21 results in Microbe-vector Interactions in Vector-borne Diseases
5 - Specificity of Borrelia–tick vector relationships
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- By Alan Barbour, Departments of Microbiology and Molecular Genetics and Medicine, University of California Irvine, B240 Medical Science I, Irvine, CA 92697–4025, USA
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Summary
BORRELIA
The genus Borrelia comprises diverse species of spirochaetes that are associated with haematophagous arthropods (Paster et al., 1991; Paster & Dewhirst, 2000). Some Borrelia species are pathogenic for humans or for livestock. Other spirochaete groups with human pathogens are the treponemes, which include the human-restricted agent of syphilis, and the leptospires, which are mostly free-living spirochaetes that infect a wide variety of animals. The spirochaete phylum also contains a number of species that are symbionts of invertebrates, such as molluscs and termites. Borrelia spirochaetes characteristically circulate in the blood of their vertebrate hosts and are transmitted between vertebrates by ticks, with the single, epidemiologically important exception of a louse-borne species. A common strategy of Borrelia spp. for prolonging spirochaetaemia – thus increasing the probability of vector transmission – is avoidance of the immune response through antigenic variation (Barbour & Restrepo, 2000; Barbour, 2002). Most types of Borrelia infections are zoonoses, but humans are the critical reservoirs for at least one species (Barbour & Hayes, 1986; Barbour, 2004).
The number of recognized Borrelia species has more than doubled over the last two decades, in part because cultivation methods improved (Barbour, 1988; Cutler et al., 1994) and technologies like PCR allowed identification and taxonomic classification without being able to culture the organism (Anda et al., 1996; Barbour et al., 1996; Kisinza et al., 2003; Scoles et al., 2001). Table 1 is a list of accepted and tentative species designations, as of early 2004. Borrelia species have been documented in the Palaearctic, Afro-Tropical, Nearctic, Neotropical and Antarctic ecological regions, and some species that use humans or livestock as reservoirs are now cosmopolitan.
Contributors
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9 - Vector competence
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- By Scott C. Weaver, Center for Biodefense and Emerging Infectious Diseases and Department of Pathology, University of Texas Medical Branch, Galveston, TX 77555–0609, USA, Lark L. Coffey, Center for Biodefense and Emerging Infectious Diseases and Department of Pathology, University of Texas Medical Branch, Galveston, TX 77555–0609, USA, Roberto Nussenzveig, Center for Biodefense and Emerging Infectious Diseases and Department of Pathology, University of Texas Medical Branch, Galveston, TX 77555–0609, USA, Diana Ortiz, Center for Biodefense and Emerging Infectious Diseases and Department of Pathology, University of Texas Medical Branch, Galveston, TX 77555–0609, USA, Darci Smith, Center for Biodefense and Emerging Infectious Diseases and Department of Pathology, University of Texas Medical Branch, Galveston, TX 77555–0609, USA
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INTRODUCTION TO THE CONCEPT OF VECTOR COMPETENCE
The importance of vector-borne diseases
Infectious diseases remain the leading causes of morbidity and mortality worldwide and arthropod-borne diseases include many of the most important, especially in the tropics and developing countries. For example, malarial parasites infect an estimated 200 million people annually in Africa alone with mortality estimates of about 1 million persons, primarily children (Greenwood, 1999). Vector-borne viruses are also important human pathogens. Approximately 2500 million people (two-fifths of the world's population) are at risk from dengue with about 50 million cases each year (www.who.int/inf-fs/ en/fact117.html). Many other vector-borne pathogens cause emerging diseases that have undergone resurgence or threaten to increase in prevalence or distribution in the coming years. However, despite the importance of vector-borne diseases, the mechanisms of transmission of many vector-borne pathogens remain poorly understood.
Here, we review the various factors that contribute to the transmission of pathogens by arthropod vectors. Due to space limitations, we focus primarily on human disease, with emphasis on mosquito-borne pathogens, especially arthropod-borne viruses (arboviruses).
Mechanisms of pathogen transmission by arthropod vectors
Vector-borne pathogens of humans and other animals circulate between their arthropod vectors and vertebrate hosts. Arboviruses represent the most basic form of vector-borne transmission, and undergo horizontal transmission between vertebrate and arthropod hosts (Fig. 1). In some cases they are maintained by vertical transmission from adult arthropod to offspring. Usually a restricted invertebrate host range is observed with only one or few vectors involved in transmission. Within the infected arthropod, the virus must undergo an extrinsic incubation period of variable duration before biological transmission to a susceptible vertebrate host (Fig. 2).
Index
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7 - How do mosquito vectors live with their viruses?
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- By Stephen Higgs, Department of Pathology, University of Texas Medical Branch, Galveston, TX 77555–0609, USA
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INTRODUCTION
The word vector is derived from the Latin and means ‘bearer’ or ‘one who does something’, but for the purposes of this chapter, we can use the term as defined by Gordh & Headrick (2001) as an arthropod that carries disease-producing organisms to a vertebrate host. A wide variety of arthropods are involved as vectors in pathogen transmission cycles but the focus of this chapter is the mosquito. To date, there are at least 537 named arthropod-borne viruses (arboviruses) (Karabatsos, 1985), encompassing species from many genera, examples of which are given in Table 1. With one exception (African swine fever virus, transmitted by ticks) all of the viruses regarded as true arboviruses have an RNA genome. The vast majority of arboviruses (e.g. Togaviridae, Flaviviridae, Bunyaviridae) are enveloped, i.e. the nucleocapsid is surrounded by a lipid-containing envelope. A few (e.g. Reoviridae) are non-enveloped. Most of these (e.g. bluetongue virus and Colorado tick fever virus) are transmitted by midges and ticks. Fewer than 20 have been isolated from mosquitoes and only lebombo and Orungo have been associated with human infections. As discussed by Mitchell (1983), because arboviruses are typically not pathogenic to their insect hosts, it has been hypothesized that they may have evolved in insects and later became infectious to vertebrates. Tick-borne arboviruses (see review by Nuttall & Labuda, 2003) and the true insect viruses (e.g. nuclear polyhedrosis viruses) that infect and replicate in mosquitoes, but are not transmitted to vertebrate hosts (Miller & Ball, 1998), are not considered in this chapter. Given that almost all mosquito-borne viruses are enveloped and have an RNA genome, unless specified otherwise, this review therefore focuses on this predominant group of arboviruses.
10 - Vector immunity
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- By Norman A. Ratcliffe, School of Biological Sciences, University of Wales Swansea, Singleton Park, Swansea, UK, Miranda M. A. Whitten, School of Biological Sciences, University of Wales Swansea, Singleton Park, Swansea, UK
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INTRODUCTION
Invertebrates, particularly insects, act as vectors of the most debilitating diseases in many already socially and economically compromised populations of the developing world. The diseases vectored include malaria, sleeping sickness, Chagas’ disease, leishmaniasis, lymphatic filariases and river blindness, dengue and yellow fever, and schistosomiasis. Only the latter is transmitted by non-insectan invertebrates, i.e. snails of the genus Biomphalaria. Hurd (2003) recently pointed out that of the ten most important tropical diseases affecting the poorer nations, eight of these are transmitted by invertebrate vectors (Table 1). In addition, and often underestimated in importance since they are probably second only to mosquitoes as vectors of human infectious diseases (Parola & Raoult, 2001), are the ticks. In fact, in the USA ticks transmit more vector-borne diseases than any other vector (US Centers for Disease Control and Prevention, 1999). The diseases transmitted by ticks include Lyme borreliosis, tick-borne encephalitis, ehrlichiosis and babesiosis (Gratz, 1999).
The reason that insects are particularly widespread vectors undoubtedly reflects the success of this group, occupying almost every habitat on earth in vast numbers. In addition, their power of flight, their extraordinarily well developed sense organs and their haematophagous habit have made them ideal vehicles for transmitting human blood-borne diseases. Since many insects live in places infested with pathogens and parasites they can only do so due to the extreme efficiency of their immune defences so that any invading parasite must counteract these defences to survive.
Thus, most parasites are not simply passively transported from human to human by their vectors, but interact intimately with their invertebrate hosts, usually undergoing significant biochemical and molecular modifications to survive, differentiate and multiply in their vectors.
16 - Vaccines targeting vectors
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- By Geoffrey A. T. Targett, Department of Infectious and Tropical Diseases, London School of Hygiene & Tropical Medicine, London WC1B 3DP, UK
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INTRODUCTION
It has been known for a long time that arthropods requiring blood meals induce pronounced immune responses in the host on which they feed (Trager, 1939). These, together with the wide range of haemostatic mechanisms that are employed by the host to avoid blood loss, present a formidable barrier to the arthropod' essential need for regular blood meals in order to moult or produce eggs.
The immune responses first recognized were in those vector–host interactions where the arthropod remains attached to its host for a long period – days or often weeks. This is true of many ticks. The much cited work of William Trager (1939) is recognized as one of the seminal studies on anti-tick resistance and, during the past 60 years, a great deal has been learned about the numerous underlying mechanisms.
It is not surprising that such a lengthy and intimate association leads to cellular and humoral responses to vector antigens. However, it is now equally clear that such responses can also develop to haematophagous arthropods that feed rapidly and then leave the host. Notable here are insects of medical importance such as mosquitoes, sandflies and black flies.
Extensive studies have shown that, in both the slow-feeding (tick) and rapid-feeding (insect) arthropods, there is an array of immunomodulatory factors to counter the induction of immune responses, to deflect the immunity away from mechanisms that are harmful to the arthropod, or to lessen the effect of these if they do occur.
2 - Evolution of tick-borne disease systems
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- By Sarah E. Randolph, Department of Zoology, South Parks Road, Oxford OX1 3PS, UK
- Edited by S. H. Gillespie, University College London, G. L. Smith, Imperial College of Science, Technology and Medicine, London, A. Osbourn
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INTRODUCTION – BIOLOGICAL CONSTRAINTS ON PATHOGENS’ USE OF TICKS AS VECTORS
All vector-borne disease systems involve a triangle of dynamic reciprocal interactions between vector, host and pathogen, which is subject to two major classes of constraints. The intrinsic physical, physiological, cellular barriers determine whether the route of transmission is possible. The pathogen must be adapted to invade, establish and survive in vertebrate and invertebrate tissues alternately, and is also at the mercy of the vector–host interaction upon which depends the passage of fluids (typically vertebrate blood and invertebrate saliva) as a vehicle for infective particles. Transmission is not simply a mechanical matter, but rather a biological process whose complexity at the cellular and molecular level is still the focus of much research aimed at developing blocking agents. These biological barriers, which constitute the innate immunity of host and vector, are not absolute, but become more or less leaky through slow evolutionary change. By comparison, the second class of constraints, ecological and epidemiological hurdles, may change much more rapidly. They depend on the quantitative balance between all the factors determining the rates of transmission: transmission may be biologically possible but may not always occur effectively enough to allow persistent cycles of transmission. Such hurdles are determined to a large extent by the impact of extrinsic multifactorial environmental factors on the vector–host–pathogen triangle, and also by the impact of intrinsic factors such as host acquired immunity.
In this essay, I suggest that the evolution of tick-borne pathogens is driven by intrinsic biological barriers, but is directed and constrained by extrinsic environmental factors.
Contents
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1 - Vector-borne diseases
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- By Brian W. J. Mahy, National Center for Infectious Diseases, Centers for Disease Control and Prevention, Atlanta, GA 30333, USA
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INTRODUCTION
The first discovery of human disease transmission by a vector species was in 1877, when Patrick Manson (who later founded the Royal Society of Tropical Medicine and Hygiene in London) was able to show that transmission of the nematode worms causing lymphatic filariasis involved haematophagous mosquitoes (Manson, 1878). Since that time, hundreds of vector-borne diseases have been described, usually involving arthropod vectors which may transmit not only helminths but also protozoa, bacteria or viruses to cause major epidemics of diseases such as malaria, plague, trypanosomiasis, leishmaniasis, louse-borne typhus, dengue fever, yellow fever, Japanese encephalitis and West Nile fever. The numbers of people who are affected by vector-borne diseases in the world belies the imagination. For example, 750 million people in 76 countries live in areas of endemic filariasis, and an estimated 118 million of them are infected with filariae (World Health Organization, 1995). In India alone, 50000–100000 people die each year from visceral leishmaniasis. Malaria, the most important protozoal disease infecting humans, occurs in areas where Anopheles mosquitoes are present, and causes some 300–500 million clinical cases and approximately 1·2 million deaths worldwide each year. Malaria is the commonest vector-borne disease imported into the USA, and vector-competent Anopheles mosquitoes exist there.
Apart from yellow fever, which can now be controlled effectively by vaccination, most prevention and control programmes have been based on control of the arthropod vector, but a number of factors such as resistance to insecticides and drugs have combined to cause a resurgence of many vector-borne diseases since the 1970s (Gubler, 1998).
This chapter provides an overview of the most important human diseases that are transmitted by an arthropod vector.
Frontmatter
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6 - Bunyavirus/mosquito interactions
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- By Richard M. Elliott, Division of Virology, Institute of Biomedical and Life Sciences, University of Glasgow, Church Street, Glasgow G11 5JR, UK, Alain Kohl, Division of Virology, Institute of Biomedical and Life Sciences, University of Glasgow, Church Street, Glasgow G11 5JR, UK
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INTRODUCTION
Bunyaviruses comprise the largest family of arthropod-transmitted viruses. Of the 300 or so viruses in the family Bunyaviridae more than half are transmitted by mosquitoes (Calisher, 1996). A number of these viruses cause disease in man or animals, although thus far many have not been associated with human illness. The family is classified into five genera, Orthobunyavirus, Hantavirus, Nairovirus, Phlebovirus and Tospovirus, and each genus is associated with a principal arthropod vector, except the hantaviruses, which have no arthropod involvement in their life cycle. Tospoviruses are transmitted by thrips to plants, nairoviruses primarily by ticks, and orthobunyaviruses predominantly by mosquitoes. The Phlebovirus genus contains viruses transmitted by phlebotomine flies, ticks and, notably for Rift Valley fever virus, mosquitoes as well. This chapter will deal with the interactions between orthobunyaviruses and mosquitoes.
BUNYAVIRUS CHARACTERISTICS
All bunyaviruses have a tri-segmented single-stranded RNA genome of negative-sense (or a variant of negative-sense, termed ambisense) polarity. The genomic RNA segments are called L (large), M (medium) and S (small). All family members encode four structural proteins – two glycoproteins called Gn and Gc according to their position within the primary gene product, a nucleoprotein, N, that encapsidates the genomic (and antigenomic) RNA segments and an RNA-dependent RNA polymerase, called L protein. The pattern of sizes of the viral proteins and the RNAs is conserved within a genus (Elliott et al., 2000). Viruses in some genera also encode non-structural proteins on their M and/or S RNA segments, termed NSm and NSs, respectively.
13 - Pathogenic strategies of Anaplasma phagocytophilum, a unique bacterium that colonizes neutrophils
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- By Jason A. Carlyon, Section of Rheumatology, Department of Internal Medicine, Yale University School of Medicine, The Anylan Center for Medical Research and Education, New Haven, CT 06520-8031, USA, Erol Fikrig, Section of Rheumatology, Department of Internal Medicine, Yale University School of Medicine, The Anylan Center for Medical Research and Education, New Haven, CT 06520-8031, USA
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INTRODUCTION
Human granulocytic ehrlichiosis (HGE) is an emerging zoonosis caused by Anaplasma phagocytophilum, an obligate intracellular bacterium with a unique tropism for neutrophils (Dumler et al., 2001; Chen et al., 1994). Since documentation of the first human case in 1994, HGE has been increasingly recognized in the United States (US) and Europe. Cases of HGE occur in areas endemic for ticks of the Ixodes persulcatus complex, which are vectors for the disease. In the US, these areas comprise the Northeast, the upper Midwest and northern California (Dumler & Bakken, 1998; Richter et al., 1996; Telford et al., 1996; Pancholi et al., 1995; Chen et al., 1994). Cases of or serological evidence for HGE have been documented in the following European countries – Slovenia, the Netherlands, Sweden, Norway, Spain, Croatia, Poland (Blanco & Oteo, 2002), the Czech Republic (Hulinska et al., 2002), Belgium (Guillaume et al., 2002), Greece (Daniel et al., 2002), Germany (Fingerle et al., 1999), Bulgaria (Christova & Dumler, 1999), Denmark (Lebech et al., 1998) and Switzerland (Pusterla et al., 1998). The annual incidence of HGE in these areas ranges from 2.3 to 16.1 cases per 100 000 persons (McQuiston et al., 1999). In one study, however, 71 of 475 Wisconsin resident serum samples contained antibodies against the HGE agent, which suggests that the annual incidence in this area may be considerably higher (Bakken et al., 1998). In nature, A. phagocytophilum cycles between its tick vector and mammalian hosts, the primary of which are the white-footed mouse, Peromyscus leucopus, and the whitetailed deer, Odocoileus virginianus. Humans are accidental hosts (Dumler & Bakken, 1998; Ogden et al., 1998).
4 - RNA-based immunity in insects
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- By Rui Lu, Center for Plant Cell Biology, Department of Plant Pathology, University of California, Riverside, CA 92521, USA, Hongwei Li, Center for Plant Cell Biology, Department of Plant Pathology, University of California, Riverside, CA 92521, USA, Wan-Xiang Li, Center for Plant Cell Biology, Department of Plant Pathology, University of California, Riverside, CA 92521, USA, Shou-Wei Ding, Center for Plant Cell Biology, Department of Plant Pathology, University of California, Riverside, CA 92521, USA
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INTRODUCTION
Drosophila has been an excellent model for the mechanistic studies of innate immunity (Hoffmann, 2003). Recently, a new RNA-based antiviral immunity with features of both innate and adaptive immunities has been described in Drosophila and Anopheles cells (Li et al., 2002, 2004). This RNA-silencing-mediated immunity is characterized by the production of pathogen-derived, 22-nt small RNAs that serve as specificity determinants inside a multi-subunit complex. Similar to innate immunity, however, the new invertebrate antiviral response is capable of a rapid virus clearance in the absence of a virus-encoded suppressor of RNA silencing. The discovery of a new antiviral pathway in insects opens up the possibility of using this pathway to prevent transmission of vector-borne virus pathogens such as dengue and West Nile viruses.
THE RNA-SILENCING PATHWAY
Homology-dependent gene silencing was discovered in transgenic plants in a form of co-suppression between introduced transgenes or between a transgene and its homologous endogenous gene (Matzke et al., 1989; Napoli et al., 1990; Van der Krol et al., 1990). Similar gene-silencing phenomena have subsequently been described in a wide range of eukaryotic organisms such as fungi, worms, flies and mammals (Denli & Hannon, 2003; Fire et al., 1998). A generic term, RNA silencing (Ding, 2000), has been used to describe these related RNA-guided gene regulatory mechanisms variously termed post-transcriptional gene silencing (PTGS) in plants, quelling in fungi and RNA interference (RNAi) in animals.
A core feature of RNA silencing detected in all organisms is the production of 21–26-nt small RNAs from structured or double-stranded RNA (dsRNA) by the endoribonuclease Dicer (Bernstein et al., 2001; Hamilton et al., 2002; Hamilton & Baulcombe, 1999; Hammond et al., 2000; Zamore et al., 2000).
Editors' Preface
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Vector-borne diseases are a major threat to public health throughout the world. They include major human diseases such as malaria, trypanosomiasis and dengue, and others that principally affect animal populations such as theileriosis and West Nile fever. The study of vector-borne diseases has gone through a period when there was little impetus for research. However, through programmes such as the ‘Great Neglected Diseases Network’ and the Tropical Diseases Research Programme of WHO in the 1980s scientific attention was redirected to vector-borne diseases. More recently initiatives such as the ‘Roll Back Malaria’ programme have moved the subject on further.
In industrialized countries vector-control programmes have eradicated the important vector-borne diseases although increasing international travel has meant that diseases acquired in the tropics are no longer rare in developed world clinical practice. Climate change, industrialization, changing land use and increasing population have all had a profound impact on vector-borne diseases, resulting in epidemics spread. The recent appearance of West Nile virus in North America and Nipah virus in Malaysia has been responsible for dramatic epidemics in animal and human populations with considerable numbers of human deaths. These epidemics have caught the attention of the public.
Vector-borne infections require an extraordinary integration of host and pathogen. Unravelling these interrelationships is a challenge for biologists. The subtle ways in which parasites control their vectors, and the filaria that contain Wolbachia spp., essential to survival and fertility, are examples of elegant and effective co-evolution.
This symposium volume brings together internationally recognized experts to explore the complexity of host–pathogen–vector interactions.
3 - Insect transmission of viruses
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- By Stéphane Blanc, UMR Biologie et Génétique des Interactions Plante-Parasite (BGPI), CIRAD-INRA-ENSAM, TA 41/K, Campus International de Baillarguet, 34398 Montpellier Cedex 5, France
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INTRODUCTION
Obligate parasites all rely on their host for survival. Transfer from one host to another is thus a key aspect of their life cycle, ensuring both their maintenance (vertical and horizontal transmission) and spread (primarily horizontal transmission) in the environment. Vertical transmission, i.e. the passage of a parasite directly from one host to its descendant(s), is a strategy that does not involve passage outside the host, and so maintains the parasite in an amiable environment. However, the success of this transmission strategy depends on the survival of the host line. Furthermore, it restricts parasite spread within the host population. In fact, the survival of the parasite population is directly related to host fitness. Thus vertical transmission may be expected to lead to parasite populations with lower virulence or to non-pathogenic interactions (Frank, 1996; Day, 2001; Weiss, 2002).
All pathogens have evolved additional (or even exclusive) strategies for horizontal transmission, with an impressive diversity of mechanisms that will be the focus of the present chapter. Pathogens are found among viruses, bacteria, fungi, protozoa and metazoa but the tremendous richness of transmission strategies mentioned above is certainly best illustrated by viruses. Examples of horizontal transmission of organisms other than viruses are described in several sections of this symposium volume. Some viruses have, like other pathogens, developed the ability to transfer from infected to healthy hosts by contact or through passage in the external medium, via a variety of propagating forms (Kuno, 2001). Most viruses, however, exploit an additional organism, itself travelling from host to host, as a transportation device (referred to as a vector).
12 - Wolbachia host–symbiont interactions
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- By Mark J. Taylor, Filariasis Research Laboratory, Molecular and Biochemical Parasitology, Liverpool School of Tropical Medicine, Pembroke Place, Liverpool L3 5QA, UK
- Edited by S. H. Gillespie, University College London, G. L. Smith, Imperial College of Science, Technology and Medicine, London, A. Osbourn
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INTRODUCTION
Wolbachia are obligate intracellular bacteria that infect a wide range of invertebrate hosts (O'Neill et al., 1997). As a genus they are most closely related to the arthropodtransmitted bacteria Ehrlichia within the Rickettsiaceae (Fig. 1a). Wolbachia are widespread throughout insects and also infect crustaceans, mites and filarial nematodes. The extent of their distribution is still not well understood but they are already among the most abundant and widespread bacterial symbionts so far described. Wolbachia have evolved a diversity of associations with their hosts, ranging from being pathogens and reproductive parasites in arthropods to mutualists in filarial nematodes. A common target of Wolbachia is the host's reproductive system, which is manipulated to enhance the spread of the symbiont throughout populations. In arthropods, the bacteria have developed a bewildering repertoire of strategies with which to manipulate host reproduction. They can affect the fertility of hosts by controlling compatibility between infected males and uninfected females (cytoplasmic incompatibility), alter sex ratios by killing males or by inducing asexual production of female offspring (parthenogenesis) and even turn males into females (feminization). Not all associations, however, are weighted in favour of the bacteria. In a species of parasitic wasp which feeds on Drosophila, egg production is dependent on Wolbachia (Dedeine et al., 2001), and in filarial nematodes the symbionts are strict mutualists, essential for larval and embryo development and the survival of adult worms (Taylor, 2002). The diversity of association and phenotype in Wolbachia provides an excellent opportunity to study the nature, mechanisms and evolution of these different host–symbiont interactions.
9 - Environmental influences on arbovirus infections and vectors
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- By P. S. Mellor, Institute for Animal Health, Pirbright Laboratory, Ash Rd, Pirbright, Woking GU24 0NF, UK
- Edited by S. H. Gillespie, University College London, G. L. Smith, Imperial College of Science, Technology and Medicine, London, A. Osbourn
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INTRODUCTION
In 430 BC, Hippocrates said ‘Whoever would study medicine aright must learn of the following subjects. First, he must consider the effect of the seasons of the year and the differences between them. Second, he must study the warm and cold winds, both those which are common to every country and those peculiar to a particular locality.’
From this we can see that the influence of climate and environmental variables on infectious diseases has been a topic of interest to medicine for over 2000 years. Recently there has been an increase in this interest spurred on by the various climate change scenarios that have been put forward and highlighted by the fact that 9 of the 10 warmest years on record have occurred since 1990, thereby supporting the concept of ongoing climate change. Furthermore, recent climate models project an increase in global mean temperature of between 1.4 and 5.8 °C during the 21st century (Intergovernmental Panel on Climate Change, 2001). It is further predicted that maximum warming will occur at high latitudes and during the winter, and that nighttime temperatures will increase more than daytime temperatures (Karl et al., 1993; Kukla & Karl, 1993; McMichael et al., 1996).
In the context of vector-borne diseases, these predicted temperature increases, and accompanying climatic changes in rainfall patterns and extreme weather events (for example increased windiness), are likely to have a variable but significant effect. By definition, vector-borne diseases possess a vector stage, usually an insect, tick, crustacean or mollusc, which is poikilothermic and therefore peculiarly liable to be profoundly influenced by climatic variables, especially temperature.
15 - Transgenic malaria
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- By Peter W. Atkinson, Department of Entomology, University of California, Riverside, CA 92521, USA, David A. O'Brochta, Center for Biosystems Research, University of Maryland Biotechnology Institute, College Park, 2MD 20742, USA
- Edited by S. H. Gillespie, University College London, G. L. Smith, Imperial College of Science, Technology and Medicine, London, A. Osbourn
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- Book:
- Microbe-vector Interactions in Vector-borne Diseases
- Published online:
- 06 July 2010
- Print publication:
- 06 May 2004, pp 345-362
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Summary
INTRODUCTION
The life cycle of the causative agent of human malaria, the protozoan Plasmodium, involves transmission through both humans and the mosquito vector. Malaria can therefore be tackled on three distinct biological fronts. The most explored is the development of vaccines or drugs that kill the parasite during its passage through the human host. Drugs such as chloroquine have been used with considerable success; however, the emergence of resistance to these drugs has compromised their effectiveness in many regions of the world. Considerable efforts have been directed towards the development of vaccines for malaria; however, the ability of the parasite to evade the human acquired immune response has so far frustrated these attempts. The publication of the complete genomic sequence of Plasmodium falciparum (Gardner et al., 2002), the major malaria pathogen in sub-Saharan Africa, has rejuvenated efforts to develop drugs that will specifically target the parasite while minimizing side effects on the human host.
Development of malaria control or eradication strategies based on mosquito vector control has traditionally involved the use of chemical insecticides with or without elimination or reduction of excess standing water. These have been successful in terms of reducing or eliminating malaria from specific geographical regions. Regions of Europe, Asia, the Americas and Australia have been rendered malaria-free as a consequence of these area-wide strategies. The effectiveness of this approach has, however, diminished with the cessation of the application of DDT, the rapid emergence of resistance to insecticides amongst mosquito populations, and the overall reluctance of communities to accept the spraying of any chemical insecticide.
14 - Interactions of Yersinia pestis with its flea vector that lead to the transmission of plague
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- By B. Joseph Hinnebusch, Laboratory of Human Bacterial Pathogenesis, Rocky Mountain Laboratories, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Hamilton, MT 59840, USA
- Edited by S. H. Gillespie, University College London, G. L. Smith, Imperial College of Science, Technology and Medicine, London, A. Osbourn
-
- Book:
- Microbe-vector Interactions in Vector-borne Diseases
- Published online:
- 06 July 2010
- Print publication:
- 06 May 2004, pp 331-344
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Summary
YERSINIA PESTIS, A NEWLY EMERGED ARTHROPOD-BORNE AGENT
Bacterial pathogens transmitted by blood-feeding arthropods present serious public health problems worldwide and new ones continue to be recognized, for example the agents of Lyme disease, ehrlichiosis and cat scratch disease. Arthropod-borne transmission is relatively rare among the prokaryotes, but has evolved independently in a phylogenetically diverse group of eubacteria, including the rickettsiae, spirochaetes in the genus Borrelia, and the Gram-negative or proteobacteria Yersinia pestis and Francisella tularensis.
Y. pestis, the agent of plague, produces a cyclic infection of rodents and their fleas in many parts of the world. Maintenance of Y. pestis in nature depends on these rodent–flea–rodent transmission cycles, although it can also be transmitted in some cases by aerosol, direct contact or ingestion (Poland & Barnes, 1979). The disease produces periodic eruptive, rapidly spreading epizootics among certain highly susceptible rodents, followed by regression to focal areas (Barnes, 1982). Historic human plague pandemics exhibit this same pattern. Other species of wild rodents are more resistant to overt disease, and these species are thought to constitute ecologically important reservoir hosts (Kartman et al., 1958). Human plague remains an international public health concern, and recent outbreaks in India and parts of Africa, where the disease had been dormant for decades, suggest a resurgence (Chanteau et al., 1998).
More troubling is the isolation of multi-drug-resistant strains of Y. pestis during the current epidemic in Madagascar (Galimand et al., 1997). These considerations, and the recognized potential of Y. pestis as a bioterrorism agent, underline the need for a detailed understanding of plague transmission and pathogenicity, which hopefully will bring about new effective vaccines and other control measures.
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