Virulence and local adaptation of a horizontally transmitted parasite

Parasites are thought to maximize the number of successfully transmitted offspring by trading off propagule production against host survival. In a horizontally transmitted microparasitic disease in Daphnia, a planktonic crustacean, increasing geographic distance between host and parasite origin was found to be correlated with a decrease in spore production and virulence. This finding indicates local adaptation of the parasite, but contradicts the hypothesis that long-standing coevolved parasites are less virulent than novel parasites. Virulence can be explained as the consequence of balancing the positive genetic correlation between host mortality and strain-specific spore production.

Benefits of host genetic diversity for resistance to infection depend on parasite diversity

Host populations with high genetic diversity are predicted to have lower levels of infection prevalence. This theory assumes that host genetic diversity results in variation in susceptibility and that parasites exhibit variation in infectivity. Empirical studies on the effects of host heterogeneity typically neglect the role of parasite diversity. We conducted three laboratory experiments designed to test if genetic variation in Daphnia magna populations and genetic variation in its parasites together influence the course of parasite spread after introduction. We found that a natural D. magna population exhibited variation in susceptibility to infection by three parasite species and had strong host clone-parasite species interactions. There was no effect of host heterogeneity in experimental host populations (polycultures and monocultures) separately exposed to single strains of three parasite species. When we manipulated the genetic diversity of a single parasite species and exposed them to host monocultures and polycultures, we found that parasite prevalence increased with the number of parasite strains. Host monocultures exposed to several parasite strains had higher mean parasite prevalence and higher variance than polycultures. These results indicate that effect of host genetic diversity on the spread of infection depends on the level of genetic diversity in the parasite population.

Spectroscopic findings in attention-deficit/hyperactivity disorder: review and meta-analysis

OBJECTIVES

The last decade has seen an increasing interest in the method of magnet resonance spectroscopy (MRS) since this is the only research tool that allows a non-invasive in vivo assessment of neurochemical aspects of ADHD without employing ionising radiation. In this paper we review published MRS results with respect to childhood, adolescence and adult ADHD.

METHOD

We searched the Medline (Pub Med) database using the key words ADHD, attention-deficit/hyperactivity disorder, magnet resonance spectroscopy, MRS and spectroscopy. Citations of identified articles were also searched for relevant studies. Meta-analyses were performed for the measured metabolites and regions of assessment.

RESULTS

Sixteen studies could be identified that used MRS to investigate the neurobiology of ADHD. Two regions could be identified as the focus of spectroscopic investigations--the frontal lobe including anterior cingulate cortex and parts of prefrontal cortex and the basal ganglia, mostly striatum, alongside the fronto-striato-thalamo-frontal circuits. As for metabolites, in the majority of studies the ratios to creatine and not absolute concentrations of metabolites were estimated. Choline compounds, N-acetyl-aspartate and glutamate/glutamine (to creatine ratios) could be identified as being altered in several studies in ADHD. The meta-analysis showed increased choline compounds in several researched regions.

DISCUSSION

MRS is a promising tool for the non-invasive in vivo assessment of the cerebral neurochemistry in ADHD. More regions of interest (ROI) like amygdala, hippocampus, thalamus and cerebellum should be assessed in future studies. Further methodological improvements of MRS are desirable in order to assess the absolute metabolite concentration of several ROIs at the same time. Such developments will open novel perspectives in spectroscopic investigations of ADHD.

Vision in depressive disorder

BACKGROUND

Reduced dopaminergic transmission has been implicated in the pathophysiology of major depression. Furthermore, dopaminergic neurotransmission plays an important role in the physiology of visual contrast sensitivity (CS). To test the hypothesis that altered dopaminergic neurotransmission plays a role in major depression we measured contrast sensitivity in patients with major depression and in healthy control subjects.

METHODS

Twenty-eight patients diagnosed with major depressive disorder were compared to 21 age-matched control subjects on their ability to detect a Gabor target with slightly elevated luminance contrast embedded in seven equi-contrast distracters.

RESULTS

Contrast discrimination thresholds were significantly elevated in unmedicated and medicated patients with major depression compared to control subjects, at all pedestal contrast levels tested.

CONCLUSIONS

Contrast discrimination performance is reduced in depressive patients and might reflect a state of altered dopaminergic neurotransmission.

Neurochemical alterations in women with borderline personality disorder and comorbid attention-deficit hyperactivity disorder

BACKGROUND

Borderline personality disorder (BPD) is associated with structural and functional brain changes. Recent models and findings refer to alterations of glutamate and total N-acetylaspartate (tNAA) in this condition.

METHODS

Absolute quantities of tNAA, creatine, glutamate, glutamine, myoinositol and total choline were measured using 3 Tesla magnetic resonance spectroscopy of the left anterior cingulate cortex and the left cerebellum in 14 unmedicated women with BPD and comorbid attention-deficit hyperactivity disorder (ADHD) and 18 healthy women. Both groups were matched with respect to age, education and premorbid intelligence.

RESULTS

In the anterior cingulate, we found significantly higher tNAA and glutamate concentrations and a trend for lower glutamine levels in women with BPD and comorbid ADHD as compared to healthy women. There were no significant group differences in cerebellar metabolite concentrations.

CONCLUSIONS

Glutamatergic changes in the anterior cingulate may be associated with BPD and comorbid ADHD. Increased anterior cingulate tNAA may indicate disturbed energy metabolism or impaired frontal maturation.

Reduced interhemispheric structural connectivity between anterior cingulate cortices in borderline personality disorder

Functional and structural alterations of the anterior cingulate cortex (ACC), a key region for emotional and cognitive processing, are associated with borderline personality disorder (BPD). However, the interhemispheric structural connectivity between the left and right ACC and between other prefrontal regions in this condition is unknown. We acquired diffusion-tensor imaging data from 20 healthy women and 19 women with BPD and comorbid attention-deficit hyperactivity disorder (ADHD). Interhemispheric structural connectivity between both sides of the ACC, dorsolateral prefrontal cortices and medial orbitofrontal cortices was assessed by a novel probabilistic diffusion tensor-based fiber tracking method. In the BPD group as compared with healthy controls, we found decreased interhemispheric structural connectivity between both ACCs in fiber tracts that pass through the anterior corpus callosum and connect dorsal areas of the ACCs. Decreased interhemispheric structural connectivity between both ACCs may be a structural correlate of BPD.

Pathogen dose infectivity curves as a method to analyze the distribution of host susceptibility: a quantitative assessment of maternal effects after food stress and pathogen exposure

Stress conditions have been found to change the susceptibility of hosts or their offspring to infection. The usual method of testing at just one parasite dose level does not allow conclusions on the distribution of susceptibility. To better understand the epidemiology and evolution of host-parasite systems, however, knowledge about the distribution of host susceptibility, the parameters that characterize it, and how it changes in response to environmental conditions is required. We investigated transgenerational effects of different stress factors by exposing Daphnia magna to standard conditions, to low food levels, or to a high dose of the bacterial pathogen Pasteuria ramosa and then measuring the susceptibility of the offspring to different spore doses of the parasite. For the analysis we used a mathematical model that predicts the fraction of infected hosts at different parasite doses, allowing us to estimate the mean and variance of host susceptibility. We find that low food levels reduce both the mean and the variance of offspring susceptibility. Parasite exposure, on the other hand, widens the offspring's susceptibility distribution without affecting its mean. Our analysis uncovered previously unknown transgenerational effects on the distribution of susceptibilities. The finding of an alteration in the variance of susceptibility to infection has implications for host and parasite dynamics and can contribute to our understanding of the stability of host-parasite interactions.

Extraction of prefronto-amygdalar pathways by combining probability maps

Many recent studies reported altered functional connectivity within the frontolimbic circuitry in a wide range of neuropsychiatric disorders. However, functional connectivity must rely on structural connections. In this study we applied a novel probabilistic fiber tracking method to assess the structural connectivity between the amygdala and different prefrontal brain regions in vivo. Twenty healthy subjects were investigated with diffusion tensor imaging. Probabilistic fiber tracking was started from the amygdala and different prefrontal brain regions. Resulting probability maps were combined using an extended multiplication of probabilistic maps to identify the most probable anatomical pathways connecting these structures. We found one ventral pathway through the uncinate fascicle, connecting the amygdala and the medial and lateral orbitofrontal cortices. In addition to this ventral pathway, we depicted distinct dorsal pathways (medial and lateral), which connect the amygdala with the anterior cingulate cortex and the dorsolateral prefrontal cortex. The dorso-medial pathway proceeds through the inferior thalamic peduncle, while the dorsolateral pathway travels through the external capsule. We believe that our approach provides a promising tool to assess the integrity of specific structural connections in patients with neuropsychiatric disorders.

Intensive fish farming and the evolution of pathogen virulence: the case of columnaris disease in Finland

Ecological changes affect pathogen epidemiology and evolution and may trigger the emergence of novel diseases. Aquaculture radically alters the ecology of fish and their pathogens. Here we show an increase in the occurrence of the bacterial fish disease Flavobacterium columnare in salmon fingerlings at a fish farm in northern Finland over 23 years. We hypothesize that this emergence was owing to evolutionary changes in bacterial virulence. We base this argument on several observations. First, the emergence was associated with increased severity of symptoms. Second, F. columnare strains vary in virulence, with more lethal strains inducing more severe symptoms prior to death. Third, more virulent strains have greater infectivity, higher tissue-degrading capacity and higher growth rates. Fourth, pathogen strains co-occur, so that strains compete. Fifth, F. columnare can transmit efficiently from dead fish, and maintain infectivity in sterilized water for months, strongly reducing the fitness cost of host death likely experienced by the pathogen in nature. Moreover, this saprophytic infectiousness means that chemotherapy strongly select for strains that rapidly kill their hosts: dead fish remain infectious; treated fish do not. Finally, high stocking densities of homogeneous subsets of fish greatly enhance transmission opportunities. We suggest that fish farms provide an environment that promotes the circulation of more virulent strains of F. columnare. This effect is intensified by the recent increases in summer water temperature. More generally, we predict that intensive fish farming will lead to the evolution of more virulent pathogens.

Draft genome sequence of the Daphnia pathogen Octosporea bayeri: insights into the gene content of a large microsporidian genome and a model for host-parasite interactions

BACKGROUND

The highly compacted 2.9-Mb genome of Encephalitozoon cuniculi placed the microsporidia in the spotlight, encoding a mere 2,000 proteins and a highly reduced suite of biochemical pathways. This extreme level of reduction is not universal across the microsporidia, with genomes known to vary up to sixfold in size, suggesting that some genomes may harbor a gene content that is not as reduced as that of Enc. cuniculi. In this study, we present an in-depth survey of the large genome of Octosporea bayeri, a pathogen of Daphnia magna, with an estimated genome size of 24 Mb, in order to shed light on the organization and content of a large microsporidian genome.

RESULTS

Using Illumina sequencing, 898 Mb of O. bayeri genome sequence was generated, resulting in 13.3 Mb of unique sequence. We annotated a total of 2,174 genes, of which 893 encodes proteins with assigned function. The gene density of the O. bayeri genome is very low on average, but also highly uneven, so gene-dense regions also occur. The data presented here suggest that the O. bayeri proteome is well represented in this analysis and is more complex that that of Enc. cuniculi. Functional annotation of O. bayeri proteins suggests that this species might be less biochemically dependent on its host for its metabolism than its more reduced relatives.

CONCLUSIONS

The combination of the data presented here, together with the imminent annotated genome of Daphnia magna, will provide a wealth of genetic and genomic tools to study host-parasite interactions in an interesting model for pathogenesis.

Intragenic tandem repeats in Daphnia magna: structure, function and distribution

Background

Expressed sequence tag (EST) databases provide a valuable source of genetic data in organisms whose genome sequence information is not yet compiled. We used a published EST database for the waterflea Daphnia magna (Crustacea:Cladocera) to isolate variable number of tandem repeat (VNTR) markers for linkage mapping, Quantitative Trait Loci (QTL), and functional studies.

Findings

Seventy-four polymorphic markers were isolated and characterised. Analyses of repeat structure, putative gene function and polymorphism indicated that intragenic tandem repeats are not distributed randomly in the mRNA sequences; instead, dinucleotides are more frequent in non-coding regions, whereas trinucleotides (and longer motifs involving multiple-of-three nucleotide repeats) are preferentially situated in coding regions. We also observed differential distribution of repeat motifs across putative genetic functions. This indicates differential selective constraints and possible functional significance of VNTR polymorphism in at least some genes.

Conclusion

Databases of VNTR markers situated in genes whose putative function can be inferred from homology searches will be a valuable resource for the genetic study of functional variation and selection.

Amygdala volume loss in patients with dysphoric disorder of epilepsy

A categorical approach to the study of amygdala volumes in specific neuropsychiatric disorders leads to contradictory findings. In an alternative dimensional approach, we tested the hypothesis that amygdala volume loss represents specific dimensions of affective syndromes in patients with epilepsy. One hundred sixty patients with chronic therapy-refractory epilepsy were carefully diagnosed for psychiatric symptoms. Fifty-three patients without any lifetime psychopathology (n=24), with dysphoric disorder of epilepsy (n=12), or with major depressive disorder (n=17) were included. Amygdala and hippocampal volumes were measured using established protocols. Amygdala volumes were significantly reduced in patients with dysphoric disorder of epilepsy and correlated significantly with core symptoms of dysphoric disorder of epilepsy, that is, emotional instability, dysphoria, irritability, and aggression. Our finding supports a dimensional concept of the meaning of brain alterations and validates the clinical concept of dysphoric disorder of epilepsy.

Comparative metagenomics of Daphnia symbionts

BACKGROUND

Shotgun sequences of DNA extracts from whole organisms allow a comprehensive assessment of possible symbionts. The current project makes use of four shotgun datasets from three species of the planktonic freshwater crustaceans Daphnia: one dataset from clones of D. pulex and D. pulicaria and two datasets from one clone of D. magna. We analyzed these datasets with three aims: First, we search for bacterial symbionts, which are present in all three species. Second, we search for evidence for Cyanobacteria and plastids, which had been suggested to occur as symbionts in a related Daphnia species. Third, we compare the metacommunities revealed by two different 454 pyrosequencing methods (GS 20 and GS FLX).

RESULTS

In all datasets we found evidence for a large number of bacteria belonging to diverse taxa. The vast majority of these were Proteobacteria. Of those, most sequences were assigned to different genera of the Betaproteobacteria family Comamonadaceae. Other taxa represented in all datasets included the genera Flavobacterium, Rhodobacter, Chromobacterium, Methylibium, Bordetella, Burkholderia and Cupriavidus. A few taxa matched sequences only from the D. pulex and the D. pulicaria datasets: Aeromonas, Pseudomonas and Delftia. Taxa with many hits specific to a single dataset were rare. For most of the identified taxa earlier studies reported the finding of related taxa in aquatic environmental samples. We found no clear evidence for the presence of symbiotic Cyanobacteria or plastids. The apparent similarity of the symbiont communities of the three Daphnia species breaks down on a species and strain level. Communities have a similar composition at a higher taxonomic level, but the actual sequences found are divergent. The two Daphnia magna datasets obtained from two different pyrosequencing platforms revealed rather similar results.

CONCLUSION

Three clones from three species of the genus Daphnia were found to harbor a rich community of symbionts. These communities are similar at the genus and higher taxonomic level, but are composed of different species. The similarity of these three symbiont communities hints that some of these associations may be stable in the long-term.

Borderline typical symptoms in adult patients with attention deficit/hyperactivity disorder

Adult attention deficit/hyperactivity disorder (ADHD) and borderline personality disorder (BPD) share several clinical features, e.g. emotional lability and impulsivity. This study aimed to delineate differences and similarities between ADHD and BPD with respect to borderline typical symptomatology and gender specifics. Borderline symptomatology was assessed in 60 adult patients with ADHD with the borderline symptom list (BSL) and compared to both 60 gender- and age-matched BPD patients and control subjects. The BSL is a standardized instrument including 95 items on 7 subscales (self-perception, affect regulation, self-destruction, dysphoria, loneliness, intrusions and hostility). Adult ADHD patients showed significantly higher BSL total scores and all of the seven subscales compared to healthy controls (p < 0.001) but lower scores than BPD patients (p < 0.001). With respect to the seven subscales, the largest differences between ADHD and BPD patients were found with respect to self-destruction (d = 1.12) and affect dysregulation (d = 0.90), whereas the smallest difference was found with respect to loneliness (d = 0.36). In females, the BSL subscales "loneliness" and "hostility" did not differentiate between BPD and ADHD. Borderline typical symptoms are common in adult patients with ADHD but seem to be less pronounced than in patients with BPD. Females with ADHD and BPD share more clinical features than males. However, symptoms of self-destruction and affect dysregulation appear to be more severe in BPD patients.

Expression of parasite virulence at different host population densities under natural conditions

It has recently been suggested that the expression of parasite virulence depends on host population density, such that infected hosts have a higher sensitivity to density, and thus reach their carrying capacity earlier than uninfected hosts. In this scenario, parasite-induced reduction in fitness (i.e., virulence) increases with host density. We tested this hypothesis experimentally, using outdoor mesocosm populations of Daphnia magna infected by the microsporidian Octosporea bayeri. Contrary to the prediction, virulence was independent of host density. In a competition experiment with initial prevalence of 50%, O. bayeri reduced the competitive ability of infected Daphnia within the asexual growth phase independent of initial host population density. In an additional experiment we set up populations with 100% and 0% prevalence and followed their population dynamics over the whole season. Consistent with the competition experiment, we found no difference in population dynamics within the asexual growth phase of the host, suggesting that infected hosts are not more sensitive to density than uninfected hosts. The additional experiment, however, included more than the initial growth phase as did the competition experiment. Eventually, after 100 days, 100% infected populations assumed a reduced carrying capacity compared to uninfected populations. We identify and discuss three reasons for the discrepancy between our experiment and the predictions.

Time-shift experiments as a tool to study antagonistic coevolution

Although understanding natural selection in antagonistic host-parasite interactions has been a challenge for many years, direct evidence for the coevolutionary process is still scarce, particularly in relation to changes in antagonist populations over time. The underlying processes of coevolution thus remain difficult to characterise. Time-shift experiments can be used to test the performance of an antagonist population from a moment in time against the other from the same and different moments in time, revealing reciprocal adaptation in host-parasite relationships. Here we discuss how time-shift experiments together with modelling can shed new insights on the underlying processes of antagonistic coevolution.

[Multimodal treatment of adult attention-deficit hyperactivity disorder]

The indication for treatment of adult attention-deficit hyperactivity disorder (adult ADHD) is derived not from the diagnosis itself but results from the severity of symptoms, comorbidities, psychosocial consequences, and a lack of defined resources for ADHD. The basis of therapy is psychoeducation that includes teaching about symptoms, models of the disorder, and options for treatment. The combination of pharmacotherapy and psychotherapy is recommended. Methylphenidate is considered the first-line therapy, because of its strong effect and modest side effects, but is not authorized in Germany ("off-label use"). Atomoxetine, which is authorized for continuing treatment into adulthood, is indicated if methylphenidate is insufficient or has unacceptable side effects and in case of comorbid substance use. Various psychotherapeutic interventions using available ADHD-typical resources have demonstrated positive effects. Psychosocial support and self-help groups complete the treatment concept. Persistence of the treatment indication has to be reevaluated at regular intervals. Disorder-specific multimodal therapy of adult ADHD conforms to the complex, primarily neurobiologic etiology and the psychosocial consequences.

Hippocampus and amygdala morphology in adults with attention-deficit hyperactivity disorder

OBJECTIVE

Attention-deficit hyperactivity disorder (ADHD) in adulthood is a serious health problem with a prevalence of up to 4%. Limbic structures have been implicated in the genesis of ADHD; it has been suggested that they mediate mood and cognitive disturbances in affected individuals. Recently, a large study involving children and adolescents with ADHD reported bilateral enlargement of the hippocampus and indirect evidence of amygdala volume loss in this patient sample. We sought to test the hypothesis that, like in pediatric patients, there might be hippocampus and amygdala volume abnormalities in adult patients with ADHD.

METHODS

We studied 27 adult patients with ADHD and 27 group-matched healthy volunteers using a 1.5 T magnetic resonance imaging scanner. We manually obtained morphometric measurements of the regions mentioned.

RESULTS

In contrast to previous findings in children and adolescents, we found no significant differences in hippocampus and amygdala volumes among adults with and without the disorder.

CONCLUSION

Findings of hippocampus enlargement and amygdala volume loss are not very stable across different samples of patients with ADHD. Contradictory findings may be related to the different locations of alterations along the complex circuits responsible for the different symptoms of ADHD. Further studies involving larger samples of adult patients with ADHD and using multimodal designs are needed.

Bacterial infection changes the elemental composition of Daphnia magna

1. An animal's elemental composition can be an important indicator of its physiological state and role in ecosystem nutrient cycling. We examined the interactive effects of bacterial (Pasteuria ramosa) infection and phosphorus (P)-poor food on the body stoichiometry of Daphnia magna. Daphnia were exposed to or held free of a bacterial parasite and fed algal food of different C:P ratios (100-500) over a 28-day period. 2. To assess the effects of exposure and infection on Daphnia stoichiometry, we measured their whole body content of carbon (C), nitrogen (N), and P on four different days (4, 8, 15, and 28) during the experiment. 3. We found strong effects of infection, food quality, and/or their interactions on the C, N, and P content of Daphnia, especially as the infectious disease progressed. At the end of the experiment, infected animals had significantly more C and less P in their bodies than uninfected conspecifics. Body N content of Daphnia consuming P-rich food was reduced by bacterial infection whereas Daphnia consuming P-poor algae showed increased body N content from infection. 4. Using a mass-balance model, we found that changes in N and P content of host bodies were largely accountable by disease-induced alterations to Daphnia reproduction (i.e. bacterial induced sterility) and the accumulation of Pasteuria spores in the body cavity. Our calculations also show that the observed increase in host C content could not be accounted for by loss of eggs or accumulation of bacterial spores. This instead must result from unidentified changes to underlying daphnid tissue C content. 5. These results demonstrate that intraspecific variation in zooplankton body stoichiometry can be caused by exposure to and infection by bacterial parasites. In addition, these effects were found to depend both upon the stage of the disease and upon the food quality consumed by the host.

Frontolimbic glutamate alterations in first episode schizophrenia: evidence from a magnetic resonance spectroscopy study

Glutamatergic dysfunction has been implicated in the pathophysiology of schizophrenia. In this study we performed absolute-quantification short-echo magnetic resonance spectroscopy (MRS) in nine patients with first episode schizophrenia and 32 group-matched control subjects to test the hypothesis of glutamatergic dysfunction at disease onset. Regions of interest were the left dorsolateral prefrontal cortex and the left hippocampus. In the patient group absolute concentrations of glutamate were significantly higher in the prefrontal cortex and near-significantly higher in the hippocampus. The glutamate signals significantly correlated with rating scores for schizophreniform symptoms. Absolute-quantification [1H]MRS can reveal glutamatergic abnormalities which might play an important role in the pathogenesis and course of schizophrenia.

Responses of a bacterial pathogen to phosphorus limitation of its aquatic invertebrate host

Host nutrition is thought to affect the establishment, persistence, and severity of pathogenic infections. Nutrient-deficient foods possibly benefit pathogens by constraining host immune function or benefit hosts by limiting parasite growth and reproduction. However, the effects of poor elemental food quality on a host's susceptibility to infection and disease have received little study. Here we show that the bacterial microparasite Pasteuria ramosa is affected by the elemental nutrition of its aquatic invertebrate host, Daphnia magna. We found that high food carbon:phosphorus (C:P) ratios significantly reduced infection rates of Pasteuria in Daphnia and led to lower within-host pathogen multiplication. In addition, greater virulent effects of bacterial infection on host reproduction were found in Daphnia-consuming P-deficient food. Poor Daphnia elemental nutrition thus reduced the growth and reproduction of its bacterial parasite, Pasteuria. The effects of poor host nutrition on the pathogen were further evidenced by Pasteuria's greater inhibition of reproduction in P-limited Daphnia. Our results provide strong evidence that elemental food quality can significantly influence the incidence and intensity of infectious disease in invertebrate hosts.