Reservoirs for rabies virus

In: Expert review of vaccines , Vol. Wong G , Kobinger G. Expert review of vaccines. Wong, Gary ; Kobinger, Gary. Rabies virus RABV does not persist in the environment as it is a very fragile agent.

The primary hosts are mammalian species in the orders Carnivora and Chiroptera. We, however, argue that reservoirs may include nonessential hosts. Excluding nonessential hosts from a reservoir causes two problems.

First, populations harboring infection may be nonessential to maintenance yet play a major role in transmitting the pathogen to the target population. For example, FMDV persists indefinitely in African buffalo herds; yet impala may constitute an important source of infection for the cattle target population 22 e.

Second, as Ashford recognized, the definition of reservoir membership becomes ultimately intractable if each constituent population in the reservoir is considered nonmaintenance. Under these circumstances, a reservoir could be composed of subsets of nonmaintenance populations in a variety of ways Figure 1D.

Although a minimal definition of a reservoir is clear, a fully inclusive definition is much less so. In Figure 1E , population V is not an essential host; nonetheless, this population must be considered a component of the reservoir because, if infection is eliminated in the rest of the reservoir, eradication would not be achieved.

For the same reason, our concept of a reservoir differs from the notion of a critical species assemblage, which is defined as the minimum set of host communities in which a parasite can persist Practical disease control requires answers to two questions: 1 Can an acceptable level of control be accomplished without consideration of a reservoir? Given a target-reservoir system, policies to manage infection may contain elements of three broadly different tactics: 1 target control: directing efforts within the target population with no reference to the reservoir e.

These three approaches require progressively increased levels of understanding of reservoir structure and function. Target control has the important advantage of requiring no knowledge of potentially complex reservoir dynamics.

A complete understanding of infection dynamics within the reservoir is also not necessary to implement blocking tactics, although identifying source populations in the reservoir is essential. The more precisely that source populations can be identified and the more quantitative data that are available on their relative contribution to transmission, the more efficient the allocation of resources is for disease control. Reservoir control tactics require a much more complete understanding of the structure and transmission processes that occur within the reservoir.

For example, efforts directed at controlling infection in nonmaintenance components of a reservoir are unlikely to be effective if infection in the maintenance component of the reservoir remains uncontrolled. The practical problem of identifying reservoirs of rabies for humans in Zimbabwe provides a useful illustration of some issues involved.

After a rise in the incidence of jackal and dog rabies in the s, debate has centered on whether jackals Canis adustus are reservoirs of this disease, an issue that has important implications for formulating national rabies-control programs 10 , In Zimbabwe, domestic dogs are a maintenance and source population of rabies for humans. Jackals may be important components of the reservoir as a maintenance or nonmaintenance population Figure 2.

Because rabies can be maintained in dogs without jackals, jackals are not an essential constituent population of the reservoir. But can infection persist in jackals without dogs Figure 2B? Jackals may constitute part of a maintenance community in conjunction with an assemblage of other wild carnivores Figure 2A. The question is important because if dogs are the only maintenance population in the reservoir, effective vaccination campaigns targeted at dogs should successfully eliminate human rabies from Zimbabwe.

If, however, jackals comprise all or part of a maintenance community independent of dogs, eliminating rabies will only be successful if jackal rabies were also controlled 10 , The recent high incidence of jackal rabies in Zimbabwe might suggest that jackals are maintenance populations. A high incidence of disease alone is neither necessary nor sufficient evidence for this claim, particularly when wide fluctuations in disease incidence occur as with jackal rabies.

Mathematical models suggest that jackals are probably unable to support infection without frequent reintroductions from outside sources However, Bingham et al.

The issue can be resolved unequivocally through implementation of a mass dog vaccination campaign, which would be a logical first phase of a national program. 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.

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