Reservoir host

Reservoir host
n., plural: reservoir hosts
[ˈɹɛz.ə(ɹ).vwɑɹ hoʊst]
Definition: A host harboring pathogen that naturally lives, grows, and reproduce, and often not causing illness to the host

Reservoir Host Definition

A reservoir host is a host that harbors the pathogen and serves as a source of the infective agent that it transmits to a potential host. Reservoir hosts may or may not show ill effects. Often, they show no symptoms (asymptomatic). The disease may spread rather easily when a pathogen is widespread and utilizes various reservoirs. Carriers of the disease are very likely to transmit the disease-causing agent because they are unaware that they are infected. Unsuspecting hosts tend to get exposed and infected, especially when unaware that their surroundings harbor the pathogen.

Examples of reservoir hosts: marmots, black rats, prairie dogs, chipmunks, and squirrels harboring the pathogen that causes bubonic plague
Synonyms: natural reservoir, nidus; asymptomatic host
Compare: paratenic host

• Definition of a Host

A host is an organism that harbors another organism (symbiont), which may be a mutualistic agent, a commensal, a parasite, or an infectious agent. The host harboring such an organism is relatively larger and may or may not benefit from the association, depending on the symbiotic relationship established. The organism that dwells on or within the host, though, is generally benefitting as it uses the host as its dwelling site (habitat). And possibly, it may derive its source of nutrients from the host.

There are two fundamental types of hosts based on where the symbiont (or agent) is in its life cycle.

• • The primary host, also called the definitive host, is a host wherein the organism it harbors includes the sexually active (mature) phase of the symbiont’s life cycle (for example, from larval to adult stages)
  • A secondary host, also called the intermediate host, is a host wherein the organism it harbors does not include the sexual phase but the sexually immature stages of its life cycle (for example, from fertilized egg to larval stages)

There are special instances when the primary host is also the secondary host of the organism. That is when both the sexually mature and immature stages occur in the same host but only differ in sites. For example, certain adult roundworms dwell in the digestive tract of their host while the larvae roundworms dwell in the muscles of the same host.

There are other types of hosts apart from the two mentioned above: reservoir hosts and paratenic hosts.

• • A reservoir host, also called the asymptomatic host, is a host that generally does not show symptoms when harboring a disease-causing pathogen or a parasite.
  • A paratenic host, also called the transport host, is a host that serves as the parasite’s “transport” from one host to another. It is regarded as the parasite’s “dumps” as the parasite does not require such a host to develop or further its life cycle. In essence, no life cycle stage developments occur in this host.

• Definition of a Reservoir

What is a reservoir? In health, medicine, and epidemiology, a reservoir is where an infectious agent or a parasite can live, grow, and reproduce until such time that it reaches its target host species where it will manifest the disease and symptoms it carries. By this definition, reservoirs can therefore be living or nonliving things. They may also be categorized into the following types: human reservoirs, animal reservoirs, and environmental reservoirs.

In epidemiology and medicine, a natural reservoir is defined as follows:

“… A reservoir (is) one or more epidemiologically connected populations or environments in which the pathogen can be permanently maintained and from which infection is transmitted to the defined target population.”

(Haydon et al., 2002)

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Types of Reservoirs

Here are the different types of reservoirs.

• Human reservoirs

Human reservoirs are when humans serve as reservoirs of the disease-causing agent. In human disease cases, humans can be both reservoirs and definitive hosts. For instance, some people are carriers of the disease and do not develop the disease, and therefore they may unknowingly transmit the pathogen to a new susceptible human host. Examples of human diseases where humans could serve as reservoirs of infection are sexually-transmitted diseases, measles, mumps, and streptococcal infection. (Principles of Epidemiology, 2023) In such a case, no intermediate host is needed.

• Animal reservoirs

Animal reservoirs may vary — from the invertebrates such as ticks, mosquitoes, and other arthropods, to vertebrates, such as mammals, birds, reptiles, etc. Many human pathogens are classified as zoonotic, which means the pathogen tends to be transmitted to humans (accidental hosts) from non-human vertebrate hosts, especially wild animals.

Such pathogens can use multiple hosts, using important animal reservoirs and humans for transmission. For example, the natural reservoirs of coronaviruses are bats but have been transmitted to and infected other vertebrate animals, including humans.

• Environmental reservoirs

The environment can also be a source of pathogens. Many disease-causing bacteria and viruses can survive in soil and water. Soil-borne bacterium, Clostridium tetani, for instance, can cause tetanus when it gains entry to an open wound and subsequently infects its host.

Common Characteristics of Reservoir Hosts

Taking into consideration animals as reservoirs, it seems there are particular species that serve as reservoirs more often than other species. Or, let’s take humans as an example. Not all of them will be susceptible to the disease. Rather, they become carriers of the pathogen. What makes them reservoirs? 

Here are some of the common characteristics that identify reservoir hosts.

• Enablers of a sustainable relationship. The host species (or an individual host for that matter) may have certain characteristics that provide a conducive habitat for the pathogen; the pathogen, in turn, tends not to bring significant harm to its reservoir. A pathogen that can replicate and live in or on its host without causing severe disease or death can have better chances of persisting and getting transmitted to other hosts.
• An apposite immune system that nurtures coevolution. The reservoir host species would typically have an immune system that is neither impaired nor overly responsive toward the pathogen. An immunocompromised host due to its impaired immune system may eventually manifest severe symptoms. Its body may fail to keep up as the pathogen makes use of the available sources from its host during its ‘stay’. Conversely, a highly responsive immune system would act by destroying the pathogen as it would not tolerate its presence upon recognition. This is where coevolution with the pathogen comes in and could be built over time.
• Efficient in transmitting the pathogen. The host species can become ideal reservoirs when they inherently have ways that enable greater transmission. If the potential reservoir hosts can transmit the pathogens not only by direct contact but also by other means, such as via aerosols or by close contact with the vectors, then, they’ll likely be “favored” over the others. They offer greater opportunities for pathogens to circulate within the host population. And with that said, a good reservoir host would also have a high population density as well as a broad geographic distribution. As these factors would be favorable to the transmission of the pathogen.

Examples of Reservoir Hosts

Here are some examples of commonly recognized reservoir host species for certain infectious diseases:

1. Birds are reservoirs of the West Nile virus but not all birds are. According to WHO, the virus is not pathogenic for birds in Europe, Africa, and Asia but highly for those in the Americas. (WHO, 2017) The West Nile virus can cause a fatal neurological disease in humans. Humans acquire the virus through infected mosquitoes (Culex sp.) that feed on human blood. Mosquitoes, in turn, get the virus when feeding on infected birds.
2. Ticks may act not only as vectors. They may also be a crucial source of disease-causing bacteria (e.g., spotted fever group rickettsiae, recurrent fever borreliae, and Francisella tularensis) in humans. (Parola & Didier Raoult, 2001)
3. Ground-dwelling birds and small mammals (e.g., rodents) serve as reservoir hosts for the Lyme disease spirochete bacterium, Borrelia burgdorferi. (Voordouw et al., 2015)
4. Wild waterfowl harboring avian livestock pathogens can transmit the virus to poultry. For example, duck and goose reservoirs may transmit the avian influenza virus to susceptible chickens through close contact while co-grazing or when sharing water sources.
5. Human carriers of sexually-transmitted diseases are reservoirs that can transmit the pathogen to another human host. (Principles of Epidemiology, 2023) Because of the lack of severe symptoms, agents of sexually-transmitted diseases are transmitted from an unsuspecting human carrier to a susceptible host through sexual contact.

Modes of Transmission

Pathogens are transmitted from reservoir hosts to susceptible hosts via direct or indirect transmission.

• Direct transmission is how the agent is transferred from the infected hosts or sources to the susceptible hosts directly, which means it can be any of these routes:
  • Person-to-person: occurs when the agent is transmitted from the infected person to another person susceptible to the infective agent, via body fluids (e.g., sexual contact, blood transfusion), physical contact (e.g., touching), and respiratory droplets (coughing and sneezing)
  • Vertical transmission: this occurs when the infective agent is transmitted during pregnancy, childbirth, or breastfeeding. Thus, this form of transmission occurs between a mother (source) and the offspring (susceptible host).
  • Direct contact with contaminated environmental sources and then the agent gains entry into the body of a susceptible host
• Indirect transmission is one in which the agent is transmitted using an intermediate agent. There is no direct contact between the source and the susceptible host.
  • Airborne transmission: when the infective agent makes use of air or atmosphere. Some pathogens (expelled from coughing, sneezing, or talking) are capable of remaining suspended in the air for an extended period of time. The portal of entry is via inhalation of a susceptible host.
  • Waterborne/Foodborne transmissions: occur via the ingestion of pathogens that have contaminated water and/or food
  • Fomite transmission: when the pathogen that has contaminated objects (fomites) has gained entry to the host, e.g., using contaminated utensils when eating
  • Vector-mediated transmission: A vector is an organism that acts as an intermediate host of a pathogen, transmitting the disease from the reservoir to the definitive or final host.

Biological Importance of Reservoir Hosts

So far, we’ve become acquainted with reservoir hosts playing a crucial role in the life history of the pathogens. With a wide host range, pathogens can exploit different host species for their transmission, thus, would have better chances of perpetuating their existence as a species than those with limited reservoir species.

What could be worse for a pathogen is if it landed on other host species that tend to limit the effective transmission of the pathogen. Such hosts are referred to as “dead-end hosts”. A possible reason for this is the lack of a compatible relationship between the host and the pathogen.

The host may not have the characteristics or traits (e.g., specific cell receptors or cellular machinery) that would have enabled a productive infection and/or transmission of the pathogen.

Another reason is the efficiency of the host’s immune system. Before the pathogen could replicate to sufficient levels and become infective, it already thwarts the pathogen with an effective immune response.

• Amplifying pathogen transmission

In a seemingly endless game of  “hide and seek”, pathogens have to acquire traits and find a way to avert the hosts’ immune system so that they could persist for a favorable period, preferably undetected or unharmed. Thus, it becomes important from the co-evolution standpoint that a pathogen ‘trades off’ virulence so as to be nonlethal toward its host. At that, it would benefit more as it could ‘stay’ in its living habitat, use the host resources for its survival and replication, and amplify its transmission potential amongst the host populations. It is, thus, fitting to refer to such a host as a maintenance host — a stable host that while harboring and maintaining the pathogen will not experience severe symptoms or develop a disease that may end up threatening its life.

• A source of spillover events

In humans, the lack of recognizable or severe symptoms often does not cause an alarm to the individual carrying the pathogen. As such, there is a high tendency for a carrier to spread the pathogen to a wide range of human hosts.

In effect, it has led to great human health concerns, especially since human hosts respond differently. While some people will not manifest symptoms, others may do and develop a disease in which the severity will depend on how well one’s immune system competently works.

Reservoir hosts serve as a source of spillover events wherein a pathogen can be transmitted from the human reservoir to other reservoirs, including animals and the environment. Such events can spark outbreaks and epidemics in the host populations.

• Huge impact on general health 

Because of the potential impact not only on human health but also on animal health and the environment, identifying reservoirs and determining the pathogen’s transmission routes (e.g., human-to-human, from animals to humans, and vice versa) are vital disease control measures. They help halt the spread. By working hand in hand, researchers and public health authorities can identify pathways for intervention, assess risks, and thereby implement strategies. For the purpose of developing treatments and vaccines, reservoir hosts have an essential role to play.

• Role in developing interventions and preventive measures

Some of the ways reservoir hosts can help are as follows:

• • By supplying a source or sample for isolating and studying the pathogen, especially in terms of the pathogen’s genetic makeup, virulence factors, and antigenic properties, which are essential information when developing an effective vaccine or treatment.
  • By providing an understanding of how the immune system works, especially coming from one that confers some level of tolerance toward the pathogen
  • By helping in studying the pathogenesis of a disease, such as how the pathogen interacts at the level of cells, tissues, organs, or systems. As such, researchers may gain insights into the disease mechanisms and then find potential therapeutic interventions.

In essence, reservoirs can contribute to the overall health of humans, animals, and the environment. They can provide helpful insights for use in designing comprehensive strategies targeting the pathogen at its source. This is vital as reservoir hosts for some pathogens are already well established but reservoirs for other pathogens remain unclear and yet to be uncovered.

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References

• Ashford, R. W. (2001). When Is a Reservoir Not a Reservoir? Emerging Infectious Diseases, 9(11), 1495–1496. https://doi.org/10.3201/eid0911.030088
• Haydon, D. T., Cleaveland, S., Taylor, L. H., & M. Karen Laurenson. (2002). Identifying Reservoirs of Infection: A Conceptual and Practical Challenge. 8(12), 1468–1473. https://doi.org/10.3201/eid0812.010317
• Principles of Epidemiology. (2023). https://www.cdc.gov/csels/dsepd/ss1978/lesson1/section10.html
• Small, H. J., & Pagenkopp, K. M. (2011). Reservoirs and alternate hosts for pathogens of commercially important crustaceans: A review. Journal of Invertebrate Pathology, 106(1), 153–164. https://doi.org/10.1016/j.jip.2010.09.016
• Smallpox. (2023). https://www.cdc.gov/smallpox/index.html.‌‌‌‌
• WHO. (2017, October 3). West Nile virus. Who.int; World Health Organization: WHO. https://www.who.int/news-room/fact-sheets/detail/west-nile-virus.
• Voordouw, M. J., Lachish, S., & Dolan, M. C. (2015). The Lyme Disease Pathogen Has No Effect on the Survival of Its Rodent Reservoir Host. 10(2), e0118265–e0118265. https://doi.org/10.1371/journal.pone.0118265
• Parola, P., & Didier Raoult. (2001). Ticks and Tickborne Bacterial Diseases in Humans: An Emerging Infectious Threat. 32(6), 897–928. https://doi.org/10.1086/319347‌‌

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