Epidemiology 101: Chain of Infection
In Epidemiology 101: Key Concepts, we discussed the all-important epidemiological triad. The agent in this triad is typically the germ, as in, germ theory of disease. In epidemiology, the transmission of an agent or germ is known as the chain of infection. In short, this is referring to the process of a microbe or other agent spreading from one host to another. In everyday language, we simply refer to it as whether or not a health condition is contagious. Communicable and infectious are other words used to describe contagious diseases.
The chain of infection documents the transmission process where a microbe exits a reservoir (see below) and enters a host (i.e. you or me). Anything that happens during that process is considered to be part of the chain. The portal of entry describes how the microbe enters the body and the portal of exit describes how it exits the body. In other words, this is how disease spreads. Easy-peasy so far.
Here is an example: many pediatric infections are spread via the fecal-oral route. In this scenario, the rectum/fecal matter would be the portal of exit and the mouth would be the portal of entry. Hence the name fecal-oral. The individual links on this chain likely include the hands and inanimate objects (toys). The chain of infection, then, is what happens when a 4 year old is sick, uses the restroom and doesn’t wash their hands, plays with a toy, then passes that toy off to another child who chews on the toy. The microbe has effectively been transmitted from the portal of exit (rectum/fecal matter) through the chain of behaviors and objects, to another child’s portal of entry (mouth). Disease = spread.
While it may seem simplistic, the chain of infection is one of the most important factors in epidemiology and involves all three elements of the triad. In the above scenario, the agent is the microbe, the environment involves shared toys and the lack of sanitation, and the host factors include the second child’s behavior (chewing on the toy), and his or her susceptibility to the illness. All three of those elements are part of the chain of infection. This is not necessarily always the case, but it is common.
What is a reservoir in epidemiology?
The reservoir is where the agent is found under normal circumstances. It is the agent’s usual home. Humans can be reservoirs, but so can animals or environmental locations such as soil or water. Within the reservoir, the agent (or microbe) thrives and replicates. The reservoir does not have to be the place where the individual actually gets sick, but it is the place where these microbes or agents are typically found.
HUMANS AS RESERVOIRS
Humans can be the reservoirs for many infectious diseases. These are illnesses which are transmitted from person to person. Measles, pertussis, hepatitis A, and poliovirus are all examples of microbes with human reservoirs. This is important because we have the potential to eradicate microbes, such as smallpox, which have human reservoirs. Tetanus, on the other hand, has a primary reservoir of the soil so it can’t be eradicated.
When infected, a human reservoir will often experience symptoms of the illness, but humans can also be carriers or have asymptomatic infections. This is particularly common will illnesses such as poliovirus. In this case, the individual does not know he or she is sick and is passing the disease to others unaware.
This is the case with the infamous Typhoid Mary. Mary Mallon (1869-1938) was a household cook for several families in New York. She is the first documented carrier of typhoid fever and caused at least 51 infections and 3 deaths. She famously refused to abide by quarantine orders or to change careers, so she frequently changed names to continue working as a cook. She transitioned from family to family, leaving the majority of each household sick or dead in her wake. Because of her frequent name and household changes, the total number of individuals who died as a result of her behavior is unknown, but estimated to be as high as 50. Eventually, she was forced to entered quarantine and was no longer a threat to public health.
Like Typhoid Mary, many carriers do not believe that they could be responsible for infecting those around them. While modern testing provides confirmed scientific evidence of the infection, it is still not unusual for carriers to place others at risk due to a refusal to take precautions. We saw many examples of this during the early stages of the COVID-19 pandemic.
ANIMALS AS RESERVOIRS
Animals are also frequent reservoirs for infectious disease which can infect humans. It’s not uncommon for humans to be accidental victims to animal diseases (zoonosis), but there are also many infections which frequently spread to humans. For example, evidence suggests that ebola can be attributed to bats. The natural reservoir of cholera is shellfish. Ticks are the natural home of Rocky Mountain spotted fever, and Bubonic plague is found in prairie dogs, black rats, chipmunks, and squirrels.
Additionally, environmental factors can be reservoirs for infectious disease. These are frequently a water supply and the soil. Tetanus and anthrax are examples of microbes found in the soil.
Methods of Transmission
The microbes leave the host or reservoir through a portal of exit, which could be cuts, lesions, respiratory excretions, urine, feces, or other pathways. They then are transmitted to the host through a variety of methods. These are classified in epidemiology as those which require direct contact and those spread via indirect contact.
DIRECT CONTACT
This requires close contact with the infected individual or reservoir through skin to skin contact of some sort, including sexual contact and infectious droplets. Droplet spread should not be confused with airborne transmission, which is discussed below. Droplet spread specifically requires close physical proximity such as sneezing, where aerosols are expelled directly onto the other individual. In this case, the droplets are launched a few feet, then they do not linger. They are heavy and fall, in which case the risk rapidly decreases.
Close contact does not have to be human contact; if the reservoir is the soil, then it requires contact with the soil. Infections which spread this way are frequently easier to prevent as prevention focuses on avoiding contact with the reservoir.
INDIRECT CONTACT
Unlike direct contact, indirect contact does not require close proximity with the infected individual or reservoir; these infectious are often riskier to public health as a result. Indirect contact includes infections which are airborne, vehicleborne, and vectorborne.
Airborne infections are similar to droplet spread, but these agents become suspended in the air, prolonging exposure time. These are less than 5 microns in size, so they can remain in the air for lengthy periods of time; they can also travel with wind currents because they are suspended in the air. This distinction caused a significant amount of confusion during the 2014 ebola outbreak as the infection could spread via droplet spread but was not airborne.
VEHICLEBORNE AND VECTORBORNE
Vectorborne infections require a living vector on the chain. Mosquitos are common vectors and they spread diseases ranging from malaria to chikungunya. Other vectors include fleas and ticks.
Vehicleborne infections include non-living vehicles of transmission. Food is a widespread example of vehicleborne infection. Water is also a vehicle for infection, as is infected clothing or other inanimate objects.
Portal of Entry
The chain of infection is not complete with mere exposure; it must come through a portal of entry into the host. Many portals of exit are also portals of entry, such as the mouth, open wounds, and open sores. A common example of this is the fecal-oral route, in which manner many small children through poor sanitation spread infectious disease rapidly through a childcare center or community.
Infection Prevention
Identifying the chain of infection is crucial for effective health prevention. Without this foundational knowledge, we can’t determine which precautions will be effective. While the chain of infection is a relatively easy concept to grasp, identifying and confirming the factors associated with this chain requires extensive scientific research, especially in the case of a novel disease.
Image Credit: Centers for Disease Control and Prevention. Principles of epidemiology, 2nd ed. Atlanta: U.S. Department of Health and Human Services;1992.