Reservoirs: Where Pathogens Persist
One of the important concerns of epidemiology is discovering where pathogens originate and how they are transmitted. As we saw in the Case Study, these aspects are intensely scrutinized from the very first signs of an outbreak.
For an infectious agent to continue to exist and be spread, it must have a permanent place to reside. The reservoir is the primary habitat in the natural world from which a pathogen originates. Often it is a human or animal carrier, although soil, water, and plants are also reservoirs. The reservoir can be distinguished from the infection source, which is the individual or object from which an infection is actually acquired. An infectious agent that does not survive well outside of its host and is usually transmitted directly tends to have the same reservoir and source, whereas one that can remain viable outside of the host can have a different reservoir and source. For example, gonorrhea has the same reservoir and source (the human body), whereas hepatitis A usually has a different reservoir (a human carrier) and source (contaminated food).
Living Reservoirs
Many pathogens continue to exist and spread because they are harbored by members of a host population. A person or animal with an obvious or overt symptomatic infection is clearly a source of infection, as opposed to a carrier who is, by definition, an individual who inconspicuously shelters a pathogen and spreads it to others unknowingly. Although human carriers are occasionally detected through routine screening (blood tests, cultures) and other epidemiological devices, they are unfortunately very difficult to discover and control. As long as a pathogenic reservoir is maintained, the disease will continue to exist in that population or location, and the potential for epidemics will be a constant threat. The duration of the carrier state can be short or long term, and the carrier may or may not have experienced disease due to the microbe.
Several situations can produce the carrier state. Asymptomatic (apparently healthy) carriers are infected, but as previously indicated, they show no outward symptoms (figure 1a). A few asymptomatic infections (such as gonorrhea and genital warts) can carry out their entire course without overt manifestations. Figure 1b demonstrates three types of carriers who show no indications of infection at the time they transmit the pathogen. Incubation carriers spread the infectious agent during the incubation period before symptoms have appeared. For example, persons living with HIV can harbor and potentially transmit the virus for months and years before their infection is evident. Recuperating patients without symptoms are considered convalescent carriers when they continue to shed viable microbes and convey the infection to others after symptoms have subsided. Norovirus, the most common cause of gastroenteritis, can be transmitted for weeks after an infection is resolved.
Fig1. Involvement of carriers in transmission of infectious agents. (a) An asymptomatic carrier is infected without symptoms. (b) Incubation, convalescent, and chronic carriers can transmit the infection either before or after the period of symptoms. (c) A passive carrier is contaminated but not infected and spreads the pathogen through physical contact.
An individual who continues to shelter the infectious agent for a long period after recovery is a chronic carrier. Patients who have recovered from tuberculosis, hepatitis, and herpes infections frequently carry the agent chronically. About one in 20 victims of typhoid fever harbors Salmonella typhi in the gallbladder for several years and sometimes for life. The most infamous of these was “Typhoid Mary,” a cook who spread the infection to hundreds of victims in the early 1900s.
The passive carrier is of great concern during patient care. Medical and dental personnel who must constantly handle materials that are heavily contaminated with body fluids and blood are at risk of picking up pathogens mechanically and accidentally transferring them to other patients (figure 1c). Proper hand washing, handling of contaminated materials, and aseptic techniques greatly reduce this likelihood.
Animals as Reservoirs and Sources Up to now we have lumped animals with humans in discussing living reservoirs or carriers, but animals deserve special consideration as vectors of infections. The word vector is used by epidemiologists to indicate a live animal that transmits an infectious agent from one host to another. (The term is sometimes misused to include any object that spreads disease.) The majority of vectors are arthropods such as fleas, mosquitoes, flies, and ticks, although larger animals can also spread infection; for example, mammals (rabies), birds (ornithosis), or lower vertebrates (salmonellosis).
By tradition, vectors are placed into one of two categories, depending upon the animal’s relationship with the microbe. A bio logical vector actively participates in a pathogen’s life cycle, serving as a site in which it can multiply or complete its development. A biological vector passes the infectious agent to the human host by biting, aerosol formation, or touch. In the case of biting vectors, the animal can inject infected saliva into the blood (mosquito), defecate around the bite wound (flea), or regurgitate blood into the wound (tsetse fly).
Mechanical vectors are not necessary to the life cycle of an infectious agent and merely transport it without being infected. The external body parts of these animals become contaminated when they come into physical contact with pathogens. The agent is subsequently transferred to humans indirectly by an intermediate such as food or occasionally by direct contact (as in certain eye infections). Houseflies are notorious mechanical vectors. They feed on decaying garbage and feces, and while they are feeding, their feet and mouthparts easily become contaminated. They also regurgitate juices onto food to soften and digest it. Flies spread more than 20 bacterial, viral, protozoan, and worm infections. Other non-biting flies transmit tropical ulcers, yaws, and trachoma. Cock roaches, which have unsavory habits similar to those of flies, play a role in the mechanical transmission of fecal pathogens, as well as contributing to allergy attacks in asthmatic children.
Many vectors and animal reservoirs spread their own infections to humans. An infection indigenous to animals but naturally transmissible to humans is a zoonosis. In these types of infections, the human is essentially a dead-end host and does not contribute to the natural persistence of the microbe. Some zoonotic infections (rabies, for instance) can have multihost involvement, and others (such as plague) can have very complex cycles in the wild. Zoonotic spread of disease is promoted by close associations of humans with animals, and people in animal-oriented or outdoor professions are at greatest risk. At least 150 zoonoses exist worldwide; the most common ones are listed in table 1. Zoonoses make up a full 70% of all new emerging diseases worldwide. It is worth noting that zoonotic infections are impossible to completely eradicate without also eradicating the animal reservoirs. Attempts have been made to eradicate mosquitoes and certain rodents. During influenza epidemics, millions of chickens and thousands of pigs are slaughtered in the attempt to control its spread.
Table1. Common Zoonotic Infections
One technique that can provide an early warning signal for the occurrence of certain mosquito-borne zoonoses has been the use of sentinel animals. These are usually domestic animals (most often chickens or horses) that can serve as hosts for diseases such as West Nile fever, various viral encephalitides, and malaria. Sentinel animals are placed at various sites throughout the community, and their blood is monitored periodically for antibodies to the infectious agents that would indicate a recent infection by means of a mosquito bite. The presence of infected animals provides useful data on the potential for human exposure, and it also helps establish the epidemiological pat tern of the zoonosis, including where it may have spread.
Nonliving Reservoirs
Clearly, microorganisms have adapted to nearly every habitat in the biosphere. They thrive in soil and water and often find their way into the air. Although most of these microbes are saprobic and cause little harm and considerable benefit to humans, some are opportunists and a few are regular pathogens. Because human hosts are in regular con tact with these environmental sources, acquisition of pathogens from natural habitats is of diagnostic and epidemiological importance.
Soil harbors the vegetative forms of bacteria, protozoa, helminths, and fungi, as well as their resistant or developmental stages such as spores, cysts, ova, and larvae. Bacterial pathogens that live in soil include the anthrax bacillus and species of Clostridium that are responsible for gas gangrene, botulism, and tetanus. Pathogenic fungi in the genera Coccidioides and Blastomyces are spread by spores in the soil and dust. The invasive stages of the hookworm Necator occur in the soil. Natural bodies of water carry fewer nutrients than soil does but still support pathogenic species such as Legionella, Cryptosporidium, and Giardia.
