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David H Walker

Publications and source records attributed to David H Walker.

At least 55 records · Page 3Linked to original sources
PubMed2004

Complete genome sequence of Rickettsia typhi and comparison with sequences of other rickettsiae.

Rickettsia typhi, the causative agent of murine typhus, is an obligate intracellular bacterium with a life cycle involving both vertebrate and invertebrate hosts. Here we present the complete genome sequence of R. typhi (1,111,496 bp) and compare it to the two published rickettsial genome sequences: R. prowazekii and R. conorii. We identified 877 genes in R. typhi encoding 3 rRNAs, 33 tRNAs, 3 noncoding RNAs, and 838 proteins, 3 of which are frameshifts. In addition, we discovered more than 40 pseudogenes, including the entire cytochrome c oxidase system. The three rickettsial genomes share 775 genes: 23 are found only in R. prowazekii and R. typhi, 15 are found only in R. conorii and R. typhi, and 24 are unique to R. typhi. Although most of the genes are colinear, there is a 35-kb inversion in gene order, which is close to the replication terminus, in R. typhi, compared to R. prowazekii and R. conorii. In addition, we found a 124-kb R. typhi-specific inversion, starting 19 kb from the origin of replication, compared to R. prowazekii and R. conorii. Inversions in this region are also seen in the unpublished genome sequences of R. sibirica and R. rickettsii, indicating that this region is a hot spot for rearrangements. Genome comparisons also revealed a 12-kb insertion in the R. prowazekii genome, relative to R. typhi and R. conorii, which appears to have occurred after the typhus (R. prowazekii and R. typhi) and spotted fever (R. conorii) groups diverged. The three-way comparison allowed further in silico analysis of the SpoT split genes, leading us to propose that the stringent response system is still functional in these rickettsiae.

Chromosome Inversion↗
PubMed2004

Rickettsia species infecting Amblyomma cooperi ticks from an area in the state of São Paulo, Brazil, where Brazilian spotted fever is endemic.

Owing to the potential role of the tick Amblyomma cooperi in the enzootic cycle of Rickettsia rickettsii, the etiologic agent of Brazilian spotted fever (BSF), this study evaluated infection by Rickettsia species in A. cooperi ticks collected from an area in Brazil where BSF is endemic. Among a total of 40 A. cooperi adult ticks collected in an area of BSF endemicity in the state of São Paulo, PCR analysis detected DNA of Rickettsia bellii in 16 ticks (40%), and 3 other ticks (7.5%) were positive for a previously unidentified spotted-fever-group (SFG) rickettsia. Cultivation in Vero cell cultures by the shell vial technique with individual A. cooperi ticks resulted in two isolates of R. bellii and one isolate genotypically characterized as an SFG rickettsia. The two R. bellii isolates were established in Vero cell cultures in the laboratory and were confirmed to be R. bellii by molecular analysis of the gltA and 17-kDa protein-encoding genes and by electron microscopic analysis. The SFG rickettsial isolate could not be stably passaged in cell culture in the laboratory, but molecular analysis of early passages suggested that it was closely related to Rickettsia parkeri, Rickettsia africae, and Rickettsia sibirica. These results do not support the role of A. cooperi in the ecology of R. rickettsii in the area studied, but they add two more species of rickettsiae to the poorly developed list of species occurring in ticks in South America.

Animals↗
PubMed2004

First serologic evidence of human spotted fever group rickettsiosis in Korea.

To investigate the prevalence of spotted fever group rickettsioses in Korea, a serosurvey of Japanese spotted fever rickettsiosis in patients with acute febrile illness was conducted with an indirect immunofluorescence assay. Overall, 19.88% of the patients were found to have polyvalent antibody against Rickettsia japonica. This study is the first documentation of spotted fever group rickettsiosis in Korea.

Adolescent↗
PubMed2004

Molecular evidence for a spotted fever group Rickettsia species in the tick Amblyomma longirostre in Brazil.

Two Amblyomma longirostre adult male ticks were collected from a Brazilian porcupine Coendou prehensilis L. in the state of Rondonia, Western Amazon, Brazil. The two ticks were pooled for DNA extraction and tested for the presence of rickettsial DNA by amplifying portions of the gltA, 17-kDa, ompA, and ompB rickettsial genes by polymerase chain reaction (PCR). Portions of the four genes were amplified from the sample and subsequently sequenced. These results indicated the presence of a Rickettsia strain infecting A. longirostre, which was designated as strain Aranha. Compared with homologous ompA rickettsial sequences, "Rickettsia amblyommii" ompA seemed to be the closest relative to Aranha (similarity values: 99.0-99.3%). Phylogenetic analyses of more conserved genes including 17-kDa and gtlA partial sequences indicated that this Rickettsia sp. is a spotted fever group rickettsia. The partial ompB sequence of strain Aranha was distinct from all homologous sequences available in GenBank. Although our ompA analysis suggested a very close molecular phylogenetic relationship of Aranha with "R. amblyommii," we cannot at this time determine if Aranha is a new strain of "R. amblyommii" or a new Rickettsia species in South America.

Animals↗
PubMed2004

Rickettsia bellii and Rickettsia amblyommii in Amblyomma ticks from the State of Rondônia, Western Amazon, Brazil.

This study evaluates the rickettsial presence in Amblyomma ticks from eight areas of the Amazon forest in Rondônia, Brazil. The following tick species (number in parentheses) were examined: Amblyomma ovale Koch (121), Amblyomma cajennense (F.) (41), Amblyomma naponense (Packard) (36), Amblyomma scalpturatum Neumann (35), Amblyomma oblongoguttatum Koch (30), Amblyomma incisum Neumann (27), Amblyomma rotundatum Koch (16), Amblyomma coelebs Neumann (10), and Amblyomma humerale Koch (6). Ticks were examined individually or in pools (2-10 ticks) by polymerase chain reaction (PCR) targeting the gltA gene. The PCR-determined minimal infection rate for each tick species was A. ovale 28%, A. cajennense 27%, A. naponense 0%, A. scalpturatum 11%, A. oblongoguttatum 3%, A. incisum 0%, A. rotundatum 87%, A. coelebs 10%, and A. humerale 50%. Partial sequences of the gltA gene of Rickettsia from A. ovale, A. scalpturatum, A. oblongoguttatum, A. rotundatum, and A. humerale were 99.9% (349/350) identical to Rickettsia bellii. DNA sequences of PCR products from A. cajennense and A. coelebs were 100% (350/350) identical to Rickettsia amblyommii. R. bellii organisms were isolated in Vero cells from A. scalpturatum, A. ovale, A. rotundatum, and A. oblongoguttatum, but only one of the isolates, cultured from A. scalpturatum, was established in continuous cell culture passage. R. amblyommii was isolated from A. cajennense and was successfully established in continuous passage in cell culture. R. amblyommii infection of Vero cells was analyzed by transmission electron microscopy. This study adds South America to the known geographic distribution of R. amblyommii and reports rickettsiae in six Amblyomma species for the first time.

Animals↗
PubMed2004

Acute spotted fever rickettsiosis among febrile patients, Cameroon.

Although potential arthropod vectors are abundant in Cameroon, acute febrile illnesses are rarely evaluated for arboviral or rickettsial infections. Serum samples from 234 acutely febrile patients at clinics in Tiko and Buea, Cameroon, were examined for antibodies to Rickettsia africae and African alphaviruses and flaviviruses. These serum samples did not contain antibodies against typhoid, and blood malarial parasites were not detected. Serum samples of 32% contained immunoglobulin M antibodies reactive with R. africae by immunofluorescence assay and were reactive with outer membrane proteins A and B of R. africae by immunoblotting. These findings established a diagnosis of acute rickettsiosis, most likely African tick-bite fever. Hemagglutination inhibition testing of the serum samples also detected antibodies to Chikungunya virus (47%) and flaviviruses (47%). High prevalence of antibodies to arboviruses may represent a major, previously unrecognized public health problem in an area where endemic malaria and typhoid fever have been the principal diagnostic considerations.

Adolescent↗
PubMed2004

Staff training and computers in respiratory care.

The respiratory therapists' (RTs') knowledge base is among a hospital's greatest assets. RTs routinely use advanced medical technologies and must understand the patient's physiology, anatomy, and condition to deliver safe and effective treatment with those technologies, especially within critical care environments. Also RTs often must educate patients and patients' families, which requires thorough understanding of the operation of medical equipment. RT education must include both tacit and explicit knowledge, so RT education can be complex, which may present challenges to clinical educators. RTs must have continuing education, and computer-based education can help meet education challenges, promote safe and effective patient care, improve patient outcomes, and improve employee satisfaction, which may improve RT recruitment and retention, while decreasing the cost of staff training. However, since computer-based education is relatively new to health care, RT educators should learn from other industries that have extensive experience with computer-based staff education.

Computer-Assisted Instruction↗
PubMed2004

Prevalence of antibodies to spotted fever group rickettsiae in humans and domestic animals in a Brazilian spotted fever-endemic area in the state of São Paulo, Brazil: serologic evidence for infection by Rickettsia rickettsii and another spotted fever group Rickettsia.

In serum samples obtained from all the healthy humans, horses, dogs, and donkeys present on three farms in the Pedreira Municipality, an endemic area for Brazilian spotted fever, an indirect immunofluorescence assay (IFA) detected antibodies against Rickettsia rickettsii in 17 (77.3%) horses, 5 (31.3%) dogs (titers ranging from 64 to 4,048), and none of 4 donkeys or 50 humans. Five canine and eight equine sera with high antibody titers to R. rickettsii were also tested by IFA against R. bellii, R. akari, and R. africae antigens. Sera from two horses and two dogs that showed similar high antibody titers against two rickettsial antigens were evaluated after cross-absorption. Sera from seven horses and two dogs contained antibodies specific for R. rickettsii, and one dog serum had antibodies against a Rickettsia species very closely related to R. africae. The latter may have been caused by infection with the recently identified COOPERI strain.

Animals↗
PubMed2004

Detection of Rickettsia africae in patients and ticks along the coastal region of Cameroon.

Rickettsia africae was identified in seven (6%) of 118 patients with acute fevers of unknown etiology proven not to be malaria or typhoid fever from clinics along the coastal region of Cameroon by polymerase chain reaction (PCR) amplification and sequencing of the citrate synthase (gltA) and outer membrane protein A (ompA) genes of Rickettsia. The majority (71%) of the patients were female. Clinical manifestations included fever (100%), headache (71%), myalgia (71%), arthralgia (43%), pulmonary involvement (29%), and diffuse rash (14%). Moreover, R. africae was detected by PCR amplification and sequence analysis of the gltA and ompA genes in 62 (75%) of 83 adult Amblyomma variegatum ticks collected from cattle in the same region. These results confirm the presence of a previously unrecognized infectious disease in the indigenous Cameroonian population, as well as extend the established range of R. africae.

Adult↗
PubMed2004

Effect of blocking the CXCL9/10-CXCR3 chemokine system in the outcome of endothelial-target rickettsial infections.

Rickettsiae cause systemic infections such as Rocky Mountain spotted fever and boutonneuse fever. The main cellular target of these obligately intracellular bacteria is the endothelium. T lymphocytes are the most important effectors of immunity, and the CXCR3 ligands CXCL9 and CXCL10 may play an important role in the T cell-mediated clearance of rickettsiae from the infected vasculature as suggested by recent expression studies. Here we showed that antibody-mediated neutralization of CXCL9 and CXCL10, and CXCR3 gene knockout, had no effect on survival or bacterial loads of mice infected with rickettsiae. We also demonstrated that rickettsiae triggered the endothelial expression of intercellular adhesion molecule 1 and vascular cell adhesion molecule 1 in vivo. These findings suggested that antigenic presentation by endothelial cells together with an endothelial inflammatory phenotype induced by the rickettsial infection may be sufficient to arrest T cells and trigger their anti-rickettsial effector mechanisms without the need for chemokines.

Animals↗
PubMed2004

Ehrlichia chaffeensis: a prevalent, life-threatening, emerging pathogen.

Ehrlichia chaffeensis are small, obligately intracellular, endosomal bacteria with tropism for macrophages. Persistent infection in reservoir white-tailed deer is transmitted by lone star ticks. Flu-like illness can progress to severe multisystem disease with toxic shock-like syndrome, meningitis, or ARDS. The case-fatality rate is 2.7%. Leukopenia and thrombocytopenia are diagnostically useful. Granulomas are associated with control of the infection. Ehrlichial proteins and glycoproteins have been sequenced and expressed for diagnostic serology and vaccine development. Mouse models (mild disease and persistent infection with E. muris and fatal monocytotropic ehrlichiosis with a Japanese tick isolate) revealed that CD4 and CD8 T type 1 lymphocyte responses, IFN-gamma, TNF-alpha, and antibodies play roles in protective immunity, while a weak CD4 T-helper response, overproduction of TNF-alpha, and very high IL-10 are associated with toxic shock-like mortality. Protection against fatal ehrlichiosis was achieved by prior infection with low virulence E. muris. Acute clinical diagnosis is difficult except by PCR. Response to doxycycline is dramatic.

Animals↗
PubMed2003

Rickettsia sibirica infection in members of scientific expeditions to northern Asia.

The risk of acquiring North Asian tick typhus (infection by Rickettsia sibirica) during travel to regions of Asia where this disease is endemic is unknown. We investigated prospectively 13 paleontologists on expedition to Mongolia. Four paleontologists had acute illness characterised by fever, rash, headache, and lymphadenopathy. All had IgM and IgG antibodies to R sibirica. Paleontologists with no illness and people who went on expeditions in other parts of the world did not have antibodies to R sibirica. Only two of the four infected persons were aware of tick bites. Travellers to regions endemic for R sibirica are at risk of contracting North Asian tick typhus even in the absence of recognised tick-bites.

Aldehyde Oxidoreductases↗
PubMed2003

Cross-protection between distantly related spotted fever group rickettsiae.

Two excellent C3H/HeN mouse models of spotted fever rickettsioses caused by the distantly related organisms, Rickettsia conorii and Rickettsia australis, were utilized to evaluate the possibility of the stimulation of broad cross-protective immunity. Sublethal infection stimulated complete immunity, that is absence of disease, after challenge with a dose of the heterologous Rickettsia that uniformly killed naïve mice. In contrast, heterologous immune sera did not protect mice against a lethal dose (two LD50) of rickettsiae in the mouse toxicity neutralization assay, the standard method for evaluation of rickettsial vaccine potency. These observations suggest that development of a broadly protective vaccine against spotted fever group (SFG) rickettsiae is feasible, and the results indicate that mouse toxicity neutralization is an inappropriate method for evaluation of candidate vaccines.

Animals↗
PubMed2003

Expression analysis of the T-cell-targeting chemokines CXCL9 and CXCL10 in mice and humans with endothelial infections caused by rickettsiae of the spotted fever group.

Rocky Mountain spotted fever and other related diseases are systemic infections caused by rickettsiae. These obligatory intracellular bacteria target the endothelium, offering an appealing model to study the interactions between endothelial cells and T lymphocytes. We investigated the mRNA expression of chemokines known to target CD8+ T cells and CD4(+) T-helper 1 cells in the lungs of C3H/HeN mice infected with Rickettsia conorii with the purpose of identifying evidence for a role of chemokines in the immune clearance of rickettsiae from the vasculature. The expression of the CXCR3 ligands CXCL9 and CXCL10 was significantly higher than the other chemokines investigated. We validated the relevance of these results in the animal model through the analysis of tissues from humans with Rocky Mountain spotted fever. We then characterized the kinetics and localization of expression of CXCL9 and CXCL10 in lungs, brain, and liver of mice infected with lethal or sublethal doses of R. conorii by a combination of quantitative real-time polymerase chain reaction and immunohistochemistry. Interestingly, the peak of expression of these chemokines occurred 4 days before CD8+ T cells infiltrated the infected tissues. Our results suggest that CXCL9 and CXCL10 may play a role early during the immune response against rickettsial infections.

Animals↗
PubMed2003

Pathogenic mechanisms of diseases caused by Rickettsia.

The specter of bioterrorism employing genetically engineered Rickettsia resistant to all antibiotics should reawaken the world's desire to elucidate the pathogenesis of typhus and spotted fever rickettsioses in a search for mechanisms vulnerable to interdiction. The pathogenetic sequence includes rickettsial entry into the dermis, hematogenous dissemination to vascular endothelial cells (most critically in brain and lungs), increased vascular permeability, edema, and immunity mediated by NK cells, IFN-gamma, TNF-alpha, RANTES, antibodies, and cytotoxic T lymphocytes. Silverman has demonstrated the role of reactive oxygen species (ROS) produced by R. rickettsii-infected endothelial cells in peroxidative damage to cell membranes in vitro, and Heinzen has described actin-based rickettsial intracellular mobility and intercellular spread. At this point the availability of sequences of rickettsial genomes and excellent animal models of rickettsioses have yielded insufficient progress towards the identification of rickettsial virulence factors and knowledge of the importance of injury mediated by ROS, phospholipase A(2), protease(s) or other mechanisms in vivo. Attention to the rickettsiosis-associated procoagulant state led to determination that hemostatic mechanisms largely prevent major hemorrhage without disseminated intravascular coagulation or thrombosis-mediated ischemia. Particularly lacking is knowledge of early events in vivo at the portal of entry in skin (or lung), of the effects of the inoculum medium (arthropod saliva or feces), mediators produced by infected endothelium under conditions of flow and of the contributions in vivo of immune effectors to pathology, of the role of apoptosis in rickettsial infection, and of the endothelial cell alterations that account for increased vascular permeability. The host cell receptor for the Rickettsia ligand and the mechanism of rickettsial escape from the phagosome need to be elucidated.

Chronic Disease↗
PubMed2003

Rickettsiosis of the genus Rickettsia in South America.

In South America, human cases of infection by the genus Rickettsia have been described in several countries in the last twenty years. The role of international organizations, such as the Centers for Disease Control and Prevention in Atlanta, Georgia, USA and the World Health Organization Collaborating Center for Tropical Diseases at the University of Texas Medical Branch at Galveston, Texas, USA, was very important in the last twenty years for the development of surveillance systems and for the increase in notification of rickettsial diseases by the countries of South America. We hope that the next goal will be prevention and control of rickettsial diseases in the countries of South America, as well as maintaining the programs developed during the last twenty years, so that a good health system and improved social conditions will be possible.

Animals↗
PubMed2003

Novel Immunoreactive glycoprotein orthologs of Ehrlichia spp.

We previously identified a strongly immunoreactive 43 kDa protein (p43) of Ehrlichia canis. As an immunodiagnostic antigen, the p43 had a 96% accuracy as compared with IFA and provided species-specific diagnosis of E. canis infections. Further investigation has revealed that the E. canis p43 represents the N-terminal portion of the largest immunoreactive protein described in Ehrlichia spp. with a predicted molecular mass of 153 kDa. Analysis of the recombinant N-terminal region (p43) of the p153 by protein gel electrophoresis demonstrated a larger than predicted molecular mass (approximately 30%), and presence of carbohydrate glycans, indicating that the p153 is a glycoprotein. A BLASTn search was performed on the E. chaffeensis genome sequence (95%), and the gene encoding the p153 ortholog was identified in E. chaffeensis. The E. canis p153 (4,263 bp) and E. chaffeensis p156 (4,389 bp) genes had similar chromosomal locations, downstream of the homologous (approximately 87%) deoxyguanosine-triphosphate triphosphohydrolase genes, and homologous (approximately 90%) intergenic sequences preceding the open reading frames. Nucleic acid sequence homology (52%) observed between the glycoprotein genes supported previous findings with regard to genetic divergence of the p43 gene fragment, and the p153 and p156 proteins had amino acid similarity of 32%. A native E. canis protein with a molecular mass of 200 kDa reacted with antisera produced against the N-terminal region (p43) of the p153, suggesting that the native protein was posttranslationally modified. Similarly, recombinant constructs of E. chaffeensis p156 migrated larger than predicted (approximately 200 kDa), and carbohydrate was detected on the recombinant proteins. The chromosomal location, amino acid homology, and biophysical properties support the conclusion that the p153 and p156 glycoproteins (designated gp200s) are species-specific immunoreactive orthologs.

Amino Acid Sequence↗