What is the Epidemiological Triad?

Parts of the Epidemiological Triad

The epidemiological triad consists of:

• External agent
• Susceptible host
• Environment

In its most simplistic form, the host and agent are brought together and cause the disease in the host.

Agent

The agent is the virus, bacteria, parasite, or other organisms that cause the disease.

Host

Hosts carry the disease after an agent infects them. Hosts can get sick from the disease or act as a carrier. Think about when someone is sick with a cold at work. You may or may not get sick, but you can carry the virus to your kids when you get home.

Environment

The environment includes the air, soil, water, climate change, and other factors that influence the spread of infectious diseases.

Evolution of the Epidemiological Triad

The epidemiological triangle can be extremely simplistic with one agent, host, and environmental factor or expanded to include a more holistic view of disease causation and transmission.

Ideally, an effective epidemiological triad includes both direct and indirect transmission methods:

Direct Transmission: Sometimes, the agent or pathogen directly infects the host and causes the disease.

Indirect Transmission: Diseases can also be transmitted indirectly through the air, soil, water, or animal disease carriers called ‘vectors’. Vectors carry diseases from one host to another but do not cause the disease.

The environment can also paint a larger picture of how climate change, new zoonotic outbreaks, deforestation, socio-political and economic factors, availability of vaccines, deep-rooted community beliefs, natural disasters, and more affect the origin and spread of disease.

Benefits of the Epidemiological Triad

The sheer amount of information about infectious diseases can be overwhelming for students starting in epidemiology. The epidemiological triangle is a great starting point to learn how existing and emerging diseases are caused and spread. This traditional concept helps budding epidemiologists break down essential components of disease transmission. Teachers often use the epidemiological triangle to enhance learning and build a solid foundation in epidemiology. 

Epidemiologists who create the visual representation of the epidemiological triangle also get an immediate snapshot of what information they have on hand and what’s left to learn. Locking down the fundamentals of an epidemiological triad can help frame the appropriate public health response during an outbreak. 

For example, when COVID-19 first hit, the world knew of the agent (a new virus) and one confirmed host (humans). There were misconceptions about the environment that were eventually cleared. Initially, we were told that the virus could live on and get transmitted through surfaces. This idea was later debunked with more data, and health officials could focus their efforts on other environmental factors. 

Examples of the Epidemiological Triad

Let’s look at some examples of the epidemiological triad in action. For this exercise, the GIDEON database was used. It is a convenient way to search for diseases and get all the information needed in one place.

Dengue

Dengue is a neglected tropical disease (NTD) but is now considered a potential pandemic threat as it spreads rapidly to non-endemic regions. So, let’s apply the epidemiological triad to dengue and learn more. 

Agent: Flaviviridae, Flavivirus, Dengue virus

Host: Humans, intermediate hosts: mosquitos (Stegomyia (Aedes) aegypti, S. albopictus, S. polynesiensis, S. Scutellaris), monkeys in Malaysia and Africa

Environment: breeding grounds for mosquitoes like stagnant water, open gutters, centralized but unclean water supply, lack of access to supportive care, urbanization, deforestation, climate change

Discussion: At its most simplistic model, the epidemiological triad for dengue will contain the dengue virus as the agent, humans as the host, and contaminated water as the environment. However, the model can (and should) be expanded to include intermediate hosts (vectors) like the aedes mosquitoes that are largely responsible for the spread.

Environmental factors must include socio-economic and infrastructure-related factors, like access to healthcare. For example, while 80% of dengue cases are mild or asymptomatic, severe dengue can develop during the critical phase (3-7 days after getting sick). Symptoms can suddenly worsen, leading to complications and even death without hospitalization and close medical monitoring. Access to healthcare nearby is a critical ‘environmental’ factor. Similarly, climate change has been responsible for dengue now spreading to non-endemic regions, making dengue a pandemic-level threat.

Polio

In September 2022, the State of New York declared an emergency due to polio cases spreading. The poliovirus was detected in sewage water samples. Polio is highly contagious; the virus can cause total paralysis within a few hours. There is no cure for polio; prevention is critical to curbing its spread. Let’s look at the epidemiological triad for polio. 

Agent: Picornaviridae, Picornavirus: Polio virus

Host: Humans

Environment: Fecal matter, contaminated water or food, unsanitary practices (not washing hands with soap), lack of access to polio vaccines, vaccine hesitancy, distrust related to the healthcare system, interruptions to vaccine drives due to the COVID-19 pandemic, and vaccine supply, storage, and distribution.

Discussion: Polio can be prevented and eradicated with the right public health responses, resources, and awareness about the power of the polio vaccine. While it has one agent and one host, the environment holds a multitude of factors to address before the world can eradicate the disease. For example, a lack of access to polio vaccines could be due to insufficient supply, a global pandemic, or a natural disaster interrupting scheduled vaccine drives. Other environmental factors include misinformation (and disinformation) about vaccines. Once these points are mapped out on the epidemiological triangle, they can be prioritized and addressed.

Ebola

Ebola is prioritized by the World Health Organization (WHO) for research and development due to its potential to cause a widespread public health crisis. The disease is also known as Ebola Virus Disease (EVD) and, previously, Ebola hemorrhagic fever. Although rare, it can cause severe illness and fatal infections. The case-fatality rate is 50% — extremely high. What would the epidemiological triangle for Ebola look like? 

 Agent: Viruses in the genus ebolavirus, including Zaire ebolavirus, Sudan ebolavirus, Taï Forest ebolavirus, and Bundibugyo ebolaviruses.

Host: Humans, primates (apes and monkeys), guinea pigs, fruit bats, porcupines, and forest antelopes

Environment: Blood or body secretions from an infected host (alive or recently dead), contaminated needles and syringes, breastfeeding from an infected parent, sexual contact, respiratory/air, access to supportive care and vaccines, PPE (protective gear) for healthcare workers, unsafe burials, unsafe sexual practices, population growth, urbanization, and raw meat consumption, globalization.

Discussion: Ebola is not a high risk for rapid outbreak spread because it is not spread through the air, water, or food. Direct contact with blood or other body fluids is how Ebola is transmitted. However, in recent years, deforestation due to human encroachment has disturbed fruit bats’ natural habitat, a common ebolavirus host. This displacement can increase the risk of zoonotic spillover of Ebola from wild animals to humans. There are two licensed vaccines against Ebola, but quantities are limited. However, in January 2021, a global stockpile of Ebola vaccines began to help mitigate the risk of a large-scale Ebola outbreak. 

As you see, epidemiological triangles are a great way to organize epidemiology information, provide a visual snapshot of infectious disease, and teach the principles of epidemiology. 

GIDEON, the world’s leading infectious diseases database, is committed to advancing the global effort against infectious diseases. A huge part of this strategy is to empower epidemiology and public health teachers and faculty with lesson plans, including a lesson plan on epidemiological triangles.