Virus reservoirs

If jackal rabies persists in the absence of dog rabies, an effective program for rabies elimination will likely need to include oral vaccination of jackals. Potential complexity of rabies reservoirs in Zimbabwe. If jackals with A or without B other wild carnivore populations constitute a maintenance community independent of dogs, then vaccination of dogs alone will not result in rabies elimination in the target.

If jackals do not constitute a maintenance community independent of dogs C , then dog vaccination should clear rabies from the reservoir symbols as in Figure 1. Rabies also provides an example of the need to identify a target population when defining reservoirs. In the Serengeti Plain in Tanzania, a distinct strain of rabies appears to be maintained independently in spotted hyenas, without causing them any clinical disease, and with no evidence of spillover infection or disease occurring in any other species within the limits of current knowledge By our definition, unless this strain is identified as the cause of disease in another species i.

Newly emerging diseases usually originate from reservoirs of infection in other host species. When such diseases first appear, only rapid, accurate identification of the reservoir will enable appraisal of the full range of disease-control options. Ring-fencing is clearly impractical when no knowledge of the reservoir populations exists, but other steps can be taken to acquire progressively more detailed information about the reservoir structure.

Accumulating epidemiologic evidence is often the best first step in identifying a reservoir. Initially, such analyses are often based on sparse data and are rarely published.

Links between target and reservoir may be particularly elusive when transmission from reservoir to target is rare or sporadic, as, for example, occurs with Ebola virus or Marburg virus Quantitative data on risk factors for infection can be obtained through more formal epidemiologic research, such as case-control and cohort studies.

For example, a case-control study of Borna disease in cats indicated that hunting mice was a risk factor and that rodents might be virus reservoirs Case-control studies have identified badgers as risk factors for M. In other cases, putative reservoirs have been ruled out. For example, a risk factor analysis of Helicobacter pylori infection in young children showed that household pets were not incriminated Although such associations may suggest a link between reservoir and target populations, further evidence is required to establish the identity of a reservoir.

Identifying natural infection is a useful step towards determining natural hosts that may constitute potential reservoirs. Natural infection may be determined in two ways: by identifying previous infection through antibody detection or by identifying current infection through isolating the infectious agent or its genes from the host.

The appropriate approach depends on the longevity of the infection in the host and the resources available. For example, very large sample sizes might be required to isolate a virus from a reservoir population; a serologic survey might be less expensive and more feasible.

In a number of studies, demonstration of natural infection has been considered strong evidence that hosts are reservoirs, e. Seropositivity indicates that infection has occurred. However, not all natural hosts are reservoir hosts, and to include a nontarget population in a reservoir, evidence of transmission to the target population, direct or indirect, must exist.

Furthermore, the level of seroprevalence does not provide information as to whether a nontarget population is a maintenance host. High seroprevalence at a single point in time may simply indicate an outbreak in the host population, rather than pathogen persistence Low seroprevalence may arise when case-mortality rates are high in the reservoir as in rabies infections , during an interepidemic trough, or when a pathogen persists at a stable but low prevalence, particularly when the duration of the infectious period is high e.

The critical issue is the persistence of infection in the reservoir, which can only be determined through longitudinal studies. Similar guidelines apply to data based on demonstration of the pathogen within a host. For example, detection of Trypanosoma brucei gambiense in wild ruminants and primates in West Africa has been taken as evidence of an animal reservoir for Gambian sleeping sickness However, as animal-to-human transmission has never been demonstrated, wildlife remain classified as potential reservoir hosts, and disease control relies on treatment of people.

In contrast, for Rhodesian sleeping sickness, isolation of T. Only in were cattle identified as reservoir hosts Current strategies focus on treating cattle with trypanocidal drugs Detecting a pathogen, particularly its transmission stage, in secretions or tissues provides supportive, but not unequivocal, evidence that transmission to the target population can occur.

Even where experiments demonstrate that transmission is possible, it may not occur in nature for a variety of behavioral or social reasons, because the population is below critical community size or because of constraints of pathogen life history.

Genetic and antigenic characterization of pathogens isolated from different populations provides a more powerful tool for identifying key components of reservoirs.

Antigenic and genetic variation of pathogens isolated from the target population within the range observed in the reservoir is consistent with reservoir-target transmission. This pattern can be demonstrated by applying phylogenetic methods to sequence, random amplified polymorphic DNA, or restriction fragment length polymorphism data, or by using serum cross-reactivity studies.

Such methods have also been used to rule out important animal reservoirs of human disease in studies of Ascaris in Guatemala 37 and Cryptosporidium in Australia Complete ring-fencing of target populations is the ultimate step in identifying the existence and structure of reservoirs.

In practice, however, ring-fencing has rarely been achieved and, as a result, even those reservoirs we consider to be most fully understood are not usually inconvertibly proven. Despite this, once a potential reservoir is identified, intervention studies can permit incidental but powerful inferences about the dynamics of infection in target-reservoir systems.

In many cases, disease-control programs can effectively act as intervention studies. Control in a reservoir host population may be achieved by reducing host or vector density e.

Alternatively, control measures may focus more directly on preventing transmission from the reservoir, e. The success of such interventions often provides reasonable confirmation of the original assumptions concerning transmission and maintenance of infection in the target-reservoir system.

We have a poorer understanding of the epidemiology of multihost pathogens than simpler single-host systems. This dearth of understanding is a particular problem with emerging diseases, since most emerging human, domestic animal, and wildlife diseases infect multiple hosts. Reservoirs must be defined with reference to particular target populations. Disappearance of the pathogen in the target population after ring-fencing provides categorical evidence of the existence of a reservoir and its possible identity.

However, exhaustive identification of all constituent populations of a reservoir may be difficult. This identification need not be a management priority if disease control is directed at the target population or at blocking transmission between reservoir and target.

For infection to be eliminated, however, disease-control measures must be directed at the reservoir. Thus, an understanding of reservoir infection dynamics is essential. When the risks and costs of control are low, circumstantial evidence may be sufficient to justify implementing control measures.

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Viral reservoirs are an anatomical site in which viruses accumulate and persist. Research 10 January Open Access. Here, Kim et al. Research 14 December Open Access. Research 22 November Open Access.

Goethert and colleagues use a host-specific retrotransposon targeted real time PCR assay on questing nymphal ticks to identify a reservoir for Powassan virus lineage 2 or deer tick virus. Infected ticks were found to have preferentially fed on shrews, as opposed to white-footed mice as was expected, suggesting that they may be a reservoir host for this virus. Reviews 19 November Frederic D. Bushman, Ph.

Core Investigator bushman pennmedicine. Beatrice H. Hahn, M. Core Investigator bhahn upenn.