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 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.
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
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 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.