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Letter
When Is a Reservoir Not a
Reservoir?
R.W. Ashford*
*Liverpool School of Tropical Medicine, Liverpool, United Kingdom
Suggested citation
for this article:
Ashford RW. When is a reservoir not a reservoir? Emerg Infect Dis [serial
online] 2003 Nov [date cited]. Available from: URL: http://www.cdc.gov/ncidod/EID/vol9no11/03-0088.htm
To the Editor: Some 80% of parasitic infections of humans are
zoonoses (1). These infections are caused by multihost
parasites for which the reservoir of infection depends on hosts other
than Homo sapiens. But what is a reservoir of infection?
Haydon et al. (2) proposed a new series of definitions
in connection with multihost pathogens, in which a target host is the
host of interest in a particular context. The reservoir of infection included,
for these authors, all hosts, whether incidental or not, that are epidemiologically
connected to (i.e., contribute to transmission to) the target host.
The availability of three terms—reservoir, reservoir of infection, and
reservoir host—frequently used interchangeably, leads to confusion. This
confusion is, in part, what prompted me (3) to slightly
redefine a reservoir (of infection) as an ecologic system in which an
infectious agent survives indefinitely. Such a system includes all the
component host populations, including that of any intermediate host or
vector, in the context of any environmental component, and any quantitative
requisite such as critical community size, which is required to maintain
the agent indefinitely.
Vertebrate hosts that form an essential part of the system are reservoir
hosts, though whether a whale or a fish is the reservoir host of Anisakis
species can be a matter of debate. A host that becomes infected, but is
not required for the maintenance of the population of a pathogen, can
usefully be called an incidental host. (Accidental host is frequently
used, but this is arguably a teleological term and therefore undesirable.)
For incidental hosts that transmit pathogens from a reservoir to another
incidental host, analogous to a pipe leading from a water reservoir, Garnham
(4) coined the useful term “liaison host.”
Haydon et al. dismiss my definition on two grounds. First, I exclude
liaison hosts from the reservoir. This distinction is valid and could
be argued either way, but I suggest that the pipes leading from a reservoir
do not form part of the reservoir and that it is both conceptually and
practically important to distinguish liaison hosts from reservoir hosts.
The second objection is that many pathogens depend on the presence of
several host species, at any given stage in the life history, for their
maintenance. This concept is clearly considered in my article: together,
such hosts collectively constitute part of the reservoir system, though
no single one may be the reservoir host in its own right.
In good scientific English, each term should have a precise definition,
and synonyms should be avoided. The Oxford English Dictionary (OED) (5)
definitions of reservoir generally refer to a place or container used
for the collection and storage of water, other fluids, or even solid material.
The OED definition of reservoir as a medical term is. “A population which
is chronically infested with the causative agent of a disease and can
infect other populations.” While one might argue with the terms chronically,
infested, and the infection of populations, this definition captures the
usual sense in which reservoir host is used.
The quotations given in OED are more helpful. The earliest one given
for reservoir in a medical context is from 1937, “For the continuous existence
of a disease there must be some reservoir of infection...The most important
reservoirs of infection are human or animal cases or carriers. Plants
may be the reservoir of infection in some of the mycoses." However,
according to OED, the compound term “reservoir host” was used earlier,
in 1913, “The monkey is most probably the normal reservoir host [for Physaloptera
mordens].”
The main conceptual difference between the proposal of Haydon et al.
and my own is that mine is more generalized: for a given pathogen in a
given place, there is a single reservoir. The proposal of Haydon et al.
is more limited to practical considerations: the reservoir for one target
host may not be the same as that for another target host in the same place.
The most important contribution of these two publications is that they
raise an issue that has confused the literature for many years. Parasitologists,
virologists, and bacteriologists should agree on a consensus set of terms
for the ecologic description of multihost systems. When we all agree on
what we are talking about, we will understand each other better.
References
- Ashford RW, Crewe W. The parasites of Homo sapiens:
an annotated checklist of the protozoa, helminths and arthropods for
which we are home. London: Taylor and Francis; 2003.
- Haydon DT, Cleaveland S, Taylor LH, Laurenson MK. Identifying
reservoirs of infection: a conceptual and practical challenge. Emerg
Infect Dis 2002;8:1468–73.
- Ashford RW. What it takes to be a reservoir host. Belgian Journal
of Zoology 1997;127(Suppl1):85–90.
- Garnham PCC. Progress in parasitology. London: Athlone Press; 1971.
- OED online. Oxford: Oxford University Press 2003. Available from:
URL: http://dictionary.oed.com
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