What is the Reservoir in the Chain of Infection?

The chain of infection explains how diseases spread from one person, animal, or environment to another. The reservoir is a key part of this chain. It refers to the source where an infectious agent lives, grows, and multiplies. Without a reservoir, the infectious agent would die off and fail to spread.

This guide covers the reservoir’s role in the chain of infection, examples of reservoirs, and how they contribute to the spread of disease.

What is the Reservoir?

The reservoir in the chain of infection is the habitat or environment where an infectious agent thrives. It can be a living host, such as humans or animals, or a non-living environmental source such as contaminated water, soil, or surfaces. Infectious agents rely on reservoirs to survive and reproduce before they move to a new host.

The reservoir acts as a holding area for pathogens (organisms that cause disease). Once conditions in the reservoir are right, the pathogen can leave and infect others. This process is critical for its life cycle and the continuation of disease transmission.

What are the Types of Reservoirs?

Reservoirs can be broadly classified into two categories: living (biotic) and non-living (abiotic). Below, we’ll explain these in detail.

Living Reservoirs

These reservoirs consist of living organisms that carry infectious agents. Examples include:

• Humans: Humans can act as reservoirs for diseases. For example, someone with the flu virus can transmit the infection to others. Some individuals, called carriers, do not show symptoms but still harbour the infectious agent and spread the disease. Diseases like tuberculosis and HIV rely on humans as reservoirs.
• Animals: Certain infectious agents live in animals and can infect humans through direct contact or vectors (organisms that transmit infections). For example, bats serve as reservoirs for rabies virus, and mosquitoes can spread malaria from infected organisms.
• Vectors: While not reservoirs themselves, vectors like ticks or mosquitoes act as intermediaries between the reservoir and host, carrying the infectious agent to humans.

Non-Living Reservoirs

These consist of environmental sources that harbour infectious agents. Examples include:

• Water: Contaminated water is a common reservoir for pathogens like Vibrio cholerae, which causes cholera, and norovirus, which can infect large groups of people.
• Soil: Soil serves as a reservoir for infectious agents such as Clostridium tetani, which causes tetanus, and Histoplasma capsulatum, a fungus responsible for histoplasmosis.
• Surfaces: Household objects, hospital equipment, or public spaces can act as reservoirs for bacteria or viruses like Staphylococcus aureus or flu viruses. Pathogens survive on these surfaces for hours or even days.

What are the Traits of Reservoirs?

Certain traits make a reservoir suitable for infectious agents. These include:

• Optimal environment: Reservoirs provide conditions where infectious agents can thrive. For example, bacteria will grow rapidly in moist environments, while others might prefer dry surfaces.
• Ability to maintain pathogens: Reservoirs often allow the infectious agent to remain undetected, which means they can act as long-term sources of infection.
• Compatibility with pathogens: Infectious agents are suited to specific reservoirs. Waterborne pathogens, for instance, cannot survive outside moist environments, while respiratory pathogens need hosts that breathe.

Reservoirs and Modes of Transmission

Reservoirs are intimately linked to how diseases are transmitted. Infectious agents leave the reservoir and spread through different methods, such as direct or indirect contact, droplets, or vectors. For example:

• Direct contact: A reservoir, such as an infected human, can spread the disease directly through physical contact (e.g., touching or sneezing on someone).
• Contaminated water or food: Non-living reservoirs like water or food can serve as intermediaries, allowing pathogens to affect individuals who consume them.
• Animal reservoirs: Zoonotic diseases, like Ebola or bird flu, involve pathogens moving from animals to humans. This often requires close proximity to infected animals or their waste products.

Human Carriers as Reservoirs

An important type of reservoir is the human carrier. Carriers are individuals who harbour infectious agents but may not show visible symptoms of disease. These carriers unknowingly spread infections to others.

Carriers can be:

• Active carriers: These individuals actively shed the pathogen while experiencing symptoms of the disease.
• Passive carriers: These individuals shed the pathogen without ever showing symptoms. For example, someone carrying Salmonella may pass it through contaminated food but feel completely well.
• Chronic carriers: These people carry the infectious agent for long periods. For example, after recovering from typhoid fever caused by Salmonella typhi, some individuals continue to shed the bacterium for years.

Identifying Reservoirs

Identifying the reservoir is key to controlling the spread of infection. Once healthcare professionals know the source of a pathogen, they can develop strategies to prevent its transmission. For example:

• In hospitals, staff analyse surfaces or equipment as potential reservoirs.
• During outbreaks, investigators scrutinise environmental reservoirs such as water or food supplies.
• For animal-borne infections, public health authorities investigate species that commonly act as carriers.

Examples of Reservoir-Related Diseases

Many diseases are closely linked to their reservoir types. Below are examples:

• Cholera: Spread via contaminated water reservoirs.
• Malaria: Transmitted when mosquitoes act as reservoirs for the Plasmodium parasites.
• Tetanus: Caused by Clostridium tetani found in soil.
• Typhoid fever: Spread as Salmonella typhi uses humans as reservoirs.

Preventing Reservoir-Related Infections

Reducing the risk of infection requires minimising exposure to reservoirs. Strategies include:

• Hand hygiene: Washing hands after contact with potential reservoirs prevents direct transmission.
• Cleaning and disinfecting surfaces: Regular cleaning removes infectious agents that might survive on equipment or furniture.
• Safe water practices: Treating and boiling drinking water in high-risk areas kills pathogens.
• Vaccination programs: Immunising people against infectious agents reduces the risk of disease from human reservoirs.
• Quarantine of infected individuals: Restricting movement reduces the risk of pathogens leaving the reservoir.

How Reservoirs Contribute to Outbreaks

Reservoirs often play a major role during outbreaks. For example, contaminated food supplies in restaurants or large gatherings can act as reservoirs for pathogens. If the problem isn’t identified early, many individuals may get sick. Similarly, an asymptomatic human carrier in close-contact settings like care homes can unknowingly spread infections to vulnerable groups.

Environmental reservoirs, like floodwaters after natural disasters, can trigger outbreaks of waterborne diseases like cholera or leptospirosis. Public health teams often focus on identifying and eliminating reservoirs to contain outbreaks promptly.

Role of Surveillance in Tackling Reservoirs

Surveillance systems help healthcare professionals track reservoirs and prevent infections. These systems collect data on outbreaks, test samples from suspected reservoirs, and monitor changes in pathogens. For example, surveillance programmes might regularly test water supplies to detect harmful bacteria early.

Using data from surveillance systems enables public health authorities to map high-risk reservoirs and focus resources to address them effectively.

Final Thoughts

The reservoir is where infectious agents survive and multiply before spreading to hosts. Without reservoirs, pathogens cannot sustain themselves or transmit infections. Reservoirs can be living organisms, like humans and animals, or environmental sources like water, soil, and surfaces.

Understanding reservoirs helps healthcare professionals curb infections through sanitation, hygiene, vaccinations, and other interventions. By addressing reservoirs as part of the chain of infection, health and social care workers play a vital role in protecting individuals and communities from the spread of disease.