In this study, we analysed and compared the whole parasite community from the fish Notothenia rossii and Notothenia coriiceps collected from Fildes Bay at King George Island, Antarctica, during January–February 2017 in a field campaign supported by the Chilean Antarctic Institute. The fish samples collected were 45 specimens of N. rossii and 22 of N. coriiceps, with total lengths averaging 29.7 ± 5.3 cm and 32.5 ± 3.2 cm, respectively. Fish were dissected to collect their internal and external parasites. All the fish were parasitized; 13 taxa were found in N. rossii and 12 taxa in N. coriiceps. Acanthocephalans, mainly Metacanthocephalus johnstoni and Aspersentis megarhynchus, were the most abundant and prevalent parasites in both fish species. The abundance and richness of the parasite infracommunity increased with the host body length only in N. rossii. Twelve parasitic taxa were shared by both notothen species. Abundance and prevalence of parasitic taxa, as well as the average richness and abundance of the parasite infracommunities were mostly similar between the two fish species. Parasite compositions of N. coriiceps reported in published studies from King George Island were relatively comparable to our sample. We concluded that the two congeneric and sympatric fish species had highly similar parasite communities, which indicates that they use resources in a similar way, thus allowing them to become parasitized with the same parasitic species and in the same abundances. All parasites recorded in this study have been found in several other fish species; therefore, parasites from notothens are considered to be generalists.

This paper deals largely with the dynamics and changes in the helminth parasite communities of fish along the trophic gradient of lakes. The use of parasitological community data as a bioindicator of environmental health underlines the need to study parasite communities at comparable localities with known pollution levels. The comparison of the conditions in different habitats might be helpful to differentiate between normal fluctuations in ambient conditions and pollution-mediated effects. Therefore, the present study was designed to examine the community structure of parasites in snow trout (Schizothorax niger Heckel) inhabiting 3 lakes of contrasting trophic status in Kashmir. The idea of selecting the lakes, namely Anchar (strongly hypereutrophic), Dal (eutrophic) and Manasbal (mesotrophic) for this study was intentional as they depict different trophic gradients and exhibit the desirable pattern which was a prerequisite for this study. The findings presented in this article suggest an apparent lake-wise gradient in community structure, as the increase in trematode and cestode infections in Anchar was markedly greater, to levels clearly distinguishable from those in the other two water bodies. We conclude that human-induced eutrophication of lakes modifies the parasite community at component level and community-level studies on parasites may provide information on health status of lakes.

The epidemiology of Bartonella species infecting Apodemus flavicollis and Myodes glareolus in a forest in Eastern Poland was followed for 2 years using mark-recapture. Infections could be acquired in any month, but prevalence, and probability of infection, peaked in the summer. There were significant differences in the pattern of infections between the two species. Both hosts were primarily infected as juveniles, but the probability of infection was highest for A. flavicollis, which, evidence suggests, experienced longer-lasting infections with a wider range of Bartonella genotypes. There was no evidence of increased host mortality associated with Bartonella, although the infection did affect the probability of recapture. Animals could become re-infected, generally by different Bartonella genotypes. Several longer lasting, poorly resolved infections of A. flavicollis involved more than 1 genotype, and may have resulted from sequential infections. Of 22 Bartonella gltA genotypes collected, only 2 (both B. grahamii) were shared between mice and voles; all others were specific either to A. flavicollis or to M. glareolus, and had their nearest relatives infecting Microtus species in neighbouring fields. This heterogeneity in the patterns of Bartonella infections in wild rodents emphasizes the need to consider variation between both, host species and Bartonella genotypes in ecological and epidemiological studies.

Predator-prey models are often applied to the interactions between host immunity and parasite growth. A key component of these models is the immune system's functional response, the relationship between immune activity and parasite load. Typically, models assume a simple, linear functional response. However, based on the mechanistic interactions between parasites and immunity we argue that alternative forms are more likely, resulting in very different predictions, ranging from parasite exclusion to chronic infection. By extending this framework to consider multiple infections we show that combinations of parasites eliciting different functional responses greatly affect community stability. Indeed, some parasites may stabilize other species that would be unstable if infecting alone. Therefore hosts' immune systems may have adapted to tolerate certain parasites, rather than clear them and risk erratic parasite dynamics. We urge for more detailed empirical information relating immune activity to parasite load to enable better predictions of the dynamic consequences of immune-mediated interspecific interactions within parasite communities.

Developments in the study of the ecology of helminth parasites of freshwater fishes over the last half century are reviewed. Most research has of necessity been field based and has involved the search for patterns in population and community dynamics that are repeatable in space and time. Mathematical models predict that under certain conditions host and parasite populations can attain equilibrial levels through operation of regulatory factors. Such factors have been identified in several host-parasite systems and some parasite populations have been shown to persist over long time-periods. However, there is no convincing evidence that fish parasite populations are stable and regulated since in all cases alternative explanations are equally acceptable and it appears that they are non-equilibrial systems. It has proved particularly difficult to detect replicable patterns in parasite communities. Inter-specific competition, evidenced by functional and numerical responses, has been detected in several communities but its occurrence is erratic and its significance unclear. Some studies have failed to find any nested patterns in parasite community structure and richness, whereas others have identified such patterns although they are seldom constant over space and time. Departures from randomness appear to be the exception and then only temporary. It appears that parasite communities are non-equilibrial, stochastic assemblages rather than structured and organized.

The search for nested subset patterns has become a powerful tool for understanding the processes shaping parasite communities. Here, we re-examine the results of past studies on nestedness in parasite communities, to assess how sensitive they are to the analytical method used. Using the metric N and the null model RANDOM1, the first method available to study nested patterns, early studies concluded that nestedness was infrequent in parasite communities. In contrast later studies, using instead the metric T and the nestedness temperature calculator (NTC), found that nested subset patterns were very common in parasite communities. Recently, a new algorithm, the binary matrix nestedness temperature calculator (BINMATNEST), has been proposed to quantify nestedness. Using data on 31 helminth communities of fish hosts, we show that applying the NTC yields consistently more significant nested patterns than when N and RANDOM1 are used on the same data. The use of BINMATNEST produced results that depend on the choice of the null model. To provide a benchmark, a straightforward comparison between the observed frequencies of co-occurrences of species with those expected from their prevalence under random assembly was also made for each community. This test indicates that random structure occurs in practically all communities, even those where one of the nestedness analyses found a significant pattern. We demonstrate that the probability of finding a nested pattern in a parasite community depends entirely on the metric and null model chosen for analysis.

Given that numerous amphibians are suffering population declines, it is becoming increasingly important to examine the relationship between disease and environmental disturbance. Indeed, while many studies relate anthropogenic activity to changes in the parasitism of snails and fishes, little is known of the impact on the parasites of amphibians, particularly from agriculture. For 2 years, the parasite communities of metamorphic northern leopard frogs from 7 agricultural wetlands were compared with those from 2 reference wetlands to study differences in parasite community diversity and abundance of various species under pristine conditions and 3 categories of disturbance: only agricultural landscape, only pesticides, and agricultural landscape with pesticides. Agricultural (and urban) area was negatively related to species richness, and associated with the near absence of adult parasites and species that infect birds or mammals. We suggest that agriculture and urbanization may hinder parasite transmission to frogs by limiting access of other vertebrate hosts of their parasites to wetlands. The only parasite found at all localities was an unidentified echinostome infecting the kidneys. This parasite dominated communities in localities surrounded by the most agricultural land, suggesting generalist parasites may persist in disrupted habitats. Community composition was associated with dissolved organic carbon and conductivity, but few links were found with pesticides. Pollution effects may be masked by a strong impact of land use on parasite transmission.

The patterns of association between parasites within a particular host are determined by a number of factors. One of these factors is whether or not infection by one parasite influences the probability of acquiring other parasite species. This study investigates the pattern of association between various parasites of the New Zealand cockle Austrovenus stutchburyi. Hundreds of cockles were collected from one locality within Otago Harbour, New Zealand and examined for trematode metacercariae and other symbionts. Two interspecific associations emerged from the study. First, the presence of the myicolid copepod Pseudomyicola spinosus was positively associated with higher infection intensity by echinostomes. The side-effect of the copepod's activities within the cockle is suggested as the proximate mechanism that facilitates infection by echinostome cercariae, leading to a greater rate of accumulation of metacercariae in cockles harbouring the copepod. Second, a positive association was also found between infection intensity of the metacercariae of foot-encysting echinostomes and that of gymnophallid metacercariae. This supports earlier findings and suggests that the gymnophallid is a hitch-hiker parasite because, in addition to the pattern of positive association, it (a) shares the same transmission route as the echinostomes, and (b) unlike the echinostomes, it is not capable of increasing the host's susceptibility to avian predation. Thus, both active hitch-hiking and incidental facilitation lead to non-random infection patterns in this parasite community.

The parasite community of animals is generally influenced by host physiology, ecology, and phylogeny. Therefore, sympatric and phylogenetically related hosts with similar ecologies should have similar parasite communities. To test this hypothesis we surveyed the endoparasites of 5 closely related cheilinine fishes (Labridae) from the Great Barrier Reef. They were Cheilinus chlorourus, C. trilobatus, C. fasciatus, Epibulus insidiator and Oxycheilinus diagramma. We examined the relationship between parasitological variables (richness, abundance and diversity) and host characteristics (body weight, diet and phylogeny). The 5 fishes had 31 parasite species with 9–18 parasite species per fish species. Cestode larvae (mostly Tetraphyllidea) were the most abundant and prevalent parasites followed by nematodes and digeneans. Parasites, body size and diet of hosts differed between fish species. In general, body weight, diet and host phylogeny each explained some of the variation in richness and composition of parasites among the fishes. The 2 most closely related species, Cheilinus chlorourus and C. trilobatus, had broadly similar parasites but the other fish species differed significantly in all variables. However, there was no all-encompassing pattern. This may be because different lineages of parasites may react differently to ecological variables. We also argue that adult parasites may respond principally to host diet. In contrast, larval parasite composition may respond both to host diet and predator-prey interactions because this is the path by which many parasites complete their life-cycles. Finally, variation in parasite phylogeny and parasite life-cycles among hosts likely increase the complexity of the system making it difficult to find all-encompassing patterns between host characteristics and parasites, particularly when all the species in rich parasite communities are considered.

Although parasite communities have been studied extensively in recent years, spatial and temporal variation in factors affecting the communities has received less attention. This paper examined the similarity of parasite assemblages of perch (Perca fluviatilis) in 18 locations within a single lake in relation to geographical distance and temporal dynamics in the host and parasite populations. We expected that in the present study-scale where distinct but potentially interacting host subpopulations could occur, similarity of the assemblages could be affected by seasonal dynamics in host movements particularly during the spawning period. Parasite species showed differences in infection levels between the sampling locations and similarity of the assemblages of autogenic parasite species in winter, measured inversely as difference in parasite numbers, was negatively affected by geographical distance between the locations. However, no such relationship was observed in the allogenic species Ichthyocotylurus variegata. Furthermore, no relationship was found either in autogenic parasites when the locations were re-sampled in summer, after the spawning period of perch. We concluded that the effect of geographical distance on the similarity of parasite assemblages is a dynamic process, which is affected by seasonal dynamics in host and parasite populations.

Three species of poeciliids (Gambusia holbrooki, Xiphophorus helleri and X. maculatus) and 15 species of ecologically similar native freshwater fishes (mainly eleotrids, ambassids, melanotaeniids and retropinnids) were examined for parasite richness to investigate parasite flux, qualitative differences, quantitative differences and the structuring factors in parasite communities in the 2 fish types in Queensland, Australia. Theory suggests that poeciliids would harbour depauperate parasite communities. Results supported this hypothesis; poeciliids harboured more species-poor parasite infracommunities and regional faunas than natives (P < 0·0001), despite greater sampling effort for the former. Cluster analysis of presence/absence data for poeciliids and the 6 most-sampled native fishes revealed that parasite communities of the 2 fish groups are qualitatively distinct; the proportion of parasite species with complex life-cycles was lower in poeciliids than in native species, and Myxosporea, Microspora, Coccidia and parasitic Crustacea were all absent from poeciliids. Limited exchange of parasite species has occurred between natives and poeciliids. Logistic ordinal regression analysis revealed that fish origin (exotic or native), environmental disturbance and host sex were all significant determinants of parasite community richness (P < 0·05). Theoretical modelling suggests that poeciliids are at a competitive advantage over native fishes because of their lack of parasites.