Cross-Immunity and Community Structure of a Multiple-Strain Pathogen in the Tick Vector

Many vector-borne pathogens consist of multiple strains that circulate in both the vertebrate host and the arthropod vector. Characterization of the community of pathogen strains in the arthropod vector is therefore important for understanding the epidemiology of mixed vector-borne infections. Borrelia afzelii and B. garinii are two species of tick-borne bacteria that cause Lyme disease in humans. These two sympatric pathogens use the same tick, Ixodes ricinus, but are adapted to different classes of vertebrate hosts. Both Borrelia species consist of multiple strains that are classified using the highly polymorphic ospC gene. Vertebrate cross-immunity against the OspC antigen is predicted to structure the community of multiple-strain Borrelia pathogens. Borrelia isolates were cultured from field-collected I. ricinus ticks over a period spanning 11 years. The Borrelia species of each isolate was identified using a reverse line blot (RLB) assay. Deep sequencing was used to characterize the ospC communities of 190 B. afzelii isolates and 193 B. garinii isolates. Infections with multiple ospC strains were common in ticks, but vertebrate cross-immunity did not influence the strain structure in the tick vector. The pattern of genetic variation at the ospC locus suggested that vertebrate cross-immunity exerts strong selection against intermediately divergent ospC alleles. Deep sequencing found that more than 50% of our isolates contained exotic ospC alleles derived from other Borrelia species. Two alternative explanations for these exotic ospC alleles are cryptic coinfections that were not detected by the RLB assay or horizontal transfer of the ospC gene between Borrelia species.     

Active Forest Management Reduces Blacklegged Tick and Tick-Borne Pathogen Exposure Risk

EcoHealth - In the northeastern USA, active forest management can include timber harvests designed to meet silvicultural objectives (i.e., harvesting trees that meet certain maturity, height, age,...     

Cross-Species Pathogen Transmission and Disease Emergence in Primates

Many of the most virulent emerging infectious diseases in humans, e.g., AIDS and Ebola, are zoonotic, having shifted from wildlife populations. Critical questions for predicting disease emergence are: (1) what determines when and where a disease will first cross from one species to another, and (2) which factors facilitate emergence after a successful host shift. In wild primates, infectious diseases most often are shared between species that are closely related and inhabit the same geographic region. Therefore, humans may be most vulnerable to diseases from the great apes, which include chimpanzees and gorillas, because these species represent our closest relatives. Geographic overlap may provide the opportunity for cross-species transmission, but successful infection and establishment will be determined by the biology of both the host and pathogen. We extrapolate the evolutionary relationship between pathogen sharing and divergence time between primate species to generate “hotspot” maps, highlighting regions where the risk of disease transfer between wild primates and from wild primates to humans is greatest. We find that central Africa and Amazonia are hotspots for cross-species transmission events between wild primates, due to a high diversity of closely related primate species. Hotspots of host shifts to humans will be most likely in the forests of central and west Africa, where humans come into frequent contact with their wild primate relatives. These areas also are likely to sustain a novel epidemic due to their rapidly growing human populations, close proximity to apes, and population centers with high density and contact rates among individuals.     

Proteomic Screening of Antigenic Proteins from the Hard Tick,Haemaphysalis longicornis (Acari: Ixodidae)

Proteomic tools allow large-scale, high-throughput analyses for the detection, identification, and functional investigation of proteome. For detection of antigens from Haemaphysalis longicornis, 1-dimensional electrophoresis (1-DE) quantitative immunoblotting technique combined with 2-dimensional electrophoresis (2-DE) immunoblotting was used for whole body proteins from unfed and partially fed female ticks. Reactivity bands and 2-DE immunoblotting were performed following 2-DE electrophoresis to identify protein spots. The proteome of the partially fed female had a larger number of lower molecular weight proteins than that of the unfed female tick. The total number of detected spots was 818 for unfed and 670 for partially fed female ticks. The 2-DE immunoblotting identified 10 antigenic spots from unfed females and 8 antigenic spots from partially fed females. Matrix Assisted Laser Desorption Ionization-Time of Flight Mass Spectrometry (MALDI-TOF) of relevant spots identified calreticulin, putative secreted WC salivary protein, and a conserved hypothetical protein from the National Center for Biotechnology Information and Swiss Prot protein sequence databases. These findings indicate that most of the whole body components of these ticks are non-immunogenic. The data reported here will provide guidance in the identification of antigenic proteins to prevent infestation and diseases transmitted by H. longicornis.     

Influence of the Biotope on the Tick Infestation of Cattle and on the Tick-Borne Pathogen Repertoire of Cattle Ticks in Ethiopia

Background

The majority of vector-borne infections occur in the tropics, including Africa, but molecular eco-epidemiological studies are seldom reported from these regions. In particular, most previously published data on ticks in Ethiopia focus on species distribution, and only a few molecular studies on the occurrence of tick-borne pathogens or on ecological factors influencing these. The present study was undertaken to evaluate, if ticks collected from cattle in different Ethiopian biotopes harbour (had access to) different pathogens.

Methods

In South-Western Ethiopia 1032 hard ticks were removed from cattle grazing in three kinds of tick biotopes. DNA was individually extracted from one specimen of both sexes of each tick species per cattle. These samples were molecularly analysed for the presence of tick-borne pathogens.

Results

Amblyomma variegatum was significantly more abundant on mid highland, than on moist highland. Rhipicephalus decoloratus was absent from savannah lowland, where virtually only A. cohaerens was found. In the ticks Coxiella burnetii had the highest prevalence on savannah lowland. PCR positivity to Theileria spp. did not appear to depend on the biotope, but some genotypes were unique to certain tick species. Significantly more A. variegatum specimens were rickettsia-positive, than those of other tick species. The presence of rickettsiae (R. africae) appeared to be associated with mid highland in case of A. variegatum and A. cohaerens. The low level of haemoplasma positivity seemed to be equally distributed among the tick species, but was restricted to one biotope type.

Conclusions

The tick biotope, in which cattle are grazed, will influence not only the tick burden of these hosts, but also the spectrum of pathogens in their ticks. Thus, the presence of pathogens with alternative (non-tick-borne) transmission routes, with transstadial or with transovarial transmission by ticks appeared to be associated with the biotope type, with the tick species, or both, respectively.     

Multiple and Diverse vsp and vlp Sequences in Borrelia miyamotoi,a Hard Tick-Borne Zoonotic Pathogen

Based on chromosome sequences, the human pathogen Borrelia miyamotoi phylogenetically clusters with species that cause relapsing fever. But atypically for relapsing fever agents, B. miyamotoi is transmitted not by soft ticks but by hard ticks, which also are vectors of Lyme disease Borrelia species. To further assess the relationships of B. miyamotoi to species that cause relapsing fever, I investigated extrachromosomal sequences of a North American strain with specific attention on plasmid-borne vsp and vlp genes, which are the underpinnings of antigenic variation during relapsing fever. For a hybrid approach to achieve assemblies that spanned more than one of the paralogous vsp and vlp genes, a database of short-reads from next-generation sequencing was supplemented with long-reads obtained with real-time DNA sequencing from single polymerase molecules. This yielded three contigs of 31, 16, and 11 kb, which each contained multiple and diverse sequences that were homologous to vsp and vlp genes of the relapsing fever agent B. hermsii. Two plasmid fragments had coding sequences for plasmid partition proteins that differed from each other from paralogous proteins for the megaplasmid and a small plasmid of B. miyamotoi. One of 4 vsp genes, vsp1, was present at two loci, one of which was downstream of a candiate prokaryotic promoter. A limited RNA-seq analysis of a population growing in the blood of mice indicated that of the 4 different vsp genes vsp1 was the one that was expressed. The findings indicate that B. miyamotoi has at least four types of plasmids, two or more of which bear vsp and vlp gene sequences that are as numerous and diverse as those of relapsing fever Borrelia. The database and insights from these findings provide a foundation for further investigations of the immune responses to this pathogen and of the capability of B. miyamotoi for antigenic variation.     

带有饱和接触率和垂直传染的两病株病原体的共存性（英文）

讨论了带有饱和接触率和垂直传染的两病株病原体的共存性,获得了共存性存在的充要条件R_2（N_2~＊）〈R_1（N_2~＊）,R_1（N_1~＊）〈R_2（N_1~＊）,并且在共存平衡点存在的条件下证明得到其局部稳定.     

Heterogeneity,Mixing, and the Spatial Scales of Mosquito-Borne Pathogen Transmission

The Ross-Macdonald model has dominated theory for mosquito-borne pathogen transmission dynamics and control for over a century. The model, like many other basic population models, makes the mathematically convenient assumption that populations are well mixed; i.e., that each mosquito is equally likely to bite any vertebrate host. This assumption raises questions about the validity and utility of current theory because it is in conflict with preponderant empirical evidence that transmission is heterogeneous. Here, we propose a new dynamic framework that is realistic enough to describe biological causes of heterogeneous transmission of mosquito-borne pathogens of humans, yet tractable enough to provide a basis for developing and improving general theory. The framework is based on the ecological context of mosquito blood meals and the fine-scale movements of individual mosquitoes and human hosts that give rise to heterogeneous transmission. Using this framework, we describe pathogen dispersion in terms of individual-level analogues of two classical quantities: vectorial capacity and the basic reproductive number, . Importantly, this framework explicitly accounts for three key components of overall heterogeneity in transmission: heterogeneous exposure, poor mixing, and finite host numbers. Using these tools, we propose two ways of characterizing the spatial scales of transmission—pathogen dispersion kernels and the evenness of mixing across scales of aggregation—and demonstrate the consequences of a model''s choice of spatial scale for epidemic dynamics and for estimation of , both by a priori model formulas and by inference of the force of infection from time-series data.     

Evaluating the Effect of Environmental Factors on Pathogen Regrowth in Compost Extract

Pathogenic microorganisms may survive the composting process in low numbers and subsequently regrow to high levels under favorable conditions. The objective of this study was to investigate the regrowth potential of Salmonella spp., Escherichia coli O157:H7, and Listeria monocytogenes in dairy-based composts under different environmental conditions. Water extract of commercially available dairy compost was used as a model system. Cocktails of five rifampin-resistant strains of each pathogen previously grown in reduced nutrient media (1/2 or 1/10 strength of tryptic soy broth, TSB) were inoculated into water extract of compost of different ratios (1:2,1:5, and 1:10, w/v), and then stored at 35°C or 22°C for 7 days. The strains exhibiting greatest survival or regrowth were identified by pulsed-field gel electrophoresis (PFGE). At 22°C, both E. coli O157:H7 and L. monocytogenes multiplied in all compost extracts, whereas Salmonella spp. regrew in both 1:2 and 1:5 compost extracts but not in 1:10. For all three pathogens, incubation at 22°C provides better conditions for regrowth than at 35°C. Both Salmonella and E. coli O157:H7 previously adapted to nutrient-limited broth (1/10 strength of TSB) regrew in compost extracts to higher populations than the control cultures grown previously in full strength of TSB. In the absence of indigenous microorganisms, all three pathogens regrew even in the most diluted sterile compost extract (1:10) with growth potentials ranging from 2.30 to 3.59 log CFU/ml. In nonsterile compost extract with ca. 5 log CFU/ml of background microorganisms, all three pathogens regrew only in the most concentrated compost extract (1:2) with much less population increases ranging from 0.70 to 1.43 log CFU/ml. Compost extract samples of all ages supported the regrowth of both Salmonella and E. coli O157:H7 with population increases ranging from 0.95 to 2.32 log CFU/ml. The PFGE patterns for E. coli O157:H7 isolates from sterile compost extracts matched with either the spinach outbreak strain or an avirulent B6914 strain. These results demonstrated that compost extract of dairy-based compost contained sufficient nutrients for pathogen regrowth. Cultures previously adapted to low nutrient media regrew to higher populations than control cultures; however, indigenous microflora suppressed the pathogen regrowth in compost extract, especially at 35°C.     

Persistence of Pathogens with Short Infectious Periods in Seasonal Tick Populations: The Relative Importance of Three Transmission Routes

Background

The flaviviruses causing tick-borne encephalitis (TBE) persist at low but consistent levels in tick populations, despite short infectious periods in their mammalian hosts and transmission periods constrained by distinctly seasonal tick life cycles. In addition to systemic and vertical transmission, cofeeding transmission has been proposed as an important route for the persistence of TBE-causing viruses. Because cofeeding transmission requires ticks to feed simultaneously, the timing of tick activity may be critical to pathogen persistence. Existing models of tick-borne diseases do not incorporate all transmission routes and tick seasonality. Our aim is to evaluate the influence of seasonality on the relative importance of different transmission routes by using a comprehensive mathematical model.

Methodology/Principal Findings

We developed a stage-structured population model that includes tick seasonality and evaluated the relative importance of the transmission routes for pathogens with short infectious periods, in particular Powassan virus (POWV) and the related “deer tick virus,” emergent encephalitis-causing flaviviruses in North America. We used the next generation matrix method to calculate the basic reproductive ratio and performed elasticity analyses. We confirmed that cofeeding transmission is critically important for such pathogens to persist in seasonal tick populations over the reasonable range of parameter values. At higher but still plausible rates of vertical transmission, our model suggests that vertical transmission can strongly enhance pathogen prevalence when it operates in combination with cofeeding transmission.

Conclusions/Significance

Our results demonstrate that the consistent prevalence of POWV observed in tick populations could be maintained by a combination of low vertical, intermediate cofeeding and high systemic transmission rates. When vertical transmission is weak, nymphal ticks support integral parts of the transmission cycle that are critical for maintaining the pathogen. We also extended the model to pathogens that cause chronic infections in hosts and found that cofeeding transmission could contribute to elevating prevalence even in these systems. Therefore, the common assumption that cofeeding transmission is not relevant in models of chronic host infection, such as Lyme disease, could lead to underestimating pathogen prevalence.