Proportion Of Infected Hosts Research Articles

Mesoscale spatiotemporal variability in a complex host-parasite system influenced by intermediate host body size.

BackgroundParasites are essential components of natural communities, but the factors that generate skewed distributions of parasite occurrences and abundances across host populations are not well understood.MethodsHere, we analyse at a seascape scale the spatiotemporal relationships of parasite exposure and host body-size with the proportion of infected hosts (i.e., prevalence) and aggregation of parasite burden across ca. 150 km of the coast and over 22 months. We predicted that the effects of parasite exposure on prevalence and aggregation are dependent on host body-sizes. We used an indirect host-parasite interaction in which migratory seagulls, sandy-shore molecrabs, and an acanthocephalan worm constitute the definitive hosts, intermediate hosts, and endoparasite, respectively. In such complex systems, increments in the abundance of definitive hosts imply increments in intermediate hosts’ exposure to the parasite’s dispersive stages.ResultsLinear mixed-effects models showed a significant, albeit highly variable, positive relationship between seagull density and prevalence. This relationship was stronger for small (cephalothorax length >15 mm) than large molecrabs (<15 mm). Independently of seagull density, large molecrabs carried significantly more parasites than small molecrabs. The analysis of the variance-to-mean ratio of per capita parasite burden showed no relationship between seagull density and mean parasite aggregation across host populations. However, the amount of unexplained variability in aggregation was strikingly higher in larger than smaller intermediate hosts. This unexplained variability was driven by a decrease in the mean-variance scaling in heavily infected large molecrabs.ConclusionsThese results show complex interdependencies between extrinsic and intrinsic population attributes on the structure of host-parasite interactions. We suggest that parasite accumulation—a characteristic of indirect host-parasite interactions—and subsequent increasing mortality rates over ontogeny underpin size-dependent host-parasite dynamics.

Helminth Diversity in Synanthropic Rodents from an Urban Ecosystem.

Richness and diversity of parasites depend on a set of interrelated factors related to the characteristics of the host, the environment and the parasites itself. In the City of Buenos Aires, rodent communities vary according to landscape structure. The goal of this paper was to study the variations of helminth richness and diversity among invasive rodent species in different landscape units of the City of Buenos Aires. 73% of the rodents were parasitized with at least one of the 10 identified helminth species. Each rodent species presented its own characteristics in terms of richness, diversity and helminth composition, keeping these characteristics still occupying more than one landscape unit. The infracommunities with greater diversity corresponded to R. norvegicus due to its high values of parasitic richness, proportion of infected hosts and parasite prevalence. Instead, R. rattus and M. musculus infracommunities had lower diversity since a high percentage of them presented a unique helminth species. Within the city, the inhabitants of shantytowns would be the most exposed to zoonotic diseases transmitted by rodents due to high abundance of rodents harboring a high parasite load, including species like Hymenolepis nana and H. diminuta, recognized worldwide from a zoonotic aspect.

A novel model fitted to multiple life stages of malaria for assessing efficacy of transmission-blocking interventions

BackgroundTransmission-blocking interventions (TBIs) aim to eliminate malaria by reducing transmission of the parasite between the host and the invertebrate vector. TBIs include transmission-blocking drugs and vaccines that, when given to humans, are taken up by mosquitoes and inhibit parasitic development within the vector. Accurate methodologies are key to assess TBI efficacy to ensure that only the most potent candidates progress to expensive and time-consuming clinical trials. Measuring intervention efficacy can be problematic because there is substantial variation in the number of parasites in both the host and vector populations, which can impact transmission even in laboratory settings.MethodsA statistically robust empirical method is introduced for estimating intervention efficacy from standardised population assay experiments. This method will be more reliable than simple summary statistics as it captures changes in parasite density in different life-stages. It also allows efficacy estimates at a finer resolution than previous methods enabling the impact of the intervention over successive generations to be tracked. A major advantage of the new methodology is that it makes no assumptions on the population dynamics of infection. This enables both host-to-vector and vector-to-host transmission to be density-dependent (or other) processes and generates easy-to-understand estimates of intervention efficacy.ResultsThis method increases the precision of intervention efficacy estimates and demonstrates that relying on changes in infection prevalence (the proportion of infected hosts) alone may be insufficient to capture the impact of TBIs, which also suppress parasite density in secondarily infected hosts.ConclusionsThe method indicates that potentially useful, partially effective TBIs may require multiple infection cycles before substantial reductions in prevalence are observed, despite more rapidly suppressing parasite density. Accurate models to quantify efficacy will have important implications for understanding how TBI candidates might perform in field situations and how they should be evaluated in clinical trials.

Disruption of Vector Host Preference with Plant Volatiles May Reduce Spread of Insect-Transmitted Plant Pathogens.

Plant pathogens can manipulate the odor of their host; the odor of an infected plant is often attractive to the plant pathogen vector. It has been suggested that this odor-mediated manipulation attracts vectors and may contribute to spread of disease; however, this requires further broad demonstration among vector-pathogen systems. In addition, disruption of this indirect chemical communication between the pathogen and the vector has not been attempted. We present a model that demonstrates how a phytophathogen (Candidatus Liberibacter asiaticus) can increase its spread by indirectly manipulating the behavior of its vector (Asian citrus psyllid, Diaphorina citri Kuwayama). The model indicates that when vectors are attracted to pathogen-infected hosts, the proportion of infected vectors increases, as well as, the proportion of infected hosts. Additionally, the peak of infected host populations occurs earlier as compared with controls. These changes in disease dynamics were more important during scenarios with higher vector mortality. Subsequently, we conducted a series of experiments to disrupt the behavior of the Asian citrus psyllid. To do so, we exposed the vector to methyl salicylate, the major compound released following host infection with the pathogen. We observed that during exposure or after pre-exposure to methyl salicylate, the host preference can be altered; indeed, the Asian citrus psyllids were unable to select infected hosts over uninfected counterparts. We suggest mechanisms to explain these interactions and potential applications of disrupting herbivore host preference with plant volatiles for sustainable management of insect vectors.

Epidemiological Implications of Host Biodiversity and Vector Biology: Key Insights from Simple Models.

Models used to investigate the relationship between biodiversity change and vector-borne disease risk often do not explicitly include the vector; they instead rely on a frequency-dependent transmission function to represent vector dynamics. However, differences between classes of vector (e.g., ticks and insects) can cause discrepancies in epidemiological responses to environmental change. Using a pair of disease models (mosquito- and tick-borne), we simulated substitutive and additive biodiversity change (where noncompetent hosts replaced or were added to competent hosts, respectively), while considering different relationships between vector and host densities. We found important differences between classes of vector, including an increased likelihood of amplified disease risk under additive biodiversity change in mosquito models, driven by higher vector biting rates. We also draw attention to more general phenomena, such as a negative relationship between initial infection prevalence in vectors and likelihood of dilution, and the potential for a rise in density of infected vectors to occur simultaneously with a decline in proportion of infected hosts. This has important implications; the density of infected vectors is the most valid metric for primarily zoonotic infections, while the proportion of infected hosts is more relevant for infections where humans are a primary host.

Reproductive cycle and parasitism in the clam Anomalocardia brasiliana (Bivalvia: Veneridae)

The reproductive cycle and parasitism in the clam Anomalocardia brasiliana were studied in two different areas, an intermediate beach (Cidade) and a tidal flat (Araçá), in Southeast Brazil. Four gametogenic stages were described for males and females in both areas. Mature and spawning individuals were present throughout the period of study at Cidade and Araçá; however, major temporal differences were recorded between sites. Whereas only a few individuals in the resting stage were recorded at Cidade throughout the study period, resting stage individuals were frequent at Araçá. Moreover, a shorter period of gametogenesis was observed at Araçá than at Cidade. Oocyte number was the most effective parameter to differentiate gametogenic stages. An unidentified digenetic trematode was the only parasite infecting A. brasiliana at the study sites, and caused castration of all hosts. Parasite prevalence (proportion of infected hosts) was similar and low (ca 7.5%) at Cidade and Araçá and therefore is not expected to compromise the reproductive output of these populations. Comparison with data from previous studies suggests a latitudinal pattern for the reproductive biology of A. brasiliana, with resting stages occurring only at two high-latitude sites; however, no evidence was found for a latitudinal pattern of trematode prevalence in this species.

Epibiotic macrofauna on common minke whales, Balaenoptera acutorostrata Lacépède, 1804, in Icelandic waters

BackgroundWhilst there is a body of scientific literature relating to the epibiotic macrofauna on large whales, there is little information on the cetaceans in Icelandic waters. Common minke whales, Balaenoptera acutorostrata Lacépède, 1804, are a common sighting between the months of April to November, however, the migration and distribution of the population in winter requires establishing. The present study provides baseline information on the species composition, geographic distribution and abundance of the epibiotic macrofauna on minke whales landed in Icelandic waters and comments on their acquisition.MethodsThe epibiotic macrofauna and skin lesions on 185 and 188 common minke whales respectively, landed in Icelandic waters between April to September 2003-2007 were determined. For each whale, the fluke and one lateral side was examined.ResultsA total of seven epibiotic species were found: the caligid copepod Caligus elongatus (prevalence (P) = 11.9%, mean intensity (M.I) = 95.5); the pennellid copepod Pennella balaenopterae (P = 10.3%, M.I = 1.6); the cyamid amphipod Cyamus balaenopterae (P = 6.5%, M.I = 37.0); the lepadid cirripedes Conchoderma virgatum (P = 0.5%, M.I = 4.0) and Conchoderma auritum (P = 0.5%, M.I = 1.0), the balanid cirriped Xenobalanus globicipitis (P = 1.6%, M.I = 5.3) and the sea lamprey Petromyzon marinus (P = 2.7%, M.I = 1.0). In addition, the hyperparasitic monogenean Udonella caligorum was found on C. elongatus (P = 6.6%) on 8 of the 22 whales infected with the copepod. No significant relationship was observed between parasite intensity and host body length for either C. balaenopterae or C. elongatus, while the proportion of infected hosts was higher in August-September than earlier in the summer for C. balaenopterae (χ2 = 13.69; p<0.01: d.f.=1) and C. elongatus (χ2 = 28.88; p<0.01: d.f.=1).ConclusionsThe higher prevalence of C. balaenopterae on male whales (χ2 = 5.08; p<0.05: d.f.=1), suggests possible different migration routes by the sexes. A likely explanation of the occurrence of P. marinus attached to the minke whales may be due to the gradually rising sea temperature in the area in recent years. This study represents the first known record of C. elongatus on a cetacean host.

Scaling up from epidemiology to biogeography: local infection patterns predict geographical distribution in fish parasites

AbstractAim We investigated how the spatial distribution of parasites, measured as either their geographical range size or their frequency of occurrence among localities, relates to either their average local abundance or the variance in their abundance among localities where they occur.Location We used data on the abundance of 46 metazoan parasite species in 66 populations of threespine sticklebacks, Gasterosteus aculeatus, from Europe and North America.Methods For each parasite species, frequency of occurrence was calculated as the proportion of stickleback populations in which it occurred, and geographical range size as the area within the smallest possible polygon delimited using the coordinates of the localities where it occurred. Generalized linear models were used to assess how these two measures of spatial distribution were influenced by several predictor variables: geographical region (North America or Europe), life cycle (simple or complex), average local abundance, the coefficient of variation in abundance across localities, and median prevalence (proportion of infected hosts within a locality).Results Our analyses uncovered four patterns. First, parasites in North America tend to have higher frequencies of occurrence among surveyed localities, but not broader geographical ranges, than those in Europe. Second, parasite species with simple life cycles have wider geographical ranges than those with complex cycles. Third, there was a positive relationship between average abundance of the different parasite species and their frequency of occurrence, but not between average abundance and geographical range size. Fourth, the coefficient of variation in abundance covaried positively with both the frequency of occurrence and geographical range size across the different parasite species. Thus, all else being equal, parasites showing greater site‐to‐site variability in abundance occur in a greater proportion of localities and over a broader geographical area than those with a more stable abundance among sites.Main conclusions Local infection patterns are linked with large‐scale distributional patterns in fish parasites, independently of host effects, such that local commonness translates into regional commonness. The mechanisms linking parasite success at both scales remain unclear, but may include those that maintain the continuum between specialist and generalist parasites. Regardless, the observed patterns have implications for the predicted changes in the geographical distributions of many parasites in response to climate change.

Infection dynamics ofCichlidogyrus tilapiaeandC. sclerosus(Monogenea, Ancyrocephalinae) in Nile tilapia (Oreochromis niloticusL.) from Uganda

The infection dynamics of the gill monogeneans Cichlidogyrus tilapiae and C. sclerosus on Oreochromis niloticus with respect to habitat type (reservoir, stream, ponds and cages), host sex, size and seasons was determined between January and November 2008. During the study period, 45.2% of the 650 fish examined were infected with Cichlidogyrus spp. The infected hosts harboured an average of 8.6 ± 3.4 parasites/fish. Across habitat types, the proportion of infected fish was not statistically different. In contrast, the number of parasites recorded on infected fish from different habitat types differed significantly. The highest parasite number was recorded in reservoir-dwelling fish and lowest in stream-dwelling hosts. Concerning sex, more female O. niloticus were infected and harboured a high number of parasites than male and sexually undifferentiated fish. A weak negative relationship was found between rainfall and monthly parasite infections. However, a higher number of parasites and proportion of infected hosts were found during dry than in wet seasons, except in ponds. Results of this study show that differential exposure due to changes in fish behaviour associated with habitat modification and sex may account for the infection difference across the sampled sites. Meanwhile, rainfall and the associated hydrological events are important factors regulating monogenean infections in tropical aquatic environments. The continuous presence of Cichlidogyrus spp. in fish provides evidence of possible parasite outbreaks, indicating the application of biosecurity measures as crucial for the success of intensive fish farming.

Parasite prevalence and sample size: misconceptions and solutions

Parasite prevalence (the proportion of infected hosts) is a common measure used to describe parasitaemias and to unravel ecological and evolutionary factors that influence host-parasite relationships. Prevalence estimates are often based on small sample sizes because of either low abundance of the hosts or logistical problems associated with their capture or laboratory analysis. Because the accuracy of prevalence estimates is lower with small sample sizes, addressing sample size has been a common problem when dealing with prevalence data. Different methods are currently being applied to overcome this statistical challenge, but far from being different correct ways of solving a same problem, some are clearly wrong, and others need improvement.

Drug resistance models for malaria

The main factors affecting the evolution of drug resistance in malaria according to theoretical models are reviewed here. The overwhelming influence on the emergence and rate of spread of drug resistance is the proportion of infected hosts that are treated with drugs. A second important effect is drug efficacy in killing parasites. Factors such as average transmission rate, recombination, the biological cost of resistance, and the mode of gene action also influence the rate of spread but have relatively minor impacts. A simple population dynamics model that captures the epidemiological effects of drug treatment and resistance, as opposed to a population genetics model that does not, is presented in order to illustrate the main conclusions.

Investigating the epidemiology of heartwater (Cowdria ruminantium infection) by means of a transmission dynamics model.

A mathematical model of the transmission dynamics of Cowdria ruminantium by the ixodid tick Amblyomma hebraeum in the bovine host is developed and used to investigate the epidemiology of heartwater across a range of vector challenge. The processes described are supported by empirical data. The pattern of outcome measures (incidence, case-fatality and proportion of infected hosts) predicted agrees with those described anecdotally from field experience and empirical observation, and demonstrates the concept of endemic stability. The underlying theory is explored and it is shown that endemic stability may be due principally to the protection of calves against disease by either innate or maternally derived factors. The role of vertical infection in the establishment and maintenance of endemic stability is also investigated. Although increasing the vertical infection proportion results in endemic stability occurring at progressively lower levels of tick challenge, the concomitant reduction in incidence and case-fatality predictions across the range of tick challenge means the endemically stable state simultaneously becomes less discernible. Model limitations and future developments are discussed. The essential role of a transmission dynamics model in assessing the impact of new vaccines in conjunction with vector control programmes is highlighted.

Factors affecting the evolution of virulence: nematode parasites of fig wasps as a case study

SUMMARYThe natural history of fig-pollinating wasps and their associated species-specific nematodes allows the measurement of many parameters which are relevant to testing hypotheses concerning host-parasite ecology and evolution. Within fig wasps species, it is possible to estimate lifetime reproductive success of foundress wasps as a function of presence or absence of nematode parasitism (virulence). Across species, there is a wide range of host population structures which, in turn, results in a range of opportunities for either horizontal or vertical nematode transmission. Therefore, estimates of virulence can be related to opportunities for transmission across a group of closely related hosts and their parasites. Further, the dynamics of the nematode infections over ecological and short-term evolutionary timescales can be monitored, giving added insight into the interpretation of the virulence estimates. Moreover, several scales of longer term evolutionary relationships are either known directly from fossil evidence or can be inferred from molecular data, providing deeper temporal context for the observed patterns. This combination of attributes permits detailed testing of hypotheses concerning the factors that potentially influence the evolution of virulence in host-parasite systems, and further, population and simulation models of the system that incorporate the parameter estimates can clarify the interpretation of how those factors act. There is little evidence suggesting that intermediate and long-term evolutionary relationships explain current levels of virulence. That is, it appears that virulence can change rapidly relative to speciation events, and that the nematodes do not tend to become ‘benign over time’. Instead, it appears that host population structure can influence the evolution of parasite virulence by affecting the relative opportunities for horizontal to vertical transmission, which, in turn, influences the relative costs and benefits of virulence to the nematodes. At one level, increased opportunities for horizontal transmission decouple the reproductive interests of the individual nematodes from those of the individual hosts that they are directly parasitizing, thereby reducing the cost of virulence to individual nematodes. At another level, increased opportunities of horizontal transmission also increases the relative frequency of hosts infected by multiple strains of nematodes. This promotes the evolution of more virulent forms by increasing the relative importance of within-host competition among nematode strains, thereby favouring strains that ‘eat more host sooner’. An interesting property of the fig-nematode systems is that the proportion of infected hosts does not change dramatically through time. This finding implies that there can be considerable negative effects on survival of infected hosts in addition to the previously documented reductions in fecundity of infected foundresses, because the latter are insufficient to account for the observed stability of wasp infection rates.