becomes ill. Individual susceptibility varies according to numerous factors, including:

• age
• immune function
• chronic illness
• pregnancy
• nutritional status
• medications
• vaccination history
• existing wounds
• stress
• overall health.

Many clients receiving occupational therapy services may have increased susceptibility due to disability, age or underlying health conditions.

Similarly, assistance animals may become more vulnerable to infection if they are very young, elderly, immunocompromised or experiencing illness.

Breaking the Chain of Infection

The primary goal of infection prevention is to interrupt one or more links within the chain of infection.

Strategies include:

• vaccination
• hand hygiene
• environmental cleaning and disinfection
• use of personal protective equipment
• respiratory hygiene
• appropriate waste disposal
• safe food handling
• routine veterinary healthcare
• parasite prevention
• isolation when required
• early identification of illness
• exclusion from work or public access when infectious.

Even simple interventions, such as effective hand hygiene, may interrupt multiple links simultaneously.

Application to Assistance Animal Practice

Occupational Therapists working with assistance animals routinely encounter situations where understanding the chain of infection informs clinical decision-making.

Examples include:

• determining whether an assistance animal with diarrhoea should attend therapy
• deciding when equipment requires cleaning or disinfection
• assessing whether an immunocompromised client requires additional precautions
• managing therapy sessions during respiratory illness outbreaks
• identifying when veterinary assessment is required
• educating clients about hand hygiene after animal handling
• planning safe participation in healthcare environments.

Rather than applying blanket restrictions, Occupational Therapists should undertake individual risk assessments based on the chain of infection and current evidence.

Common Misconceptions About Assistance Animals and Infection

One of the greatest challenges facing assistance animal teams is the misconception that animals automatically represent a significant infection risk.

Research consistently demonstrates that healthy, well-groomed assistance animals receiving appropriate veterinary care present a very low risk of transmitting infectious diseases in public settings. In many situations, the greatest risk of disease transmission comes from human-to-human contact rather than from the animal itself.

Occupational Therapists have an important role in correcting misinformation by explaining that infection prevention should always be based on evidence rather than assumptions. Decisions regarding access should consider the health of the animal, the health status of the client, the environment, and the presence of any specific biological hazards. Excluding an assistance animal solely because it is an animal, without conducting an appropriate risk assessment, may unnecessarily restrict participation and could constitute discriminatory practice under disability legislation.

Key Practice Principle

Occupational Therapists should understand the chain of infection and the various modes of disease transmission to effectively identify biological hazards, undertake proportionate risk assessments and implement evidence-informed infection prevention strategies. By breaking one or more links in the chain of infection, practitioners can protect clients, assistance animals and the wider community while supporting safe, inclusive and meaningful occupational participation.