MicrobiologyBytes

This article reviews recent research on Rift Valley fever virus (RVFV) infection, encompassing four main areas: epidemiology and outbreak prediction, viral pathogenesis, human diagnostics and therapeutics, and vaccine and therapeutic candidates. RVFV continues to extend its range in Africa and the Middle East. Better definition of RVFV-related clinical syndromes and human risk factors for severe disease, combined with early-warning systems based on remote-sensing, simplified rapid diagnostics, and tele-epidemiology, hold promise for earlier deployment of effective outbreak control measures. Advances in understanding of viral replication pathways and host cell-related pathogenesis suggest means for antiviral therapeutics and for more effective vaccination strategies based on genetically engineered virus strains or subunit vaccines. RVFV is a significant health and economic burden in many areas of Africa, and remains a serious threat to other parts of the world. Development of more effective methods for RVFV outbreak prevention and control remains a global health priority.

Advances in Rift Valley fever research: insights for disease prevention. Curr Opin Infect Dis. Jul 6 2010 doi: 10.1097/QCO.0b013e32833c3da6

Sleeping sickness, or Human African Trypanosomiasis, is a disease affecting the health and productivity of poor people in many rural areas of sub-Saharan Africa. The disease is caused by a single-celled flagellate, Trypanosoma brucei, which evades the immune system by periodically switching the proteins on its surface. Researchers have produced a genome sequence for T. brucei gambiense, which is the particular subspecies causing most disease in humans. They compared this with an existing reference genome for a non-human infecting strain to identify genes in T. b. gambiense that might explain its ability to infect humans and to assess how well the reference performs as a universal plan for all T. brucei. The genome sequences differ only due to rare insertions and duplications and homologous genes are over 95% identical on average. The archive of surface antigens that enable the parasite to switch its protein coat is remarkably consistent, even though it evolves very quickly. They also identified genes with predicted cell surface functions that are only present in T. b. brucei and have evolved rapidly in recent time. These genes might help to explain variation in disease pathology between different T. brucei strains in different hosts.

The team wanted to answer two questions: Is the existing T. b. brucei sequence representative of the full diversity of T. brucei parasites? And, is there anything in the T. b. gambiense genome that might explain its ability to infect and thrive in human populations? Historically, sleeping sickness has been a severely neglected disease, with considerable impact on human health and the well-being, and prosperity of communities. The genome comparison revealed a remarkable level of similarity between T. b. brucei and T. b. gambiense – just a single locus was unique to T. b. brucei. Moreover, the sequences of comparable genes were, on average, 98.2% identical. Because the genomes were so similar, the team could say with confidence that the T. b. brucei parasite and its genome are good models for future experiments to understand the biology of T. b. gambiense. The similarity between the two genomes also suggested that the source of T. b. gambiense’s ability to infect humans cannot be explained simply by the addition or removal of a few genes. Changes in the phenotype – the physical characteristics – seem to be down to more subtle changes in genetic information. Single letter changes in the genome; differences in the number of copies of genes; changes in how the activity of genes is regulated – all of these genetic nuances could play that crucial role in determining why T. b. gambiense behaves so differently to T. b. brucei. With two high-quality reference genome sequences in place for the T. brucei strains, the search for those small genetic differences is given a boost. It is this search that will fuel the pursuit of targeted drug treatments to tackle T. b. gambiense.

The Genome Sequence of Trypanosoma brucei gambiense, Causative Agent of Chronic Human African Trypanosomiasis. 2010 PLoS Negl Trop Dis 4(4): e658. doi:10.1371/journal.pntd.0000658

Related:

• Antigenic Variation in African Trypanosomes
• BILBO and the trypanosomes
• The Trypanosoma brucei flagellum

Nearly half of adolescents admitted to two public hospitals in Zimbabwe are HIV positive according to a new paper in PLoS Medicine. This study examines the causes of unselected acute hospitalization in adolescents, and serves to highlight the growing crisis of HIV acquired at birth in teenagers and young adults in the developing world. Each 10-18 year old participant completed a questionnaire about themselves and their health and underwent standard investigations, including HIV testing. Nearly half of participants were HIV positive; they were more likely to be stunted, to have pubertal delay, and to be maternal orphans or have an HIV-infected mother than HIV-negative adolescents. 69% of HIV-positive participants were admitted to hospital because of infections such as tuberculosis or pneumonia whereas only 19% of the HIV-negative participants were admitted for infections. 22% of the HIV-positive participants died while in hospital compared to only 7% of the HIV-negative participants.

Few studies have addressed the prevalence of perinatally-acquired HIV in older children and adolescence, perhaps because it was thought that children infected at birth were unlikely to survive beyond 5 years of age. Additionally, while the researchers assessed causes of hospitalization such as infectious disease or chronic underlying disease, they did not assess underlying mental health conditions. In the developed world, the most common reasons for psychiatric hospitalization for HIV-infected children were for depression or behavioral disorders and calls for the impact of death and chronic ill health of caregivers or siblings on HIV-infected adolescents in the developing world require investigation.

Causes of Acute Hospitalization in Adolescence: Burden and Spectrum of HIV Related Morbidity in a Country with an Early-Onset and Severe HIV Epidemic: A Prospective Survey. 2010 PLoS Med 7(2): e1000178

Related:

• What do we really know about the spread of AIDS in Africa?
• GSK to provide cheap drugs to the developing world
• Global Fund grants Zimbabwe $37.9m to fight AIDS
• HIV treatment reduces death rates in Africa

Trypanosoma brucei and related subspecies are protozoan parasites that cause African trypanosomiasis in humans and animals, a deadly disease with devastating health and economic consequences. The T. brucei flagellum and flagellar motility are central to disease pathogenesis in the mammalian host and parasite development in the tsetse fly vector. Recent functional studies have revealed that the T. brucei flagellum is an essential and multifunctional organelle with critical roles in motility, cellular morphogenesis, cell division, immune evasion, and potentially, sensory perception. Concurrently, genomic and proteomic studies have significantly expanded the inventory of known trypanosome flagellar proteins. Because the flagellum is an essential organelle, understanding unique aspects of the T. brucei flagellum may uncover novel drug targets. In addition, T. brucei is emerging as a powerful experimental system for studies of conserved aspects of flagellum/cilium biology (flagellum and cilium are interchangeable terms for the same organelle), with direct relevance to other eukaryotes, including humans, in which flagellum/cilium defects underlie many heritable, fatal, and debilitating diseases.

African trypanosomes are devastating human and animal pathogens. Trypanosoma brucei rhodesiense and T. b. gambiense subspecies cause the fatal human disease known as African sleeping sickness. It is estimated that several hundred thousand new infections occur annually and the disease is fatal if untreated. T. brucei is transmitted by the tsetse fly and alternates between bloodstream-form and insect-form life cycle stages that are adapted to survive in the mammalian host and the insect vector, respectively. The importance of the flagellum for parasite motility and attachment to the tsetse fly salivary gland epithelium has been appreciated for many years. Recent studies have revealed both conserved and novel features of T. brucei flagellum structure and composition, as well as surprising new functions that are outlined in this review. These discoveries are important from the standpoint of understanding trypanosome biology and identifying novel drug targets, as well as for advancing our understanding of fundamental aspects of eukaryotic flagellum structure and function.

The Trypanosoma brucei flagellum: moving parasites in new directions. Ann Rev Microbiol. 2009 63: 335-362

Related:

• Antigenic Variation in African Trypanosomes
• Trypanosomes and immune evasion
• BILBO and the trypanosomes

Anopheles gambiae mosquitoes are the principal vectors of human malaria, a disease with devastating consequences for public health and the economic development of disease-endemic countries. The creation of new tools to control vector populations is a focal point of intensive efforts to eradicate the burden of malaria. As mosquitoes generally copulate only once during their lives, interfering with the mating process is a promising avenue for research into vector control. Unfortunately, very little is known about the molecular or physiological basis of mating and insemination in malaria vectors. Of particular concern is our lack of knowledge about factors and pathways ensuring male reproductive success, such as those that result in sperm storage, oviposition, and the inhibition of remating in females. Improving our understanding of mating biology might not only inform currently proposed strategies for vector control, but could potentially allow the development of entirely novel tools for combating malaria.

Stopping male mosquitoes from sealing their sperm inside females with a ‘mating plug’ could prevent mosquitoes from reproducing, and offer a potential new way to combat malaria. The new study focuses on An. gambiae, the species of mosquito primarily responsible for the transmission of malaria in Africa. These mosquitoes mate only once in their lives, which means that disrupting the reproductive process offers a good way of dramatically reducing their populations. When these mosquitos mate, the male transfers sperm to the female and then afterwards transfers a coagulating mass of proteins and seminal fluids known as a mating plug. This plug is not found in any other species of mosquito and until now, very little was known about the role it plays in An. gambiae reproduction. The authors show that the mating plug is essential for ensuring that sperm is correctly retained in the female’s sperm storage organ, from where she can fertilise eggs over the course of her lifetime. Without the mating plug, sperm is not stored correctly, and fertilisation cannot occur.

The researchers analysed the composition of the protein-rich mating plug and discovered that it is formed when an enzyme called transglutaminase interacts with other proteins in the male mosquito’s seminal fluid. This interaction causes the seminal fluids to coagulate into a gelatinous solid mass. When the research team removed this enzyme in male mosquitoes in the lab, the fluids failed to coagulate and form the plug. Furthermore, when these males, lacking the key protein and therefore the plug, mated with females, reproduction was not successful. The male mating plug is not a simple barrier to insemination from rival males, as has been previously suggested. Instead, the plug plays an important role in allowing the female to successfully store sperm in the correct way inside her, and as such is vital for successful reproduction. If in the future we can develop an inhibitor that prevents the coagulating enzyme doing its job inside male An. gambiae mosquitoes in such a way that can be deployed easily in the field – for example in the form of a spray as it is done with insecticides – then we could effectively induce sterility in female mosquitoes in the wild. This could provide a new way of limiting the population of this species of mosquito, and could be one more weapon in the arsenal against malaria.

Transglutaminase-Mediated Semen Coagulation Controls Sperm Storage in the Malaria Mosquito. 2009 PLoS Biol 7(12): e1000272. doi:10.1371/journal.pbio.1000272
Insect seminal fluid proteins are powerful modulators of many aspects of female physiology and behaviour including longevity, egg production, sperm storage, and remating. The crucial role of these proteins in reproduction makes them promising targets for developing tools aimed at reducing the population sizes of vectors of disease. In the malaria mosquito Anopheles gambiae, seminal secretions produced by the male accessory glands (MAGs) are transferred to females in the form of a coagulated mass called the mating plug. The potential of seminal fluid proteins as tools for mosquito control demands that we improve our limited understanding of the composition and function of the plug. Here, we show that the plug is a key determinant of An. gambiae reproductive success. We uncover the composition of the plug and demonstrate it is formed through the cross-linking of seminal proteins mediated by a MAG-specific transglutaminase (TGase), a mechanism remarkably similar to mammalian semen coagulation. Interfering with TGase expression in males inhibits plug formation and transfer, and prevents females from storing sperm with obvious consequences for fertility. Moreover, we show that the MAG-specific TGase is restricted to the anopheline lineage, where it functions to promote sperm storage rather than as a mechanical barrier to re-insemination. Taken together, these data represent a major advance in our understanding of the factors shaping Anopheles reproductive biology.

Related:

• New methods of biocontrol for malaria
• Evolution-proof insecticides against malaria
• Malaria, mosquitoes and the legacy of Ronald Ross

HIV infection was first detected in the United Kingdom (as AIDS) in 1981–2 among men who have sex with men (MSM). Early outbreaks with UK sources include Scottish intravenous drug users (IDUs) dated to 1983 and haemophiliacs to 1984. All strains isolated initially were of the B subtype, both in MSM and IDUs and also in the small number of individuals infected through heterosexual contact during that decade. However within 10 years, multiple virus subtypes had been detected within the UK. From the mid 1990s increasing numbers of HIV infections in the UK were being found in heterosexuals, until the current situation was attained whereby this risk group comprises the majority of new HIV diagnoses. This increase coincided with increasing immigration from southern and Eastern Africa, particularly from South Africa, Uganda and Zimbabwe. Genetic characterisation of viruses from infected heterosexuals revealed that while subtype B was still observed in the majority of samples obtained during 1996/7, by the year 2000, subtype C was most common (35%) with subtype A at 15%, reflecting the main subtypes in those countries. Subtype B was present in only 25% of individuals. Thus, the heterosexual risk group in the UK has become strongly associated with non-B HIV subtypes. Recently there has been some evidence of limited crossover among risk groups.

Since 1995, HIV among heterosexuals in the UK increased to the point where the total number of heterosexuals infected with HIV, predominantly of non-B subtypes, exceeds the number of HIV-positive homosexual men. To understand the dynamics of this epidemic, researchers applied phylodynamic techniques to the analysis of viral sequences taken in the course of routine clinical care from approximately 40% of the HIV-infected heterosexual population in the UK. Phylodynamics reconstructs the pattern of viral sequence divergence in time, revealing the size of transmission clusters and the dynamics of transmission within them.

Of 11,071 patients studied, 296 were linked to at least two others in the UK. There were 8 clusters comprising 10 or more individuals among these, yielding a total of 143 or 5% of all individuals with links, much lower than seen earlier among homosexual men (25%). HIV transmissions within clusters also occurred less rapidly, only 2% being dated to the first 6 months of infection, compared to 25% among homosexual men. Overall, transmission clusters exist in the UK heterosexual HIV epidemic but they are generally smaller than among homosexuals; onward transmission occurs less rapidly and is not associated with acute HIV infection. The study concludes that heterosexual transmission could be significantly reduced by early diagnosis and treatment. The slower dynamics of the heterosexual epidemic thus offer more opportunity for successful intervention, but it is essential that diagnosis is achieved as early as possible.

Molecular Phylodynamics of the Heterosexual HIV Epidemic in the United Kingdom. 2009 PLoS Pathog 5(9): e1000590 doi:10.1371/journal.ppat.1000590
The heterosexual risk group has become the largest HIV infected group in the United Kingdom during the last 10 years, but little is known of the network structure and dynamics of viral transmission in this group. The overwhelming majority of UK heterosexual infections are of non-B HIV subtypes, indicating viruses originating among immigrants from sub-Saharan Africa. The high rate of HIV evolution, combined with the availability of a very high density sample of viral sequences from routine clinical care has allowed the phylodynamics of the epidemic to be investigated for the first time. Sequences of the viral protease and partial reverse transcriptase coding regions from 11,071 patients infected with HIV of non-B subtypes were studied. Of these, 2774 were closely linked to at least one other sequence by nucleotide distance. Including the closest sequences from the global HIV database identified 296 individuals that were in UK-based groups of 3 or more individuals. There were a total of 8 UK-based clusters of 10 or more, comprising 143/2774 (5%) individuals, much lower than the figure of 25% obtained earlier for men who have sex with men (MSM). Sample dates were incorporated into relaxed clock phylogenetic analyses to estimate the dates of internal nodes. From the resulting time-resolved phylogenies, the internode lengths, used as estimates of maximum transmission intervals, had a median of 27 months overall, over twice as long as obtained for MSM (14 months), with only 2% of transmissions occurring in the first 6 months after infection. This phylodynamic analysis of non-B subtype HIV sequences representing over 40% of the estimated UK HIV-infected heterosexual population has revealed heterosexual HIV transmission in the UK is clustered, but on average in smaller groups and is transmitted with slower dynamics than among MSM. More effective intervention to restrict the epidemic may therefore be feasible, given effective diagnosis programmes.

Related:

• AVERT: United Kingdom HIV statistics by race, age and gender
• Web 2.0 and sexually transmitted infections
• Health Risk Behaviors on MySpace
• Visualizing the HIV pandemic

Dr Helen McShane at the University of Oxford and the Oxford-Emergent Tuberculosis Consortium Ltd have received a Wellcome trust Strategic Award to part-fund the first trial to test efficacy of the new TB vaccine candidate, MVA85A, for potential efficacy in South African infants. In this film Dr McShane discusses the new vaccine candidate and these trials. www.wellcome.ac.uk

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• Origin and spread of Mycobacterium tuberculosis
• Tuberculosis – is the white plague winning?
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Neglected tropical diseases (NTDs) are the most common conditions affecting the poorest 500 million people living in sub-Saharan Africa (SSA), and together produce a burden of disease that may be equivalent to up to one-half of SSA’s malaria disease burden and more than double that caused by tuberculosis. Approximately 85% of the NTD disease burden results from helminth infections. Hookworm infection occurs in almost half of SSA’s poorest people, including 40–50 million school-aged children and 7 million pregnant women in whom it is a leading cause of anaemia. Schistosomiasis is the second most prevalent NTD after hookworm (192 million cases), accounting for 93% of the world’s number of cases and possibly associated with increased horizontal transmission of HIV/AIDS. Lymphatic filariasis (46–51 million cases) and onchocerciasis (37 million cases) are also widespread in SSA, each disease representing a significant cause of disability and reduction in the region’s agricultural productivity. There is a dearth of information on Africa’s non-helminth NTDs. The protozoan infections, human African trypanosomiasis and visceral leishmaniasis, affect almost 100,000 people, primarily in areas of conflict in SSA where they cause high mortality, and where trachoma is the most prevalent bacterial NTD (30 million cases). However, there are little or no data on some very important protozoan infections, e.g., amebiasis and toxoplasmosis; bacterial infections, e.g. typhoid fever and non-typhoidal salmonellosis, the tick-borne bacterial zoonoses, and non-tuberculosis mycobaterial infections; and arboviral infections. Thus, the overall burden of Africa’s NTDs may be severely underestimated. A full assessment is an important step for disease control priorities, particularly in Nigeria and the Democratic Republic of Congo, where the greatest number of NTDs may occur.

If you want to make a difference, start here.

Neglected Tropical Diseases in Sub-Saharan Africa: Review of Their Prevalence, Distribution, and Disease Burden. 2009 PLoS Negl Trop Dis 3(8): e412. doi:10.1371/journal.pntd.0000412

Related:

• Schistosomiasis control in Mali – or not
• 7 million pregnant women in sub-Saharan Africa infected with hookworm
• Parasitic worm may increase susceptibility to AIDS virus
• Neglected Tropical Diseases in Latin America and the Caribbean