Exposure to viral and bacterial pathogens among Soay sheep (Ovis aries) of the St Kilda archipelago.

We assessed evidence of exposure to viruses and bacteria in an unmanaged and long-isolated population of Soay sheep (Ovis aries) inhabiting Hirta, in the St Kilda archipelago, 65 km west of Benbecula in the Outer Hebrides of Scotland. The sheep harbour many metazoan and protozoan parasites but their exposure to viral and bacterial pathogens is unknown. We tested for herpes viral DNA in leucocytes and found that 21 of 42 tested sheep were infected with ovine herpesvirus 2 (OHV-2). We also tested 750 plasma samples collected between 1997 and 2010 for evidence of exposure to seven other viral and bacterial agents common in domestic Scottish sheep. We found evidence of exposure to Leptospira spp., with overall seroprevalence of 6·5%. However, serological evidence indicated that the population had not been exposed to border disease, parainfluenza, maedi-visna, or orf viruses, nor to Chlamydia abortus. Some sheep tested positive for antibodies against Mycobacterium avium subsp. paratuberculosis (MAP) but, in the absence of retrospective faecal samples, the presence of this infection could not be confirmed. The roles of importation, the pathogen-host interaction, nematode co-infection and local transmission warrant future investigation, to elucidate the transmission ecology and fitness effects of the few viral and bacterial pathogens on Hirta.

Five challenges in evolution and infectious diseases.

Evolution is a key aspect of the biology of many pathogens, driving processes ranging from immune escape to changes in virulence. Because evolution is inherently subject to feedbacks, and because pathogen evolution plays out at scales ranging from within-host to between-host and beyond, evolutionary questions provide special challenges to the modelling community. In this article, we provide an overview of five challenges in modelling the evolution of pathogens and their hosts, and point to areas for development, focussing in particular on the issue of linking theory and data.

Comparing methods for estimating R0 from the size distribution of subcritical transmission chains.

Many diseases exhibit subcritical transmission (i.e. 0

The Potential Impact of Male Circumcision on HIV in Sub-Saharan Africa

Background: A randomized controlled trial (RCT) has shown that male circumcision (MC) reduces sexual transmission of HIV from women to men by 60(32?76; 95CI) offering an intervention of proven efficacy for reducing the sexual spread of HIV. We explore the implications of this finding for the promotion of MC as a public health intervention to control HIV in sub-Saharan Africa. Methods and Findings :Using dynamical simulation models we consider the impact of MC on the relative prevalence of HIV in men and women and in circumcised and uncircumcised men. Using country level data on HIV prevalence and MC; we estimate the impact of increasing MC coverage on HIV incidence; HIV prevalence; and HIV-related deaths over the next ten; twenty; and thirty years in sub-Saharan Africa. Assuming that full coverage of MC is achieved over the next ten years; we consider three scenarios in which the reduction in transmission is given by the best estimate and the upper and lower 95confidence limits of the reduction in transmission observed in the RCT. MC could avert 2.0 (1.1?3.8) million new HIV infections and 0.3 (0.1?0.5) million deaths over the next ten years in sub-Saharan Africa. In the ten years after that; it could avert a further 3.7 (1.9?7.5) million new HIV infections and 2.7 (1.5?5.3) million deaths; with about one quarter of all the incident cases prevented and the deaths averted occurring in South Africa. We show that a) MC will increase the proportion of infected people who are women from about 52to 58; b) where there is homogenous mixing but not all men are circumcised; the prevalence of infection in circumcised men is likely to be about 80of that in uncircumcised men; c) MC is equivalent to an intervention; such as a vaccine or increased condom use; that reduces transmission in both directions by 37. Conclusions: This analysis is based on the result of just one RCT; but if the results of that trial are confirmed we suggest that MC could substantially reduce the burden of HIV in Africa; especially in southern Africa where the prevalence of MC is low and the prevalence of HIV is high. While the protective benefit to HIV-negative men will be immediate; the full impact of MC on HIV-related illness and death will only be apparent in ten to twenty years

Superspreading and the effect of individual variation on disease emergence.

Population-level analyses often use average quantities to describe heterogeneous systems, particularly when variation does not arise from identifiable groups. A prominent example, central to our current understanding of epidemic spread, is the basic reproductive number, R(0), which is defined as the mean number of infections caused by an infected individual in a susceptible population. Population estimates of R(0) can obscure considerable individual variation in infectiousness, as highlighted during the global emergence of severe acute respiratory syndrome (SARS) by numerous 'superspreading events' in which certain individuals infected unusually large numbers of secondary cases. For diseases transmitted by non-sexual direct contacts, such as SARS or smallpox, individual variation is difficult to measure empirically, and thus its importance for outbreak dynamics has been unclear. Here we present an integrated theoretical and statistical analysis of the influence of individual variation in infectiousness on disease emergence. Using contact tracing data from eight directly transmitted diseases, we show that the distribution of individual infectiousness around R(0) is often highly skewed. Model predictions accounting for this variation differ sharply from average-based approaches, with disease extinction more likely and outbreaks rarer but more explosive. Using these models, we explore implications for outbreak control, showing that individual-specific control measures outperform population-wide measures. Moreover, the dramatic improvements achieved through targeted control policies emphasize the need to identify predictive correlates of higher infectiousness. Our findings indicate that superspreading is a normal feature of disease spread, and to frame ongoing discussion we propose a rigorous definition for superspreading events and a method to predict their frequency.

Changes in fibroglandular volume and water content of breast tissue during the menstrual cycle observed by MR imaging at 1.5 T.

MR imaging at 1.5 T was used to investigate variations in breast parenchyma during the menstrual cycle. Seven subjects were examined twice weekly over at least one menstrual cycle. A three-point Dixon technique (TE = 19 msec, TR = 2000 msec) provided images of fat, water, and static magnetic field (Bo), from which two quantitative whole breast parameters were calculated: the mean relative volumetric water content, , and the mean volumetric fibroglandular fraction, . Four of seven subjects showed unequivocal cyclic variations in and consistent with expected histologic changes; and values were elevated during menses and reduced in mid-cycle. The maximum deviation measured for each of the four subjects was < or = 10% in and units. These variations probably do not influence significantly the clinical interpretation of unenhanced MR breast images. Quantitative measurements of breast parenchyma, however, should recognize these effects.