Ehrlichia chaffeensis: a prototypical emerging pathogen

Lymph node hemophagocytosis in rickettsial diseases: a pathogenetic role for CD8 T lymphocytes in human monocytic ehrlichiosis (HME)?

BACKGROUND

Human monocytic ehrlichiosis (HME) and Rocky Mountain spotted fever (RMSF) are caused by Ehrlichia chaffeensis and Rickettsia rickettsii, respectively. The pathogenesis of RMSF relates to rickettsia-mediated vascular injury, but it is unclear in HME.

METHODS

To study histopathologic responses in the lymphatic system for correlates of immune injury, lymph nodes from patients with HME (n = 6) and RMSF (n = 5) were examined. H&E-stained lymph node tissues were examined for five histopathologic features, including hemophagocytosis, cellularity, necrosis, and vascular congestion and edema. The relative proportions of CD68 macrophages, CD8 and CD4 T lymphocytes, and CD20 B lymphocytes were evaluated by immunohistochemical staining.

RESULTS

Hemophagocytosis was similar in HME and RMSF, and was greater than in control cases (p = .015). Cellularity in HME was not different from controls, whereas RMSF lymph nodes were markedly less cellular (p < 0.002). E. chaffeensis-infected mononuclear phagocytes were infrequent compared to R. rickettsii-infected endothelial cells. More CD8 cells in lymph nodes were observed with HME (p < .001), but no quantitative differences in CD4 lymphocytes, macrophages, or B lymphocytes were identified.

CONCLUSION

Hemophagocytosis, CD8 T cell expansion, and the paucity of infected cells in HME, suggest that E. chaffeensis infection leads to macrophage activation and immune-mediated injury.

Susceptibility and resistance to monocytic ehrlichiosis in the mouse

To address the role of cellular immunity during ehrlichia infection, we have utilized a model of monocytic ehrlichiosis that results from infection of mice by Ixodes ovatus ehrlichia (IOE). Although ehrlichiosis in humans is largely a disease of immunocompromised individuals, the use of the IOE model has allowed us to identify factors required for host defense in normal mice. Using a low-dose infection C57BL/6 mouse model, we have demonstrated that host defense requires immune mechanisms involving CD4 T cell-mediated, TNF-alpha-, IL-12-, and IFN-gamma-dependent, macrophage activation. We have also provided formal evidence that IFN-gamma produced by CD4 Th1 cells is sufficient for protective immunity. Our recent studies have demonstrated, in addition, an essential role for IL-10, which is probably important in inhibiting immunopathological responses, and for inducible nitric oxide synthase. The latter observation establishes an important role for reactive nitrogen intermediates in bacterial elimination in vivo. In contrast, evaluation of mice carrying wild-type and mutant alleles of Nramp1 revealed at most a modest role for this gene in resistance to fatal IOE infection. Other studies in low-dose infected mice have indicated that the generation of immunological memory may be impaired during low-dose IOE infection, possibly due to bacterial immune subversion. These studies highlight the utility of the IOE mouse model in identifying important parameters of the immune response during ehrlichiosis.

Co-infection of White-Tailed Deer with Multiple Strains ofEhrlichia chaffeensis

We investigated the effect of exposing deer to multiple strains of Ehrlichia chaffeensis that differed in number of tandem repeats in either the variable-length PCR target (VLPT) gene or 120 kDa antigen gene. We hypothesized that infection with one strain would provide immunity to infection with other strains of E. chaffeensis. All deer initially exposed to strain A (604-2) became PCR and culture positive by 10 days post-infection (DPI). Three deer infected with strain A and subsequently inoculated with strain B (623-4) became infected with strain B. Two deer infected with strain A and subsequently inoculated with strain C (125B) became infected with strain C. Of three deer, each infected with strain B and subsequently inoculated with strain C, one was PCR positive for strain C. Of three deer previously inoculated with both strains A and B, and subsequently inoculated with strain C, one showed delayed evidence of strain C. Western blot analysis demonstrated that deer sera reacted differently to antigens from each exposed strain. A complementary in vitro study demonstrated that exposure to two strains differing in VLPT repeats may lead to co-infection of DH82 cells. These results complement a previous study and further show that deer can become sequentially infected with up to three strains of E. chaffeensis. This suggests that competitive exclusion, a phenomenon described in related organisms such as Anaplasma marginale whereby infection with one strain precludes subsequent infection by a second, distinct strain of the same species, may not occur with E. chaffeensis.

[Human ehrlichiosis]

BACKGROUND

Human ehrlichiosis is a newly recognized disease. It is a tick-borne disease caused by several bacterial species of the genhus Erlichia. These are small gram-negative pleomorphic cocci, that are obligatory intracellular bacteria. Tick Ixodes is the principle vector in Europe, and Amblyomma amenicanum in the United States. Bacterial organisms replicate in a tick, and are transmited from infected cells in a vector to the blood cells of animals or humans. Human ehrlichiosis is a name for a group of diseases caused by different species of Ehrlichia. One of them is the disease named human monocytic ehrlichiosis, caused by Ehrlichia chaffeensis, and the other is a human granulocytic ehrlichiosis caused by Anaplasma phagocytophilia.

CASE REPORT

We reported a 23-year-old patient admitted for the clinical treatment with the symptoms of high febrility (above 40 degrees C), headache, vomiting, general weakness and exhaustion, but without data on a tick bite. The patient was treated with trimetoprim-sulfamethoxazole for a week when Ehrlichia chaffeensis was confirmed by the immunofluoroscence test, and the therapy contimed with doxacyclin.

CONCLUSION

Human ehrlichiosis is also present in our country, so this disease should be considered everyday, especially in infectology practice.

Ehrlichia infection of the central nervous system

Ehrlichiosis in the United States is caused by three closely related bacterial species (Ehrlichia chaffeensis, Ehrlichia ewingii, and Anaplasma phagocytophilum), all transmitted through tick bite. Although there is variation with respect to geography and tick vector, the clinical manifestations are similar, and treatment of these infections is identical. Ehrlichiosis can present with a spectrum of neurologic manifestations, ranging in severity from headache to meningoencephalitis. Treatment is straightforward if the diagnosis is suspected, but antibiotic therapy should not be delayed pending laboratory confirmation. Doxycycline, the treatment of choice for adults and children with suspected ehrlichiosis, has high bioavailability and can be administered orally in most cases. Therapy is typically continued at least 3 days after the last documented fever. Although there have been no studies specifically evaluating duration or dosing of doxycycline for Ehrlichia meningoencephalitis, anecdotal reports suggest 100 mg doxycycline administered twice daily is effective, despite limited penetration into the cerebrospinal fluid. Because doxycycline interacts with CYP3A4 enzymes, there is potential for drug interactions with a number of medications. In endemic areas, documentation of coinfection with Borrelia burgdorferi, the etiologic agent of Lyme disease, may require prolonging the duration of doxycycline therapy.

Novel genetic variants of Anaplasma phagocytophilum, Anaplasma bovis, Anaplasma centrale, and a novel Ehrlichia sp. in wild deer and ticks on two major islands in Japan

Wild deer are one of the important natural reservoir hosts of several species of Ehrlichia and Anaplasma that cause human ehrlichiosis or anaplasmosis in the United States and Europe. The primary aim of the present study was to determine whether and what species of Ehrlichia and Anaplasma naturally infect deer in Japan. Blood samples obtained from wild deer on two major Japanese islands, Hokkaido and Honshu, were tested for the presence of Ehrlichia and Anaplasma by PCR assays and sequencing of the 16S rRNA genes, major outer membrane protein p44 genes, and groESL. DNA representing four species and two genera of Ehrlichia and Anaplasma was identified in 33 of 126 wild deer (26%). DNA sequence analysis revealed novel strains of Anaplasma phagocytophilum, a novel Ehrlichia sp., Anaplasma centrale, and Anaplasma bovis in the blood samples from deer. None of these have been found previously in deer. The new Ehrlichia sp., A. bovis, and A. centrale were also detected in Hemaphysalis longicornis ticks from Honshu Island. These results suggest that enzootic cycles of Ehrlichia and Anaplasma species distinct from those found in the United States or Europe have been established in wild deer and ticks in Japan.

Detection of Ehrlichia chaffeensis in Brazilian marsh deer (Blastocerus dichotomus)

Ehrlichia chaffeensis was detected for the first time in blood samples from Brazilian marsh deers (Blastocerus dichotomus) captured in the marshes of Parana River in Southeast Brazil in 1998. Seven EDTA-blood samples from deers were analyzed by PCR and nested PCR for presence of Ehrlichia chaffeensis, Ehrlichia ewingii, Ehrlichia canis, Neoriickettsia risticii, Anaplasma phagocytophilum and Anaplasma marginale. Three samples showed positive reactions for E. chaffeensis and Anaplasma marginale. None contained detectable A. phagocytophilum, E. ewingii, E. canis or Neorickettsia risticii DNA. In Brazil, the wild marsh deer may be a natural reservoir of the agents that cause human monocytotropic ehrlichiosis and ruminant erythrocytic anaplasmosis.

Comparative genomics of emerging human ehrlichiosis agents

Anaplasma (formerly Ehrlichia) phagocytophilum, Ehrlichia chaffeensis, and Neorickettsia (formerly Ehrlichia) sennetsu are intracellular vector-borne pathogens that cause human ehrlichiosis, an emerging infectious disease. We present the complete genome sequences of these organisms along with comparisons to other organisms in the Rickettsiales order. Ehrlichia spp. and Anaplasma spp. display a unique large expansion of immunodominant outer membrane proteins facilitating antigenic variation. All Rickettsiales have a diminished ability to synthesize amino acids compared to their closest free-living relatives. Unlike members of the Rickettsiaceae family, these pathogenic Anaplasmataceae are capable of making all major vitamins, cofactors, and nucleotides, which could confer a beneficial role in the invertebrate vector or the vertebrate host. Further analysis identified proteins potentially involved in vacuole confinement of the Anaplasmataceae, a life cycle involving a hematophagous vector, vertebrate pathogenesis, human pathogenesis, and lack of transovarial transmission. These discoveries provide significant insights into the biology of these obligate intracellular pathogens.

Ehrlichiosis is an acute disease that triggers flu-like symptoms in both humans and animals. It is caused by a range of bacteria transmitted by ticks or flukes. Because these bacteria are difficult to culture, however, the organisms are poorly understood. The genomes of three emerging human pathogens causing ehrlichiosis were sequenced. A database was designed to allow the comparison of these three genomes to sixteen other bacteria with similar lifestyles. Analysis from this database reveals new species-specific and disease-specific genes indicating niche adaptations, pathogenic traits, and other features. In particular, one of the organisms contains more than 100 copies of a single gene involved in interactions with the host(s). These comparisons also enabled a reconstruction of the metabolic potential of five representative genomes from these bacteria and their close relatives. With this work, scientists can study these emerging pathogens in earnest.

Differentially expressed and secreted major immunoreactive protein orthologs of Ehrlichia canis and E. chaffeensis elicit early antibody responses to epitopes on glycosylated tandem repeats

Ehrlichia canis major immunoreactive proteins of 36 and 19 kDa elicit the earliest detectable antibody responses during the acute phase of canine monocytic ehrlichiosis. Genes encoding the major immunoreactive 36-kDa protein of E. canis and the corresponding ortholog of E. chaffeensis (47 kDa) were identified and the proteins characterized. The molecular masses of the strongly immunoreactive recombinant proteins were larger than predicted (26.7 and 32.9 kDa, respectively) but were consistent with those of the corresponding native proteins (36 and 47 kDa). Similar to other reported ehrlichial immunoreactive glycoproteins, carbohydrate was detected on the recombinant expressed proteins, indicating that they were glycoproteins. Both glycoproteins (gp36 and gp47) have carboxy-terminal serine/threonine-rich tandem repeat regions containing repeats that vary in number (4 to 16 repeats) and amino acid sequence among different isolates of each species. E. canis gp36 was recognized by early acute-phase antibodies (day 14), and species-specific antibody epitopes were mapped to C-terminal nonhomologous repeat units of gp36 and gp47. Periodate treatment of recombinant gp36 reduced the antibody reactivity, and nonglycosylated synthetic peptide repeat units from E. canis gp36 and E. chaffeensis gp47 were substantially less immunoreactive than corresponding recombinant peptides, demonstrating that glycans are important epitope determinants that are structurally conserved on the recombinant proteins expressed in Escherichia coli. E. canis gp36 and E. chaffeensis gp47 were differentially expressed only on the surface of dense-cored ehrlichiae and detected in the Ehrlichia-free supernatants, indicating that these proteins are released extracellularly during infection.

Ehrlichia subversion of host innate responses

Anaplasma (formerly Ehrlichia) phagocytophilum and Ehrlichia chaffeensis, upon infection of humans, replicate in host leukocyte granulocytes and monocytes/macrophages, respectively. These unusual Gram-negative bacteria lack genes for biosynthesis of the lipopolysaccharide and peptidoglycan that activate host leukocytes. Caveolae-mediated endocytosis directs A. phagocytophilum and E. chaffeensis to an intracellular compartment secluded from oxygen-dependent and -independent killing. Furthermore, these bacteria orchestrate a remarkable series of events that culminate in suppression of NADPH oxidase, phagocyte activation and differentiation pathways, apoptosis, and interferon-gamma signaling in host leukocytes. They offer a fascinating example of how pathogens employ intricate strategies to usurp and subvert host cell function.

Experimental Ehrlichia canis infection in the dog does not cause immunosuppression

A carrier state develops in some Ehrlichia canis-infected dogs due to ineffective host defenses. The subsequent development of immune-mediated diseases or opportunistic infections in chronic ehrlichiosis suggests dysregulation of immunity; however, the immunobiology of this infection has not been well characterized. In this study, eight dogs were infected with E. canis, and changes in seroreactivity, serum immunoglobulin (Ig) concentrations, peripheral blood T cell subsets, lymphocyte blastogenesis (LBT), and lymphokine-activated killer (LAK) activity were evaluated over 4 months. Infection, which was documented by seroconversion, polymerase chain reaction, and blood culture, caused self-limiting fever and thrombocytopenia. Infected dogs developed an anti-E. canis antibody response but were not immune to re-infection. Serum IgM, IgG, and IgA concentrations were unaffected by E. canis. The percentage of circulating CD4(+) T cells was similar in uninfected and infected dogs at all points. Infected dogs developed a CD8(+) lymphocytosis 6 weeks after inoculation that subsequently subsided, despite organism persistence. Functional defects of cell-mediated immunity, measured as suppression of LAK activity or mitogen-driven LBT, were not observed. These results suggest that immune responses are not grossly impaired in young dogs during the first several months following experimental E. canis infection.

Anaplasma and Ehrlichia Infection

Human and animal infections by Anaplasmataceae are increasingly recognized as important and potentially fatal arthropod-transmitted diseases. Since the first recognition and implementation of diagnostic methods for human infection by Ehrlichia chaffeensis and Anaplasma phagocytophilum, the incidence of infections has linearly increased. Moreover, diagnostic and epidemiological testing indicates that "ehrlichia" infections are globally distributed and suggests that additional agents of human and veterinary "ehrlichiosis" will be identified. With increasing disease identification has come recognition that the infections can be severe, with approximately one-half of patients requiring hospitalization for complications including respiratory distress, myocarditis, neurological complications, hepatitis, a septic or toxic shock-like disease, opportunistic infections, and death in 0.5 to 3.0%. An understanding of the diseases, pathophysiology, pathogenesis, control, and management will best be developed through fundamental investigations. Advances in comprehension as to the separate contributions of bacteria and host are crucial since most data now suggest that alterations in host neutrophil function and protection from innate and adaptive immunity also contribute to disease manifestations. It is reasonable to operate from the hypothesis that these host cell functional changes ultimately benefit bacterial survival while inadvertently eliciting clinical disease in poorly adapted hosts. A firmer basis for the scientific understanding of Anaplasmataceae biology will allow logical and rational approaches toward infection control, prevention, and treatment.

Antibodies to Rickettsia rickettsii, Rickettsia typhi, Coxiella burnetii, Bartonella henselae, Bartonella quintana, and Ehrlichia chaffeensis among healthy population in Minas Gerais, Brazil

Rickettsial diseases except those belonging to spotted fever group rickettsioses are poorly studied in South America particularly in Brazil where few epidemiological reports have been published. We describe a serosurvey for Rickettsia rickettsii, R. typhi, Coxiella burnetii, Bartonella henselae, B. quintana, and Ehrlichia chaffeensis in 437 healthy people from a Brazilian rural community. The serum samples were tested by indirected micro-immunoflourescence technique and a cutoff titer of 1:64 was used. The seroprevalence rates for R. rickettsii, R. typhi, C. burnetii, B. henselae, B. quintana, and E. chaffeensis were respectively 1.6% (7 samples); 1.1% (5 samples); 3.9% (17 samples); 13.7% (60 samples); 12.8% (56 samples), and 10.5% (46 samples). Frequent multiple/cross-reactivity was observed in this study. Age over 40 years old, urban profession, and rural residence were significantly associated with some but not all infections rate. Low seropositivity rates for R. rickettsii, R. typhi, and C. burnetii contrasted with higher rates of seropositivity for B. quintana, B. henselae, and E. chaffeensis. These results show that all tested rickettsial species or antigenically closely related possible exist in this particular region.

Detection of medically important Ehrlichia by quantitative multicolor TaqMan real-time polymerase chain reaction of the dsb gene

Ehrlichia species are the etiological agents of emerging and life-threatening tick-borne human zoonoses, in addition to causing serious and fatal infections in companion animals and livestock. We developed the first tricolor TaqMan real-time polymerase chain reaction assay capable of simultaneously detecting and discriminating medically important ehrlichiae in a single reaction. Analytical sensitivity of 50 copies per reaction was attained with templates from Ehrlichia chaffeensis, Ehrlichia ewingii, and Ehrlichia canis by amplifying the genus-specific disulfide bond formation protein gene (dsb). Ehrlichia genus-specific dsb primers amplified DNA from all known Ehrlichia species but not from other rickettsial organisms including Anaplasma platys, Anaplasma phagocytophilum, Rickettsia conorii, or Rickettsia typhi. High species specificity was attained as each species-specific TaqMan probe (E. chaffeensis, E. ewingii, and E. canis) identified homologous templates but did not cross-hybridize with heterologous Ehrlichia templates at concentrations as high as 10(8) copies. Identification of E. chaffeensis, E. ewingii, and E. canis from natural and experimental infections, previously confirmed by polymerase chain reaction and serological or microscopic evidence, demonstrated the comparable specificity and sensitivity of the dsb real-time assay. This assay provides a powerful tool for prospective medical diagnosis for human and canine ehrlichioses and for ecologic and epidemiological studies involving arthropod and mammalian hosts.

Transstadial and intrastadial experimental transmission of Ehrlichia canis by male Rhipicephalus sanguineus

The acquisition and transmission of rickettsial pathogens by different tick developmental stages has important epidemiological implications. The purpose of this study was to determine if male Rhipicephalus sanguineus can experimentally acquire and transmit Ehrlichia canis in the absence of female ticks. Two trials were performed where nymphal and male R. sanguineus were simultaneously acquisition fed on the same infected donor hosts, and transstadially or intrastadially exposed male ticks were fed on separate pathogen-free dogs as a test for transmission. A single-step p30-based PCR assay was used to test canine and tick hosts for E. canis infections before and after tick feeding. E. canis was detected after either intrastadial or transstadial passage in male ticks, the organism remained detectable in both tick groups after transmission feeding, and both tick groups transmitted the rickettsia to susceptible dogs. Infection of dogs via tick feeding resulted in milder clinical signs and lower antibody titers than intravenous inoculation of carrier blood, but further investigation is needed to understand the mechanisms responsible for this observation. These results demonstrate that male R. sanguineus can take multiple feedings, and that they can both acquire and transmit E. canis in the absence of female ticks. This tick development stage could be important in transmission of E. canis, and perhaps related pathogens, between vertebrate hosts under natural and experimental conditions.

Immunity to the ehrlichiae: new tools and recent developments

PURPOSE OF REVIEW

Discusses recent developments in the study of immunity and host defense against the monocytic ehrlichiae in 2003 and 2004. The review does not address anaplasmoses, as the anaplasmae were recently re-classified into the genus Anaplasma, and are distinct in cell tropism from the ehrlichiae.

RECENT FINDINGS

The features of the immune responses against these emerging Gram-negative obligate intracellular pathogens are only beginning to be understood. Important advances in our ability to study host defense include the development of new experimental mouse models. Recent studies have defined possible mechanisms of innate immune subversion in human monocytes, as well as roles for lymphocyte subsets and type I cytokines during mouse infection. Other studies in the mouse suggest that cytokine production by CD8 T cells may contribute to immunopathology. New data also support a role for humoral immunity during host defense against these intracellular pathogens.

SUMMARY

The use of new animal models will facilitate research of the mechanisms of innate, adaptive, and pathological immune responses, and will enhance our understanding of human immunity to the ehrlichiae as well as to other pathogenic intracellular bacteria.

Multiplex detection of Ehrlichia and Anaplasma species pathogens in peripheral blood by real-time reverse transcriptase-polymerase chain reaction

Tick-borne infections are responsible for many emerging diseases in humans and several vertebrates. These include human infections with Anaplasma phagocytophilum, Ehrlichia chaffeensis, and Ehrlichia ewingii. Because single or co-infections can result from tick bites, the availability of a rapid, multiplex molecular test will be valuable for timely diagnosis and treatment. Here, we describe a multiplex molecular test that can detect single or co-infections with up to five Ehrlichia and Anaplasma species. The test protocol includes the magnetic capture-based purification of 16S ribosomal RNA, its enrichment, and specific-pathogen(s) detection by real-time reverse transcriptase-polymerase chain reaction. We also report a unique cloning strategy to develop positive controls in the absence of a pathogen's genomic DNA. The test was assessed by examining blood samples from dogs suspected to be positive for ehrlichiosis. The dog was chosen as the model system because it is susceptible to acquire infections with up to five pathogens of the genera Ehrlichia and Anaplasma. The test identified single infections in the canine host with E. chaffeensis, E. canis, E. ewingii, A. phagocytophilum, and A. platys and co-infection with E. canis and A. platys. The multipathogen detection and novel positive control development procedures described here will be valuable in monitoring infections in people, other vertebrates, and ticks.

Primary and Secondary Infection withEhrlichia chaffeensisin White-Tailed Deer (Odocoileus virginianus)

White-tailed deer (Odocoileus virginianus) are the principal reservoir host for Ehrlichia chaffeensis, causative agent of human monocytic ehrlichiosis (HME). Because white-tailed deer maintain a long-term infection with E. chaffeensis and because deer can be naturally exposed to multiple strains of E. chaffeensis, we evaluated the response to secondary infection of E. chaffeensis in deer. For primary infection, six white-tailed deer were injected with 5.4 x 10(6) DH82 cells infected with the Arkansas strain of E. chaffeensis (Ark) and two control deer were injected with noninfected DH82 cells. On post-infection day 54, three E. chaffeensis (Ark) infected deer and one naive deer were injected with 4.2 x 10(6) cells infected with strain WTD-6045B E. chaffeensis, which differs from the Arkansas strain by number of nucleotide repeats in the variable length PCR target (VLPT) gene; three other Arkansas strain infected deer were injected with noninfected DH82 cells. All animals were monitored for 31 additional days. All deer in the primary infection became positive by PCR amplification of the 16S rRNA or VLPT genes and/or cell culture by DPI-8. PCR amplification of the VLPT gene on whole blood, cell culture, and tissues detected primary and/or secondary strains in all deer exposed to both primary and secondary strains; in one deer, the primary strain was cultured from the lymph node. Our culture results demonstrated that both strains were present; however, PCR detection suggests that the secondary strain may have been circulating in blood at higher levels. In conclusion, this study provides evidence that primary infection of deer with E. chaffeensis does not protect against subsequent exposure and confirms that deer can be simultaneously coinfected with at least two different strains of E. chaffeensis.

Tick-Borne Bacterial, Rickettsial, Spirochetal, and Protozoal Infectious Diseases in the United States: A Comprehensive Review

Approximately 900 tick species exist worldwide, and they parasitize a variety of mammals, including humans; thus, ticks play a significant role in the transmission of infectious diseases. In the United States, tick-borne diseases are seasonally and geographically distributed; they typically occur during spring and summer but can occur throughout the year. Tick-borne diseases are endemic to a variety of geographic regions of the United States, depending on the species of tick commonly found in a specific locale. Specific tick-borne diseases are difficult to diagnose. Most patients have vague constitutional symptoms and nonspecific laboratory findings. Initially, serologic methods are of little benefit because they lack sensitivity early in the disease course. Therefore, a thorough history and physical examination are necessary for establishing a diagnosis. Antimicrobial regimens for tick-borne infections are poorly studied but well established. Tetracyclines and rifampin form the cornerstones of therapy for most tick-borne infections, but these agents may not be suitable for all patient populations. Therefore, no single agent can be chosen empirically to treat all tick-borne diseases. Because pharmacists are the most accessible health care providers, they are often asked how to treat tick-borne diseases. Thus, practitioners should be familiar with the ticks that inhabit their locale.

Emerging intracellular bacterial infections

Human infections by intracellular bacteria have been recognized for many years, but much of what we know about the pathogenesis of these diseases and their etiologic organisms has emerged within the past few years as a result of improved molecular-based means for their detection and classification. New insights concerning the epidemiology and pathogenesis of intracellular bacterial infections and methods for the detection of Chlamydophila pneumoniae, Ehrlichia chaffeensis, Anaplasma phagocytophilum, and Rickettsia species have made an impact on how we view them as agents of human disease. Emerging evidence suggesting a possible intracellular existence for another organism, Mycoplasma pneumoniae, may explain how this organism interacts with the host to induce chronic inflammatory conditions of the respiratory tract.

Treatment and prevention of rickettsial and ehrlichial infections

Rickettsial and ehrlichial infections are both carried by arthropod vectors. Both Rickettsia and Ehrlichia are small intracellular gram-negative coccobacilli. Clinical manifestations of Rickettsia range from spotted fevers to various forms of typhus. Human ehrlichiosis can present as monocytic ehrlichiosis or granulocytic anaplasmosis. Prevention is by avoidance of the responsible vectors. Therapy is usually with doxycycline, but chloramphenicol can also be used.

Detection of Ehrlichia chaffeensis in adult and nymphal Amblyomma americanum (Acari: Ixodidae) ticks from Long Island, New York

The lone star tick, Amblyomma americanum (L.), has increased in abundance in several regions of the northeastern United States, including areas of Long Island, NY. Adult and nymphal stage A. americanum collected from several sites on Long Island were evaluated for infection with Ehrlichia chaffeensis, the causative agent of human monocytic ehrlichiosis (HME), by using a nested polymerase chain reaction assay. Fifty-nine (12.5%) of 473 adults and eight of 113 pools of five nymphs each (estimated minimum prevalence of infection 1.4%) contained DNA of E. chaffeensis. These data, coupled with the documented expansion of lone star tick populations in the northeastern United States, confirm that E. chaffeensis is endemic to many areas of Long Island and that HME should be considered among the differential diagnoses of the many distinct tick-borne diseases that occur in this region.

Development of a p28-based PCR assay for Ehrlichia chaffeensis

Detection of Ehrlichia chaffeensis is necessary to study interactions between the parasite and its vertebrate and invertebrate hosts. The purpose of this study was to develop a sensitive, specific PCR assay for E. chaffeensis based on the outer membrane protein gene, p28. Candidate primer sets were identified and ranked based on annealing scores, similarities to three major p28 sequence clusters, dissimilarity to E. canis p30, an ortholog of p28, and the proximities of flanking primer sequences for nested PCR. The relative sensitivities of five optimized single-step and two nested PCR assays were determined, and the most sensitive assay was found to be a single-step PCR that was as much as 1000-fold more sensitive than a standard 16S rDNA-based nested PCR assay. This p28-based PCR assay amplified the target amplicon from isolates representative of all three major clusters of known p28 sequences, and this assay did not amplify template prepared from either of the two species most closely related to E. chaffeensis, E. canis and E. muris. These results indicate that this sensitive, specific and isolate-universal single-step PCR assay will be a useful tool in characterizing the transmission of this important zoonotic pathogen.

Histologic, serologic, and molecular analysis of persistent ehrlichiosis in a murine model

Human monocytotropic ehrlichiosis caused by Ehrlichia chaffeensis was reported in 1987. An animal model to study acute fatal ehrlichiosis in mice that has been developed closely resembles the fatal form of human monocytotropic ehrlichiosis. However, animal models for persistent infection in the genus Ehrlichia in immunocompetent mice have not been characterized. We report the histopathological progression of Ehrlichia muris infection in immunocompetent mice (AKR and C57BL/6 strains) correlated with their antibody response determined by indirect immunofluorescence and Western immunoblotting, and the distribution and quantity of the ehrlichial load by immunohistochemistry, polymerase chain reaction (PCR), and real-time PCR in lungs, liver, and spleen. Mild to moderate correlation was observed between histopathological grading in these organs and relative ehrlichial loads. The highest ehrlichial loads were present between days 4 and 14 after infection. E. muris was detected in tissues examined up to 150 days after infection by real-time PCR. Analysis of the serological response revealed several immunodominant antigens, including 200-, 180-, 100-, 73/75-, 45-, and 28-kd proteins. In conclusion, we have provided for the first time a complete histopathological, serological, immunohistochemical, and quantitative analysis of an animal model for the study of persistent ehrlichial infection.

Production of IFN-gamma by CD4 T cells is essential for resolving ehrlichia infection

To address the role of cellular immunity during ehrlichia infection, we have used a newly described model of monocytic ehrlichiosis that results from infection of mice by an ehrlichia that was isolated from an Ixodes ovatus tick (Ixodes ovatus ehrlichia, IOE). Immunocompetent C57BL/6 and BALB/c mice exhibited a dose-dependent susceptibility to IOE infection. Mice infected with a high dose inoculum ( approximately 1000 organisms) exhibited pronounced thrombocytopenia, lymphopenia, anemia, and morbidity within 12 days postinfection. Infection was associated with bacterial colonization of a number of tissues. In contrast, mice infected with a low dose inoculum ( approximately 100 organisms) exhibited only transient disease and were able to resolve the infection. SCID mice were highly susceptible to low-dose infection, indicating that adaptive immunity was required. Resistance to sublethal challenge in both C57BL/6 and BALB/c mice was CD4-, but not CD8-, dependent and required IL-12p40-dependent cytokines, IFN-gamma, and TNF-alpha, but not IL-4. CD4 T cells purified from infected mice proliferated in vitro in response to IOE Ags. T cell proliferation was associated with production of IFN-gamma, and the production of this cytokine by CD4 T cells rescued IFN-gamma-deficient mice from fatal infection. Exogenous IFN-gamma was capable of inducing microbiocidal activity in infected macrophages. The data suggest that classical immune mechanisms involving CD4 cells and type 1 cytokines are responsible for macrophage activation and for elimination of this intracellular bacterial pathogen.

Obligatory intracellular parasitism by Ehrlichia chaffeensis and Anaplasma phagocytophilum involves caveolae and glycosylphosphatidylinositol-anchored proteins

Obligatory intracellular, human ehrlichiosis agents Ehrlichia chaffeensis and Anaplasma phagocytophilum create unique replicative compartments devoid of lysosomal markers in monocytes/macrophages and granulocytes respectively. The entry of these bacteria requires host phospholipase C (PLC)-gamma2 and protein tyrosine kinases, but their entry route is still unclear. Here, using specific inhibitors, double immunofluorescence labelling and the fractionation of lipid rafts, we demonstrate that bacterial entry and intracellular infection involve cholesterol-rich lipid rafts or caveolae and glycosylphosphatidylinositol (GPI)-anchored proteins. By fluorescence microscopy, caveolar marker protein caveolin-1 was co-localized with both early and replicative bacterial inclusions. Additionally, tyrosine-phosphorylated proteins and PLC-gamma2 were found in bacterial early inclusions. In contrast, clathrin was not found in any inclusions from either bacterium. An early endosomal marker, transferrin receptor, was not present in the early inclusions of E. chaffeensis, but was found in replicative inclusions of E. chaffeensis. Furthermore, several bacterial proteins from E. chaffeensis and A. phagocytophilum were co-fractionated with Triton X-100-insoluble raft fractions. The formation of bacteria-encapsulating caveolae, which assemble and retain signalling molecules essential for bacterial entry and interact with the recycling endosome pathway, may ensure the survival of these obligatory intracellular bacteria in primary host defensive cells.

Mechanisms of immunity to Ehrlichia muris: a model of monocytotropic ehrlichiosis

Ehrlichia species can cause life-threatening infections or chronic persistent infections. Mechanisms of protective immunity were examined in an Ehrlichia muris mouse model of monocytotropic ehrlichiosis. C57BL/6 mice possessed strong genetic resistance to E. muris of an undetermined mechanism. CD8 T lymphocytes were particularly important, as revealed by 81% fatalities for E. muris-infected, major histocompatibility complex class I gene knockout mice compared with no deaths for wild-type C3H mice. Moreover, 80% of C3H mice depleted of CD8 and CD4 cells died of E. muris infection compared with only 44% of CD4 cell-depleted mice. CD8 T lymphocytes were demonstrated for the first time in an Ehrlichia infection to exhibit cytotoxic T-lymphocyte activity against Ehrlichia-infected target cells. Both gamma interferon and tumor necrosis factor were shown to play synergistic roles in protective immunity in vivo for the first time, as demonstrated by 75% fatalities when both cytokines were neutralized compared with minimal mortality when they were depleted separately. Passive transfer of antibodies, but not Fab fragments, to E. muris protected C3H/SCID mice against lethal infection. The mechanism of increased susceptibility (22% lethality) of C57BL/6 major histocompatibility complex class II gene knockout mice and CD4 cell-depleted C3H mice (i.e., through a gamma interferon or antibody mechanism), as well as the more important role of CD8 T lymphocytes (in the form of cytotoxic T-lymphocyte activity and/or gamma interferon production), remains to be elucidated. Protective immunity against monocytotropic E. muris is mediated by a combination of CD8 and CD4 T lymphocytes, gamma interferon, tumor necrosis factor alpha, and antibodies.

Survival strategy of obligately intracellular Ehrlichia chaffeensis: novel modulation of immune response and host cell cycles

Ehrlichia chaffeensis is an obligatory intracellular bacterium which resides in an early endosome in monocytes. E. chaffeensis infection in a human monocyte cell line (THP1) significantly altered the transcriptional levels of 4.5% of host genes, including those coding for apoptosis inhibitors, proteins regulating cell differentiation, signal transduction, proinflammatory cytokines, biosynthetic and metabolic proteins, and membrane trafficking proteins. The transcriptional profile of the host cell revealed key themes in the pathogenesis of Ehrlichia. First, E. chaffeensis avoided stimulation of or repressed the transcription of cytokines involved in the early innate immune response and cell-mediated immune response to intracellular microbes, such as the interleukin-12 (IL-12), IL-15, and IL-18 genes, which might make Ehrlichia a stealth organism for the macrophage. Second, E. chaffeensis up-regulated NF-kappaB and apoptosis inhibitors and differentially regulated cell cyclins and CDK expression, which may enhance host cell survival. Third, E. chaffeensis also inhibited the gene transcription of RAB5A, SNAP23, and STX16, which are involved in membrane trafficking. By comparing the transcriptional response of macrophages infected with other bacteria and that of macrophages infected with E. chaffeensis, we have identified few genes that are commonly induced and no commonly repressed genes. These results illustrate the stereotyped macrophage response to other pathogens, in contrast with the novel host response to obligate intracellular Ehrlichia, whose survival depends entirely on a long evolutionary process of outmaneuvering macrophages.

Molecular variation in the variable-length PCR target and 120-kilodalton antigen genes of Ehrlichia chaffeensis from white-tailed deer (Odocoileus virginianus)

Genes encoding two surface-expressed antigens of Ehrlichia chaffeensis, the variable-length PCR target (VLPT) and the 120-kDa antigen, which contain variable numbers of tandem repeats, were characterized for E. chaffeensis from white-tailed deer (Odocoileus virginianus). Both genes from infected deer contained numbers of repeats similar to those reported in genes from humans and ticks, although a new variant of the 120-kDa antigen gene containing five repeat units and coinfection with multiple VLPT and 120-kDa antigen gene genetic types were detected. Sequence analysis of both genes revealed more nucleotide variation than previously reported for E. chaffeensis from infected humans or ticks. This is the most extensive study of E. chaffeensis VLPT and 120-kDa antigen gene genetic variation to date and is the first to examine genetic variation in E. chaffeensis from a nonhuman vertebrate host.

Divergent interactions of Ehrlichia chaffeensis- and Anaplasma phagocytophilum-infected leukocytes with endothelial cell barriers

Human anaplasmosis (formerly human granulocytic ehrlichiosis) and human monocytic ehrlichiosis (HME) are emerging tick-borne infections caused by obligate intracellular bacteria in the family Anaplasmataceae. Clinical findings include fever, headache, myalgia, leukopenia, thrombocytopenia, and hepatic inflammatory injury. Whereas Ehrlichia chaffeensis (HME) often causes meningoencephalitis, this is rare with Anaplasma phagocytophilum infection. The abilities of infected primary host monocytes and neutrophils and of infected HL-60 cells to cross human umbilical vein endothelial cell-derived EA.hy926 cell barriers and human brain microvascular cells (BMEC), a human blood-brain barrier model, were studied. Uninfected monocyte/macrophages crossed endothelial cell barriers six times more efficiently than neutrophils. More E. chaffeensis-infected monocytes transmigrated than uninfected monocytes, whereas A. phagocytophilum suppressed neutrophil transmigration. Differences were not due to barrier dysfunction, as transendothelial cell resistivities were the same for uninfected cell controls. Similar results were obtained for HL-60 cells used as hosts for E. chaffeensis and A. phagocytophilum. Differential transmigration of E. chaffeensis- and A. phagocytophilum-infected leukocytes and HL-60 cells confirmed a role for the pathogen in modifying cell migratory capacity. These results support the hypothesis that Anaplasmataceae intracellular infections lead to unique pathogen-specific host cell functional alterations that are likely important for pathogen survival, pathogenesis, and disease induction.

Scrub typhus and tropical rickettsioses

PURPOSE OF REVIEW

Recent developments in molecular taxonomic methods have led to a reclassification of rickettsial diseases. The agent responsible for scrub typhus (Orientia tsutsugamushi ) has been removed from the genus Rickettsia and a bewildering array of new rickettsial pathogens have been described. An update of recent research findings is therefore particularly timely for the nonspecialist physician.

RECENT FINDINGS

An estimated one billion people are at risk for scrub typhus and an estimated one million cases occur annually. The disease appears to be re-emerging in Japan, with seasonal transmission. O. tsutsugamushi has evolved a variety of mechanisms to remain viable in its intracellular habitat. Slowing the release of intracellular calcium inhibits apoptosis of macrophages. Subsets of chemokine genes are induced in infected cells, some in response to transcription factor activator protein 1. Cardiac involvement is uncommon and clinical complications are predominantly pulmonary. Serious pneumonitis occurred in 22% of Chinese patients. Dual infections with leptospirosis have been reported. Standardized diagnostic tests are being developed and attempts to improve treatment of women and children are being made. Of the numerous tick-borne rickettsioses identified in recent years, African tick-bite fever appears to be of particular importance to travellers. The newly described flea-borne spotted fever caused by Rickettsia felis may be global in distribution.

SUMMARY

Rash and fever in a returning traveler could be rickettsial and presumptive doxycycline treatment can be curative. Recent research findings raise more questions than answers and should stimulate much needed research.

Evaluation of a PrototypeEhrlichia chaffeensisSurveillance System using White-Tailed Deer (Odocoileus virginianus) as Natural Sentinels

The natural history of Ehrlichia chaffeensis, the causative agent of human monocytotropic ehrlichiosis, includes the lone star tick (LST, Amblyomma americanum) as a vector and white-tailed deer (WTD; Odocoileus virginianus) as both a natural reservoir of E. chaffeensis and a major host of LST. The goal of the current study was to implement and evaluate a prototype surveillance system to delineate the geographic distribution of E. chaffeensis using WTD as natural sentinels. To accomplish this goal, serologic testing using the indirect immunofluorescent antibody (IFA) test was performed on WTD serum samples, and to confirm serologic results, polymerase chain reaction (PCR) assays and culture isolation were conducted. Considerations relevant to the applicability of a surveillance system utilizing WTD were analyzed (e.g., age and gender relationships to serologic status, adequacy of sample sizes needed to distinguish between uninfected and infected populations, presence of LST, and ability to detect stability and spread of E. chaffeensis in WTD populations). Of 3275 WTD serologically tested, 549 (47%) from 17 of 18 states had antibodies reactive to E. chaffeensis (IFA titer > or = 1:128). No difference between age groups or gender was noted with serologic testing, thus these variables would not be a concern for a surveillance system using WTD. Significantly more deer in younger age groups (< or = 1.5 yr) were PCR and culture positive, and 46% of 122 seropositive WTD populations were confirmed positive by PCR or culture isolation. A significant association between LST infestation and E. chaffeensis seroreactivity was noted. Furthermore, the surveillance system was able to detect stability of E. chaffeensis within WTD populations and also spread to new populations, both of which were associated with LST status. These data clearly demonstrate that WTD are useful as natural sentinels for this emerging human pathogen, and establish a prototypical framework for a WTD surveillance system.

Clinical manifestations, epidemiology, and laboratory diagnosis of human monocytotropic ehrlichiosis in a commercial laboratory setting

Clinical, epidemiological, and laboratory diagnostic issues of human monocytotropic ehrlichiosis (HME) were investigated in a retrospective case study conducted at a national reference laboratory (Focus Technologies, formerly MRL Reference Laboratory), and at the University of Texas Medical Branch at Galveston, Texas, during 1997 and 1998. Standard questionnaires were sent to physicians for each laboratory-diagnosed patient 2 days to 2 weeks after immunofluorescent antibody assay results were available. Among the 41 cases for which data were obtained, 32 (78%) were definite cases of HME, and 9 (22%) were probable cases of HME. Tick bite or exposure to ticks was recorded in more than 97% of cases. The most prominent clinical findings were fever, abdominal tenderness, and regional lymphadenopathy. There was an association between age and severity of illness. The main laboratory findings included leukopenia, thrombocytopenia, and elevated aspartate aminotransferase and alanine aminotransferase. Clinical and laboratory findings were nonspecific and were not good predictors of the severity of illness. The 90% of patients who received doxycycline treatment underwent rapid clinical improvement with a favorable outcome. The usual duration of effective treatment with doxycycline was 7 to 10 days. This retrospective study is unique because it was based in a commercial reference laboratory setting that receives specimens from different geographic locations. The clinical and laboratory information from 41 patients provides insight into the epidemiological, clinical, and laboratory characteristics of HME.

Ehrlichia chaffeensis and Anaplasma phagocytophilum lack genes for lipid A biosynthesis and incorporate cholesterol for their survival

Ehrlichia chaffeensis and Anaplasma phagocytophilum are agents of human monocytic and granulocytic ehrlichioses, respectively. They are extremely sensitive to mechanical stress and are pleomorphic gram-negative bacteria. Membrane incorporation of cholesterol from the eukaryotic host is known to be essential for other fragile and pleomorphic bacteria and mycoplasmas that lack a cell wall. Thus, we tested whether cholesterol is required for E. chaffeensis and A. phagocytophilum. Using a freeze fracture technique and biochemical analysis, these bacteria were found to contain significant levels of membrane cholesterol. These bacteria lack genes for cholesterol biosynthesis or modification. However, host cell-free bacteria had the ability to take up directly exogenous cholesterol or NBD-cholesterol, a fluorescent cholesterol derivative. Treatment of the bacteria with cholesterol extraction reagent methyl-beta-cyclodextrin caused their ultrastructural changes. Furthermore, pretreatment of the bacteria with methyl-beta-cyclodextrin or NBD-cholesterol deprived these bacteria of the ability to infect leukocytes, thus killing these obligate intracellular bacteria. Analysis of E. chaffeensis and A. phagocytophilum genome sequences revealed that these bacteria lack all genes for the biosynthesis of lipid A and most genes for the biosynthesis of peptidoglycan, which confer structural strength to gram-negative bacteria. Taken together, these results suggest that human ehrlichiosis agents became cholesterol dependent due to the loss of these genes. As the first report of gram-negative bacteria incorporating cholesterol for survival, these findings offer insight into the unique nature of their parasitism and imply that cholesterol is important in the control of human ehrlichioses.