Vectors — animals, insects, other humans
Vehicles — water, food, air, medical devices, various other Inanimate objects
Figure 3-2 Summary of microbial reservoirs and modes of transmission to humans.
tion regarding a patient's exposure to animals is often a key component to diagnosing these infections. Some microorganisms primarily infect animal populations and on occasion accidentally encounter and infect humans. Such infectious diseases are classified as zoonoses; when a human infection results from such an encounter it is referred to as a zoonotic infection.
The most common role of insects (arthropods) in the transmission of infectious diseases is as vectors rather than as reservoirs. A wide variety of arthropods transmit viral, parasitic, and bacterial diseases from animals to humans, whereas others transmit microorganisms between human hosts without an animal reservoir being involved. Malaria, a leading cause"of death from infection, is a prime example of a disease that is maintained in the human population by the activities of an insect (Le., mosquito) vector. Still other arthropods may themselves be agents of disease. These include organisms such as lice and scabies that are spread directly between humans and cause skin irritations but do not penetrate the body. Because they are able to survive on the skin of the host without usually gaining access to internal tissues,- they are referred to as ectoparasites.
The soil and natural environmental debris are reservoirs for countless types of microorganisms. Therefore, it is not surprising that these also serve as reservoirs for microorganisms that can cause infections in humans. Many of the fungal agents (see Chapter 50) are acquired by inhalation of soil and dust particles containing the microorganisms (e.g., San Joaquin Valley fever). Other, nonfungal infections (e.g., tetanus) may result when microbial agents in the environment are introduced into the human body as a result of a penetrating wound.
THE MICROORGANISM'S PERSPECTIVE
Clearly, numerous activities can result in human encounters with many microorganisms. Because humans are engaged in all of life's complex activities, the tendency is to perceive the microorganism as having a passive role in the encounter process. However, this assumption is a gross oversimplification.
Microorganisms are also driven by survival, and the environments of the reservoirs they occupy must allow their metabolic and genetic needs to be fulfilled. Furthermore, most reservoirs are usually inhabited by hundreds, if not thousands, of different species. Yet human encounters with the reservoirs, either directly or indirectly, do not result in all species establishing an association with the human host. Although some spedes have evolved strategies that do not involve the human host to ensure survival, others have included humans to a greater or lesser extent as part of their survival tactics. Therefore, the latter type of organism often has mechanisms that enhance its chances for human encounter.
Depending on factors associated with both the human host and the microorganism involved, the encounter may have a benefidal, disastrous, or inconsequential impact on each of the participants.
Was this article helpful?