CSEP421 - Chain of Infection Notes

The chain of infection consists of:

Infectious agent: Bacteria, viruses, fungi, protozoa, and parasites.
Reservoir: People, equipment, and water.
Portal of exit: Excretions, secretions, and droplets.
Mode of transmission: Contact (direct, indirect, droplet), airborne, and vectorborne.
Portal of entry: Respiratory tract, GI tract, mucous membrane, and skin.
Susceptible host: People with chronic disease, immunocompromised individuals, the very young, and the elderly.

A reservoir is a continual source of a disease-causing organism, which can be a living organism or an environment that allows the pathogen to grow and be transmitted.
Reservoirs may be:
- Human
- Other animals
- Non-living environment (soil and water)
In healthcare, humans are the primary reservoir.

Infected people constantly produce and shed pathogens.
Infection can spread during incubation and recovery periods.
Subclinical infections, latent viral infections, and chronic carriers are important reservoirs without symptoms.
Normal/resident human flora can be a source of potential pathogens (e.g., urinary tract infections).

Soil:
- May contain endospores of Clostridium species (tetanus, gas gangrene).
- Legionella in potting mixes (Legionnaires’ disease).
- Soil fungi and protozoa can also cause infections.
Water:
- Can be a reservoir for pathogens found in feces or urine (giardiasis, cryptosporidiosis, leptospirosis, polio, hepatitis A).
- Reservoir for gastro-intestinal viral pathogens.

The portal of exit is the path by which an infectious microorganism leaves the reservoir.
Pathogens can exit the body through:
- Excretions: Eliminating waste matter.
- Secretions: Substances produced and discharged from cells, glands, or organs.
- Droplets: Propelled out of the mouth/nose during coughing, sneezing, or medical procedures.
Exposure to blood, tissues, and body fluids can transmit bloodborne viruses.
Main human portals of exit:
- Respiratory system
- Digestive system
- Reproductive system
- Urinary system
- Skin
- Blood

Standard precautions are basic infection control practices to reduce transmission of pathogens from known and unknown sources.
Designed to protect healthcare workers (HCW) and prevent HCW from spreading infections among patients.
Based on the principle that all body fluids pose a potential risk for transmission of bloodborne viruses or other transmissible infectious micro-organisms.
Body fluids include blood and body fluids, secretions, excretions (except sweat), non-intact skin, and mucous membranes.
Include:
- Hand washing.
- Wearing Personal Protective Equipment (PPE) such as gloves, protective masks, clothing, eye/face protection shields.

Main modes (categories) of transmission:
1. Contact
2. Common Vehicle
3. Vector

Direct contact: Close or intimate contact between infected host and susceptible individual (e.g., touch, sexual contact).
Indirect Contact: Microbes transferred via a non-living object (fomite) e.g. door handle
Droplet transmission: Droplets falling less than 1 meter (e.g., sneezing, coughing, laughing, speaking).

An inanimate object that can transmit an infectious agent.
In hospitals, fomites include patient care items, medical equipment, and environmental surfaces.
Transmission occurs via direct contact, indirectly via hands/gloves of healthcare personnel, or less commonly via aerosols, water, or food.

Airborne Transmission
- Organisms carried in air currents for more than 1 metre and can circulate for hours.
- Organisms must survive outside the host and tolerate dry conditions.
- Examples include rubeola (measles) and varicella-zoster (chickenpox).
- SARS-CoV-2 (COVID-19) is spread by droplet and airborne transmission.
Water-borne Transmission
- Occurs when there is fecal contamination of water used for drinking and bathing.
- Includes most organisms responsible for diarrhea.
- Hepatitis A is transmitted by this ‘faecal-oral route’ transmission.
Food-borne Transmission
- Occurs when organisms are introduced by poor hygiene, lack of sanitation, and/or poor food handling.
- Example: Salmonella from poor hand washing after toileting (faecal-oral route).

Mechanical Transmission
- Passive transport of microorganisms on the exterior of an insect’s body.
Biological Transmission
- An insect vector bites a host, ingests blood containing the microorganisms, and transmits them when it bites another host.
- The microorganism requires both the vector and the host to complete its life cycle (e.g., the malarial parasite Plasmodium).
Life cycle of Plasmodium - causes malaria

Portal of Entry: is the path by which an infectious microorganism gains entry to a new host
For example:
- inhalation (e.g. Respiratory system)
- ingestion (e.g. Digestive system)
- absorption (e.g. mucous membranes of eyes)
- break in skin (e.g. needle stick, cut)
- introduction by medical procedures (e.g. catheters)

Susceptible hosts are common in healthcare settings, as those accessing the healthcare system often have compromised immune systems
This may be due to other illnesses, treatments or medications leaving them vulnerable to infectious agents
Susceptible hosts may include:
- Young children
- The elderly
- Chronically ill people
- People receiving treatment such as chemotherapy or high doses of steroids
- People with open wounds e.g. burn victims
Protecting Susceptible Hosts from potential infection can include:
- Ensuring effective use of Standard Precautions
- Treating the underlying disease/condition
- Ensuring they have appropriate nutrition
- Vaccination (where applicable)