Natural host

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 is asymptomatic and non-lethal. Once discovered, natural reservoirs elucidate the complete life cycle of infectious diseases, providing effective prevention and control.

Some viruses have no non-human reservoir: poliomyelitis and smallpox are prominent examples. The lack of a non-human reservoir makes these viruses good candidates for eradication efforts.

The natural reservoir of some diseases remain unclear. This is the case of the Ebola virus disease.

One study between 1976 and 1998, from 30,000 mammals, birds, reptiles, amphibians, and arthropods sampled from outbreak regions, no ebolavirus (EBOV) 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.

However, a subsequent study, in a later outbreak, found 31.8% of the dogs closest to an outbreak contained antigens that indicate a previous active viral load. Whether dogs passed the virus to humans, or both were infected by a third species is unknown.

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.

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