Populations of northern bobwhites (Colinus virginianus; hereafter bobwhite) have been declining across their geographic range in North America, prompting consideration of the role parasites may play. We conducted this study to learn about the helminth fauna in South Texas, a region that supports a sustainable bobwhite population. Helminths were examined from 356 bobwhites collected during the 2014–2015 (n = 124) and 2015–2016 (n = 232) hunting seasons, when increasing trends in precipitation were observed in comparison with the previous two years. Ten helminth species were found, consisting of 14,127 individuals. Of these, all are heteroxenous parasites and three are pathogenic (Dispharynx nasuta, Tetrameres pattersoni and Oxyspirura petrowi). Aulonocephalus pennula numerically dominated the component community (81% prevalence, 99% of the total helminths found), whereas each of the remaining species occurred rarely (≤9% prevalence) and contributed few individuals (≤0.4%) to the helminth community. Prevalence and abundance of A. pennula were not influenced by host age, sex or body mass, but abundance was higher during the 2014–2015 than the 2015–2016 hunting season. Our findings indicate that the helminth community in bobwhites from South Texas can vary during long-term, highly variable precipitation conditions and these communities are more similar to those found in the Rolling Plains of Texas than those found in the eastern part of the bobwhite's geographic range in the US.

Urban slums provide suitable conditions for infestation by rats, which harbour and shed a wide diversity of zoonotic pathogens including helminths. We aimed to identify risk factors associated with the probability and intensity of infection of helminths of the digestive tract in an urban slum population of Rattus norvegicus. Among 299 rats, eleven species/groups of helminths were identified, of which Strongyloides sp., Nippostrongylus brasiliensis and, the human pathogen, Angiostrongylus cantonensis were the most frequent (97, 41 and 39%, respectively). Sex interactions highlighted behavioural differences between males and females, as eg males were more likely to be infected with N. brasiliensis where rat signs were present, and males presented more intense infections of Strongyloides sp. Moreover, rats in poor body condition had higher intensities of N. brasiliensis. We describe a high global richness of parasites in R. norvegicus, including five species known to cause disease in humans. Among these, A. cantonensis was found in high prevalence and it was ubiquitous in the study area – knowledge which is of public health importance. A variety of environmental, demographic and body condition variables were associated with helminth species infection of rats, suggesting a comparable variety of risk factors for humans.

This paper describes a mathematical model which allows us to compare the data collected from short-term cross-sectional surveys with the population dynamics of host and parasite populations over longer periods of time. The model extends an earlier framework for two parasite species in one host, to one for an arbitrary number of parasite species. We show that the conditions necessary for the coexistence of two parasite species extend to expressions for the coexistence of three or more parasite species. Furthermore, the model suggests that those species which form the ‘core’ of the parasite community are those whose high fecundity and transmission efficiency permit them to colonize hosts readily. In contrast, those species which are classified as ‘satellite’ species of the community are either species with low fecundity, or low transmission efficiencies. This work confirms earlier studies that suggest that increasing degrees of aggregation are crucial in allowing several species of parasites to coexist in the same species of hosts.

The properties of the model are compared with patterns observed in data collected for helminth parasites of Anolis lizards, wood mice and eels. This combined theoretical and empirical approach confirms the importance of the life history strategies of the parasite in determining the abundance of each species in the community. It suggests that studies of parasite community structure have to pay more attention to the strategies pursued by each individual species before interactions between species are considered.

The prediction that species richness and diversity of intestinal helminth communities in eels would change over time in response to habitat changes was tested over a period of 13 years in a small stream subjected to extensive human management. Nearly all measures of helminth community structure adopted indicated a decline in richness and diversity over the first 6 years followed by a recovery over the last few years to levels unexpectedly close to those at the start of the investigation. Changes in total number of species suggested that the component community was far richer (from 3–9 species) at the end of the period. By contrast, changes in diversity and dominance measures revealed less variation than expected and suggested that there was an underlying stability of community structure characterized by high dominance by a single species, although the identity of this changed, low diversity and a large proportion of the eel population harbouring 0 or only 1 species. A similar pattern of changes was recorded in the infra-communities, where values of species richness and diversity were very similar at the commencement and termination of the study. It appeared that those helminths that colonized in the recovery period contributed to community richness but had little impact on community structure. The helminth communities clearly did change in response to habitat changes, and the evidence for a fixed number of niches and an underlying constancy in helminth community structure in eels is evaluated.

The suggestion that there may be a limit to the number of niches available to helminth species in the intestine of Anguilla anguilla was investigated by examining the frequency distributions of the number of helminth species per eel and the relationships between maximum and mean infracommunity richness and component community richness in 1 locality over 17 years and in 64 localities throughout Ireland and England. The maximum number of species per eel did not exceed 4 in the 1 locality, or 3 in the 64 localities. In both the single and the several localities, the relationship between maximum and mean infracommunity richness and component community richness was curvilinear and best described by a power or polynomial function. This was interpreted to mean that infracommunity richness became increasingly independent of component community richness, and that infracommunities were saturated at values well below the higher level of helminth richness immediately available for colonization i.e. component community richness. It is argued that these findings cannot be explained by supply-side ecology, pool exhaustion or transmission rates, but only by infracommunity processes acting to impose a fixed limit to the number of species in an infracommunity. Most infracommunities are species poor, and the limiting factors will only operate as species richness rises to determine a maximum. Acceptance of a limit to the number of niches available also resolves the apparent inconsistency between the occurrence and importance of interspecific competition and the nature of isolationist communities.

The possibility that much of the spatial and temporal variation in helminth communities in freshwater fish can be explained in terms of metapopulation theory was tested by examining helminth component communities of eels sampled from 3 localities in the River Exe and 5 of its tributaries on 2 occasions at an interval of 3 years. Four specific predictions of metapopulation theory in relation to community dynamics were tested. The prediction that parasite species could be recognized as common and rare and that this was sensitive to scale was met. The prediction that core species would be more widely distributed and have a higher dispersal ability was met only in part. The prediction that similarity between communities would increase with connectivity and decrease with distance apart was not met. The final prediction that species would go locally extinct and be replaced by rescue effects was not met. There was evidence of local extinctions; not of eel specialists but of species using other fish species as their principal definitive hosts. Eel helminth communities were heavily dominated by 3 species of eel specialists which were relatively stable and responsible for most of the similarity and its changes in space and time. It was concluded that metapopulation theory did not provide a very satisfactory explanation for the changes in the helminth communities in eels and reasons for this included the inherent difficulty of applying metapopulation theory to parasites and the fact that the helminth communities in eels in the River Exe were atypical in being dominated by eel specialists.

The hypothesis that intestinal helminth communities in freshwater brown trout are dissimilar in composition and structure to those in the European eel was tested by an analysis of component communities from 72 localities and of infracommunities from 34 localities in the British Isles and Norway. Derived indices were then compared with published data from eels. Composition of helminth communities differed considerably between the two hosts as a group of 4 species occurred commonly in trout and so gave greater predictability to the community composition. These 4 species were trout specialists and in 97% of the localities a trout specialist dominated the community rather than a generalist acanthocephalan as is typical for eels. By contrast all measures of community structure and indices of richness and diversity indicated that helminth communities in trout were isolationist in character, species poor and exhibited low diversity at both component and infracommunity levels. All values of indices for trout helminth communities were strikingly similar to those obtained from eels. Evidence of interspecific interactions within the trout helminth communities and a limit of 4 to infracommunity species richness further enhanced the similarities and suggested a common determinant of community structure. The hypothesis was thus supported in respect of species composition but refuted in respect of community structure.

The composition, richness and similarity of helminth communities in a tropical freshwater fish were determined in samples of Cichlasoma urophthalmus collected from 7 localities of broadly similar age and character situated along the northern coast of the Yucatan peninsula, Mexico. The component communities exhibited a unique combination of characteristics for a freshwater fish. They were dominated by digeneans, with all other helminth groups being numerically inferior. A common suite of species could be recognized, but many of its members were generalists and not cichlid specialists. Species richness and number of individuals per host were high, but diversity was low, reflecting high dominance by one species. Intra- and inter-locality similarity levels were low, and local variation high. In respect of species richness and digenean dominance, the communities resembled those in Australian tropical anguillids, but in respect of low diversity, similarity and high dominance they are more similar to helminth communities of northern temperate fish.