David H Walker

University of Texas Medical Branch at Galveston, Galveston, Texas, United States

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Publications (156)876.38 Total impact

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    Marcelo B Labruna, David H Walker
    Emerging infectious diseases 10/2014; 20(10):1768-1769. · 5.99 Impact Factor
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    ABSTRACT: Scrub typhus is a neglected, but important, tropical disease, which puts one-third of the world's population at risk. The disease is caused by Orientia tsutsugamushi, an obligately intracellular Gram-negative bacterium. Dysregulation in immune responses is known to contribute to disease pathogenesis; however, the nature and molecular basis of immune alterations are poorly defined. This study made use of a newly developed murine model of severe scrub typhus and focused on innate regulators and vascular growth factors in O. tsutsugamushi-infected liver, lungs and spleen. We found no activation or even reduction in base-line expression for multiple molecules (IL-7, IL-4, IL-13, GATA3, ROR-ct, and CXCL12) at 2, 6 and 10 days post-infection. This selective impairment in type 2-related immune responses correlated with a significant activation of the genes for IL-1b, IL-6, IL-10, TNF-a, IFN-c, as well as CXCR3-and CXCR1-related chemokines in inflamed tissues. The elevated angiopoietin (Ang)-2 expression and Ang-2/Ang-1 ratios suggested excessive inflammation and the loss of endothelial integrity. These alterations, together with extensive recruitment of myeloperoxidase (MPO)-expressing neutrophils and the influx of CD3 + T cells, contributed to acute tissue damage and animal death. This is the first report of selective alterations in a panel of immune regulators during early O. tsutsugamushi infection in intravenously inoculated C57BL/6 mice. Our findings shed new light on the pathogenic mechanisms associated with severe scrub typhus and suggest potential targets for therapeutic investigation. Citation: Soong L, Wang H, Shelite TR, Liang Y, Mendell NL, et al. (2014) Strong Type 1, but Impaired Type 2, Immune Responses Contribute to Orientia tsutsugamushi-Induced Pathology in Mice. PLoS Negl Trop Dis 8(9): e3191. doi:10.1371/journal.pntd.0003191 Copyright: ß 2014 Soong et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability: The authors confirm that all data underlying the findings are fully available without restriction. All relevant data are within the paper and its Supporting Information files. Funding: This work was supported in part by a pilot grant from the University of Texas Medical Branch Center for Biodefense and Emerging Infectious Diseases and start-up funds (to LS), the Carmage and Martha Walls Distinguished University Chair in Tropical Diseases (to DHW), as well as the McLaughlin post-doctoral fellowship (to TRS). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist.
    PLoS Neglected Tropical Diseases 09/2014; 8(9):e3191. · 4.57 Impact Factor
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    ABSTRACT: Rickettsia massiliae, belonging to the spotted fever group of Rickettsia, is a human pathogen causing a similar course of disease to that caused by R. conorii, the originally recognized etiologic agent of Mediterranean spotted fever. In view of this similarity, we performed an ultrastructural study of R. massiliae in organs of Rhipicephalus sanguineus ticks, in order to advance knowledge of the complex dynamics at the tick-pathogen interface in rickettsioses. Adult R. massiliae-infected Rh. sanguineus ticks were fed on uninfected Hartley strain guinea pigs, and five females were collected daily throughout their feeding period up to day 6, and analyzed by quantitative real-time PCR and electron microscopy. An increase in rickettsial content was observed in the salivary glands, particularly in the first two days of feeding, and a plateau was observed between days 3 and 6. Rickettsial organisms were observed in all tick organs analyzed, in higher numbers in the fed state, and statistically significant differences were observed in measurements of the periplasmic layer of R. massiliae in salivary glands of fed and unfed Rh. sanguineus ticks, with increased thickness in the former case. This study provides insight into the interface between R. massiliae and Rh. sanguineus ticks, highlighting the need for analysis of R. massiliae to fully ascertain its place as an important pathogenic agent of a spotted fever rickettsiosis.
    Ticks and Tick-borne Diseases 08/2014; · 2.35 Impact Factor
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    ABSTRACT: Abstract Rocky Mountain spotted fever (RMSF) is a severe illness caused by Rickettsia rickettsii for which there is no available vaccine. We hypothesize that exposure to the highly prevalent, relatively nonpathogenic "Rickettsia amblyommii" protects against R. rickettsii challenge. To test this hypothesis, guinea pigs were inoculated with "R. amblyommii." After inoculation, the animals showed no signs of illness. When later challenged with lethal doses of R. rickettsii, those previously exposed to "R. amblyommii" remained well, whereas unimmunized controls developed severe illness and died. We conclude that "R. amblyommii" induces an immune response that protects from illness and death in the guinea pig model of RMSF. These results provide a basis for exploring the use of low-virulence rickettsiae as a platform to develop live attenuated vaccine candidates to prevent severe rickettsioses.
    Vector borne and zoonotic diseases (Larchmont, N.Y.) 08/2014; 14(8):557-62. · 2.61 Impact Factor
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    ABSTRACT: Orientia tsutsugamushi, the etiologic agent of scrub typhus, is a mite-borne rickettsia transmitted by the parasitic larval stage of trombiculid mites. Approximately one-third of the world's population is at risk of infection with Orientia tsutsugamushi, emphasizing its importance in global health. In order to study scrub typhus, Orientia tsutsugamushi Karp strain has been used extensively in mouse studies with various inoculation strategies and little success in inducing disease progression similar to that of human scrub typhus. The objective of this project was to develop a disease model with pathology and target cells similar to those of severe human scrub typhus. This study reports an intravenous infection model of scrub typhus in C57BL/6 mice. This mouse strain was susceptible to intravenous challenge, and lethal infection occurred after intravenous inoculation of 1.25610 6 focus (FFU) forming units. Signs of illness in lethally infected mice appeared on day 6 with death occurring ,6 days later. Immunohistochemical staining for Orientia antigens demonstrated extensive endothelial infection, most notably in the lungs and brain. Histopathological analysis revealed cerebral perivascular, lymphohistiocytic infiltrates, focal hemorrhages, meningoencephalitis, and interstitial pneumonia. Disseminated infection of endothelial cells with Orientia in C57BL/6 mice resulted in pathology resembling that of human scrub typhus. The use of this model will allow detailed characterization of the mechanisms of immunity to and pathogenesis of O. tsutsugamushi infection. Copyright: ß 2014 Shelite et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability: The authors confirm that all data underlying the findings are fully available without restriction. All relevant data are within the paper and its Supporting Information files. Funding: This project was funded by the T32-AI060549 Biodefense Training Grant and the Carmage and Martha Walls Distinguished University Chair in Tropical Diseases. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist.
    PLoS Neglected Tropical Diseases 07/2014; 8(7):e2966. · 4.57 Impact Factor
  • The Journal of Infectious Diseases 07/2014; · 5.85 Impact Factor
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    David H Walker
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    ABSTRACT: Presidential address given at the November 2013 ASTMH annual meeting in Washington DC.
    The American journal of tropical medicine and hygiene 03/2014; · 2.53 Impact Factor
  • Lucas S Blanton, David H Walker, Donald H Bouyer
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    ABSTRACT: Abstract Tick-borne diseases, such as spotted fever rickettsioses and ehrlichioses, are potentially severe and life-threatening infections. The incidences of these infections increase during warm weather months as ticks become active. Clinicians often consider outdoor activities in rural areas to be a risk factor for exposure to ticks and the pathogens they carry, but are those who live, work, and play within an urban environment excluded from this risk? In this study, we collected ticks from two urban parks in Little Rock, AR, to assess the presence of rickettsiae and ehrlichiae within an urban setting. A total of 273 ticks were collected during July, 2011. Amblyomma americanum was the predominant tick species, with 255 (93%) of those collected. The remaining 18 (7%) were Dermacentor variabilis. Ticks were separated and pooled into groups for further testing. Forty-two of the 43 (98%) A. americanum pools demonstrated molecular evidence for the presence of rickettsiae. None of the D. variabilis contained rickettsiae. Restriction enzyme fragment length polymorphism analysis and DNA sequencing revealed Rickettsia amblyommii to be the species present. One A. americanum pool from park A demonstrated the presence of Ehrlichia chaffeensis, the pathogen responsible for human monocytotropic ehrlichiosis. These results indicate that tick-borne pathogens are not limited to rural or suburban areas.
    Vector borne and zoonotic diseases (Larchmont, N.Y.) 02/2014; 14(2):168-70. · 2.61 Impact Factor
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    ABSTRACT: We report here the complete genome sequence of Ehrlichia muris strain AS145(T), which was isolated from a wild mouse in 1983 in Japan. E. muris establishes persistent infections in laboratory mice and is widely used as a surrogate pathogen in a murine model of ehrlichiosis.
    Genome announcements. 01/2014; 2(1).
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    ABSTRACT: Louse borne typhus (also called epidemic typhus) was one of man's major scourges, and epidemics of the disease can be reignited when social, economic, or political systems are disrupted. The fear of a bioterrorist attack using the etiologic agent of typhus, Rickettsia prowazekii, was a reality. An attenuated typhus vaccine, R. prowazekii Madrid E strain, was observed to revert to virulence as demonstrated by isolation of the virulent revertant Evir strain from animals which were inoculated with Madrid E strain. The mechanism of the mutation in R. prowazekii that affects the virulence of the vaccine was not known. We sequenced the genome of the virulent revertant Evir strain and compared its genome sequence with the genome sequences of its parental strain, Madrid E. We found that only a single nucleotide in the entire genome was different between the vaccine strain Madrid E and its virulent revertant strain Evir. The mutation is a single nucleotide insertion in the methyltransferase gene (also known as PR028) in the vaccine strain that inactivated the gene. We also confirmed that the vaccine strain E did not cause fever in guinea pigs and the virulent revertant strain Evir caused fever in guinea pigs. We concluded that a single nucleotide insertion in the methyltransferase gene of R. prowazekii attenuated the R. prowazekii vaccine strain E. This suggested that an irreversible insertion or deletion mutation in the methyl transferase gene of R. prowazekii is required for Madrid E to be considered a safe vaccine.
    PLoS ONE 01/2014; 9(11):e113285. · 3.53 Impact Factor
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    ABSTRACT: Severe fever with thrombocytopenia syndrome virus (SFTSV) is a newly discovered Phlebovirus causing an emerging hemorrhagic fever in East Asia with reported case fatality rates up to 30%. Despite the high case fatality rate and large number of persons at risk of infection, the pathobiology of the disease is unknown, and no effective animal model has been available for investigating the pathogenesis of the disease. We have studied mice and hamsters as potential small animal models of SFTSV infection following subcutaneous, intraperitoneal or intracerebral inoculation. Animal tissues were processed for viral load, histopathology, immunohistochemistry, confocal and electron microscopic studies. We found that immunocompetent adult mice and hamsters did not become ill after SFTSV infection. However, alpha/beta interferon receptor (IFNAR(-/-)) knockout mice were highly susceptible to SFTSV infection, and all mice died within 3-4 days after subcutaneous inoculation of 10(6) ffu of SFTSV. Histologic examination of tissues of IFNAR(-/-)mice infected with SFTSV showed no detectable lesions. In contrast, by immunohistochemistry virus antigen was found in liver, intestine, kidney, spleen, lymphoid tissue, and brain, but not in the lungs. Mesenteric lymph nodes and spleen were the most heavily infected tissues. Quantitative RT-PCR confirmed the presence of virus in these same tissues. Confocal microscopy showed that SFTSV colocalized with reticular cells, but did not colocalize with dendritic cells, monocytes/macrophages, neutrophils or endothelium. Our results indicate that SFTSV multiplied in all organs except for lungs and that mesenteric lymph nodes and spleen were the most heavily infected tissues. The major target cells of SFTSV appear to be reticular cells in lymphoid tissues of intestine and spleen.
    Journal of Virology 11/2013; · 5.08 Impact Factor
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    ABSTRACT: Rickettsiae are responsible for some of the most devastating human infections. A high infectivity and severe illness after inhalation make some rickettsiae bioterrorism threats. We report that deletion of the exchange protein directly activated by cAMP (Epac) gene, Epac1, in mice protects them from an ordinarily lethal dose of rickettsiae. Inhibition of Epac1 suppresses bacterial adhesion and invasion. Most importantly, pharmacological inhibition of Epac1 in vivo using an Epac-specific small-molecule inhibitor, ESI-09, completely recapitulates the Epac1 knockout phenotype. ESI-09 treatment dramatically decreases the morbidity and mortality associated with fatal spotted fever rickettsiosis. Our results demonstrate that Epac1-mediated signaling represents a mechanism for host-pathogen interactions and that Epac1 is a potential target for the prevention and treatment of fatal rickettsioses.
    Proceedings of the National Academy of Sciences 11/2013; · 9.81 Impact Factor
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    ABSTRACT: Ehrlichioses are emerging tick-borne bacterial diseases of humans and animals for which no vaccines are available. The diseases are caused by obligately intracellular bacteria belonging to the genus Ehrlichia. Several immunoreactive proteins of ehrlichiae have been identified based on their reactivity with immune sera from human patients and animals. These include the major outer membrane proteins, ankyrin repeat proteins and tandem repeat proteins (TRP). Polyclonal antibodies directed against the tandem repeats (TRs) of Ehrlichia chaffeensis TRP32, TRP47 and TRP120 have been shown to provide protection in mice. In the present study, we evaluated E. muris P29, which is the ortholog of E. chaffeensis TRP47 and E. canis TRP36, as a subunit vaccine in a mouse model of ehrlichiosis. Our study indicated that unlike E. chaffeensis TRP47 and E. canis TRP36, orthologs of E. muris (P29) and E. muris-like agent (EMLA) do not contain tandem repeats. Immunization of mice with recombinant E. muris P29 induced significant protection against a challenge infection. The protection induced by E. muris P29 was associated with induction of strong antibody responses. In contrast to development of P29-specific IgG antibodies following immunization, development of P29-specific IgG antibodies, but not IgM antibodies, was impaired during persistent E. muris infection. Furthermore, our study indicated that CD4+ T cells target P29 during E. muris infection and differentiate into IFN-γ-producing Th1 effector/memory cells. In conclusion, our study indicated that orthologs of E. muris P29 showed considerable variation in the central tandem repeat region among different species, induction of P29-specific IgG antibody response was impaired during persistent E. muris infection, and rP29 induced protective immune responses.
    Vaccine 10/2013; · 3.77 Impact Factor
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    ABSTRACT: Human infections with arthropod-borne Rickettsia species remain a major global health issue, causing significant morbidity and mortality. Epidemic typhus due to Rickettsia prowazekii has an established reputation as the 'scourge of armies', and as a major determinant of significant 'historical turning points'. No suitable vaccines for human use are currently available to prevent rickettsial diseases. The unique lifestyle features of rickettsiae include obligate intracellular parasitism, intracytoplasmic niche within the host cell, predilection for infection of microvascular endothelium in mammalian hosts, association with arthropods and the tendency for genomic reduction. The fundamental research in the field of Rickettsiology has witnessed significant recent progress in the areas of pathogen adhesion/invasion and host immune responses, as well as the genomics, proteomics, metabolomics, phylogenetics, motility and molecular manipulation of important rickettsial pathogens. The focus of this review article is to capture a snapshot of the latest developments pertaining to the mechanisms of rickettsial pathogenesis and immunity.
    Future Microbiology 10/2013; 8:1265-88. · 4.02 Impact Factor
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    The American journal of tropical medicine and hygiene 08/2013; 89(2):301-7. · 2.53 Impact Factor
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    ABSTRACT: Lassa fever (LF) is a potentially lethal human disease that is caused by the arenavirus, Lassa virus (LASV). Annually, around 300,000 infections with up to 10,000 deaths occur in endemic regions of West Africa. Here we demonstrate that mice lacking a functional STAT1 pathway are highly susceptible to infection with LASV and develop lethal disease with pathology similar to that reported in humans.
    Journal of Virology 07/2013; · 5.08 Impact Factor
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    ABSTRACT: Microvascular endothelial barrier dysfunction is the central enigma in spotted fever group (SFG) rickettsioses. Angiogenin (ANG) is one of the earliest identified angiogenic factors, of which some are relevant to the phosphorylation of VE-cadherins that serve as endothelial adherens proteins. Although exogenous ANG is known to translocate into the nucleus of growing endothelial cells (ECs) where it plays a functional role, nuclear ANG is not detected in quiescent ECs. Besides its nuclear role, ANG is thought to play a cytoplasmic role, owing to its RNase activity that cleaves tRNA to produce small RNAs. Recently, such tRNA-derived RNA fragments (tRFs) have been shown to be induced under stress conditions. All these observations raise an intriguing hypothesis about a novel cytoplasmic role of ANG, which is induced upon infection with Rickettsia and generates tRFs that may play roles in SFG rickettsioses. C3H/HeN mice were infected intravenously with a sublethal dose of R. conorii. At days 1, 3, and 5 post infection (p.i.), liver, lung and brain were collected for immunofluorescence (IF) studies of R. conorii and angiogenin (ANG). Human umbilical vein endothelial cells (HUVECs) were infected with R. conorii for 24, 48, and 72 hrs before incubation with 1mug/ml recombinant human ANG (rANG) in normal medium for 2 hrs. HUVEC samples were subjected to IF, exogenous ANG translocation, endothelial permeability, and immunoprecipitation phosphorylation assays. To identify small non-coding RNAs (sncRNAs) upon rickettsial infection, RNAs from pulverized mouse lung tissues and HUVECs were subjected to library preparation and deep sequencing analysis using an Illumina 2000 instrument. Identified sncRNAs were confirmed by Northern hybridization, and their target mRNAs were predicted in silico using BLAST and RNA hybrid programs. In the present study, we have demonstrated endothelial up-regulation of ANG, co-localized with SFG rickettsial infection in vivo. We also have provided direct evidence that rickettsial infection sensitizes human ECs to the translocation of exogenous ANG in a compartmentalized pattern at different times post-infection. Typically, exogenous ANG translocates into the nucleus at 24 hrs and to the cytoplasm at 72 hrs post-infection. The ANG cytoplasmic translocation enhances phosphorylation and destabilization of VE-cadherin and attenuates endothelial barrier function. Of note, deep sequencing analysis detected tRFs, mostly derived from the 5'-halves of host tRNAs, that are induced by ANG. Northern hybridization validates the two most abundantly cloned tRFs derived from tRNA-ValGTG and tRNA-GlyGCC, in both mouse tissues and human cells. Bioinformatics analysis predicted that these tRFs may interact with transcripts associated with the endothelial barrier, the host cell inflammatory response, and autophagy. Our data provide new insight into the role of compartmentalized ANG during SFG rickettsioses, and highlight its possible mediation through tRFs.
    BMC Infectious Diseases 06/2013; 13(1):285. · 3.03 Impact Factor
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    ABSTRACT: We investigated the humoral immune response against different species of Rickettsia in serum samples from small rodents collected in two areas of a silent focus for Brazilian spotted fever in the eastern region of Minas Gerais State, Brazil. Sera samples were analyzed by indirect immunofluorescence assay using antigens from Rickettsia species of the spotted fever, ancestral, and transition groups. Titers ≥ 1:64 were considered positive. In Santa Cruz do Escalvado, 94% (30 of 32) of the samples collected from Rattus rattus, 22% (5 of 23) from Nectomys squamipes, and 80% (4 of 5) from Akodon sp., reacted by indirect immunofluorescence assay with Rickettsia antigens of the spotted fever group. In the municipality of Pingo D'Água, 84% (26 of 31) of the samples collected from R. rattus, 86% (6 of 7) of the samples from Oryzomys subflavus, 86% (6 of 7) from N. squamipes, and 100% (1 of 1) from Bolomys sp. contained antibodies that reacted with rickettsial antigens of the spotted fever group. These results demonstrated the previous exposure of small rodents to spotted fever group Rickettsia, suggesting the participation of these animals in the natural history of these rickettsiae in this region.
    The American journal of tropical medicine and hygiene 03/2013; · 2.53 Impact Factor
  • Slobodan Paessler, David H Walker
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    ABSTRACT: Four families of enveloped RNA viruses, filoviruses, flaviviruses, arenaviruses, and bunyaviruses, cause hemorrhagic fevers. These viruses are maintained in specific natural cycles involving nonhuman primates, bats, rodents, domestic ruminants, humans, mosquitoes, and ticks. Vascular instability varies from mild to fatal shock, and hemorrhage ranges from none to life threatening. The pathogenic mechanisms are extremely diverse and include deficiency of hepatic synthesis of coagulation factors owing to hepatocellular necrosis, cytokine storm, increased permeability by vascular endothelial growth factor, complement activation, and disseminated intravascular coagulation in one or more hemorrhagic fevers. The severity of disease caused by these agents varies tremendously; there are extremely high fatality rates in Ebola and Marburg hemorrhagic fevers, and asymptomatic infection predominates in yellow fever and dengue viral infections. Although ineffective immunity and high viral loads are characteristic of several viral hemorrhagic fevers, severe plasma leakage occurs at the time of viral clearance and defervescence in dengue hemorrhagic fever. Expected final online publication date for the Annual Review of Pathology: Mechanisms of Disease Volume 8 is January 24, 2013. Please see http://www.annualreviews.org/catalog/pubdates.aspx for revised estimates.
    Annual Review of Pathology Mechanisms of Disease 11/2012; · 25.79 Impact Factor

Publication Stats

3k Citations
876.38 Total Impact Points


  • 1997–2014
    • University of Texas Medical Branch at Galveston
      • Department of Pathology
      Galveston, Texas, United States
  • 2013
    • Mahidol University
      Krung Thep, Bangkok, Thailand
  • 2012
    • Università degli studi di Palermo
      • Dipartimento di Biomedico di Medicina Interna e Specialistica (Di.Bi.M.I.S.)
      Palermo, Sicily, Italy
  • 2009–2011
    • Meharry Medical College
      Nashville, Tennessee, United States
    • Anhui Center for Disease Control and Prevention
      Luchow, Anhui Sheng, China
  • 2010
    • Beijing Centers for Disease Control and Prevention
      Peping, Beijing, China
  • 2004–2009
    • University of Buea
      • Department of Biochemistry and Microbiology
      Buea, South-West Region, Cameroon
    • Fundação Ezequiel Dias
      Camelleira, Pernambuco, Brazil
  • 2008
    • Texas A&M University - Galveston
      Galveston, Texas, United States
    • National Institute of Health Dr. Ricardo Jorge
      Oporto, Porto, Portugal
  • 2006
    • Autonomous University of Nuevo León
      San Nicolás de los Garza, Nuevo León, Mexico
  • 2005–2006
    • Los Andes University (Colombia)
      Μπογκοτά, Bogota D.C., Colombia
    • Johns Hopkins University
      • Department of Pathology
      Baltimore, MD, United States
    • Universidad Autónoma de Yucatán
      • Centro de Investigaciones Regionales Dr. Hideyo Noguchi
      Mérida, Yucatan, Mexico
  • 2004–2005
    • University of São Paulo
      • Departamento de Medicina Veterinária Preventiva e Saúde Animal (VPS) (Sao Paulo)
      Ribeirão Preto, Estado de Sao Paulo, Brazil
  • 2003
    • Geelong Hospital
      Geelong, Victoria, Australia
  • 2002–2003
    • Universidade Federal de Ouro Preto
      • Department of Social and Clinical Nutrition(DENCS)
      Ouro Preto, Minas Gerais, Brazil