Prognostic relevance of mitotic activity in patients with node-negative breast cancer.

The prognostic relevance of mitotic activity was analyzed in a series of 306 patients with node-negative breast cancer treated with locoregional therapy alone, until early relapse. Mitotic activity was evaluated as the number of mitotic figures per 10 high-power fields (mitotic activity index) or per 1000 tumor cells (mitotic index). Counting was carried out blindly by two observers. A high correlation was observed between the two determinations (r(s) =.96, P <.001). For clinical analysis, three mitotic activity index subgroups (mitotic figures/field 相似文献   

In vitro metabolism of a nitroderivative of acetylsalicylic acid (NCX4016) by rat liver: LC and LC–MS studies

The metabolism of a nitroderivative of acetylsalicylic acid, benzoic acid, 2-(acetyloxy)-3-[(nitrooxy)methyl]phenyl ester (NCX4016), the lead compound of a new class of NO-releasing non steroidal-antiinflammatory drugs has been studied in vitro in rat liver subcellular fractions (S 9000×g, microsomes, cytosol). Samples were extracted with CH₃CN (2 vol.) containing 1% H₃PO₄ (2 M), vortexed for 3 min and then centrifuged for 5 min at 5000 rpm. Supernatants were diluted with 0.02 M phosphoric acid and analysed by reverse-phase LC. Linearity of calibration for NCX4016 and metabolites was observed over the range 0.25–50 μg/ml with coefficients of determination greater than 0.9996. Extraction efficiency from spiked liver samples ranged from 85 to 95% for all the analytes. In the S 9000×g fraction, NCX4016 undergoes rapid metabolization, with the formation of salicylic acid (SA) and [3-(nitrooxymethyl)phenol] (HBN). HBN is then rapidly metabolised to 3-hydroxybenzylalcohol (HBA), and mainly to a new metabolic species, whose formation takes place specifically in the liver cell cytosol. LC–MS analysis (electrospray ionisation) of the cytosol extract in negative and positive-ion modes furnished deprotonated [M−H]⁻ and protonated [M+H]⁺ molecular ions at m/z 412 and 414, respectively, accompanied by the typical clusters with sodium. MS/MS analysis in negative-ion mode, by selection and collision of the ion at m/z 412, gave a fragmentation pattern characterized by the ions at m/z 272 and 254, which allowed to assign the structure of 1-(glutathion-S-yl)methylene-3-hydroxy-benzene, a conjugated product between GSH and the benzyl carbon atom of HBN. In rat liver cytosol HBN is completely metabolised to this thioether adduct within 30 min incubation; the process is enzymatically mediated by GSH transferase and strictly dependent on GSH availability. The relevance of this new metabolic pathway in NCX4016 detoxification by rat liver is discussed.     

Significance of plasma B-type natriuretic peptide in hemodialysis patients: blood sample timing and comorbidity burden

Plasma B-type natriuretic peptide (BNP) concentration was evaluated in end-stage renal disease patients to verify if measurements before or after the session could furnish different information. BNP levels in plasma from 52 hemodialysis (HD) patients were measured both before and after the first session of the week. Echocardiographic studies were also performed and patients were followed over a period of 28 months. BNP removal from plasma was influenced by equilibrated Kt/V and patient characteristics. Initial plasma BNP concentration was correlated both with cardiac systolic function (LVEF) and mortality rate, independent of blood sample timing (before or after HD). A relative risk of death of 2.67 was found for plasma BNP levels above 335 pg/mL or 232 pg/mL, before and after HD, respectively. Higher BNP levels were observed in patients with higher burden of comorbidity, as measured by the Charlson Comorbidity Index; however, statistical significance was obtained only for BNP measured before HD. In conclusion, measurement of plasma BNP could give a valuable risk stratification of HD patients while cutting costs, by confining echocardiographic studies only to cases with BNP levels above the established cutoff values.     

Non-Latin European descent could be a requirement for association of NTDs and MTHFR variant 677C > T: a meta-analysis

There are several studies that have found a positive association between neural tube defects (NTDs) and the common mutation 677C > T of 5,10-methylenetetrahydrofolate reductase (MTHFR), and others that have not found such an association. We updated the meta-analyses of the published data about NTDs and MTHFR 677C > T variant from January 1994 to October 2005 identifying 170 potentially relevant studies. After applying pertinent exclusion criteria, 37 different populations from 32 studies were included in the meta-analysis, with a total of 3,530 cases and 6,296 controls. Further we stratified the data according to geographical region and ethnicity, and produced two separated meta-analyses for non-Latin European and Latin European descent populations. The general (odds ratio 1.41; 95% confidence interval 1.24-1.59), and the non-Latin European meta-analyses (1.62; 1.38-1.90) indicate an association of TT genotype and NTDs; no association was demonstrated for Latin European populations (1.16; 0.95-1.43). The examination of non-Latin European studies revealed that the association of TT genotype with NTD has only been proven for Irish populations, both by case-control studies, and by family-based tests, such as the allele transmission disequilibrium test (TDT).     

First Polish experience with enzyme replacement therapy in patients with Fabry disease

We report here the course and outcomes of 18-month enzyme replacement therapy in two 43 and 41-year-old brothers with Fabry disease. At 18 months of recombinant alpha-galactosidase A (Fabrazyme) infusions, we observed in the older patient: weight gain, decreased proteinuria (from 4 to 1.5 g/d), stabilization of creatinine clearance, much lower frequency and intensity of angina, and in the younger brother: weight gain, stabilization of creatinine clearance and proteinuria, prolongation of PQ interval and improvement of hearing. However, neurologic manifestations deteriorated over treatment period in both patients. No serious infusion-related side effects were observed.     

Giant reversible,facet-dependent,structural changes in a correlated-electron insulator induced by ionic liquid gating

The use of electric fields to alter the conductivity of correlated electron oxides is a powerful tool to probe their fundamental nature as well as for the possibility of developing novel electronic devices. Vanadium dioxide (VO₂) is an archetypical correlated electron system that displays a temperature-controlled insulating to metal phase transition near room temperature. Recently, ionic liquid gating, which allows for very high electric fields, has been shown to induce a metallic state to low temperatures in the insulating phase of epitaxially grown thin films of VO₂. Surprisingly, the entire film becomes electrically conducting. Here, we show, from in situ synchrotron X-ray diffraction and absorption experiments, that the whole film undergoes giant, structural changes on gating in which the lattice expands by up to ∼3% near room temperature, in contrast to the 10 times smaller (∼0.3%) contraction when the system is thermally metallized. Remarkably, these structural changes are fully reversible on reverse gating. Moreover, we find these structural changes and the concomitant metallization are highly dependent on the VO₂ crystal facet, which we relate to the ease of electric-field–induced motion of oxygen ions along chains of edge-sharing VO₆ octahedra that exist along the (rutile) c axis.The use of electric fields to influence the transport properties of various materials by electrostatic injection of charge at an interface is the foundation of much of modern day electronics (1). Using a three-terminal field-effect transistor geometry, the magnitude of the electric fields provided by conventional gate dielectrics is limited by their dielectric properties. Much higher electric fields are possible by replacing the conventional gate material with an ionic liquid. Consequently, much higher electrostatically induced charge densities are possible, leading to the control or creation of novel metallic (2–3) and superconducting phases (4–7). Materials that are insulating by virtue of strong electron–electron correlations, namely Mott–Hubbard and charge-transfer insulators (8), are anticipated to be particularly sensitive to the injection of small numbers of carriers that could result in their metallization (9–11). Often these materials exhibit a thermally driven insulator to metal transition: one of these, VO₂, exhibits such a transition near room temperature (12, 13) and, for this reason, has been extensively studied (14–16). In VO₂ the metal to insulator transition (MIT) is accompanied by a structural phase transition (SPT) in which the monoclinic insulating phase transforms to a rutile metallic phase (17). Recently, both Nakano et al. (18) and Jeong et al. (19) showed that ionic liquid (IL) gating of thin films of VO₂ results in the suppression of the MIT to temperatures below ∼10 K and, moreover, that the entire film becomes metallic even though gating takes place at the top surface of the film in contact with the IL. However, whereas Nakano et al. (18) claimed the metallization phenomenon was a direct result of electrostatic carrier injection, Jeong et al. (19) presented clear evidence that the metallic state was rather induced by the electric-field–induced migration of oxygen from the film into the IL. An important question is whether the IL gating results in a structural phase transition, as supposed by Nakano et al. (18), or whether the initially insulating film remains in the monoclinic phase and the metallization results rather from the formation of oxygen vacancies (19). Here, we show, using in situ X-ray diffraction and absorption, that IL gating induces massive, reversible structural changes in which the VO₂ (001) film expands and contracts along its thickness by up to 3%, but that the film remains in the monoclinic phase. Furthermore, we identify a remarkable dependence of the IL gating phenomenon on the crystal facet of the VO₂ films. Whereas the (001) and (101) facets exhibit similar IL gate-induced metallization, almost no effect is observed for films grown with (100) and (110) facets. Because there are open channels in the VO₂ crystal structure along the rutile c axis, we associate the IL gating phenomenon with the ease of migration of oxygen along these channels under the influence of electric field. Previously, clear evidence for the formation and refilling of oxygen vacancies under ionic liquid gating of VO₂ (001) has been reported (19).The facet-dependent IL-induced metallization and associated structural changes of VO₂ were studied using two types of devices shown in Fig. 1 A and B, respectively. We label these devices as type T and X, respectively. In Fig. 1A a typical device type T with a channel area of 200 × 20 µm² defined by optical lithography is shown (see Methods for details). The channel conductance is measured using the source (S) and drain (D) contacts that are shown in the figure. A drop of the IL (∼100 nl) fully covers the channel and a significant part of the lateral gate electrode (19). Such devices T are suitable for detailed transport studies as a function of temperature and environment. Fig. 1B shows a cell (20) that was specially designed for in situ synchrotron-based X-ray measurements. Device X, used in this cell, is much larger than that of Fig. 1A and indeed is almost as large as the substrate itself (1 × 1 cm²) with S and D gold contacts, defined by shadow masks, along opposite edges of the substrate. The device and the gate electrode, which is formed from a coiled Au wire that surrounds the device, are immersed in ∼2 mL of IL which is introduced through Teflon tubes and which is contained by a 7.5-µm-thick Kapton sheet, sealed with Viton O-rings, that allows for transmission of the incident and diffracted X-ray beams and fluorescent X-rays. The cell is attached to a four-circle X-ray goniometer for the X-ray diffraction studies. Incorporated within the cell is a heater and a Peltier cooler that allows for operation at temperatures ranging from ∼250 K to ∼400 K. Pulsed laser deposition was used to deposit 10-nm-thick VO₂ films with four different crystal facets, (001), (101), (100), and (110) on single-crystalline substrates of TiO₂ with the same respective crystal orientations, and 20-nm-thick VO₂ (001) films on Al₂O₃(101¯0)(see Methods for details).Open in a separate windowFig. 1.Two different types of ionic liquid devices and facet dependence of gating effect. (A) Optical image of a device with a droplet of HMIM-TFSI with channel size of 200 × 20 µm². (B) Optical image of an ionic liquid device for in situ X-ray measurement. The entire film surface (10 × 10-mm² area) is covered with ionic liquid surrounded by a Au wire used as a gate electrode. Source–drain (Top) and gate (Bottom) current versus V_G for small size device (C) and large size device (D) fabricated from VO₂ films grown on TiO₂ substrates of different orientations. Gate voltages were swept at a rate of 3 mV/s (0.3 mV/s for D) and source–drain voltage (V_SD) of 100 mV (300 mV for D) is applied.Fig. 1 C and D compares the gate voltage (V_G) dependence of the source–drain current I_SD and the gate current I_G at 270 K for devices X and T. Initially the devices are in the insulating phase but above a certain threshold gate voltage I_SD increases substantially. When V_G is decreased to zero the devices remain conducting and revert back to their original state only when reverse gated by applying a negative voltage. I_G remains below 5 nA for all V_G for device T. For device X, I_G is much larger but only because of the much larger gated area: the leakage current per unit area of the gated VO₂ is similar for both devices (see SI Appendix for a detailed comparison). In the fully gated state, device T is metallic to low temperatures as shown earlier (19) but device X was only measured to 250 K, due to limitations of the X-ray cell, where it remained metallic. An important result is the dramatic dependence of the IL gating on the VO₂ crystal facet, as is clearly shown in Fig. 1 C and D.X-ray diffraction θ–2θ curves from a VO₂ (001) device X in the insulating phase and the thermally induced metallic phase are shown in Fig. 2A. The unit cell (all peaks are indexed throughout the paper with respect to the rutile unit cell, for simplicity) contracts along the c axis as evidenced by the shift of the VO₂ (002) peak to higher 2θ as VO₂ transforms from the monoclinic to a rutile phase. Fig. 2B shows a sequence of X-ray θ–2θ curves for the device in different gated states as V_G was systematically ramped in steps from 0 to +2.2 V to −2.2 V and back to zero. The X-ray data were collected after V_G was fixed at each step for 30 min. Fig. 2C shows the corresponding values of I_SD for these data. The gate voltage-induced increase in I_SD is ∼3 orders of magnitude. The X-ray data show very large shifts in the VO₂ (002) peak position which, however, are opposite to that seen for the temperature-driven MIT shown in Fig. 2A. The VO₂ (002) peak rather shifts to smaller 2θ values. This corresponds, as shown in Fig. 2D, to an expansion of the c-axis parameter in the fully gated metallic state by a factor which is 10 times larger than the contraction in the c-lattice parameter that is observed for the temperature-driven SPT. We note that the threshold voltages at which the lattice changes are observed appear to be smaller than that at which I_SD changes. We find that no structural changes are observed when the same experiment is carried out on a 10-nm-thick VO₂ film grown with a (110) facet on TiO₂(110). X-ray diffraction θ–2θ curves taken during gating at gate voltages of up to 2.8 V show no shift in the VO₂ (220) peak position nor any other changes in the X-ray diffraction curves. As shown in Fig. 1D there are similarly no changes in I_SD during gating up to V_G = 3 V.Open in a separate windowFig. 2.Structural changes of VO₂/TiO₂(001) by electrolyte gating as a function of gate voltages. (A) XRD patterns of insulating (red) phase at 270 K and metallic phase (black) at 300 K for 10-nm VO₂/TiO₂(001) with 12-keV photon energy. (B) XRD pattern for in situ X-ray measurements and (C) source–drain current (I_SD) versus gate voltage (V_G). Both XRD and I_SD were measured ∼30 min after V_G was applied. (D) c-axis lattice parameter extracted from B versus gate voltage. The error bars are from the nonlinear least-squares fitting algorithm and in many cases are smaller than the symbols.The structural changes that we find on gating VO₂ (001) are similar to those found by growing VO₂ (001) films of comparable thickness at lower oxygen pressures during pulsed laser deposition. Fig. 3A shows X-ray diffraction θ–2θ curves for a series of five samples, each ∼10 nm thick, prepared at oxygen pressures of 9, 7, 5, 3, and 1 mTorr. As the oxygen pressure is reduced the c-lattice parameter systematically increases, as shown in Fig. 3B. The MIT transition is systematically broadened and suppressed to low temperatures as the oxygen pressure is reduced (19). The c-lattice parameter expansion caused by the introduction of oxygen vacancies by modifying the film growth conditions are similar to those that IL gating induces, and the resulting metallization of the VO₂ films is comparable. We presume that the thickness oscillations in the θ–2θ curves from VO₂/TiO₂(001) in Fig. 2B that disappear on gating result from the loss of coherence in the film structure due to gating and the subsequent formation of oxygen vacancies.Open in a separate windowFig. 3.Crystal structure of oxygen-deficient and gated VO₂/TiO₂(001). Dependence of (A) XRD θ–2θ curves, and (B) c-lattice parameter and T_MIT on oxygen pressure during growth. (C) Reciprocal space maps of VO₂(202) peak versus oxygen pressure during film growth and comparison with those for the pristine and gated states of a device formed from a film grown at 9 mtorr. (D) Structure of the monoclinic phase of VO₂ looking along the <001> and <110> axes.The crystal facet of the VO₂ film is determined by epitaxial growth onto the respective facet of the TiO₂ substrate. This also results in clamping of the 10-nm-thick VO₂ films to the corresponding TiO₂ lattices by coherent strain such that their in-plane unit cell parameters are very similar. This is illustrated in Fig. 3C, which shows reciprocal lattice maps centered near TiO₂ (202) in the k = 0 plane for the five films prepared in different oxygen ambients in Fig. 3A. The maps show along the [20l] direction a very sharp, intense TiO₂ (202) peak together with a weaker and broader VO₂ (202) peak and associated Kiessig fringes (21). The VO₂ (202) peak systematically shifts to lower l as the oxygen pressure is reduced. Along the [h02] direction, by contrast, the VO₂ and TiO₂ peaks have similar narrow widths that indicate in-plane clamping of the VO₂ lattice to that of the TiO₂ substrate. Thus, during IL gating it is anticipated that only the out-of-plane VO₂ lattice parameter can be significantly changed. This is confirmed in the reciprocal space map for a sample that was gated at V_G = 3 V for 10 h and the IL removed before the measurement using a laboratory X-ray source.The clamping of the VO₂ lattice to that of the TiO₂ lattice could offer an explanation for the lack of any significant IL gating response for facets of VO₂ for which the c axis lies in plane. It could be that to remove significant amounts of oxygen the lattice needs to expand along the c direction. It is along the rutile c direction that the structure comprises one-dimensional chains of edge-sharing VO₆ octahedra that allow for the expansion and contraction of the VO₂ lattice during IL gating (Fig. 3D).To inspect the local environment of V we performed in situ X-ray absorption spectroscopy (XAS) at the V K edge using VO₂ (001) films grown on Al₂O₃(101¯0)rather than TiO₂ to avoid the otherwise significant fluorescence from Ti in the substrate. The sample was gated and the XAS data were measured at ambient temperature well below the MIT of the pristine film. The X-ray absorption near-edge spectra (XANES) are shown in Fig. 4A for a pristine sample and the same sample after gating (V_G = 3 V, 1 h) to a conducting state. The XANES data remain largely unchanged with two exceptions. Firstly, there is a small shift in the position of the inflection point of the V 1s→3d preedge transition and a small decrease in the intensity of the preedge peak that suggest a reduction in the valence state of the V ions [by ∼0.2 electrons per V (22)]. The decrease in the intensity of this preedge feature is also consistent with a decrease in the degree of distortion of the VO₆ octahedra (22). Secondly, there is a small shift to lower photon energies in the position of the main V K edge and the white line (V1s → V4p transition) at this edge which is also consistent with a reduced V valence on gating (22). A change in V valence was previously observed in X-ray photoelectron spectroscopy measurements performed on electrolyte-gated VO₂ that suggested the formation of oxygen vacancies on gating (19).Open in a separate windowFig. 4.XAS of VO₂/Al₂O₃(101¯0). (A) Vanadium K-edge XANES for a pristine and gated device X. (B and C) χ(R) curves for the data and corresponding fits. The individual contributions to these fits from respective V–O and V–V shells are shown (inverted) in the bottom halves of B and C. The experimental data for the pristine and gated states are shown in blue and red, respectively; the fits to these data are shown in green; the differences between the experimental data and fits are shown in magenta; the contributions from V–O shells are shown in purple; the contributions from V–V shells along the dimer axis are brown and perpendicular to the dimer axis are dark yellow. The brown horizontal lines in B and C are aids to the eye, showing the degree of dimerization, namely, the difference between the intra- and inter-V–V dimer distances. The solid and dashed curves are the moduli and real parts of the Fourier transform of the EXAFS data and the fits, respectively. (D) Table of fitted V–O and V–V bond distances.Much larger changes are observed in the extended X-ray absorption fine structure (EXAFS) (23), χ(k), that is most readily seen in its Fourier transform (FT), namely, χ(R) = FT(k³ χ(k)) (23), that is presented in Fig. 4 B and C. Detailed information on the types of locally ordered neighbor shells and their metrical parameters was obtained by nonlinear least-squares curve fits using calculated amplitudes and phases (24). The k³-weighted data were fit for k varying from 2.6 to 10.3 Å^–1 so that shells are distinguishable only if separated by more than ∼0.15 Å. The spectra were well fit (Fig. 4 B and C) with a limited number of shells relative to the crystal structure (SI Appendix). The distances found for the pristine samples are consistent with the monoclinic phase of VO₂ below its MIT in which the shorter V–V distances that correspond to those in the (rutile) c direction that is normal to the film plane are split because of the dimerization of the V–V atoms along this direction. In the pristine sample these V–V distances of 2.61 and 3.03 Å differ by 0.42 Å, whereas in the gated sample these distances become 2.94 and 3.16 Å and differ by only 0.22 Å. Note that the almost complete loss of the peak in the χ(R) spectra near R−ϕ = 2.0 Å on gating is not because of a radical change in the V–V chain ordering and departure from the V–V dimerized monoclinic structure, but is because the decreased separation between the short and long dimer V–V pairs causes their individual EXAFS waves to destructively interfere, reducing their combined amplitude in the FT. Thus, a crucial result is that the V–V dimerization, although reduced, is nevertheless retained in the gated state. Moreover, the average V–V distance in the c direction normal to the film planes increases by a much larger amount than is found by diffraction that could indicate rotation of the VO₆ octahedra. On the other hand, the V–V distance within the ab plane (∼3.5 Å) changes little on gating (Fig. 4D), suggesting that the structure in the plane is largely unaffected, consistent with the X-ray diffraction data in Fig. 3C.Our in situ X-ray diffraction (XRD) and XAS measurements clearly indicate a giant expansion of the VO₂ unit cell that is clearly inconsistent with the formation of the rutile phase that the thermally induced metallic phase exhibits. Moreover, we find these reversible structural changes only in films in which channels formed from chains of edge-sharing VO₆ octahedra do not lie in the plane of the films, strongly suggesting that these channels are the paths along which the gate-induced oxygen migration takes place. These gate-induced changes in structure and conductivity are likely to be common to many ionic liquid gated systems, opening the way to a potential future of “liquid electronics.”     

Local host specialization,host-switching,and dispersal shape the regional distributions of avian haemosporidian parasites

The drivers of regional parasite distributions are poorly understood, especially in comparison with those of free-living species. For vector-transmitted parasites, in particular, distributions might be influenced by host-switching and by parasite dispersal with primary hosts and vectors. We surveyed haemosporidian blood parasites (Plasmodium and Haemoproteus) of small land birds in eastern North America to characterize a regional parasite community. Distributions of parasite populations generally reflected distributions of their hosts across the region. However, when the interdependence between hosts and parasites was controlled statistically, local host assemblages were related to regional climatic gradients, but parasite assemblages were not. Moreover, because parasite assemblage similarity does not decrease with distance when controlling for host assemblages and climate, parasites evidently disperse readily within the distributions of their hosts. The degree of specialization on hosts varied in some parasite lineages over short periods and small geographic distances independently of the diversity of available hosts and potentially competing parasite lineages. Nonrandom spatial turnover was apparent in parasite lineages infecting one host species that was well-sampled within a single year across its range, plausibly reflecting localized adaptations of hosts and parasites. Overall, populations of avian hosts generally determine the geographic distributions of haemosporidian parasites. However, parasites are not dispersal-limited within their host distributions, and they may switch hosts readily.A regional community can be thought of as a set of species whose distributions partially overlap within a large geographic area (1, 2). The structure of the regional community (i.e., the relative abundances of species across space and the degree to which populations cooccur) is governed by local (e.g., interspecific competition) and regional (e.g., species diversification and dispersal) processes (3). Although regional communities include all species, parasites and pathogens are rarely considered integral community members (4). Indeed, impacts of parasites on community structure are frequently associated with epidemics—often following introductions to nonnative regions—that have driven naïve hosts to extinction or near extinction (5–7). However, parasites likely play a critical role in shaping regional community structure. Parasites can comprise a large proportion of the community biomass (8), form the majority of links in a community food web (9), and influence regional diversity by variously accelerating (10) or slowing (11) host diversification.Nevertheless, few studies have investigated the processes influencing the regional community structure of both parasites and their hosts. Parasite populations are integrated into community studies with difficulty, partly because these populations are distributed across multiple dimensions—space, host species, and host individuals (12)—and also because parasites are difficult to sample. Moreover, although parasites tend to specialize on one or a few host species, host-breadth may vary across a parasite’s range (13).Regional studies of birds and their dipteran-vectored haemosporidian (“malaria”) blood parasites (14–19) have shown that many parasites are heterogeneously distributed across space despite the availability of suitable hosts. Specialized associations between specific parasites and vectors (20–22) may drive such heterogeneity, although a recent analysis suggests that parasite–host compatibility is also important (23), and local coevolutionary relationships between parasites and their hosts likely influence geographic distributions of both host and parasite populations (11, 14, 15). However, most regional studies of these parasites have focused on individual host species (24–30).Here, we investigate the regional community structure of avian hosts and their haemosporidian parasites with respect to abiotic and biotic drivers of both host and parasite distributions. We surveyed local assemblages of avian haemosporidian parasites across eastern North America and related the distributions of individual parasite lineages to regional climate variation and to the distributions and abundances of their avian hosts. Community dissimilarities between sampling locations based on host assemblage structure (i.e., the relative abundances of potential host species) were positively correlated with those based on parasite assemblage structure, suggesting interdependence of host and parasite population distributions. However, when controlling statistically for that interdependence, local host assemblages responded strongly to environmental gradients and differed more with increasing geographic separation, whereas parasite assemblages did not. This finding suggests that haemosporidian parasites disperse readily across the distributions of their host populations in eastern North America, independently of difference in climate and geographic distance. The degree to which some parasite lineages specialized on particular hosts varied across years and locations, and the nonrandom parasite lineage turnover across the distribution of one well-sampled host species suggested that adaptations of hosts and parasites may also shape regional community structure. Despite evidence of pathogenicity of haemosporidian parasites in birds (31), correlations between host abundances and parasite relative abundances across the region were statistically indistinguishable from random. Taken together, these results suggest that the distributions of parasite populations largely follow the distributions of their hosts but that parasites readily switch hosts and may replace each other across the ranges of individual hosts, resulting in a complex and dynamic regional community.