A natural reservoir or nidus (the latter from the Latin word for "nest") is the long-term host of a pathogen of an infectious disease. Hosts often do not get the disease carried by the pathogen or it is carried as a subclinical infection and so asymptomatic and non-lethal. Once discovered, natural reservoirs elucidate the complete life cycle of infectious diseases, providing effective prevention and control.
Examples of natural reservoirs are:
- Field mice, for hantaviruses and Lassa fever
- Marmots, black rats, prairie dogs, chipmunks and squirrels for bubonic plague
- Armadillos and opossums for Chagas disease and several species of New World Leishmania
- Ticks for babesiosis and Rocky Mountain spotted fever
- Ground squirrels, porcupines, and chipmunks for Colorado tick fever
- Snails for schistosomiasis and swimmer's itch
- Pigs for cestode worm infections
- Raccoons, skunks, foxes and bats for rabies
- Shellfish for cholera
- Fowl (ducks and geese) for avian influenza
- Bats, the reservoir for Nipah and Hendra virus, rabies and severe acute respiratory syndrome (SARS)
- Dogs and wild canids for Leishmania infantum, the cause of infantile visceral leishmaniasis
- Cats, for Bartonella (aka Cat scratch disease)
- Gerbils for Leishmania major, the causative agent of cutaneous leishmaniasis in the Old World
- Rock hyrax for Leishmania aethiopica and, probably, certain strains of Leishmania tropica, the causative agents of cutaneous leishmaniasis in the Old World
- Mosquitoes, for malaria, West Nile virus, Yellow fever and Dengue fever
Between 1976 and 1998, from 30,000 mammals, birds, reptiles, amphibians, and arthropods sampled from outbreak regions, no ebolavirus was detected apart from some genetic traces found in six rodents (Mus setulosus and Praomys) and one shrew (Sylvisorex ollula) collected from the Central African Republic. Traces of EBOV were detected in the carcasses of gorillas and chimpanzees during outbreaks in 2001 and 2003, which later became the source of human infections. However, the high lethality from infection in these species makes them unlikely as a natural reservoir.
Plants, arthropods, and birds have also been considered as possible reservoirs; however, bats are considered the most likely candidate. Bats were known to reside in the cotton factory in which the index cases for the 1976 and 1979 outbreaks were employed, and they have also been implicated in Marburg virus infections in 1975 and 1980. Of 24 plant species and 19 vertebrate species experimentally inoculated with EBOV, only bats became infected.
The absence of clinical signs in these bats is characteristic of a reservoir species. In a 2002–2003 survey of 1,030 animals including 679 bats from Gabon and the Republic of the Congo, 13 fruit bats were found to contain EBOV RNA fragments. As of 2005, three types of fruit bats (Hypsignathus monstrosus, Epomops franqueti, and Myonycteris torquata) have been identified as being in contact with EBOV. They are now suspected to represent the EBOV reservoir hosts.
The existence of integrated genes of filoviruses in some genomes of small rodents, insectivorous bats, shrews, tenrecs, and marsupials indicates a history of infection with filoviruses in these groups as well. However, it has to be stressed that infectious ebolaviruses have not yet been isolated from any nonhuman animal.
Bats drop partially eaten fruits and pulp, then terrestrial mammals such as gorillas and duikers feed on these fallen fruits. This chain of events forms a possible indirect means of transmission from the natural host to animal populations, which have led to research towards viral shedding in the saliva of bats. Fruit production, animal behavior, and other factors vary at different times and places that may trigger outbreaks among animal populations. Transmission between natural reservoirs and humans are rare, and outbreaks are usually traceable to a single index case where an individual has handled the carcass of gorilla, chimpanzee, or duiker. The virus then spreads person-to-person, especially within families, hospitals, and during some mortuary rituals where contact among individuals becomes more likely.
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