Human plague in the USA: the importance of regional and local climate

A 56-year time series of human plague cases (Yersinia pestis) in the western United States was used to explore the effects of climatic patterns on plague levels. We found that the Pacific Decadal Oscillation (PDO), together with previous plague levels and above-normal temperatures, explained much of the plague variability. We propose that the PDO's impact on plague is conveyed via its effect on precipitation and temperature and the effect of precipitation and temperature on plague hosts and vectors: warmer and wetter climate leading to increased plague activity and thus an increased number of human cases. Our analysis furthermore provides insights into the consistency of plague mechanisms at larger scales.     

Repeatability of Cerebral Perfusion Using Dynamic Susceptibility Contrast MRI in Glioblastoma Patients

OBJECTIVES

This study evaluates the repeatability of brain perfusion using dynamic susceptibility contrast magnetic resonance imaging (DSC-MRI) with a variety of post-processing methods.

METHODS

Thirty-two patients with newly diagnosed glioblastoma were recruited. On a 3-T MRI using a dual-echo, gradient-echo spin-echo DSC-MRI protocol, the patients were scanned twice 1 to 5 days apart. Perfusion maps including cerebral blood volume (CBV) and cerebral blood flow (CBF) were generated using two contrast agent leakage correction methods, along with testing normalization to reference tissue, and application of arterial input function (AIF). Repeatability of CBV and CBF within tumor regions and healthy tissues, identified by structural images, was assessed with intra-class correlation coefficients (ICCs) and repeatability coefficients (RCs). Coefficients of variation (CVs) were reported for selected methods.

RESULTS

CBV and CBF were highly repeatable within tumor with ICC values up to 0.97. However, both CBV and CBF showed lower ICCs for healthy cortical tissues (up to 0.83), healthy gray matter (up to 0.95), and healthy white matter (WM; up to 0.93). The values of CV ranged from 6% to 10% in tumor and 3% to 11% in healthy tissues. The values of RC relative to the mean value of measurement within healthy WM ranged from 22% to 42% in tumor and 7% to 43% in healthy tissues. These percentages show how much variation in perfusion parameter, relative to that in healthy WM, we expect to observe to consider it statistically significant. We also found that normalization improved repeatability, but AIF deconvolution did not.

CONCLUSIONS

DSC-MRI is highly repeatable in high-grade glioma patients.     

Population dynamics in changing environments: the case of an eruptive forest pest species

In recent decades we have seen rapid and co‐occurring changes in landscape structure, species distributions and even climate as consequences of human activity. Such changes affect the dynamics of the interaction between major forest pest species, such as bark beetles (Coleoptera: Curculionidae, Scolytinae), and their host trees. Normally breeding mostly in broken or severely stressed spruce; at high population densities some bark beetle species can colonise and kill healthy trees on scales ranging from single trees in a stand to multi‐annual landscape‐wide outbreaks. In Eurasia, the largest outbreaks are caused by the spruce bark beetle, Ips typographus (Linnaeus), which is common and shares a wide distribution with its main host, Norway spruce (Picea abies Karst.). A large literature is now available, from which this review aims to synthesize research relevant for the population dynamics of I. typographus and co‐occurring species under changing conditions. We find that spruce bark beetle population dynamics tend to be metastable, but that mixed‐species and age‐heterogeneous forests with good site‐matching tend to be less susceptible to large‐scale outbreaks. While large accumulations of logs should be removed and/or debarked before the next swarming period, intensive removal of all coarse dead wood may be counterproductive, as it reduces the diversity of predators that in some areas may play a role in keeping I. typographus populations below the outbreak threshold, and sanitary logging frequently causes edge effects and root damage, reducing the resistance of remaining trees. It is very hard to predict the outcome of interspecific interactions due to invading beetle species or I. typographus establishing outside its current range, as they can be of varying sign and strength and may fluctuate depending on environmental factors and population phase. Most research indicates that beetle outbreaks will increase in frequency and magnitude as temperature, wind speed and precipitation variability increases, and that mitigating forestry practices should be adopted as soon as possible considering the time lags involved.     

Environmental forcing as a main determinant of bloom dynamics of the Chrysochromulina algae

In this paper, we demonstrate that the seasonal dynamics in the abiotic factors, without including seasonal changes in the biological relationships, can appropriately account for the seasonal dynamics of Chrysochromulina spp. This is through the analysis of data on the population dynamics of Chrysochromulina spp. off southern Norway that is evaluated in relation to environmental factors and season by the analyses of 12 year monthly time-series. Chrysochromulina spp. abundance, nutrient concentrations, hydrographical properties, as well as current and wind data were analysed on a monthly scale by means of autoregressive moving average models, principal component analyses (PCA), and linear and nonlinear regression models. Seasonal development of the Chrysochromulina assemblage was well predicted from regression models forced with two PCA components representing seasonal variation in nutrient and chlorophyll a levels and ratios, inflow of North Seawater to the Skagerrak and northeasterly wind along the Norwegian coast. Assuming these to be general results, we might hypothesis that marine algal communities are governed by seasonally varying abiotic factors to a large extent.     

Modeling the epidemiological history of plague in Central Asia: Palaeoclimatic forcing on a disease system over the past millennium

Background

Human cases of plague (Yersinia pestis) infection originate, ultimately, in the bacterium's wildlife host populations. The epidemiological dynamics of the wildlife reservoir therefore determine the abundance, distribution and evolution of the pathogen, which in turn shape the frequency, distribution and virulence of human cases. Earlier studies have shown clear evidence of climatic forcing on contemporary plague abundance in rodents and humans.

Results

We find that high-resolution palaeoclimatic indices correlate with plague prevalence and population density in a major plague host species, the great gerbil (Rhombomys opimus), over 1949-1995. Climate-driven models trained on these data predict independent data on human plague cases in early 20th-century Kazakhstan from 1904-1948, suggesting a consistent impact of climate on large-scale wildlife reservoir dynamics influencing human epidemics. Extending the models further back in time, we also find correspondence between their predictions and qualitative records of plague epidemics over the past 1500 years.

Conclusions

Central Asian climate fluctuations appear to have had significant influences on regional human plague frequency in the first part of the 20th century, and probably over the past 1500 years. This first attempt at ecoepidemiological reconstruction of historical disease activity may shed some light on how long-term plague epidemiology interacts with human activity. As plague activity in Central Asia seems to have followed climate fluctuations over the past centuries, we may expect global warming to have an impact upon future plague epidemiology, probably sustaining or increasing plague activity in the region, at least in the rodent reservoirs, in the coming decades. See commentary: http://www.biomedcentral.com/1741-7007/8/108     

Density-dependent foraging behaviour of sheep on alpine pastures: effects of scale

Foraging patterns of large herbivores may give important clues as to why their life history varies depending on population density. In this landscape-scale experiment, domestic sheep Ovis aries were kept at high (80 sheep km⁻²) and low (25 sheep km⁻²) population densities during summer in high mountain pastures in Hol, Norway. We predicted an increasing use of less preferred plant species or habitat types with increasing sheep population density. Foraging behaviour was investigated by direct observations of individually marked sheep on different spatial scales, and diet composition was also assessed with microhistological analysis of faecal samples from known individuals. We found that the effects of density on foraging behaviour depended on scale and were only detected at the scale of diet choice. Use of the common grass species Deschampsia flexuosa , which provided the bulk forage (10–65% of the diet), remained constant throughout the season at low densities, but increased significantly over time at high densities. On a coarser spatial scale, neither within vegetation type nor between vegetation types, selection was affected by density, but vegetation type selection differed depending on whether the sheep were grazing or resting. Our study provides evidence of density dependence in foraging behaviour, but only at the finest spatial scale (diet choice).     

Trees wanted--dead or alive! Host selection and population dynamics in tree-killing bark beetles

Bark beetles (Coleoptera: Curculionidae, Scolytinae) feed and breed in dead or severely weakened host trees. When their population densities are high, some species aggregate on healthy host trees so that their defences may be exhausted and the inner bark successfully colonized, killing the tree in the process. Here we investigate under what conditions participating with unrelated conspecifics in risky mass attacks on living trees is an adaptive strategy, and what this can tell us about bark beetle outbreak dynamics. We find that the outcome of individual host selection may deviate from the ideal free distribution in a way that facilitates the emergence of tree-killing (aggressive) behavior, and that any heritability on traits governing aggressiveness seems likely to exist in a state of flux or cycles consistent with variability observed in natural populations. This may have implications for how economically and ecologically important species respond to environmental changes in climate and landscape (forest) structure. The population dynamics emerging from individual behavior are complex, capable of switching between "endemic" and "epidemic" regimes spontaneously or following changes in host availability or resistance. Model predictions are compared to empirical observations, and we identify some factors determining the occurrence and self-limitation of epidemics.     

Fatal attraction: vegetation responses to nutrient inputs attract herbivores to infectious anthrax carcass sites

Parasites can shape the foraging behaviour of their hosts through cues indicating risk of infection. When cues for risk co-occur with desired traits such as forage quality, individuals face a trade-off between nutrient acquisition and parasite exposure. We evaluated how this trade-off may influence disease transmission in a 3-year experimental study of anthrax in a guild of mammalian herbivores in Etosha National Park, Namibia. At plains zebra (Equus quagga) carcass sites we assessed (i) carcass nutrient effects on soils and grasses, (ii) concentrations of Bacillus anthracis (BA) on grasses and in soils, and (iii) herbivore grazing behaviour, compared with control sites, using motion-sensing camera traps. We found that carcass-mediated nutrient pulses improved soil and vegetation, and that BA is found on grasses up to 2 years after death. Host foraging responses to carcass sites shifted from avoidance to attraction, and ultimately to no preference, with the strength and duration of these behavioural responses varying among herbivore species. Our results demonstrate that animal carcasses alter the environment and attract grazing hosts to parasite aggregations. This attraction may enhance transmission rates, suggesting that hosts are limited in their ability to trade off nutrient intake with parasite avoidance when relying on indirect cues.     

Regulation of species richness by advection and richness-dependent processes in a coastal fish community

The WWF Oasis of Palo, 38 kms north of Rome, is one of the last remnants of the Mediterranean plain forest formerly covering the coastal areas of Latium. Within this preserved area, nine temporary waterbodies were periodically visited during 2001 to obtain information on their morphology, water cycle and faunal composition. In total, 25 crustacean species were listed and monthly changes in their abundance were recorded. Cladoceran species prevailed in all waterbodies (11 species, 7 genera, 2 families). Copepoda were represented by 8 species, 7 genera, 2 families. Ostracoda included 5 species, 5 genera, 3 families. Anostracans were recorded in only two pools and were represented by Chirocephalus diaphanus Prévost, 1803, the most common species in Italy. Chirocephalus kerkyrensis Pesta, 1936 usually occurring in the waterbodies of the Mediterranean maquis, was missing. Species richness was maximal in late autumn and winter, and correlated positively with pond duration, flooded area, water level and number of rainpools. The species most frequently recorded were Chydorus sphaericus O.F. Müller, 1785 and Daphnia curvirostris Eylmann, 1887 for Cladocera, Megacyclops viridis Jurine, 1820, Diacyclops bicuspidatus odessanus Shmankevitch, 1875 and Mixodiaptomus kupelwieseri Brehm, 1907 for Copepoda, Cyclocypris ovum Jurine, 1820 and Eucypris virens Jurine, 1820 for Ostracoda. Others, like Alona affinis Leydig, 1860, Ceriodaphnia pulchella Sars, 1862 (Cladocera), Macrocyclops albidus Jurine, 1820 (Copepoda), Herpetocypris reptans Baird, 1835 Fabaeformiscandona fabaeformis Fischer, 1854 (Ostracoda) were found in only one pool.     

Richness dependence and climatic forcing as regulating processes of coastal fish-species richness

The maintenance of species diversity is influenced by processes originating both within and outside the ecological community of concern. Along the Norwegian Skagerrak coast, the number of fish species displays variability around a rather stable long‐time average. Using a mechanistically based first‐order autoregressive model with environmental forcing for the species number, we demonstrate for this system the presence of richness‐dependent regulation of species diversity. We explicitly model the niche space as a carrying capacity of species diversity and the intrinsic community feedback as a richness‐dependent regulation. Wind stress, temperature and the North Atlantic Oscillation (NAO) are used as environmental variables. This general and flexible model successfully captures the community dynamics of fish in the coastal areas of the Norwegian Skagerrak coast and thereby demonstrates the validity of developing general models of biodiversity patterns.