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Structure in parasite component communities
in wild rodents: predictability, stability, associations
and interactions .... or pure randomness?
J. M. BEHNKE*
School of Biology, University of Nottingham, University Park, Nottingham NG2 7TR, UK
(Received 8 November 2007; revised 19 January 2008 and 22 January 2008; accepted 30 January 2008; first published online 27 March 2008)
SUMMARY
Experimental data establish that interactions exist between species of intestinal helminths during concurrent infections
in rodents, the strongest effects being mediated through the host’s immune responses. Detecting immune-mediated
relationships in wild rodent populations has been fraught with problems and published data do not support a major role
for interactions in structuring helminth communities. Helminths in wild rodents show predictable patterns of seasonal,
host age-dependent and spatial variation in species richness and in abundance of core species. When these are controlled
for, patterns of co-infection compatible with synergistic interactions can be demonstrated. At least one of these, the positive
relationship between Heligmosomoides polygyrus and species richness of other helminths has been demonstrated in three
totally independent data-sets. Collectively, they explain only a small percentage of the variance/deviance in abundance data
and at this level are unlikely to play a major role in structuring helminth communities, although they may be important
in the more heavily infected wood mice. Current worm burdens underestimate the possibility that earlier interactions
through the immune system have taken place, and therefore interactions may have a greater role to play than is immediately
evident from current worm burdens. Longitudinal studies are proposed to resolve this issue.
Key words: rodents, bank voles, wood mice, helminths, nematodes, interactions, associations, co-infections, immunity.
INTRODUCTION
Organisms in wild, natural ecosystems live in com-
munities, where they interact with the environment
around them and with other organisms (Tokeshi,
1999). Their abundance is subject to influences from
extrinsic or abiotic factors, such as the precise
location and nature of the habitat, the regional
climate, and numbers fluctuate between years and
seasons, depending on circumstances (Elton, 1927 ;
Begon et al. 2005) . They also interact with other
living organisms, including members of their own
species, in order to reproduce or for protection, and
they compete for food and other resources, leading
to density-dependent regulation of population sizes.
Interactions occur also with members of other
species, not the least in predator-prey relationships
in food chains (Elton, 1927; Begon, Townsend and
Harper, 2005).
Parasites likewise interact with the external
environment : for example, their abundance may
be dependent on climatic factors such as ambient
temperature and humidity. Both are known to influ-
ence the development of free-living stages and affect
the survival of transmission stages outside the host,
as well as affecting the hosts themselves (Haukisalmi
and Henttonen, 1990; Guernier, Hochberg and
Guegan, 2004; Hudson et al. 2006). They also in-
teract with other living organisms, foremost among
which are the hosts within which they live, that
provide their immediate environment. A unique
facet of parasitism is that parasites are parcelled
within hosts in infrapopulations and infrac ommuni-
ties, a concept originally developed by Bush and
Holmes (198 6) (See also later in this review and Esch
et al. 1990). In some cases they may also be trans-
mitted in packages, as for example when heavi ly
infected intermediate hosts are consumed by preda-
tory definitive hosts (Bush, Heard and Overstreet,
1993; Pou lin, 2001). Each in frapopulation lives in
a restricted environment delimited by the host’s
body. Within this space, each can affect the host
in different ways, with consequences for the other
co-infecting species (e.g. through the host immune
response, pathology or nutrient limitation). Not
surprisingly, therefore, in discrete populations of
hosts, intrinsic factors such as host age, sex, diet,
genetic status and social position, including life-
history strategy, become varyingly important at in-
fluencing susceptibility to infection with parasitic
and other invasive organisms (Barnard et al. 2002).
Parasites within hosts interact with members of their
own species (hence density-dependent regulation of
infrapopulations is an important process limiting
abundance and reproductive effort among parasites;
Keymer, 1982) and finally they interact with other
* Corresponding author. Tel : +44 (0)115 951 3208. Fax:
+44(0)115 951 3251. E-mail : jerzy.behnke@nottingham.
ac.uk
751
Parasitology (2008), 135, 751–766. f 2008 Cambridge University Press
doi:10.1017/S0031182008000334 Printed in the United Kingdom
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parasites exploiting the same host, and it is this, co-
infection or polyparasitism, that constitutes the key
theme of this issue.
Interactions between species within hosts can
range from the mere coincidence of more than one
species in the same host, through one species influ-
encing the environment within that host in a way
that benefits or impairs the survival of the second
species, to species actually interacting directly. The
latter may arise physically because parasites are
crowded into a space-restricted site, through com-
petition for resources, or via chemical mediators (e.g.
‘allomone-like’ substances, metabolic by-products –
Roberts, 2000; Pedersen and Fenton, 2006). Added
complexity in co-infections is gener ated by correlated
exposure, by parasite longevity, by differences in
the genetic constitution of individual hosts, and by
broader effects on the hosts’ immune system through
commitment to a particular response phenotype (e.g.
Th1 versus Th2). The latter feature in particular is
a fertile field of current interest (Graham, 2002 ;
other contributors to this special issue). There can be
profound conse quences from such co-infections,
with one species enhancing the susceptibility of
the host to other life-threatening infections (Booth
and Bundy, 1992 ; Nacher, 2002 ; Druilhe, Tall and
Sokhna, 2005; Pedersen and Fenton, 2006), and
concern has been expressed about how little we
actually know about the cumulative effect of co-
infections on the health of the host (Buck, Anderson
and MacRae, 1978 b ; Keusch and Migasena, 1982;
Ezeamama et al. 2005). This paper will focus almost
entirely, but not exclusively, on co-infections of
helminths in laboratory and in wild rodents. After
reviewing some of the most persuasive evidence for
the existence of interactions between species, based
on laborato ry studies, I hope to demonstrate that
relationships bet ween species compatible with an
underlying interaction can be detected in field data,
but that they need to be interpreted with due
caution. In order to detect them a sound, compre-
hensive knowledge of the structure, predictability
and composition of helminth communities is an
essential pre-requirement. I make no apology fo r the
selective use of the literature, since several compre-
hensive reviews of the subject have been published
already, to which the reader is referred (Dobson,
1985; Christensen et al. 1987 ; Petney and Andrew s,
1998; Behnke et al. 2000), and for focusing particu-
larly on our own work : such was my remit.
TYPES OF INTERACTIONS AND SOURCES
OF ERROR
Interactions between species can be synergistic, that
is positive, one species benefiting another. A good
example of such an interaction among free-living
animals can be found in the relationship between the
large blue butterfly (Maculinea arion) and rabbits
(Oryctolagus cuniculus). However, this is not a simple
relationship. High rabbit density does not directly
create larg e butterfly populations or vice versa, but
rather rabbits keep the grass low, allowing ants and
their colonies to proliferate, providing a secure
environment for the larvae of the large blue butter-
fly (Warren and Wigglesworth, 2007). This first
example, therefore provides us with due warning
that the interpretation of analyses of interactions
between organisms needs to be tackled with extreme
caution, because of the scope for jumping to over-
simplistic and erroneous conclusions.
Some associations may benefit both species, as in
the case of the well known relationships between ants
and aph ids (Stadler and Dixon, 2005). Synergistic
relationships can range from the loose commensal,
with little specificity, as in the case of bees pollinating
a wide range of plants (Biesmeijer et al. 2006), to the
absolutely specific and obligatory as in the case
Darwin’s hawkmoth (Xanthopan morganii) and the
orchid Angraecum sesquipedale (Nilsson, 1998).
As already intimated, analyses of co-ocurrences
of animals, with the intention of discovering under-
lying interactions between species, are fraught with
problems, not least statistical problems and the risk
of false conclusions through failure to control for
environmental factors that concentrate animals to-
gether in some subsets of the population (Haukisalmi
and Henttonen, 1998; Howard, Donnelly and
Chan, 2001; Behnke et al. 2005). Indeed, population
structure often appears to generate the spurious
appearance of interactions through genetic, spatial,
age-related and ecological effects, and I would like
to illustrate this with a very simple example.
In a survey of animals living on a golf course, frogs
and damselflies occur in certain sectors of the total
area surveyed but not in others. Moreover, sectors
with large numbers of frogs also have many
damselflies, whilst those with few frogs have low
numbers of damselflies. It may be tempting to in-
terpret this as reflecting interactions between the
species with high abundance of frogs generating a
high abundance of damselflies or vice versa. How-
ever, the true explanation is much simpler. In this
case the golf course has many temporary and larger
permanent ponds, and some are richer in biodiver-
sity than others, supporting larger populations of
both species. So, indirect effects can stem from
some quality of the environment, which benefits
both species (Wootton, 1994) and encourages co-
occurrence, rather than because of any specific
interactions between frogs and damselflies. This
self-evident truth of free-living community ecology
underlines the importance of distinguishing between
co-occurrence of species based on convergence
of common ecological processes that bring species
together or separate them, and co-occurrence based
on direct interactions between species (Howard et al.
2001, 2002). It can be easily obscured in parasitology,
J. M. Behnke 752
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where interactions have to be identified from co-
occurrence in the host, and co-occurrence of the
free-living and infective stages as well.
Interactions between species can also be antagon-
istic, often referred to as negative interactions, for
example, exploitative or food chain relationships
when the abundance of one species lower s that of
the other. They can be competitive, based on ex-
ploitation of the same resource, when both species
may suffer if the resource is limited (Begon et al.
2005). They can also be indirect, for example when
a common pathogen lowers the fitness of both
species in situations where they co-occur. So on
damp pasture grazed by sheep, the amphibious snail
Lymnea truncatula may occur, and if it is infected
with Fasciola hepatica this is likely to impact on
abundance/fitness of both hosts in the cycle.
Co-occurrence of parasites, or polyparasitism, is
well known in the medical (Buck et al. 1978a, b;
Ashford, 1991), veterinary (Diez-Ban
ˇ
os, Cabaret
and Diez-Ban
ˇ
os, 1992) and zoological literature
(Haukisalmi and Henttonen, 1993). The majority
of animals in naturally existing wild communities
(Lotz and Font, 1994; Nilssen, Haugerud and
Folstad, 1998 ; Lello et al. 2004), and humans living
under conditions of exposure, carry several species
of parasites concurrently (Buck et al. 1978a; Keusch
and Migasena, 1982; Kvalsvig, 1988 ; Ashford,
Craig and Oppenheimer, 1992 ; Booth et al. 1998 ;
Bottomley, Ishan and Basanez, 2005). Some of these
are more often encountered together than expected
by chance, and among co-occurring species there
may be quantitative associations, such that heavy
infections with one species seemingly predispose
to heavy infections with the other. To give one well
known exampl e among human parasites, Ascaris
lumbricoides and Trichuris trichiura often co-occur in
infected individuals. Many different studies have
highlighted their co-occurrence in human popu-
lations living in developing count ries (Holland et al.
1989; Booth and Bundy, 1992 ; Howard et al. 2002).
Tchuem Tchuente
´
et al. (2003) found a highly sig-
nificant positive relationship between the intensity
of both species among those childr en that were
passing eggs of both species (after exclusion of all
double and single negatives). However, the data were
generated through a survey of 5 schools, in which
there were both boys and girls in 12 age classes.
Abundance differed between schools and age
classes, so these had to be taken into account to avoid
erroneous conclusions. This was achieved by fitting
minimum sufficient statistical models in general-
ized linear models to the data and examining the
relationship between the residuals. Having con-
trolled for differences between schools and age
classes, the relationship was now weaker, indicating
that some degree of correlated exposure was in-
volved. Nevertheless the relationship survived the
analysis and was still significant.
But even when all these relevant controls have
been considered, does co-occu rrence of species
mean that the one species facilitates or antagonises
the other ? Not necessarily. Both A. lumbricoides and
T. trichiura have robust, well-protected eggs, thus
individuals in the data subsets who showed poor
hygiene would be likely to be exposed to both. Co-
abundance is therefore not necessarily attributable
only to interactions with one species predisposing
to infection with the other, but may more likely re-
flect the coincidence of similar transmis sion strat-
egies. In the majority of co-infection studies on these
two species correlated exposure has not been taken
into account. The challenge now is to quantify human
behaviour that enhances transmission in these chil-
dren, as for example by measurement of ingestion
of silica (Bundy, 1988) and to control for that. My
prediction is that the quantitative relationship be-
tween the intensity of infection with A. lumbricoides
and T. trichiura will largely disappear when this
additional control is brought into the analysis.
LABORATORY MODELS OF POLYPARASITISM
How can we explore the underlying processes, causes
and effects, in more detail? My initial strategy was
to explore interactions in laboratory models under
the precisely controlled conditions of laboratory
experimentation first, and then to test the predictions
of laboratory-generated data in the field. A wide
range of useful and relevant model systems is avail-
able (Table 1) for such work and these have been
used extensively, although the work was largely con-
ducted in the 1970s and is mostly forgotten.
The classic work of John Holmes (1961, 1962a, b)
is seldom cited these days. Hymenolepis diminuta
and Moniliformis moniliformis both usually occupy
the anterior region of the gut of rats but Holmes
found that when rats were co-infected, M. monili-
formis was able to hold its own, staying in the pre-
ferred site, while H. diminuta was ‘pushed’ back to
Table 1. Helminth parasites of rodents that
have been used in studies of interactions between
intestine dwelling species
Nematodes Trichinella spiralis
Trichuris muris
Nippostrongylus brasiliensis
Heligmosomoides bakeri*
Strongyloides ratti
Cestodes Hymenolepis diminuta
Hymenolepis citelli
Rodentolepis microstoma**
Acanthocephala Moniliformis moniliformis***
* Previously known as H. polygyrus and Nematospiroides
dubuis.
** Previously known as Hymenolepis microstoma.
*** Previously known as M. dubius.
Community structure and interactions in helminths of rodents 753
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non-optimal distal regions of the small intestine. But
this is not the end of the story. There are two further
twists, because H. diminu ta is far longer lived than
M. moniliformis, and as the latter dies of senility, the
former moves back up into its preferred site. Equally
of interest, Holmes (1962 b) failed to find the same
relationship between these species in a different host,
the hamster. It is also relevant that H. diminuta is
a species that is particularly sensitive to crowding
effects (density-dependent regulation), its size and
fecundity being severely affected even when just
two worms, rather than a single worm, are present
(Read, 1951; Roberts, 2000). These papers by
Holmes are widely regarded as the start of the field
of quantitative helminth community ecology (Esch
et al. 1990; Janovy, 2002).
Holmes (1973) hypothesi sed that where competi-
tive interactions occur in ecol ogical time, the inter-
acting species that is the superior competitor would
bring about the extinction of the weaker competitor,
or force it to survive as an opportunistic organism,
exploiting hosts only in situations where the superior
competitor was absent. Over evolutionary time, we
may expect some stabilisation by specialization to
eliminate the competition, leading to selective seg-
regation.
Gerry Schad (1963) made a seminal contribution
to this concept. Schad worked on the pinworms of
Greek tortoises, which were parasitized by 8 species
of oxyuroid nematodes of the genus Tachygonetria .
Hosts can be very heavily parasitized, and the
8 species have become specialists to avoid compe-
tition. Thus T. uncinata and T. numidica show vir-
tually identical longitudinal distribution in the colon
of tortoises, but it turns out that the former is a
mucosal species, whereas the latter lives in the lumen.
When Schad looked at the other species, it was evi-
dent that half were mucosal and half lumen dwellers,
and he was able to match up approximately simi-
lar longitudinal distributions with this mucosal/
luminal separation. Where both longitudinal and
radial distributions were similar (e.g. T. robusta
and T. stylosa), their feeding strategies differed.
Thus T. robusta is an indiscriminate feeder, whereas
T. stylosa feeds on fine particles such as bacteria. As
a consequence of such adaptations over time site,
resource and temporal segregation have evolved to
minimise competition and niche overlap.
However, the situation may be even more complex
and controversy has continued to this day. Some
workers have argued that in particular hosts there
may be many vacant niches and that the driving
forces for niche segregation and speciation are re-
inforcement of reproductive barriers and maximum
utilisation of food resources (Price, 1980 ; Rohde,
1991 and see Esch (1990) and Janovy (2002) for
reviews of the conflicting debate in this field). There
is also the possibility that the assemblage of Tachy-
gonetria spp. resident in Schad’s tortoises was
acquired by host capture from other reptiles, in
which they had evolved to exploit slightly different
host resources.
THEIMMUNERESPONSEASAN
ENVIRONMENTAL STRESSOR
Parasitism, of course, has unique ecological features
in so far as free living organisms do not have to face
the threat of the environment consp iring specifi-
cally to destroy them. As with all other ecological
problems, parasites have to adapt or die and over
evolutionary time hosts refine their mechanisms
of defence, whil st parasites develop new counter
measures (Behnke, Barnard and Wakelin, 1992 ;
Maizels et al. 2004) and this generates the Arms
Race, also known as the gene-for-gene or Red Queen
hypotheses (Dawkins and Krebs, 1979 ; Anderson
and May, 1982 ; Behnke and Barnard, 1990).
The details of the immune system are given
elsewhere (Murphy, Travers and Walport, 2008).
However, the features of relevance to what follows
are summarised in Fig. 1. Initially the adaptive
immune response of vertebrates involves a covert
afferent phase during which recognition of foreign
invaders takes place and, eventually, this is translated
into an effector phase during whi ch the actua l de-
fences are unleashed. The latter can be pathogenic
for the host and come at a cost (Castro, Olson and
Baker, 1967; Mercer et al. 2000), but it is during this
effector phase that worms can be expelled from the
host. We have been arguing for some time about
whether a magic bullet exists within the compl ex
array of effectors (Pemberton et al. 2004; Anthony
et al. 2006 ; Artis, 2006 ; Artis et al. 2004), but the
fact is that once unleashed some components of the
response to intestinal helminths, whether effective
Worm antigen
Presentation to T cells
Amplification of response
Cytokine signals to myeloid cells
Intestinal
inflammation
Proliferation/differentiation
of inflammatory cells
Worm expulsion
Specific
afferent
phase
Non-
specific
efferent
phase
Worms enter host
Fig. 1. Schematic diagram of the major stages of
an adaptive immune response to parasitic infection
in vertebrates.
J. M. Behnke 754
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against the triggering species or not, can be detri-
mental to other species.
Five broad categories of such interactions arising
from this immune interaction can be envisaged.
Species such as Trichinella spiralis generate an in-
testinal immune response during which gut function
is severely affected and, as the immune response
intensifies, so worm numbers decline. Concurrently
marked changes in a range of cellular and soluble
mediators can be detected, reflected for example in
mastocytosis on the one hand (mast cell numbers
increase and degra nulate, releasing their highly toxic
compounds) and in changes in the expression of re-
ceptors of signals from cytokines, including adhesion
molecules, on the other hand (Else and Finkelman,
1999; Gause, Urban and Stadecker, 2003; Nair,
Guild and Artis, 2006).
If a lumen dweller such as H. diminuta is present
during this expulsion phase, it is severely affected
(Behnke, Bland and Wakelin, 1977; Christie,
Wakelin and Wilson, 1979; Silver, Dick and Welch,
1980). Fig. 2 shows that in control mice 90 % of
worms established and survived, and grew to about
1–2 mg in weight by day 8, whilst in mice respond-
ing to T. spiralis, survival was just 7
.
5% and the
biomass was only 0
.
1 mg. However, when the im-
mune response was suppressed, in this case by
treatment with the wide-spectrum steroid immuno-
suppressant cortisone, H. diminuta could survive
and grow alongside T. spiralis without any evident
detrimental consequences. Table 2 summarises
many similar examples, all work conducted in the
1970s and early 1980s. As with all biological pro-
cesses, however, there are some exceptions.
Hookworms were able to survive the acute phase
of the inflammatory response to T. spiralis in con-
currently infected hamsters without any loss
(Behnke, Rose and Little, 1994), a reflec tion of their
remarkable resilience in the face of the host’s
immune response (Louk as and Prociv, 2001).
In some cases there may be genuine cross im-
munity between species, a topic of immense im-
portance in the race to develop vaccines against
gastro-intestinal (GI) nematodes of ruminants,
where cross-immunity would be a practical asset,
protecting not only against the inducing species,
but against other co-infecting nematodes. There
have been disappointments on this front (Adams,
Anderson and Windon, 1989), but one example of
the successful demonstration of cross immunity
between species is in the interaction between
T. spiralis and Strongyloides ratti. Moqbel and
Wakelin (1979) showed that even when rats were
challenged with S. ratti 1
.
5 months after T. spiralis
infection, establishment and survival of S. ratti were
severely affected, long after the inflammation stimu-
lated by the initial T. spiralis infection would have
died down (See also Nawa et al . 1982 ; Mimori et al.
1983).
0.01
0.1
1
10
100
1000
H. diminuta alone H. diminuta +
T. spiralis
H. diminuta +
T. spiralis + cortisone
Treatment
Mean biomass (mg dry wt.) +/- S.D.
90% 90%7.5%
Percentage of worms recovered
Fig. 2. Loss of Hymenolepis diminuta from mice
concurrently infected with Trichinella spiralis at the
height of the response to the latter species. The figure
shows the percentage survival of worms and the mean
dry biomass (¡standard deviation), following
administration of 5 cysticercoids to each animal, eight
days before autopsy. The mice were given 450 larvae
of T. spiralis one day before infection with H. diminuta.
Note that the error bars cannot be seen in the middle
column because they are too small to be illustrated on
the scale used. For further details of methods used
including details of treatment with cortisone see Behnke
et al. 1977.
Table 2. Antagonistic interactions between intestinal nematodes. In
these examples one species induces an immune response in the host, and a
second co-infecting species is severely imp aired, and even expelled by the
non-specific consequences of the effector mechanisms induced
Inducing parasite Affected parasite Reference
Trichinella spiralis Trichuris muris Bruce and Wakelin (1977)
T. spiralis Hymenolepis diminuta Christie (1979),
Silver et al. (1980)
T. spiralis Rodentolepis microstoma Howard et al. (1978)
Nippostrongylus
brasiliensis
T. spiralis Kennedy (1980)
T. spiralis N. brasiliensis Kennedy (1980)
For comprehensive list of other combinations see Christensen et al. (1987).
Community structure and interactions in helminths of rodents 755
