Dengue is a mosquito-borne disease caused by four serotypes of dengue virus (DENV1–DENV4) and is currently the most common arbovirus (arthropod-transmitted) disease worldwide. Primary infection with any of the four DV serotypes typically results in dengue fever (DF), a relatively mild influenza-like illness which subsequently provides lifelong immunity to the infecting strain. However, the bad news is that secondary infection with different DV serotype is associated with an increased risk of developing more serious conditions such as dengue haemorrhagic fever (DHF) and the life-threatening dengue shock syndrome (DSS).
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The first well documented outbreaks of dengue occurred in the eighteenth century, although the disease may have been around in China eight hundred years earlier. Dengue virus was first isolated by Japanese and American scientists during World War II. Dengue is now a major public health problem, with approximately 50 million people infected each year (of whom around 20,000 die) and nearly half the world’s population, about 3.5 billion people, at risk of infection. Unfortunately, no dengue virus-specific therapies or vaccines are currently available. The incidence of dengue infection has increased dramatically in the past 50 years. This is due in part to population growth and urbanization in tropical and subtropical countries. Originally found in the jungles and rural areas of Southeast Asia, dengue virus is now maintained primarily in an urban cycle involving human hosts and Aedes aegypti and A. albopictus mosquitoes. Urban areas frequently contain many breeding sites for the mosquitoes that transmit the virus, such as rain-filled old tyres. Successful mosquito control has also been problematic. Dengue viruses have evolved rapidly as they have spread worldwide, and genotypes associated with increased virulence have expanded from South and Southeast Asia into the Pacific and the Americas.
The pathogenesis of dengue haemorrhagic fever and dengue shock syndrome remain unclear. The requirement for a second infection with a different serotype of the virus suggested that antibody-dependent enhancement is involved in these more serious conditions. After an initial period of protection, antibodies from the primary infection can cross-react with other dengue virus serotypes but have waned to non-neutralizing levels. These non-neutralizing antibodies could then mediate an increased uptake of virus into monocyte/macrophage cells via Fc receptors, leading to increased virus replication and immune activation including massive cytokine release (known as a “cytokine storm”). An alternative theory involves reactivation of cross-reactive memory T cells specific for the previous rather than the current virus strain, resulting in delayed virus clearance and/or increased cytokine secretion along with increased apoptosis of both infected and uninfected bystander cells (known as “original antigenic sin”).
With only around 65% homology based on amino acid sequence, the four dengue viruses could have been classified as separate virus groups but instead are treated as four serotypes belonging to a single group. It appears that there may be differences between the viruses, with DENV2 most commonly been associated with DHF/DSS and DENV4 the least likely to cause the more serious infections, but all serotypes can cause all of the conditions.
Because of the nature of dengue virus pathogenesis, a tetravalent vaccine effective against all four dengue virus serotypes is urgently needed. Vaccines which induce weak immune responses below protective levels over time are not acceptable because of the severe consequences of secondary DENV infections. Efforts to develop a dengue vaccine have encompassed live attenuated virus vaccines, inactivated virus vaccines, subunit vaccines and DNA vaccines. Vaccines of each type are currently or have been subjected to clinical trials, but none has yet been approved for use. Travelers to affected regions should take precautions against being bitten by mosquitos, use insect repellent day and night and check that hotels provide mosquito nets. Just another joy of those long-haul holidays.
In 1807, Alexander von Humboldt wrote “The nearer we approach the tropics, the greater the increase in the variety of structure, grace of form, and mixture of colors, as also in perpetual youth and vigor of organic life”. The increase in numbers of animal and plant species from the poles toward the equator is one of the most pervasive patterns of life on earth. Although known at least since the early 1800s, this pattern still lacks a consensus explanation. And although well documented in large, multicellular animals and plants, this pattern is reported to be relatively weak or absent in morphospecies of unicellular organisms. The lack of apparent geographic pattern has been attributed to high abundances, frequent and long-distance dispersal, and low extinction rates. This argument would suggest that bacteria, even smaller, more abundant, and more readily dispersed than protists, would also show little or no latitudinal gradient of diversity. This idea is expressed in microbiology as “Everything is everywhere; the environment selects“. But is everything everywhere?
A latitudinal diversity gradient in planktonic marine bacteria. PNAS USA, May 28, 2008
For two centuries, biologists have documented a gradient of animal and plant biodiversity from the tropics to the poles but have been unable to agree whether it is controlled primarily by productivity, temperature, or historical factors. Recent reports that find latitudinal diversity gradients to be reduced or absent in some unicellular organisms and attribute this to their high abundance and dispersal capabilities would suggest that bacteria, the smallest and most abundant organisms, should exhibit no latitudinal pattern of diversity. We used amplified ribosomal intergenic spacer analysis whole-assemblage genetic fingerprinting to quantify species richness in 103 near-surface samples of marine bacterial plankton, taken from tropical to polar in both hemispheres. We found a significant latitudinal gradient in richness. The data can help to evaluate hypotheses about the cause of the gradient. The correlations of richness with latitude and temperature were similarly strong, whereas correlations with parameters relating to productivity (chlorophyll, annual primary productivity, bacterial abundance) and other variables (salinity and distance to shore) were much weaker. Despite the high abundance and potentially high dispersal of bacteria, they exhibit geographic patterns of species diversity that are similar to those seen in other organisms. The latitudinal gradient in marine bacteria supports the hypothesis that the kinetics of metabolism, setting the pace for life, has strong influence on diversity.
There is life on Mars - almost certainly we have delivered terrestrial bacterial spores via probes. But is there indigenous life on Mars? In the May 2008 edition of Microbiology Today, Charles Cockell answers this hotly debated question:
It is not often apparent to microbiologists or members of the public that we know for certain that there has been life on Mars. Since the crash of the Soviet’s Mars 2 lander on the surface of the planet in 1971, a diversity of landed and crashed probes of various kinds, many of them not sterilized, have been delivered to the surface of Mars by the world’s space-faring nations. Only the Viking spacecraft, which landed in 1976, were completely heat sterilized to kill spores. Many of these spacecraft have delivered an inventory of spores found in spacecraft assembly facilities, including Bacillus species. A fascinating scientific question is whether there is, on the surface of Mars today, a viable spore hidden and shielded from Mars’ intense UV radiation in one of these various contraptions. There seems to no reason why a spore, cooled to Mars’ average temperature of –60 °C should not have survived since the 1970s. So on the face of it the answer to the question is likely to be ‘yes’.
Haemorrhagic disease, encephalitis, biphasic fever, flaccid paralysis, and jaundice are typical manifestations of diseases in human beings after infections by mosquito-borne or tick-borne flaviviruses such as yellow fever, dengue, West Nile, St Louis encephalitis, Japanese encephalitis, tick-borne encephalitis, Kyasanur Forest disease, and Omsk haemorrhagic fever. Although the characteristics of these viruses are well defined, they are still unpredictable with increases in disease severity, unusual clinical manifestations, unexpected methods of transmission, long-term persistence, and the discovery of new species. This paper compares the epidemiological and clinical features of the medically important flaviviruses, consider the effect of human activity on their evolution and dispersal, and draw attention to new findings and some of the unanswered questions, unresolved issues, and controversies that remain.
While Mars Phoenix looks for life on Mars, we’ve just found life on earth - but possibly not in the place you might have expected. Researchers have just found bacteria in searing hot sediments over 1.6 kilometres under the Atlantic seafloor off Newfoundland in Canada. The microbes were found in cores of sediment 111 million years old and at temperatures of 60 to 100° Celsius. This discovery doubles the previous record depth of 842 metres for living organisms. So while we wait to find out if there is life 1 metre under the Martian surface, marvel at the life here on Earth.
Extending the Sub-Sea-Floor Biosphere. Science 2008 320: 1046
Sub-sea-floor sediments may contain two-thirds of Earth’s total prokaryotic biomass. However, this has its basis in data extrapolation from ~500-meter to 4-kilometer depths, whereas the deepest documented prokaryotes are from only 842 meters. Here, we provide evidence for low concentrations of living prokaryotic cells in the deepest (1626 meters below the sea floor), oldest (111 million years old), and potentially hottest (~100°C) marine sediments investigated. These Newfoundland margin sediments also have DNA sequences related to thermophilic and/or hyperthermophilic Archaea. These form two unique clusters within Pyrococcus and Thermococcus genera, suggesting unknown, uncultured groups are present in deep, hot, marine sediments (~54° to 100°C). Sequences of anaerobic methane-oxidizing Archaea were also present, suggesting a deep biosphere partly supported by methane. These findings demonstrate that the sub-sea-floor biosphere extends to at least 1600 meters below the sea floor and probably deeper, given an upper temperature limit for prokaryotic life of at least 113°C and increasing thermogenic energy supply with depth.
On 25th May 2008 the NASA Phoenix mission is due to land on Mars. Phoenix is designed to study the history of water and habitability potential in the Martian arctic’s ice-rich soil.
Pathogenic microbes are emerging and re-emerging at an alarming rate. Of the 175 emerging infectious diseases of humans, 132 are zoonotic, residing in wildlife reservoir species and occasionally transmitted to humans either directly or via an intermediate vector. Zoonotic pathogens are not specialists on any single species, they can infect humans and at least one non-human animal. A robust understanding of the transmission cycle in nature that governs the distribution and abundance of pathogens, and thus contact with humans, is essential for the effective control of emerging infectious diseases. This paper investigates the degree to which the zoonotic pathogen that causes Lyme disease, Borrelia burgdorferi, is maintained and amplified by multiple reservoir host species.
Conspicuous impacts of inconspicuous hosts on the Lyme disease epidemic
Proc Biol Sci. 2008 275: 227-235
Emerging zoonotic pathogens are a constant threat to human health throughout the world. Control strategies to protect public health regularly fail, due in part to the tendency to focus on a single host species assumed to be the primary reservoir for a pathogen. Here, we present evidence that a diverse set of species can play an important role in determining disease risk to humans using Lyme disease as a model. Host-targeted public health strategies to control the Lyme disease epidemic in North America have focused on interrupting Borrelia burgdorferi transmission between blacklegged ticks and the putative dominant reservoir species, white-footed mice. However, B. burgdorferi infects more than a dozen vertebrate species, any of which could transmit the pathogen to feeding ticks and increase the density of infected ticks and Lyme disease risk. Using genetic and ecological data, we demonstrate that mice are neither the primary host for ticks nor the primary reservoir for B. burgdorferi, feeding 10% of all ticks and 25% of B. burgdorferi-infected ticks. Inconspicuous shrews feed 35% of all ticks and 55% of infected ticks. Because several important host species influence Lyme disease risk, interventions directed at a multiple host species will be required to control this epidemic.
Bacillus cereus is widespread in nature and frequently isolated from soil and growing plants, but it is also well adapted for growth in the intestinal tract of insects and mammals. From these habitats it is easily spread to foods, where it may cause an emetic or a diarrhoeal type of food-associated illness that is becoming increasingly important in the industrialized world. The emetic disease is a food intoxication caused by cereulide, a small ring-formed peptide. Similar to the virulence determinants that distinguish Bacillus thuringiensis and Bacillus anthracis from B. cereus, the genetic determinants of cereulide are plasmid-borne.
The diarrhoeal syndrome of B. cereus is an infection caused by vegetative cells, ingested as viable cells or spores, thought to produce protein enterotoxins in the small intestine. Three pore-forming cytotoxins have been associated with diarrhoeal disease. This review focuses on the toxins associated with foodborne diseases frequently caused by B. cereus. The disease characteristics are described, and recent findings regarding the associated toxins are discussed, as well as the present knowledge on virulence regulation.
Dengue is a mosquito-borne disease caused by four serotypes of dengue virus (DENV1–DENV4) and is currently the most common arbovirus (arthropod-transmitted) disease worldwide. Primary infection with any of the four DV serotypes typically results in dengue fever (DF), a relatively mild influenza-like illness which subsequently provides lifelong immunity to the infecting strain. However, the bad news is that secondary infection with different DV serotype is associated with an increased risk of developing more serious conditions such as dengue haemorrhagic fever (DHF) and the life-threatening dengue shock syndrome (DSS).
