Pathogen survival in the external environment and the evolution of virulence

Starvation can diversify the population structure and virulence strategies of an environmentally transmitting fish pathogen

Background

Generalist bacterial pathogens, with the ability for environmental survival and growth, often face variable conditions during their outside-host period. Abiotic factors (such as nutrient deprivation) act as selection pressures for bacterial characteristics, but their effect on virulence is not entirely understood. “Sit and wait” hypothesis expects that long outside-host survival selects for increased virulence, but maintaining virulence in the absence of hosts is generally expected to be costly if active investments are needed. We analysed how long term starvation influences bacterial population structure and virulence of an environmentally transmitting fish pathogen Flavobacterium columnare.

Results

F. columnare populations in distilled water and in lake water were monitored for 5 months. During the experiment, the population structure of F. columnare diversified by rough and soft colony morphotypes appearing among the ancestral rhizoid ones. After 5 months starvation in lake water, the virulence of the starved and ancestral bacterial isolates was tested. The starved rhizoid isolates had significantly higher virulence than the ancestral rhizoid, whereas the virulence of the rough isolates was low.

Conclusions

We suggest that F. columnare population diversification is an adaptation to tolerate unpredictable environment, but may also have other biological significance. Maintaining and increasing virulence ensures efficient invasion into the host especially under circumstances when the host density is low or the outside-host period is long. Changing from rhizoid into a rough morphotype has trade-offs in making bacteria less virulent and unable to exploit the host, but may ensure bacterial survival under unpredictable conditions. Our study gives an example how abiotic selection can diversify virulence of environmentally transmitting bacterial pathogen.

Loss of competition in the outside host environment generates outbreaks of environmental opportunist pathogens

Environmentally transmitted pathogens face ecological interactions (e.g., competition, predation, parasitism) in the outside-host environment and host immune system during infection. Despite the ubiquitousness of environmental opportunist pathogens, traditional epidemiology focuses on obligatory pathogens incapable of environmental growth. Here we ask how competitive interactions in the outside-host environment affect the dynamics of an opportunist pathogen. We present a model coupling the classical SI and Lotka–Volterra competition models. In this model we compare a linear infectivity response and a sigmoidal infectivity response. An important assumption is that pathogen virulence is traded off with competitive ability in the environment. Removing this trade-off easily results in host extinction. The sigmoidal response is associated with catastrophic appearances of disease outbreaks when outside-host species richness, or overall competition pressure, decreases. This indicates that alleviating outside-host competition with antibacterial substances that also target the competitors can have unexpected outcomes by providing benefits for opportunist pathogens. These findings may help in developing alternative ways of controlling environmental opportunist pathogens.

Pathogen-host associations and predicted range shifts of human monkeypox in response to climate change in central Africa

Climate change is predicted to result in changes in the geographic ranges and local prevalence of infectious diseases, either through direct effects on the pathogen, or indirectly through range shifts in vector and reservoir species. To better understand the occurrence of monkeypox virus (MPXV), an emerging Orthopoxvirus in humans, under contemporary and future climate conditions, we used ecological niche modeling techniques in conjunction with climate and remote-sensing variables. We first created spatially explicit probability distributions of its candidate reservoir species in Africa's Congo Basin. Reservoir species distributions were subsequently used to model current and projected future distributions of human monkeypox (MPX). Results indicate that forest clearing and climate are significant driving factors of the transmission of MPX from wildlife to humans under current climate conditions. Models under contemporary climate conditions performed well, as indicated by high values for the area under the receiver operator curve (AUC), and tests on spatially randomly and non-randomly omitted test data. Future projections were made on IPCC 4(th) Assessment climate change scenarios for 2050 and 2080, ranging from more conservative to more aggressive, and representing the potential variation within which range shifts can be expected to occur. Future projections showed range shifts into regions where MPX has not been recorded previously. Increased suitability for MPX was predicted in eastern Democratic Republic of Congo. Models developed here are useful for identifying areas where environmental conditions may become more suitable for human MPX; targeting candidate reservoir species for future screening efforts; and prioritizing regions for future MPX surveillance efforts.

Environmental survival of Neisseria meningitidis

Neisseria meningitidis is transmitted through the inhalation of large human respiratory droplets, but the risk from contaminated environmental surfaces is controversial. Compared to Streptococcus pneumoniae and Acinetobacter baumanni, meningococcal viability after desiccation on plastic, glass or metal surfaces decreased rapidly, but viable meningococci were present for up to 72 h. Encapsulation did not provide an advantage for meningococcal environmental survival on environmental surfaces.

Study on the interzonal migration of airborne infectious particles in an isolation ward using benign bacteria

Negative pressure isolation wards are essential infection control facilities against airborne transmissible diseases. Airborne infectious particles are supposed to be contained in the isolation room. However, negative pressure may break down by door-opening action or by human movement. Understanding the interzonal transport of airborne infectious particles in the isolation wards can aid the design and operation strategy of isolation facilities. In this work, the interzonal migration of airborne infectious particles by human movement was studied experimentally in an isolation ward. Artificial saliva solution with benign E. coli bacteria was aerosolized to simulate bacterium-laden infectious particles. The interzonal migration of aerosolized bacteria was characterized by biological air sampling. Less than 1% of airborne infectious particles were transported to the higher pressure zone when door was closed. With human movement, 2.7% of the particles were transported from the anteroom to the corridor. From high-to-low pressure zones, as much as 20.7% of airborne infectious particles were migrated. Only a minimal amount of particles was transported from the corridor to the positive pressure nurses' station. Infection risk of tuberculosis of the healthcare workers and other occupants in the isolation wards were also assessed based on the measured migration ratios.

PRACTICAL IMPLICATIONS

Human movement is an important factor governing interzonal migration. It is the main cause of migration of airborne infectious particles to a relatively negative pressure zone. This study provides a set of experimentally obtained particle migration ratios by human movement. Other than serving as empirical data for further studies on the mechanics, these migration ratios can also be used to assess the infection risk for occupants in the isolation ward.

A multi-scale analysis of influenza A virus fitness trade-offs due to temperature-dependent virus persistence

Successful replication within an infected host and successful transmission between hosts are key to the continued spread of most pathogens. Competing selection pressures exerted at these different scales can lead to evolutionary trade-offs between the determinants of fitness within and between hosts. Here, we examine such a trade-off in the context of influenza A viruses and the differential pressures exerted by temperature-dependent virus persistence. For a panel of avian influenza A virus strains, we find evidence for a trade-off between the persistence at high versus low temperatures. Combining a within-host model of influenza infection dynamics with a between-host transmission model, we study how such a trade-off affects virus fitness on the host population level. We show that conclusions regarding overall fitness are affected by the type of link assumed between the within- and between-host levels and the main route of transmission (direct or environmental). The relative importance of virulence and immune response mediated virus clearance are also found to influence the fitness impacts of virus persistence at low versus high temperatures. Based on our results, we predict that if transmission occurs mainly directly and scales linearly with virus load, and virulence or immune responses are negligible, the evolutionary pressure for influenza viruses to evolve toward good persistence at high within-host temperatures dominates. For all other scenarios, influenza viruses with good environmental persistence at low temperatures seem to be favored.

It has recently been suggested that for avian influenza viruses, prolonged persistence in the environment plays an important role in the transmission between birds. In such situations, influenza virus strains may face a trade-off: they need to persist well in the environment at low temperatures, but they also need to do well inside an infected bird at higher temperatures. Here, we analyze how potential trade-offs on these two scales interact to determine overall fitness of the virus. We find that the link between infection dynamics within a host and virus shedding and transmission is crucial in determining the relative advantage of good low-temperature versus high-temperature persistence. We also find that the role of virus-induced mortality, the immune response and the route of transmission affect the balance between optimal low-temperature and high-temperature persistence.

Evolution of increased survival in RNA viruses specialized on cancer-derived cells

Viruses and other pathogens can diverge in their evolved host-use strategies because of exposure to different host types and conflicts between within-host reproduction and between-host survival. Most host-pathogen studies have emphasized the role of intrahost reproduction in the evolution of pathogen virulence, whereas the role of extra-host survival has received less attention. Here, we examine the evolution of free-living virion survival in RNA virus populations differing in their histories of host use. To do so, we used lineages of vesicular stomatitis virus (VSV) that were experimentally evolved in laboratory tissue culture for 100 generations on cancer-derived cells, noncancerous cells, or alternating passages of the two host types. We observed that free-living survival improved when VSV populations specialized on human epithelial carcinoma (HeLa) cells, whereas this trait was not associated with selection on noncancer cells or combinations of the cell types. We attributed this finding to shorter-lived HeLa monolayers and/or rapid cell-to-cell spread of viruses on HeLa cells in tissue culture, both of which could select for enhanced virus stability between host-cell replenishment. We also showed evidence that increases in virion survival were associated with decreases in virulence, which suggests a trade-off between survival and virulence for the VSV populations on one cell type. Our results shed new light on the causes and consequences of "sit and wait" infection strategies in RNA viruses.

Hypovirus virulence and vegetative incompatibility in populations of the chestnut blight fungus

Cryphonectria hypovirus 1 hyperparasitizes the chestnut blight fungus Cryphonectria parasitica and acts as a biocontrol agent for this serious tree disease. The virus is transmitted cytoplasmatically between fungal individuals. However, highly virulent viruses strongly debilitate their host and, thus, reduce their own transmission probability. Furthermore, vegetative incompatibility between fungi is an important transmission barrier. Therefore, virulent viruses are expected to be strongly selected against in fungal populations with high levels of vegetative incompatibility, eventually leading to the erosion of biocontrol. To test this prediction, we assessed the virulence of the virus in four European C. parasitica populations with high diversity of vegetative compatibility types and in four populations with low diversity. We expected the degree of virus virulence to be lower in fungal populations with high levels of vegetative incompatibility. However, our results did not reveal such a trend. No significant differences in virus virulence between populations with low versus high diversity of vegetative compatibility types were observed. There was no evidence for an erosion of disease control due to the presence of these transmission barriers. Thus, the findings of this study are promising for the sustainability of Cryphonectria hypovirus 1 as a biocontrol agent for chestnut blight in Europe.

Outside-host growth of pathogens attenuates epidemiological outbreaks

Opportunist saprotrophic pathogens differ from obligatory pathogens due to their capability in host-independent growth in environmental reservoirs. Thus, the outside-host environment potentially influences host-pathogen dynamics. Despite the socio-economical importance of these pathogens, theory on their dynamics is practically missing. We analyzed a novel epidemiological model that couples outside-host density-dependent growth to host-pathogen dynamics. Parameterization was based on columnaris disease, a major hazard in fresh water fish farms caused by saprotrophic Flavobacterium columnare. Stability analysis and numerical simulations revealed that the outside-host growth maintains high proportion of infected individuals, and under some conditions can drive host extinct. The model can show stable or cyclic dynamics, and the outside-host growth regulates the frequency and intensity of outbreaks. This result emerges because the density-dependence stabilizes dynamics. Our analysis demonstrates that coupling of outside-host growth and traditional host-pathogen dynamics has profound influence on disease prevalence and dynamics. This also has implications on the control of these diseases.

Infectious disease emergence and global change: thinking systemically in a shrinking world

BACKGROUND

Concern intensifying that emerging infectious diseases and global environmental changes that could generate major future human pandemics.

METHOD

A focused literature review was undertaken, partly informed by a forthcoming report on environment, agriculture and infectious diseases of poverty, facilitated by the Special Programme for Tropical Diseases.

RESULTS

More than ten categories of infectious disease emergence exist, but none formally analyse past, current or future burden of disease. Other evidence suggests that the dominant public health concern focuses on two informal groupings. Most important is the perceived threat of newly recognised infections, especially viruses that arise or are newly discovered in developing countries that originate in species exotic to developed countries, such as non-human primates, bats and rodents. These pathogens may be transmitted by insects or bats, or via direct human contact with bushmeat. The second group is new strains of influenza arising from intensively farmed chickens or pigs, or emerging from Asian "wet markets" where several bird species have close contact. Both forms appear justified because of two great pandemics: HIV/AIDS (which appears to have originated from bushmeat hunting in Africa before emerging globally) and Spanish influenza, which killed up to 2.5% of the human population around the end of World War I. Insufficiently appreciated is the contribution of the milieu which appeared to facilitate the high disease burden in these pandemics. Additionally, excess anxiety over emerging infectious diseases diverts attention from issues of greater public health importance, especially: (i) existing (including neglected) infectious diseases and (ii) the changing milieu that is eroding the determinants of immunity and public health, caused by adverse global environmental changes, including climate change and other components of stressed life and civilisation-supporting systems.

CONCLUSIONS

The focus on novel pathogens and minor forms of anti-microbial resistance in emerging disease literature is unjustified by their burden of disease, actual and potential, and diverts attention from far more important health problems and determinants. There is insufficient understanding of systemic factors that promote pandemics. Adverse global change could generate circumstances conducive to future pandemics with a high burden of disease, arising via anti-microbial and insecticidal resistance, under-nutrition, conflict, and public health breakdown.

Experimental evolution of a bacteriophage virus reveals the trajectory of adaptation across a fecundity/longevity trade-off

Life history theory attempts to account for how organisms lead their lives, balancing the conflicting demands of reproduction and survival. Here, we track the genomic and phenotypic evolution of the bacteriophage virus T7 across a postulated fecundity/longevity constraint. We adapted T7 to a challenging survival environment (6M urea). Our evolved strain displayed a significant improvement in propagule survival, coupled with a significant loss of fecundity (reduced growth rate on host cells). However, the increased resistance to urea did not generalise to increased resistance against temperature stress, highlighting that propagule durability is environment dependent. Previous comparative studies predicted that changes in propagule resistance would be mediated by changes in capsid proteins or gene deletions. In contrast, we found that point mutations in internal core protein genes (6.7 and 16) were responsible for the increased urea resistance of our evolved strain. Prior to the emergence of the 6.7 and 16 mutations, a distinct set of 5-point mutations peaked at over 20% prevalence before attenuating, suggestive of negative epistatic interactions during adaptation. Our results illustrate that parasites can adapt to specific transmission environments, and that this adaptation can impose costs on the subsequent ability to exploit host cells, potentially constraining durable parasites to lower virulence.

Interactive effects between diet and genotypes of host and pathogen define the severity of infection

Host resistance and parasite virulence are influenced by multiple interacting factors in complex natural communities. Yet, these interactive effects are seldom studied concurrently, resulting in poor understanding of host-pathogen-environment dynamics. Here, we investigated how the level of opportunist pathogen virulence, strength of host immunity and the host condition manipulated via diet affect the survival of wood tiger moth Parasemia plantaginis (Arctidae). Larvae from "low cuticular melanin" and "high cuticular melanin" (considered as low and high pathogen resistance, respectively) selection lines were infected with moderately and highly virulent bacteria strains of Serratia marcescens, while simultaneously manipulating host diet (with or without antibacterial compounds). We measured host survival and food preference before and after infection to test whether the larvae "self-medicate" by choosing an anti-infection diet (Plantago major, i.e., plantain leaf) over lettuce (Lactuca sativa). "High melanin" larvae were more resistant than "low melanin" larvae to the less virulent strain that had slower growth and colonization rate compared with the more virulent strain. Cuticular melanin did not enhance survival when the larvae were infected with the highly virulent strain. Anti-infection diet enhanced survival of the "high melanin" but not the "low melanin" hosts. Survival was dependent on family origin even within the melanin selection lines. Despite the intrinsic preference for lettuce, no evidence of self-medication was found. These results demonstrate that the relative benefit of host cuticular melanin depends on both diet and pathogen virulence: plantain diet only boosted the immunity of already resistant "high melanin" hosts, and cuticular melanin increased host survival only when infected with moderately virulent pathogen. Moreover, there was considerable variation in host survival between families within both melanin lines suggesting genetic basis for resistance. These results indicate that although melanin is an important predictor of insect immunity, its effect on disease outcomes greatly depends on other interacting factors.

A construction method to study the role of incidence in the adaptive dynamics of pathogens with direct and environmental transmission

We study the adaptive dynamics of virulence of a pathogen transmitted both via direct contacts between hosts and via free pathogens that survive in the environment. The model is very flexible with a number of trade-off functions linking virulence to other pathogen-related parameters and with two incidence functions that describe the contact rates between hosts and between a host and free pathogens. Instead of making a priori particular assumptions about the shapes of these functions, we introduce a construction method to create specific pairs of incidence functions such that the model becomes an optimization model. Unfolding the optimization model leads to coexistence of pathogen strains and evolutionary branching of virulence. The construction method is applicable to a wide range of eco-evolutionary models.

Virulence not only costs but also benefits the transmission of a fungal virus

Current theory suggests that cost-benefit relationships govern the evolution of parasite virulence. The cost of virulence is expected to be high for fungal viruses, which are obligate parasites and completely dependent on their hosts. The majority of fungal viruses infect their hosts without any apparent symptoms. Cryphonectria hypovirus 1 (CHV-1), in contrast, is virulent and debilitates its host, Cryphonectria parasitica. However, the virulence of CHV-1 is associated with high costs for virus transmission, such as an attenuated fungal growth and reduced production of the fungal spores spreading the virus. In this study, we tested the hypothesis that virulence may not only have costs but also benefits for transmitting CHV-1 across vegetative incompatibility barriers between fungi. We investigated viruses with low, medium, and high virulence, and determined their transmission rate per host-to-host contact (transmissibility). The average transmission rate across all combinations tested was 53% for the most virulent virus, 37% for the virus with intermediate virulence, and 20% for the virus with lowest virulence. These results showed that increased virulence was strongly correlated with increased transmissibility, potentially counterbalancing virulence costs. This association of virulence and transmissibility may explain why CHV-1 spread widely and evolved higher virulence than most other fungal viruses.

Trade-offs shape the evolution of the vector-borne insect pathogen Xenorhabdus nematophila

Our current understanding on how pathogens evolve relies on the hypothesis that pathogens' transmission is traded off against host exploitation. In this study, we surveyed the possibility that trade-offs determine the evolution of the bacterial insect pathogen, Xenorhabdus nematophila. This bacterium rapidly kills the hosts it infects and is transmitted from host cadavers to new insects by a nematode vector, Steinernema carpocapsae. In order to detect trade-offs in this biological system, we produced 20 bacterial lineages using an experimental evolution protocol. These lineages differ, among other things, in their virulence towards the insect host. We found that nematode parasitic success increases with bacteria virulence, but their survival during dispersal decreases with the number of bacteria they carry. Other bacterial traits, such as production of the haemolytic protein XaxAB, have a strong impact on nematode reproduction. We then combined the result of our measurements with an estimate of bacteria fitness, which was divided into a parasitic component and a dispersal component. Contrary to what was expected in the trade-off hypothesis, we found no significant negative correlation between the two components of bacteria fitness. Still, we found that bacteria fitness is maximized when nematodes carry an intermediate number of cells. Our results therefore demonstrate the existence of a trade-off in X. nematophila, which is caused, in part, by the reduction in survival this bacterium causes to its nematode vectors.

Using child health outcomes to identify effective measures of handwashing

We assessed which practical handwashing indicators were independently associated with reduced child diarrhea or respiratory disease. Fieldworkers collected 33 indicators of handwashing at baseline in 498 households in 50 villages in rural Bangladesh. Community monitors visited households monthly and asked standard questions about diarrhea and symptoms of respiratory illness among children under 5 years of age. In multivariate analysis, three handwashing indicators were independently associated with less child diarrhea-mothers reporting usually washing hands with soap before feeding a child, mothers using soap when asked to show how they usually washed their hands after defecation, and children having visibly clean finger pads. Two indicators were independently associated with fewer respiratory infections-mothers allowing their hands to air dry after the handwashing demonstration and the presence of water where the respondents usually wash hands after defecation. These rapid handwashing indicators should be considered for inclusion in handwashing assessments.

Evolution of virulence, environmental change, and the threat posed by emerging and chronic diseases

Assessments of future threats posed by infection have focused largely on zoonotic, acute disease, under the rubric "emerging diseases." Evolutionary and epidemiological studies indicate, however, that particular aspects of infrastructure, such as protected water supplies, vector-proof housing, and health care facilities, protect against the emergence of zoonotic, acute infectious diseases. While attention in the global health community has focused on emerging diseases, there has been a concurrent, growing recognition that important chronic diseases, such as cancer, are often caused by infectious agents that are already widespread in human populations. For economically prosperous countries, the immediacy of this threat contrasts with their infrastructural protection from severe acute infectious disease. This reasoning leads to the conclusion that chronic infectious diseases pose a more significant threat to economically prosperous countries than zoonotic, acute infectious diseases. Research efforts directed at threats posed by infection may therefore be more effective overall if increased efforts are directed toward understanding and preventing infectious causes of chronic diseases across the spectrum of economic prosperity, as well as toward specific infrastructural improvements in less prosperous countries to protect against virulent, acute infectious diseases.

Streptococcus pneumoniae is desiccation tolerant and infectious upon rehydration

Streptococcus pneumoniae (pneumococcus) is a frequent colonizer of the nasopharynx and one of the leading causative agents of otitis media, pneumonia, and meningitis. The current literature asserts that S. pneumoniae is transmitted person to person via respiratory droplets; however, environmental surfaces (fomites) have been linked to the spread of other respiratory pathogens. Desiccation tolerance has been to shown to be essential for long-term survival on dry surfaces. This study investigated the survival and infectivity of S. pneumoniae following desiccation under ambient conditions. We recovered viable bacteria after all desiccation periods tested, ranging from 1 h to 4 weeks. Experiments conducted under nutrient limitation indicate that desiccation is a condition separate from starvation. Desiccation of an acapsular mutant and 15 different clinical isolates shows that S. pneumoniae desiccation tolerance is independent of the polysaccharide capsule and is a species-wide phenomenon, respectively. Experiments demonstrating that nondesiccated and desiccated S. pneumoniae strains colonize the nasopharynx at comparable levels, combined with their ability to survive long-term desiccation, suggest that fomites may serve as alternate sources of pneumococcal infection.

Even with the advent of multivalent capsular polysaccharide conjugate vaccines, S. pneumoniae continues to be a major cause of morbidity and mortality worldwide. Every year, there are approximately 7 million cases of pneumococcus-based otitis media in the United States alone, while pneumococcal invasive diseases are responsible for more than 1 million deaths globally. It is believed that the human upper respiratory tract is the sole niche of S. pneumoniae and, thus, that spread occurs via close contact with an infected individual. In this study, we characterized the desiccation tolerance of S. pneumoniae and found that it can survive for many weeks postdehydration and retain infectivity. Our results suggest that desiccation tolerance is an inherent trait of this genetically variable species and that fomites may be a source of transmission.

The curse of the Pharaoh revisited: evolutionary bi-stability in environmentally transmitted pathogens

It is increasingly evident that for a number of high-profile pathogens, transmission involves both direct and environmental pathways. Much of the distinguished evolutionary theory has, however, focused on each of transmission component separately. Herein, we use the framework of adaptive dynamics to study the evolutionary consequences of mixed transmission. We find that environmental transmission can select for increased virulence when direct transmission is low. Increasing the efficiency of direct transmission gives rise to an evolutionary bi-stability, with coexistence of different levels of virulence. We conclude that the overlooked contribution of environmental transmission may explain the curious appearance of high virulence in pathogens that are typically only moderately pathogenic, as observed for avian influenza viruses and cholera.

The rise and fall of the Mycobacterium tuberculosis genome

When studied from the perspective of non-tuberculous mycobacteria (NTM) it is apparent that Mycobacterium tuberculosis has undergone a biphasic evolutionary process involving genome expansion (gene acquisition and duplication) and reductive evolution (deletions). This scheme can instruct descriptive and experimental studies that determine the importance of ancestral events (including horizontal gene transfer) in shaping the present-day pathogen. For example, heterologous complementation in an NTM can test the functional importance of M. tuberculosis-specific genetic insertions. An appreciation of both phases of M. tuberculosis evolution is expected to improve our fundamental understanding of its pathogenicity and facilitate the evaluation of novel diagnostics and vaccines.

A prospective study of the case-notes of MRSA-positive patients: a vehicle of MRSA spread

INTRODUCTION

Methicillin-resistant Staphylococcus aureus (MRSA) has received much publicity and remains a major problem for the health service. The aim of this study was to determine whether case-notes of MRSA-positive patients can act as a vehicle for MRSA transmission.

PATIENTS AND METHODS

A prospective study was performed of patients with active MRSA infection identified from a microbiology department database. Two swabs were taken from the cover of the case-notes and the page with the most recent entry for identification of MRSA using MRSA ID (Chromogenic agar medium), Pastorex and DNase tests. Positive case-notes were re-swabbed 96 h later.

RESULTS

A total of 50 MRSA-positive patients were identified from medical, surgical, high dependency and intensive care wards. Three sets of notes were MRSA positive (3/50; 6%). None of the positive case-notes was positive on re-swabbing at 96 h.

CONCLUSIONS

Case-notes of MRSA-positive patients can act as carriers of MRSA with the potential to spread MRSA to both healthcare and non-healthcare workers without direct contact with infected patients. We recommend extending the universal precautions to the handling of case-notes belonging to MRSA-infected patients.

Parasite replication and the evolutionary epidemiology of parasite virulence

Parasite virulence evolution is shaped by both within-host and population-level processes yet the link between these differing scales of infection is often neglected. Population structure and heterogeneity in both parasites and hosts will affect how hosts are exploited by pathogens and the intensity of infection. Here, it is shown how the degree of relatedness among parasites together with epidemiological parameters such as pathogen yield and longevity influence the evolution of virulence. Furthermore, the role of kin competition and the degree of cheating within highly structured parasite populations also influences parasite fitness and infectivity patterns. Understanding how the effects of within-host processes scale up to affect the epidemiology has importance for understanding host-pathogen interactions.

Interspecies and intraspecies transmission of influenza A viruses: viral, host and environmental factors

Influenza A viruses are enveloped viruses belonging to the familyOrthomyxoviridaethat encompasses four more genera: Influenza B, Influenza C, Isavirus and Thogotovirus. Type A viruses belong to the only genus that is highly infectious to a variety of mammalian and avian species. They are divided into subtypes based on two surface glycoproteins, the hemagglutinin (HA) and neuraminidase (NA). So far, 16 HA and 9 NA subtypes have been identified worldwide, making a possible combination of 144 subtypes between both proteins. Generally, individual viruses are host-specific, however, interspecies transmission of influenza A viruses is not uncommon. All of the HA and NA subtypes have been isolated from wild birds; however, infections in humans and other mammalian species are limited to a few subtypes. The replication of individual influenza A virus in a specific host is dependent on many factors including, viral proteins, host system and environmental conditions. In this review, the key findings that contribute to the transmission of influenza A viruses amongst different species are summarized.

Common garden experiment reveals pathogen isolate but no host genetic diversity effect on the dynamics of an emerging wildlife disease

Host genetic diversity can mediate pathogen resistance within and among populations. Here we test whether the lower prevalence of Mycoplasmal conjunctivitis in native North American house finch populations results from greater resistance to the causative agent, Mycoplasma gallisepticum (MG), than introduced, recently-bottlenecked populations that lack genetic diversity. In a common garden experiment, we challenged wild-caught western (native) and eastern (introduced) North American finches with a representative eastern or western MG isolate. Although introduced finches in our study had lower neutral genetic diversity than native finches, we found no support for a population-level genetic diversity effect on host resistance. Instead we detected strong support for isolate differences: the MG isolate circulating in western house finch populations produced lower virulence, but higher pathogen loads, in both native and introduced hosts. Our results indicate that contemporary differences in host genetic diversity likely do not explain the lower conjunctivitis prevalence in native house finches, but isolate-level differences in virulence may play an important role.

Tuberculosis and leprosy in perspective

Two of humankind's most socially and psychologically devastating diseases, tuberculosis and leprosy, have been the subject of intensive paleopathological research due to their antiquity, a presumed association with human settlement and subsistence patterns, and their propensity to leave characteristic lesions on skeletal and mummified remains. Despite a long history of medical research and the development of effective chemotherapy, these diseases remain global health threats even in the 21st century, and as such, their causative agents Mycobacterium tuberculosis and M. leprae, respectively, have recently been the subject of molecular genetics research. The new genome-level data for several mycobacterial species have informed extensive phylogenetic analyses that call into question previously accepted theories concerning the origins and antiquity of these diseases. Of special note is the fact that all new models are in broad agreement that human TB predated that in other animals, including cattle and other domesticates, and that this disease originated at least 35,000 years ago and probably closer to 2.6 million years ago. In this work, we review current phylogenetic and biogeographic models derived from molecular biology and explore their implications for the global development of TB and leprosy, past and present. In so doing, we also briefly review the skeletal evidence for TB and leprosy, explore the current status of these pathogens, critically consider current methods for identifying ancient mycobacterial DNA, and evaluate coevolutionary models.

Review and comparison between the Wells-Riley and dose-response approaches to risk assessment of infectious respiratory diseases

Infection risk assessment is very useful in understanding the transmission dynamics of infectious diseases and in predicting the risk of these diseases to the public. Quantitative infection risk assessment can provide quantitative analysis of disease transmission and the effectiveness of infection control measures. The Wells-Riley model has been extensively used for quantitative infection risk assessment of respiratory infectious diseases in indoor premises. Some newer studies have also proposed the use of dose-response models for such purpose. This study reviews and compares these two approaches to infection risk assessment of respiratory infectious diseases. The Wells-Riley model allows quick assessment and does not require interspecies extrapolation of infectivity. Dose-response models can consider other disease transmission routes in addition to airborne route and can calculate the infectious source strength of an outbreak in terms of the quantity of the pathogen rather than a hypothetical unit. Spatial distribution of airborne pathogens is one of the most important factors in infection risk assessment of respiratory disease. Respiratory deposition of aerosol induces heterogeneous infectivity of intake pathogens and randomness on the intake dose, which are not being well accounted for in current risk models. Some suggestions for further development of the risk assessment models are proposed.

PRACTICAL IMPLICATIONS

This review article summarizes the strengths and limitations of the Wells-Riley and the dose-response models for risk assessment of respiratory diseases. Even with many efforts by various investigators to develop and modify the risk assessment models, some limitations still persist. This review serves as a reference for further development of infection risk assessment models of respiratory diseases. The Wells-Riley model and dose-response model offer specific advantages. Risk assessors can select the approach that is suitable to their particular conditions to perform risk assessment.

Environmental reservoirs of Vibrio cholerae and their role in cholera

In the aquatic environment, Vibrio cholerae has been reported to be associated with a variety of living organisms, including animals with an exoskeleton of chitin, aquatic plants, protozoa, bivalves, waterbirds, as well as abiotic substrates (e.g. sediments). Most of these are well-known or putative environmental reservoirs for the bacterium, defined as places where the pathogen lives over time, with the potential to be released and to cause human infection. Environmental reservoirs also serve as V. cholerae disseminators and vectors. They can be responsible for the start of an epidemic, may be critical to cholera endemicity, and affect the evolution of pathogen virulence. To date, in addition to the generally recognized role of zooplankton as the largest environmental reservoir for V. cholerae, other environmental reservoirs play some role in cholera epidemiology by favouring persistence of the pathogen during inter-epidemic periods. Little is known about the ecological factors affecting V. cholerae survival in association with aquatic substrates. Studies aimed at these aspects, i.e. understanding how environmental reservoirs interact, are affected by climate, and contribute to disease epidemiology, will be useful for understanding global implications of V. cholerae and the disease cholera.

Bordetella bronchiseptica infection in cats. ABCD guidelines on prevention and management

OVERVIEW

Bordetella bronchiseptica is a Gram-negative bacterium that colonises the respiratory tract of mammals and is considered to be a primary pathogen of domestic cats. It is sensible to consider B bronchiseptica as a rare cause of zoonotic infections. The bacterium is susceptible to common disinfectants.

INFECTION

The bacterium is shed in oral and nasal secretions of infected cats. Dogs with respiratory disease are an infection risk for cats. The microorganism colonises the ciliated epithelium of the respiratory tract of the host, establishing chronic infections.

DISEASE SIGNS

A wide range of respiratory signs has been associated with B bronchiseptica infection, from a mild illness with fever, coughing, sneezing, ocular discharge and lymphadenopathy to severe pneumonia with dyspnoea, cyanosis and death.

DIAGNOSIS

Bacterial culture and PCR lack sensitivity. Samples for isolation can be obtained from the oropharynx (swabs) or via transtracheal wash/ bronchoalveolar lavage.

DISEASE MANAGEMENT

Antibacterial therapy is indicated, even if the signs are mild. Where sensitivity data are unavailable, tetracyclines are recommended. Doxycycline is the antimicrobial of choice. Cats with severe B bronchiseptica infection require supportive therapy and intensive nursing care.

VACCINATION RECOMMENDATIONS

In some European countries an intranasal modified-live virus vaccine is available. The modified-live product is licensed for use as a single vaccination with annual boosters. Cats should not be routinely vaccinated against B bronchiseptica (non-core), since the infection generally causes only a mild disease.

Environmental transmission of low pathogenicity avian influenza viruses and its implications for pathogen invasion

Understanding the transmission dynamics and persistence of avian influenza viruses (AIVs) in the wild is an important scientific and public health challenge because this system represents both a reservoir for recombination and a source of novel, potentially human-pathogenic strains. The current paradigm locates all important transmission events on the nearly direct fecal/oral bird-to-bird pathway. In this article, on the basis of overlooked evidence, we propose that an environmental virus reservoir gives rise to indirect transmission. This transmission mode could play an important epidemiological role. Using a stochastic model, we demonstrate how neglecting environmentally generated transmission chains could underestimate the explosiveness and duration of AIV epidemics. We show the important pathogen invasion implications of this phenomenon: the nonnegligible probability of outbreak even when direct transmission is absent, the long-term infectivity of locations of prior outbreaks, and the role of environmental heterogeneity in risk.

Inactivation of influenza A viruses in the environment and modes of transmission: a critical review

OBJECTIVES

The relative importance of airborne, droplet and contact transmission of influenza A virus and the efficiency of control measures depends among other factors on the inactivation of viruses in different environmental media.

METHODS

We systematically review available information on the environmental inactivation of influenza A viruses and employ information on infectious dose and results from mathematical models to assess transmission modes.

RESULTS

Daily inactivation rate constants differ by several orders of magnitude: on inanimate surfaces and in aerosols daily inactivation rates are in the order of 1-10(2), on hands in the order of 10(3). Influenza virus can survive in aerosols for several hours, on hands for a few minutes. Nasal infectious dose of influenza A is several orders of magnitude larger than airborne infectious dose.

CONCLUSIONS

The airborne route is a potentially important transmission pathway for influenza in indoor environments. The importance of droplet transmission has to be reassessed. Contact transmission can be limited by fast inactivation of influenza virus on hands and is more so than airborne transmission dependent on behavioral parameters. However, the potentially large inocula deposited in the environment through sneezing and the protective effect of nasal mucus on virus survival could make contact transmission a key transmission mode.

Transmission-recovery trade-offs to study parasite evolution

Parasite evolution is mainly studied through a trade-off involving host death (i.e., virulence) and transmission. In addition to the lack of evidence, this trade-off largely fails to understand the evolution of sublethal parasite effects. Here, I argue that considering host recovery as a main selection pressure faced by the parasite helps to address these problems and opens new perspectives for the study of parasite evolution. Using an embedded model, I show how a trade-off between transmission and recovery may emerge from within-host dynamics if immune activation is assumed to depend on the parasite's overall growth rate. I also show that the value of the parasite's optimal growth rate strongly depends on the immunological state of the host. Transmission-recovery trade-offs are of particular interest to the study of the evolution of human pathogens because of the use of antipathogen treatments, which strengthens the recovery constraint.

Surviving the bottleneck: transmission mutants and the evolution of microbial populations

The ability of microbial populations to increase fitness through fixation of mutants with an increased growth rate has been well described. In experimental studies, this is often the only way fitness can be increased. In natural settings, however, fitness can also be improved by increasing the ability of the microbe to transmit from one host to the next. For many pathogens, transmission includes a phase outside the host during which they need to survive before the chance of reinfecting a new host occurs. In such a situation, a reduced death rate during this phase will lead to improved fitness. Here, we compute the fixation probability of mutants that better survive the transmission bottleneck during the evolution of microbial populations. We derive analytical results that show that transmission mutants are often likely to occur and that their importance relative to growth mutants increases as the population decline during the transmission phase increases. We confirm our theoretical results with numerical simulations and suggest specific experiments that can be done to test our predictions.

Long-term persistence of virulent Yersinia pestis in soil

Plague is characterized by geographical foci from which it re-emerges after decades of silence, a fact currently explained by enzootic and epizootic cycles between plague-susceptible and plague-resistant rodents. To assess the potential role of soil in plague epidemiology, we experimentally investigated whether Yersinia pestis could persist alive and virulent in soil. Sterilized soil inoculated with virulent Y. pestis biotype Orientalis was regularly sampled for 40 weeks in duplicate. Each sample was observed by acridine orange staining and immunofluorescence using an anti-Y. pestis polyclonal antibody, and DNA was extracted for PCR amplification and sequencing of the Y. pestis ureD, caf1 and pla genes. All samples were inoculated onto selective agar, and samples from soil that had been incubated for 10, 60, 165, 210 and 280 days were also inoculated into each of two BALB/c female mice. The mouse experiment was performed in triplicate. Non-inoculated, sterilized soil samples were used as negative controls. Micro-organisms fluorescing orange and detected by immunofluorescence were identified as Y. pestis in all samples. They were recovered in pure agar cultures for up to 30 weeks but thereafter were contaminated with Pseudomonas spp. Soil that had been inoculated with Y. pestis proved to be fully virulent in mice, which died with Y. pestis septicaemia and multiple organ involvement. Negative control mice showed no signs of disease. These data indicate that Y. pestis biotype Orientalis can remain viable and fully virulent after 40 weeks in soil. This study is a first step on which to base further investigations of a potential telluric reservoir for Y. pestis, which could represent an alternative mechanism for the maintenance of plague foci.

Research priorities for coordinating management of food safety and water quality

Efforts to exclude disease organisms from farms growing irrigated lettuce and leafy vegetables on California's central coast are conflicting with traditionally accepted strategies to protect surface water quality. To begin resolving this dilemma, over 100 officials, researchers, and industry representatives gathered in April 2007 to set research priorities that could lead to effective co-management of both food safety and water quality. Following the meeting, research priorities were refined and ordered by way of a Delphi process completed by 35 meeting participants. Although water quality and food safety experts conceptualized the issues differently, there were no deep disagreements with respect to research needs. Top priority was given to investigating the fate of pathogens potentially present on farms. Intermediate priorities included characterizing the influence of specific farm management practices on food safety and improving our understanding of vector processes. A scientific subdiscipline focusing on competing risks is needed to characterize and resolve conflicts between human and environmental health.

Evolution of pathogens in a man-made world

Human activities have resulted in substantial, large-scale environmental modifications, especially in the past century. Ecologists and evolutionary biologists are increasingly coming to realize that parasites and pathogens, like free-living organisms, evolve as the consequence of these anthropogenic changes. Although this area now commands the attention of a variety of researchers, a broad predictive framework is lacking, mainly because the links between human activities, the environment and parasite evolution are complex. From empirical and theoretical examples chosen in the literature, we give an overview of the ways in which humans can directly or indirectly influence the evolution of different traits in parasites (e.g. specificity, virulence, polymorphism). We discuss the role of direct and indirect factors as diverse as habitat fragmentation, pollution, biodiversity loss, climate change, introduction of species, use of vaccines and antibiotics, ageing of the population, etc. We also present challenging questions for further research. Understanding the links between anthropogenic changes and parasite evolution needs to become a cornerstone of public health planning, economic development and conservation biology.

Designability and disease

Structural designability is the number of ways it is possible to encode for structure. A protein's designability has been equated with the size of sequence space encoding for the protein's structure, a measure that reflects the structure's robustness to mutation. Current evidence suggests that designability is fundamental to our understanding of the evolvability and distribution of structures in nature and is a significant factor associated with human disease. Here, we describe definitions and principles underlying the concept of designability and discuss its relation to disease.

An ecological and evolutionary perspective on human-microbe mutualism and disease

The microbial communities of humans are characteristic and complex mixtures of microorganisms that have co-evolved with their human hosts. The species that make up these communities vary between hosts as a result of restricted migration of microorganisms between hosts and strong ecological interactions within hosts, as well as host variability in terms of diet, genotype and colonization history. The shared evolutionary fate of humans and their symbiotic bacteria has selected for mutualistic interactions that are essential for human health, and ecological or genetic changes that uncouple this shared fate can result in disease. In this way, looking to ecological and evolutionary principles might provide new strategies for restoring and maintaining human health.

On the origin of smallpox: correlating variola phylogenics with historical smallpox records

Human disease likely attributable to variola virus (VARV), the etiologic agent of smallpox, has been reported in human populations for >2,000 years. VARV is unique among orthopoxviruses in that it is an exclusively human pathogen. Because VARV has a large, slowly evolving DNA genome, we were able to construct a robust phylogeny of VARV by analyzing concatenated single nucleotide polymorphisms (SNPs) from genome sequences of 47 VARV isolates with broad geographic distributions. Our results show two primary VARV clades, which likely diverged from an ancestral African rodent-borne variola-like virus either approximately 16,000 or approximately 68,000 years before present (YBP), depending on which historical records (East Asian or African) are used to calibrate the molecular clock. One primary clade was represented by the Asian VARV major strains, the more clinically severe form of smallpox, which spread from Asia either 400 or 1,600 YBP. Another primary clade included both alastrim minor, a phenotypically mild smallpox described from the American continents, and isolates from West Africa. This clade diverged from an ancestral VARV either 1,400 or 6,300 YBP, and then further diverged into two subclades at least 800 YBP. All of these analyses indicate that the divergence of alastrim and variola major occurred earlier than previously believed.

Outside opportunities and costs incurred by others

Descriptions of interactions between ants and their 'guests' serve to illustrate the thesis that Ewald's theory of the 'evolution of virulence' not only applies to interactions between micro-organisms causing infectious diseases and their hosts, but also to interactions between individuals belonging to differing species. For instance, the prediction is put forward and discussed that guests of army ants are, relative to guests of other species of ants, more often parasitic. A key variable in Ewald's theory is 'transmissibility'. It shows some resemblance to similar variables used in micro-economic theory and in Emerson's sociological Power-Dependence Relations theory. In this article, this variable is called 'outside opportunities'. In an A-B relation, an outside opportunity for A is anything which constitutes an alternative to what B can provide. It is concluded that in A-B interactions, the more outside opportunities are available to A, the more costs are incurred by B. Differences and similarities between this idea and Game Theory are discussed.

Molecular basis for a lack of correlation between viral fitness and cell killing capacity

The relationship between parasite fitness and virulence has been the object of experimental and theoretical studies often with conflicting conclusions. Here, we provide direct experimental evidence that viral fitness and virulence, both measured in the same biological environment provided by host cells in culture, can be two unrelated traits. A biological clone of foot-and-mouth disease virus acquired high fitness and virulence (cell killing capacity) upon large population passages in cell culture. However, subsequent plaque-to-plaque transfers resulted in profound fitness loss, but only a minimal decrease of virulence. While fitness-decreasing mutations have been mapped throughout the genome, virulence determinants-studied here with mutant and chimeric viruses-were multigenic, but concentrated on some genomic regions. Therefore, we propose a model in which viral virulence is more robust to mutation than viral fitness. As a consequence, depending on the passage regime, viral fitness and virulence can follow different evolutionary trajectories. This lack of correlation is relevant to current models of attenuation and virulence in that virus de-adaptation need not entail a decrease of virulence.

Survival of epidemic strains of nosocomial- and community-acquired methicillin-resistant Staphylococcus aureus on coins

BACKGROUND

The aim of this study was to examine the survival dynamics of several epidemic nosocomial and community-acquired methicillin-resistant Staphylococcus aureus (MRSA) strains on copper alloy coins.

METHODS

Six different phage types of nosocomial MRSA (Irish 1, Irish 2, EMRSA 15, EMRSA 16, distinct type, and untypeable), as well as community-acquired MRSA, were examined in this study. Two isolates of each type were studied, resulting in a total of 14 organisms being examined. Cells were harvested from overnight cultures of Columbia blood agar (Oxoid) supplemented with 5% (wt/vol) defibrinated blood to make a 0.5 McFarland inoculum standard. An inoculum of each MRSA isolate in 5-microl volume was added to washed, dried, and presterilized 1-penny copper-plated steel coins, equating to log(10) 5 colony-forming units (cfu) and allowed to dry naturally at ambient temperatures in the dark.

RESULTS

Recovery experiments were unable to isolate any of the inoculated organisms 4 hours postinoculation. To ascertain whether this was a toxic effect from the copper alloy of the coins or a physical desiccation effect, experiments were repeated on glass and plastic, and similar results were demonstrated. The effect of soil was investigated by repeating the experiment with (60)Co irradiated pus and sterile blood, and we were able to demonstrate the survival of all organisms after at least 2 weeks storage in the dark at ambient temperature, during which the quantitative counts were reduced by approximately 1-log unit and 2-log units for blood and pus, respectively.

CONCLUSION

This study demonstrates that all epidemic nosocomial- and community-acquired MRSA do not survive when no organic protection is offered but survive well when soil (pus and blood) is present, thus offering protection from drying. This study indicates that contaminated coins may serve as potential vehicles for MRSA.

The importance of parasite life history and host density in predicting the impact of infections in red deer

We studied a macroparasite (Elaphostrongylus cervi, Nematoda) and a microparasite infection (tuberculosis, TB) in red deer (Cervus elaphus) across different populations where managers manipulated host condition, density and aggregation by providing supplemental food. We aimed to test whether and, if so, how persistence and transmission of both parasites differentially varied as host body condition and population density varied. We took account of sex, as red deer life history greatly concerns sex-related traits. Changes in host factors had different consequences for the spread of each parasite type. Individual presence of tuberculosis was positively associated with host density, whereas E. cervi abundance negatively related to host density and enhanced body condition. There was lack of body condition density dependence; and body condition was mainly dependent on the amount of supplemental food provided, but also on habitat quality descriptors. Overall, our results suggest that body condition was improved at the cost of increased host contact rates, which implied an ecological trade-off between acquiring resources to cope with E. cervi, a macroparasite, and concurrent exposure to mycobacteria. By the simultaneous study of both infections, this research suggests that the effects of changes in host number and population structure on disease spread and persistence need to take into account variation in life histories of the parasites. These findings also raise concern about the ecological consequences of diseases and management of wildlife on host life history.