Discussion and analysis

Different authors define reservoir and carrier as sources of infection, responsible for maintaining an agent in a given individual/environment/ecosystem.

The definition of carrier is not found in the official glossary of the World Organization for Animal Health¹, but in the literature as "any host that maintains an infectious agent in its organism", citing 3 types of carriers, the healthy or sub-clinical, the convalescent, and the incubation period.

Jaramillo Arango & Martínez Maya² define a reservoir as: "any human being, animal, arthropod, plant, soil or matter capable of maintaining an agent for a prolonged period of time in a given area. The natural habitat in which it lives, multiplies, grows or both, on which its survival depends and on which it can be transmitted to a susceptible host".

A carrier is defined as: "an individual harboring a specific infectious agent of a disease, without showing signs or clinical symptoms of the disease, or both, and in a position to transmit the agent".

As can be seen, it does not differ much from the definition of reservoir, which leads to confusion between the two terms.

Thrusfield³ defines a carrier as "an animal that eliminates an infectious agent without manifesting clinical symptoms", this author refers to the reservoir as a synonym of "host", or suffixes reservoir as "reservoir host", as "one in which an infectious agent normally lives and multiplies and which, therefore, is a usual source of infection for other animals".

Toma et al.⁴ explain that "sometimes an organism, after its cure, can play a role as a source of infection for another organism as a carrier of germs".

On the other hand, they explain that "in certain cases, organisms infected inapparently represent, at the individual level, a very important source of pathogens, taking into account the high titer reached by the infectious agent in certain tissues, especially blood, and constituting the reservoir of the disease".

As can be seen, one definition does not differ much from the other in its contextualization and lends itself to confusion, with the "individual" always being taken as the basis of explanation.

Jenicek & Clèroux⁵ define a germ carrier as "an individual harboring a pathogenic agent and susceptible to transmission to others. In practice, it is usually reserved for subjects without apparent clinical symptoms".

When explaining the term reservoir, they do so with a projection towards the ecosystem, encompassing a much broader explanation, incorporating other actions such as the circulation of the agent in different species, and terms such as nest, biotope and biome.

These authors refer thus, "pathogens for other animal species can persist in the environment without infecting humans. The set of these species and their ecosystem and the elements that ensure circulation are designated by various expressions: natural reservoir (nest) of infection, biotope, biome".

Rosemberg⁶, is quite similar to all the above-mentioned definitions of carrier, stating that it is "any host that maintains in its organism an infectious agent without showing signs of the disease".

As for the reservoir of an agent, it also considers it to be "any individual capable of maintaining an agent for a prolonged period of time in a given area".

However, in the definition of types of reservoirs, it refers to ecological reservoirs as "an animal species not susceptible to the disease produced by it, but in which said agent can multiply and be eliminated in such a way as to allow its transmission to any susceptible host that comes into contact with it".

Also, when referring to epidemiological reservoirs, it defines them as "any animal species that, because it is not subject to control and observation, can maintain an agent and transmit it to susceptible domestic species of sanitary interest with which it comes into contact".

With this conceptual vision, it can be affirmed that an individual belonging to a species considered a reservoir of a given agent can behave as a carrier of that agent, as a unit. In fact, the behavior of this carrier, in interaction with the agent, will determine, in its projection to the population, the characteristics of the reservoir species to which it belongs.

From all these expressions it is clear that the definitions of both terms can easily be confused, since when referring to the carrier and the reservoir, they allude to an individual, with the exception of some authors mentioned above, thus also confusing their functions.

Conclusion

The carrier status, therefore, results from an interaction between the pathogen and the individual or host.

The reservoir status implies the interaction of the pathogenic agent, the species and the ecosystem, the environment, both biological, physical and economic-social.

Thus, the definition of carrier is clear, it definitely refers to the individual in its different states, incubation, convalescent, subclinical and that its probability of transmitting the pathogenic agent is temporary, for periods of time, variable according to the type of disease.

The definition of reservoir is the species or (are) the species capable of maintaining the circulation of an agent in a given environment for an indefinite period of time, incorporating the elements of the ecosystem in which it is found.

These clear statements will have a fundamental and extremely important contribution to understand the epidemiology of diseases, to define the epidemiological chain, therefore, the epidemiological model and finally the appropriate strategies to combat diseases.

In summary, over the last few years, scientific research has contributed significantly to clarifying the complex dynamics of infectious diseases, particularly with regard to the notions of carrier and reservoir. These definitions, based on empirical evidence and epidemiological theory, have evolved our understanding of how pathogens interact with individuals and their environment.

The clear distinction between carrier and reservoir has become fundamental to the design of effective disease control and prevention strategies. We now know that a carrier, although it can transmit the pathogen temporarily, does not maintain it in a sustained manner in the population. In contrast, the reservoir, which involves the interaction of species and ecosystems, is the key to the long-term persistence of an infectious agent in a specific environment.

These refined definitions not only contribute to the understanding of disease epidemiology, but also provide a solid basis for the identification of epidemiology and the formulation of accurate epidemiological models. As a result, disease control strategies benefit from this knowledge, enabling a more effective response tailored to the specific characteristics of each disease.

In a world where public health threats are increasingly diverse and complex, these research-backed findings are essential to inform health policies and prevention strategies that safeguard the health of global communities. Our evolving understanding of carriers and reservoirs in infectious diseases promises a safer and healthier future for all.