Vector-borne parasitic diseases--an overview of recent changes

Discovery of Rickettsia spp. in mosquitoes collected in Georgia by metagenomics analysis and molecular characterization

Arthropods have a broad and expanding worldwide presence and can transmit a variety of viral, bacterial, and parasite pathogens. A number of Rickettsia and Orientia species associated with ticks, fleas, lice, and mites have been detected in, or isolated from, patients with febrile illness and/or animal reservoirs throughout the world. Mosquitoes are not currently considered vectors for Rickettsia spp. pathogens to humans or to animals. In this study, we conducted a random metagenome next-generation sequencing (NGS) of 475 pools of Aedes, Culex, and Culiseta species of mosquitoes collected in Georgia from 2018 to 2019, identifying rickettsial gene sequences in 33 pools of mosquitoes. We further confirmed the findings of the Rickettsia by genus-specific quantitative PCR (qPCR) and multi-locus sequence typing (MLST). The NGS and MLST results indicate that Rickettsia spp. are closely related to Rickettsia bellii, which is not known to be pathogenic in humans. The results, together with other reports of Rickettsia spp. in mosquitoes and the susceptibility and transmissibility experiments, suggest that mosquitoes may play a role in the transmission cycle of Rickettsia spp.

Core components, concepts and strategies for parasitic and vector-borne disease elimination with a focus on schistosomiasis: A landscape analysis

Efforts to control and eliminate human schistosomiasis have accelerated over the past decade. In a number of endemic countries and settings, interruption of schistosome transmission has been achieved. In others, Schistosoma infections continue to challenge program managers at different levels, from the complexity of the transmission cycle, over limited treatment options and lack of field-friendly accurate diagnostics, to controversy around adequate intervention strategies. We conducted a landscape analysis on parasitic and vector-borne disease elimination approaches with the aim to identify evidence-based strategies, core components and key concepts for achieving and sustaining schistosomiasis control and for progressing elimination efforts towards interruption of transmission in sub-Saharan Africa. A total of 118 relevant publications were identified from Web of Science, Pubmed and the grey literature and reviewed for their content. In addition, we conducted in-depth interviews with 23 epidemiologists, program managers, policymakers, donors and field researchers. Available evidence emphasizes the need for comprehensive, multipronged and long-term strategies consisting of multiple complementary interventions that must be sustained over time by political commitment and adequate funding in order to reach interruption of transmission. Based on the findings of this landscape analysis, we propose a comprehensive set of intervention strategies for schistosomiasis control and elimination. Before deployment, the proposed interventions will require review, evaluation and validation in the frame of an expert consultation as a step towards adaptation to specific contexts, conditions and settings. Field testing to ensure local relevance and effectiveness is paramount given the diversity of socio-ecological and epidemiological contexts.

This landscape analysis explored successful concepts, approaches and interventions of past and ongoing parasitic and vector-borne disease elimination efforts and programs with regard to relevance for progress in the elimination of human schistosome infections. Schistosomiasis is a disabling, water borne parasitic disease of public health concern with an estimated 250 million people infected worldwide. The long-term morbidity of this neglected tropical disease significantly impacts growth, cognition and socioeconomic development at all ages. Despite increased global efforts to control morbidity and advance elimination, challenges in view of the complex life cycle which involves freshwater sources, intermediate snail hosts and humans, remain. This calls for targeted interventions and concerted programs. According to the evidence from the literature and as proposed by a wide range of key informants, comprehensive, multipronged and long-term strategies supported by strong political commitment and adequate funding are required in order to achieve and sustain the set goals. Based on the findings, we propose here a comprehensive set of intervention strategies for schistosomiasis control and elimination for review and evaluation to inform implementation research needs and elimination program design.

Resurgence of cutaneous leishmaniasis in Israel, 2001-2012

Cutaneous leishmaniasis has long been endemic in Israel. After a 15-year period of moderate illness rates, reported incidence increased from 0.4 cases per 100,000 population in 2001 to 4.4 cases per 100,000 population in 2012, and the disease emerged in areas where its presence had previously been minimal. We analyzed all cases reported to the national surveillance system and found that outbreak patterns revealed an expansion of Leishmania major infections over large areas in the southern part of the country and the occurrence of spatially focused L. tropica outbreaks in the northern part of the country. Outbreaks often followed new construction in populated areas. Further study of factors affecting the transmission of cutaneous leishmaniasis is needed in Israel, as well as the development of effective methods to control the disease, an increase in awareness among health care professionals, and intensive public education regarding control measures in areas of known leishmaniasis foci.

Malaria entomological risk factors in relation to land cover in the Lower Caura River Basin, Venezuela

To explore the effects of deforestation and resulting differences in vegetation and land cover on entomological parameters, such as anopheline species composition, abundance, biting rate, parity and entomological inoculation rate (EIR), three villages were selected in the Lower Caura River Basin, state of Bolívar, Venezuela. All-night mosquito collections were conducted between March 2008-January 2009 using CDC light traps and Mosquito Magnet® Liberty Plus. Human landing catches were performed between 06:00 pm-10:00 pm, when anophelines were most active. Four types of vegetation were identified. The Annual Parasite Index was not correlated with the type of vegetation. The least abundantly forested village had the highest anopheline abundance, biting rate and species diversity. Anopheles darlingi and Anopheles nuneztovari were the most abundant species and were collected in all three villages. Both species showed unique biting cycles. The more abundantly forested village of El Palmar reported the highest EIR. The results confirmed previous observations that the impacts of deforestation and resulting changes in vegetation cover on malaria transmission are complex and vary locally.

Spatio-temporal clustering of American Cutaneous Leishmaniasis in a rural municipality of Venezuela

BACKGROUND

American Cutaneous Leishmaniasis (ACL) is an endemic disease in most Latin-American countries and a public health problem. The number of new cases in the world is thought to be about 1.5 million each year. A new epidemiologic pattern has been observed in the last years, in this sense, Geographic Information Systems (GIS) combined with methods of spatial analysis provide powerful new tools for understanding it.

STUDY OBJECTIVE

To investigate the spatial and temporal features of American Cutaneous Leishmaniasis, in an area known to be endemic.

METHODS

We retrospectively conducted a space-time cluster analysis of incident cases of ACL using cross sectional data recorded from 1348 confirmed cases from 1992 to 2007 in a rural municipality in Venezuela, to test whether the cases were distributed randomly over space and time. We used the space-time permutation scan statistic and GIS. The identified clusters were analyzed for age, sex and job. These allowed us to investigate transmission patterns of ACL without an explicit entomological study.

RESULTS

The analysis showed statistically significant space-time clusters of ACL (p-value<0.01). The most likely cluster contained 35.47% of the total cases (n=177) from 1995 to 1998. Four secondary clusters were identified for different periods. There was an indication of ACL cluster spread from the northeast to other points of the municipality. Three transmission patterns (domiciliary, peri-domiciliary and sylvatic environments) were identified along the study area.

CONCLUSION

The transmission of ACL has a spatial and temporal pattern in the studied area which is related to a complex cycle where the environment and other factors have a significant influence.

Exploring the Linkages Between Ecosystems and Human Health

The linkages between human health and ecosystems are complex, dynamic, and political. For millennia ecosystems have provided humans with essential services such as food, water, shelter and medicine. At the same time, they have mediated the transmission of many diseases and posed a number of health risks. The vitality of ecosystem services for human health and well-being is well captured by Bernard Abraham, President of Weskit-Chi Aboriginal Trappers Association, when he commented on the importance of forest ecosystems to Aboriginal people. He observed that many Aboriginal people consider the forest as: “their food bank, drugstore, meat market, bakery, fruit and vegetable stand, building material centre, beverage supply, and the habitat for all of the creator’s creatures.”¹

Shading by napier grass reduces malaria vector larvae in natural habitats in Western Kenya highlands

Increased human population in the Western Kenya highlands has led to reclamation of natural swamps resulting in the creation of habitats suitable for the breeding of Anopheles gambiae, the major malaria vector in the region. Here we report on a study to restore the reclaimed swamp and reverse its suitability as a habitat for malaria vectors. Napier grass-shaded and non-shaded water channels in reclaimed sites in Western Kenya highlands were studied for the presence and density of mosquito larvae, mosquito species composition, and daily variation in water temperature. Shading was associated with 75.5% and 88.4% (P < 0.0001) reduction in anopheline larvae densities and 78.1% and 88% (P < 0.0001) reduction in Anopheles gambiae sensu lato (s.l.) densities in two sites, respectively. Shading was associated with a 5.7°C, 5.0°C, and 4.7°C, and 1.6°C, 3.9°C, and 2.8°C (for maximum, minimum, and average temperatures, respectively) reduction (P < 0.0001) in water temperatures in the two locations, respectively. An. gambiae s.l. was the dominant species, constituting 83.2% and 73.1%, and 44.5% and 42.3%, of anophelines in non-shaded and shaded channels, respectively, in the two sites, respectively. An. gambiae sensu stricto (s.s.) constituted the majority (97.4%) of An. gambiae s.l., while the rest (2.6%) comprised of Anopheles arabiensis. Minimum water temperature decreased with increasing grass height (P = 0.0039 and P = 0.0415 for Lunyerere and Emutete sites, respectively). The results demonstrate how simple environmental strategies can have a strong impact on vector densities.

Global stressors and the global decline of amphibians: tipping the stress immunocompetency axis

There is a widespread consensus that the earth is experiencing a mass extinction event and at the forefront are amphibians, the most threatened of all vertebrate taxa. A recent assessment found that nearly one-third (32%, 1,856 species) of the world's amphibian species are threatened. Amphibians have existed on the earth for over 300 million years, yet in just the last two decades there have been an alarming number of extinctions, nearly 168 species are believed to have gone extinct and at least 2,469 (43%) more have populations that are declining. Infectious diseases have been recognized as one major cause of worldwide amphibian population declines. This could be the result of the appearance of novel pathogens, or it could be that exposure to environmental stressors is increasing the susceptibility of amphibians to opportunistic pathogens. Here I review the potential effects of stressors on disease susceptibility in amphibians and relate this to disease emergence in human and other wildlife populations. I will present a series of case studies that illustrate the role of stress in disease outbreaks that have resulted in amphibian declines. First, I will examine how elevated sea-surface temperatures in the tropical Pacific since the mid-1970s have affected climate over much of the world and could be setting the stage for pathogen-mediated amphibian declines in many regions. Finally, I will discuss how the apparently rapid increase in the prevalence of amphibian limb deformities is linked to the synergistic effects of trematode infection and exposure to chemical contaminants.

Geostatistical analysis of data on air temperature and plant phenology from Baden-Württemberg (Germany) as a basis for regional scaled models of climate change

The rise of the air temperature is assured to be part of the global climatic change, but there is still a lack of knowledge about its effects at a regional scale. The article tackles the correlation of air temperature with the phenology of selected plants by the example of Baden-Württemberg to provide a spatial valid data base for regional climate change models. To this end, the data on air temperature and plant phenology, gathered from measurement sites without congruent coverage, were correlated after performing geostatistical analysis and estimation. In addition, geostatistics are used to analyze and cartographically depict the spatial structure of the phenology of plants in spring and in summer. The statistical analysis reveals a significant relationship between the rising air temperature and the earlier beginning of phenological phases like blooming or fruit maturation: From 1991 to 1999 spring time, as indicated by plant phenology, has begun up to 15 days earlier than from 1961 to 1990. As shown by geostatistics, this holds true for the whole territory of Baden-Württemberg. The effects of the rise of air temperature should be investigated not only by monitoring biological individuals, as for example plants, but on an ecosystem level as well. In Germany, the environmental monitoring should be supplemented by the study of the effects of the climatic change in ecosystems. Because air temperature and humidity have a great influence on the temporal and spatial distribution of pathogen carriers (vectors) and pathogens, mapping of the environmental determinants of vector and pathogen distribution in space and time should be performed in order to identify hot spots for risk assessment and further detailed epidemiological studies.

Tick immunobiology

Ticks are of vast medical and veterinary public health importance due to direct damage caused by feeding and their roles in transmitting well known and emerging infectious agents. Ticks and tick-borne pathogens stimulate the immune system of the host. Those immune interactions are of importance in tick biology, pathogen transmission and control of ticks and tick-borne diseases. Both innate and specific acquired immune defenses are involved in the responses of vertebrate hosts to infestation. Ticks have evolved countermeasures to circumvent host immune defenses. This review addresses the immunobiology of the tick-host interface from the perspectives of the pharmacology of tick saliva; relationship of tick saliva to pathogen transmission; host immune responses to infestation; tick modulation of host immune defences; and genomic/proteomic strategies for studying tick salivary gland molecules.

Are New World leishmaniases becoming anthroponoses?

In the New World, leishmaniases are originally wild exoanthropic zoonoses developing in sylvatic ecotopes. For a long time, Leishmania parasites have shown a remarkable plasticity to face modifications in their environment. Now, both geographical extension and numerical increase of leishmaniasis cases in the New World are giving cause for concern. These circumstances might have been provoked by the simple invasion of zoonotic foci by humans. However, dramatic evolutionary mechanisms are also at work in the New World: (i) the reduction of biodiversity associated with anthropogenic environmental changes (deforestation and urbanization); and (ii) the subsequent adaptations and interactions of new vectors and reservoir hosts at the interface with humans. This paper considers that these processes could result in new pathogenic complexes tending to synanthropic zoonoses, if not anthroponoses. Increasing man-made risk factors could thus possibly make leishmaniases a growing public health concern in the New World.

GIS, geostatistics, metadata banking, and tree-based models for data analysis and mapping in environmental monitoring and epidemiology

By the example of environmental monitoring, some applications of geographic information systems (GIS), geostatistics, metadata banking, and Classification and Regression Trees (CART) are presented. These tools are recommended for mapping statistically estimated hot spots of vectors and pathogens. GIS were introduced as tools for spatially modelling the real world. The modelling can be done by mapping objects according to the spatial information content of data. Additionally, this can be supported by geostatistical and multivariate statistical modelling. This is demonstrated by the example of modelling marine habitats of benthic communities and of terrestrial ecoregions. Such ecoregionalisations may be used to predict phenomena based on the statistical relation between measurements of an interesting phenomenon such as, e.g., the incidence of medically relevant species and correlated characteristics of the ecoregions. The combination of meteorological data and data on plant phenology can enhance the spatial resolution of the information on climate change. To this end, meteorological and phenological data have to be correlated. To enable this, both data sets which are from disparate monitoring networks have to be spatially connected by means of geostatistical estimation. This is demonstrated by the example of transformation of site-specific data on plant phenology into surface data. The analysis allows for spatial comparison of the phenology during the two periods 1961-1990 and 1991-2002 covering whole Germany. The changes in both plant phenology and air temperature were proved to be statistically significant. Thus, they can be combined by GIS overlay technique to enhance the spatial resolution of the information on the climate change and use them for the prediction of vector incidences at the regional scale. The localisation of such risk hot spots can be done by geometrically merging surface data on promoting factors. This is demonstrated by the example of the transfer of heavy metals through soils. The predicted hot spots of heavy metal transfer can be validated empirically by measurement data which can be inquired by a metadata base linked with a geographic information system. A corresponding strategy for the detection of vector hot spots in medical epidemiology is recommended. Data on incidences and habitats of the Anophelinae in the marsh regions of Lower Saxony (Germany) were used to calculate a habitat model by CART, which together with climate data and data on ecoregions can be further used for the prediction of habitats of medically relevant vector species. In the future, this approach should be supported by an internet-based information system consisting of three components: metadata questionnaire, metadata base, and GIS to link metadata, surface data, and measurement data on incidences and habitats of medically relevant species and related data on climate, phenology, and ecoregional characteristic conditions.

Emerging parasitic infections

Parasitic infections are felt by most individuals to have little impact on the health and well being of most inhabitants of Canada, Western Europe, and the United States. As the authors show in this article, parasites are always "emerging" somewhere and have a significant impact on those areas of the world. Moreover, as we are becoming an ever-smaller global village, catastrophes and instability in the Third World affect control of parasitic diseases endemic to those areas, ensuring greater chances of transmission to visitors there. The foundation of successful limitation of parasitic diseases in both developing and developed regions is still accurate and rapid diagnosis.

Trypanosome microtubule-associated protein p15 as a vaccine for the prevention of African sleeping sickness

Trypanosomes cause African sleeping sickness, affecting millions of humans and animals. We tested trypanosome microtubule-associate protein (MAP p15) as a vaccine in mice, and show that p15 (native or recombinant) generated up to 100% protection from an otherwise lethal challenge of a heterologous strain of Trypanosoma brucei. We also tested the adenovirus as a vaccine delivery system and show that both adenoviral vector containing p15 gene or control adenovirus containing lacZ gene generated a protective response and exhibited strong CD8+ T-cell proliferation. These results suggest that the p15 protein itself is an effective vaccine and that the adenovirus may be used to mount a non-specific cellular immune response.

Synergism between trematode infection and pesticide exposure: a link to amphibian limb deformities in nature?

The apparently rapid increase in the prevalence of amphibian limb deformities has led to substantial interest from ecologists and public health professionals. Hypotheses proposed to explain the deformities fall into two broad categories: chemical contaminants and trematode infection. Although there are convincing experimental demonstrations that certain factors can lead to some deformities, the causes for recent increases in amphibian malformation remain controversial. Moreover, no experimental studies on amphibian deformities have been conducted in the field, and no studies have attempted to examine the synergistic effects of trematode infection and exposure to chemical contaminants. Here, I present the results of field and laboratory experiments that link increased trematode infection, and increased limb deformities, to pesticide exposure. Field experiments conclusively demonstrated that exposure to trematode infection was required for the development of limb deformities in wood frogs, Rana sylvatica. However, deformities were more common at sites adjacent to agricultural runoff. Laboratory experiments corroborated the association between pesticide exposure and increased infection with pesticide-mediated immunocompetency as the apparent mechanism. Given the conservative contaminant exposure levels used [Environmental Protection Agency (EPA) drinking water standards] and the widespread use of many pesticides, these negative impacts may help to explain pathogen-mediated amphibian declines in many regions.

Implications of climate change for parasitism of animals in the aquatic environment

Climate change can occur over evolutionary and ecological time scales as a result of natural and anthropogenic causes. Considerable attention has been focused in recent years on the biological consequences of global warming. However, aside from studies on those deleterious parasites that cause disease in man, little effort has been dedicated to understanding the potential changes in the parasite fauna of animal populations, especially those in aquatic systems. Predictions using General Circulation Models, among others, are examined in terms of their consequences for parasite populations in freshwater and marine ecosystems, concentrating on the temperate and boreal regions of eastern North America. Biological effects due to global warming are not predictable simply in terms of temperature response. It is also essential to explore the effects on aquatic parasites of alterations in host distribution, water levels, eutrophication, stratification, ice cover, acidification, oceanic currents, ultraviolet-light penetration, weather extremes, and human interference. Evaluation of the potential response of parasites of aquatic organisms to climate change illustrates the complexity of host–parasite systems and the difficulty of making accurate predictions for biological systems. Parasites in aquatic systems will respond directly to changes in temperature but also indirectly to changes in other abiotic parameters that are mediated indirectly through changes in the distribution and abundance of their hosts. Local extirpations and introductions may be expected as a result. In the long term, climatic change may influence selection of different life-history traits, affecting parasite transmission and, potentially, virulence.

Zoonotic infections in Nigeria: overview from a medical perspective

Infections of domestic and wild animals that are transmitted directly or by an arthropod vector to humans are a major cause of morbidity and mortality worldwide and particularly in Nigeria. With a population of over 100 million and the need for improved health care delivery, Nigerians are at considerable risk considering the seriousness of these infections. Zoonotic infections that are endemic in Nigeria include tuberculosis, trypanosomiasis, toxoplasmosis, taeniasis, rabies, lassa fever and yellow fever. Zoonotic food-borne infections (caused by Campylobacter, Salmonella and Escherichia coli O157:H7) and cryptosporidiosis are emerging. Sporadic cases such as strongyloidiasis, ascariasis, leptospirosis, scabies, pentastomiasis and African histoplasmosis have been reported. There is a need to determine the prevalence of tick-borne zoonoses. Prevention and control of zoonoses in humans is by vaccination, treatment and health education. As a first measure to improve control, the link between veterinary and medical officers, which is presently very weak, needs to be strengthened. Furthermore, regional multidisciplinary approaches to the control of zoonotic infections should be adopted in West Africa, which take into consideration the huge inter-border traffic.

Tick modulation of host immunity: an important factor in pathogen transmission

Immunological interactions at the tick host interface involve innate and specific acquired host immune defenses and immunomodulatory countermeasures by the tick. Tick feeding stimulates host immune response pathways involving antigen-presenting cells, cytokines, B-cells, T-cells, circulating and homocytotropic antibodies, granulocytes, and an array of biologically active molecules. In response to host immune defenses, tick-mediated host immunosuppressive countermeasures inhibit: host antibody responses; complement activation; T-cell proliferation; and cytokine elaboration by macrophages and Th1-lymphocytes. Immunosuppressive proteins identified in tick salivary glands and saliva have been partially characterised. Tick-induced host immunosuppression facilitates blood meal acquisition and is an important factor in the transmission/establishment of the tick-borne disease-causing agent, Borrelia burgdorferi. A novel strategy for control of tick-borne pathogens is proposed.