Ehrlichia canis-like agent isolated from a man in Venezuela: antigenic and genetic characterization

Antigenic variation of Ehrlichia chaffeensis resulting from differential expression of the 28-kilodalton protein gene family

The transcriptional activity and allele variation of the 28-kDa outer membrane protein gene (p28) of Ehrlichia chaffeensis were analyzed to determine the mechanism of the antigenic variation of the 28-kDa outer membrane proteins. Reverse transcriptase PCR amplification of mRNA indicated that 16 of the 22 members of the p28 multigene family were transcribed. Amino acid sequence analysis indicated that the p28-19 protein was produced in vitro in the Arkansas strain. The p28-19 gene and its promoter region were sequenced and compared in 12 clinical isolates of E. chaffeensis to determine allele variation. The variation of the p28-19 gene among the isolates is limited to three types represented by strains Arkansas, 91HE17, and Sapulpa, respectively. These results indicate that the majority of the p28 genes are active genes and that antigenic variation of the E. chaffeensis 28-kDa proteins may result from differential expression of the p28 gene family members rather than gene conversion.

Human ehrlichioses

Human ehrlichioses represent one of the best examples of newly emergent infectious diseases in which the classic triad of host, infectious agent, and environment are intertwined closely. These pathogens have existed for eons on the planet, and some were described as veterinary pathogens decades ago. Because of dramatic increases of deer and small mammal populations in certain areas and the subsequent increased populations of particular blood-feeding ticks, the risk of developing these diseases is higher than before. Increasing human populations in suburban areas and increased immunosuppressed populations (transplant patients, human immunodeficiency virus patients, and cancer survivors) also have increased risk of developing severe forms of these diseases.

Detection of Ehrlichia canis in canine carrier blood and in individual experimentally infected ticks with a p30-based PCR assay

Detection of vector-borne pathogens is necessary for investigation of their association with vertebrate and invertebrate hosts. The ability to detect Ehrlichia spp. within individual experimentally infected ticks would be valuable for studies to evaluate the relative competence of different vector species and transmission scenarios. The purpose of this study was to develop a sensitive PCR assay based on oligonucleotide sequences from the unique Ehrlichia canis gene, p30, to facilitate studies that require monitoring this pathogen in canine and tick hosts during experimental transmission. Homologous sequences for Ehrlichia chaffeensis p28 were compared to sequences of primers derived from a sequence conserved among E. canis isolates. Criteria for primer selection included annealing scores, identity of the primers to homologous E. chaffeensis sequences, and the availability of similarly optimal primers that were nested within the target template sequence. The p30-based assay was at least 100-fold more sensitive than a previously reported nested 16S ribosomal DNA (rDNA)-based assay and did not amplify the 200-bp target amplicon from E. chaffeensis, the human granulocytic ehrlichiosis agent, or Ehrlichia muris DNA. The assay was used to detect E. canis in canine carrier blood and in experimentally infected Rhipicephalus sanguineus ticks. Optimized procedures for preparing tissues from these hosts for PCR assay are described. Our results indicated that this p30-based PCR assay will be useful for experimental investigations, that it has potential as a routine test, and that this approach to PCR assay design may be applicable to other pathogens that occur at low levels in affected hosts.

Molecular and antigenic comparison of Ehrlichia canis isolates from dogs, ticks, and a human in Venezuela

We previously culture isolated a strain of Ehrlichia canis, the causative agent of canine ehrlichiosis, from a human in Venezuela. In the present study, we examined whether dogs and ticks are infected with E. canis in Venezuela and, if so, whether this is the same strain as the human isolate. PCR analysis using E. canis-specific primers revealed that 17 of the 55 dog blood samples (31%) and all three pools of four Rhipicephalus sanguineus ticks each were positive. An ehrlichial agent (Venezuelan dog Ehrlichia [VDE]) was isolated and propagated in cell culture from one dog sample and was further analyzed to determine its molecular and antigenic characteristics. The 16S rRNA 1,408-bp sequence of the new VDE isolate was identical to that of the previously reported Venezuelan human Ehrlichia isolate (VHE) and was closely related (99.9%) to that of E. canis Oklahoma. The 5' (333-bp) and 3' (653-bp) sequences of the variable regions of the 16S rRNA genes from six additional E. canis-positive dog blood specimens and from three pooled-tick specimens were also identical to those of VHE. Western blot analysis of serum samples from three dogs infected with VDE by using several ehrlichial antigens revealed that the antigenic profile of the VDE was similar to the profiles of VHE and E. canis Oklahoma. Identical 16S rRNA gene sequences among ehrlichial organisms from dogs, ticks, and a human in the same geographic region in Venezuela and similar antigenic profiles between the dog and human isolates suggest that dogs serve as a reservoir of human E. canis infection and that R. sanguineus, which occasionally bites humans residing or traveling in this region, serves as a vector. This is the first report of culture isolation and antigenic characterization of an ehrlichial agent from a dog in South America, as well as the first molecular characterization of E. canis directly from naturally infected ticks.

Analysis of transcriptionally active gene clusters of major outer membrane protein multigene family in Ehrlichia canis and E. chaffeensis

Ehrlichia canis and E. chaffeensis are tick-borne obligatory intramonocytic ehrlichiae that cause febrile systemic illness in humans and dogs, respectively. The current study analyzed the pleomorphic multigene family encoding approximately 30-kDa major outer membrane proteins (OMPs) of E. canis and E. chaffeensis. Upstream from secA and downstream of hypothetical transcriptional regulator, 22 paralogs of the omp gene family were found to be tandemly arranged except for one or two genes with opposite orientations in a 28- and a 27-kb locus in the E. canis and E. chaffeensis genomes, respectively. Each locus consisted of three highly repetitive regions with four nonrepetitive intervening regions. E. canis, in addition, had a 6.9-kb locus which contained a repeat of three tandem paralogs in the 28-kb locus. These total 47 paralogous and orthologous genes encoded OMPs of approximately 30 to 35 kDa consisting of several hypervariable regions alternating with conserved regions. In the 5'-end half of the 27-kb locus or the 28-kb locus of each Ehrlichia species, 14 paralogs were linked by short intergenic spaces ranging from -8 bp (overlapped) to 27 bp, and 8 remaining paralogs in the 3'-end half were connected by longer intergenic spaces ranging from 213 to 632 bp. All 22 paralogs, five unknown genes, and secA in the omp cluster in E. canis were transcriptionally active in the monocyte culture, and the paralogs with short intergenic spaces were cotranscribed with their adjacent genes, including the respective intergenic spaces at both the 5' and the 3' sides. Although omp genes are diverse, our results suggest that the gene organization of the clusters and the gene locus are conserved between two species of Ehrlichia to maintain a unique transcriptional mechanism for adaptation to environmental changes common to them.

Significance of serological testing for ehrlichial diseases in dogs with special emphasis on the diagnosis of canine monocytic ehrlichiosis caused by Ehrlichia canis

Dogs are susceptible to a number of ehrlichial diseases. Among them, canine monocytic ehrlichiosis is an important and potentially fatal disease of dogs caused by the rickettsia Ehrlichia canis. Diagnosis of the disease relies heavily on the detection of antibodies and is usually carried out using the indirect immunofluoresence antibody (IFA) test. The IFA test may be confounded by cross-reactivities between a number of the canine ehrlichial pathogens. This article presents a review of the ehrlichial diseases affecting dogs with reference to their immune responses, host specificities, cross-reactivites and diagnosis. Diagnostic means such as Western immunblot, dot-blot and PCR are discussed. The use of the IFA test as a diagnostic means for E. canis is presented along with its potential pitfalls. The review emphasizes that the disease process, cross-reactivites with other ehrlichial species, multiple tick-borne infections and persistent IFA antibody titers post-treatment, should all be considered when interpreting E. canis serological results.

Animal model of fatal human monocytotropic ehrlichiosis

Human monocytotropic ehrlichiosis caused by Ehrlichia chaffeensis is a life-threatening, tick-borne, emerging infectious disease for which no satisfactory animal model has been developed. Strain HF565, an ehrlichial organism closely related to E. chaffeensis isolated from Ixodes ovatus ticks in Japan, causes fatal infection of mice. C57BL/6 mice became ill on day 7 after inoculation and died on day 9. The liver revealed confluent necrosis, ballooning cell injury, apoptosis, poorly formed granulomas, Kupffer cell hyperplasia, erythrophagocytosis, and microvesicular fatty metamorphosis. The other significant histological findings consisted of marked expansion of the marginal zone and infiltration of the red pulp of the spleen by macrophages, interstitial pneumonitis, and increased numbers of immature myeloid cells and areas of necrosis in the bone marrow. Ehrlichiae were detected by immunohistology and electron microscopy in the liver, lungs, and spleen. The main target cells were macrophages, including Kupffer cells, hepatocytes, and endothelial cells. Apoptosis was detected in Kupffer cells, hepatocytes, and macrophages in the lungs and spleen. This tropism for macrophages and the pathological lesions closely resemble those of human monocytotropic ehrlichiosis for which it is a promising model for investigation of immunity and pathogenesis.

Survival of Ehrlichia chaffeensis in refrigerated, ADSOL-treated RBCs

BACKGROUND

The purpose of this study was to investigate the persistence of viable Ehrlichia chaffeensis in ADSOL-treated RBCs stored at 4 to 6 degrees C.

STUDY DESIGN AND METHODS

The continuous monocytic cell lines THP-1 and DH82 were infected with E. chaffeensis (St. Vincent isolate). Packed RBC units were inoculated in separate experiments with E. chaffeensis-infected cells as final concentrations of 8.02 x 10(4) (DH82) and 1.43 x 10(4) (THP-1) infected cells per mL. Aliquots were stored at 4 to 6 degrees C for 1 to 42 days. At selected intervals, nucleated cells from the RBC aliquots were obtained by using a ficoll-isopaque separation procedure. Uninfected DH82 cell cultures were inoculated with the harvested nucleated cells or supernatant. The cell cultures were evaluated for infection by weekly examination of Wright's (Diff-Quik) stained cytocentrifuged slides. PCR amplification was also used to test the harvested nucleated cells or supernatant for the presence of E. chaffeensis DNA.

RESULTS

In both types of infected cell lines, E. chaffeensis was reisolated in DH82 cells for as long as 11 days from the cellular fraction and for up to 5 days from the supernatant fraction. PCR results were positive throughout the 42-day testing period.

CONCLUSION

Cell-associated E. chaffeensis remains viable in ADSOL-treated RBCs stored at 4 to 6 degrees C for at least 11 days. These data suggest that transfusion-acquired infection is possible. Successful reisolation was achieved from the supernatant fraction, which suggests that RBC products treated with a WBC-reduction procedure may still present a risk for transfusion transmission. No correlation between PCR positivity and viability of bacteria was noted.

New Ehrlichia species closely related to Ehrlichia chaffeensis isolated from Ixodes ovatus ticks in Japan

Seven Ehrlichia strains (six HF strains and one Anan strain) that were obtained from laboratory mice by intraperitoneally inoculating homogenates of adult Ixodes ovatus collected in Japan were characterized. 16S rRNA sequences of all six HF strains were identical, and the sequences were 99.7, 98.2, and 97.7% identical to those of Anan strain, Ehrlichia chaffeensis (human monocytic ehrlichiosis agent), and E. muris, respectively. Partial GroEL amino acid sequencing also revealed that the six HF strains had identical sequences, which were 99.0, 98.5, and 97.3% identical to those of E. chaffeensis, the Anan strain, and E. canis, respectively. All HF strains were lethal to mice at higher dosages and intraperitoneal inoculation, whereas the Anan or E. muris strain induced only mild clinical signs. Light and electron microscopy of moribund mice inoculated with one of the HF strains revealed severe liver necrosis and the presence of numerous ehrlichial inclusions (morulae) in various organs. The study revealed that members of E. canis genogroup are naturally present in Ixodes ticks. HF strains that can cause severe illness in immunocompetent laboratory mice would be valuable in studying the pathogenesis and the roles of both cellular and humoral immune responses in ehrlichiosis caused by E. canis genogroup.

Molecular cloning and characterization of the 120-kilodalton protein gene of Ehrlichia canis and application of the recombinant 120-kilodalton protein for serodiagnosis of canine ehrlichiosis

The 120-kDa outer membrane protein (p120) is a potential adhesin of Ehrlichia chaffeensis, and recombinant p120 is very useful for serodiagnosis of human monocytotropic ehrlichiosis. The analogous gene of p120 in Ehrlichia canis was cloned, sequenced, and expressed. Like the E. chaffeensis p120, the E. canis p120 contains tandem repeat units. However, neither the repeat number nor the amino acid sequences in the repeats are identical in the two Ehrlichia species. The repeat units are hydrophilic and by probability analysis are predicted to be surface exposed in both species. The repeat regions of the p120s of the two species have common amino acid sequences that are predicted to be surface exposed. The overall amino acid sequence of the E. canis p120 is 30% homologous to that of E. chaffeensis p120. Protein immunoblotting demonstrated that the recombinant E. canis p120 reacted with convalescent sera from dogs with canine ehrlichiosis. These results indicate that the recombinant p120 is a potential antigen for the serodiagnosis of canine ehrlichiosis.

Ehrlichia ewingii, a newly recognized agent of human ehrlichiosis

BACKGROUND

Human ehrlichiosis is a recently recognized tick-borne infection. Four species infect humans: Ehrlichia chaffeensis, E. sennetsu, E. canis, and the agent of human granulocytic ehrlichiosis.

METHODS

We tested peripheral-blood leukocytes from 413 patients with possible ehrlichiosis by broad-range and species-specific polymerase-chain-reaction (PCR) assays for ehrlichia. The species present were identified by species-specific PCR assays and nucleotide sequencing of the gene encoding ehrlichia 16S ribosomal RNA. Western blot analysis was used to study serologic responses.

RESULTS

In four patients, ehrlichia DNA was detected in leukocytes by a broad-range PCR assay, but not by assays specific for E. chaffeensis or the agent of human granulocytic ehrlichiosis. The nucleotide sequences of these PCR products matched that of E. ewingii, an agent previously reported as a cause of granulocytic ehrlichiosis in dogs. These four patients, all from Missouri, presented between May and August 1996, 1997, or 1998 with fever, headache, and thrombocytopenia, with or without leukopenia. All had been exposed to ticks, and three were receiving immunosuppressive therapy. Serum samples obtained from three of these patients during convalescence contained antibodies that reacted with E. chaffeensis and E. canis antigens in a pattern different from that of humans with E. chaffeensis infection but similar to that of a dog experimentally infected with E. ewingii. Morulae were identified in neutrophils from two patients. All four patients were successfully treated with doxycycline.

CONCLUSIONS

These findings provide evidence of E. ewingii infection in humans. The associated disease may be clinically indistinguishable from infection caused by E. chaffeensis or the agent of human granulocytic ehrlichiosis.

Comparison of Ehrlichia muris strains isolated from wild mice and ticks and serologic survey of humans and animals with E. muris as antigen

In metropolitan Tokyo, the Ehrlichia muris seropositivity rate of 24 wild mice was 63% in Hinohara Village, but in the surrounding areas, it was 0 to 5%. This finding suggests that the reservoir of E. muris is focal. Among the 15 seropositive mice, ehrlichiae were isolated from 9 Apodemus speciosus mice and 1 A. argenteus mouse, respectively. Five ehrlichial isolates were obtained from 10 ticks (Haemaphysalis flava) collected in Asuke Town, Aichi Prefecture, where the E. muris type strain had been isolated. These new isolates were compared with the E. muris type strain. The mouse virulence and ultrastructure of the new isolates were similar to those of the type strain, and all of them were cross-reactive with each other, as well as with the type strain, by indirect immunofluorescent-antibody test. The levels of similarity of the base sequences of the 16S rRNA gene of one of the A. speciosus isolates and one of the tick isolates to that of the E. muris type strain were 99.79 and 99.93%, respectively. We suggest that all of these isolates are E. muris; that E. muris is not limited to Eothenomys kageus but infects other species of mice; and that E. muris is present at locations other than Aichi Prefecture. It appears that H. flava is a potential vector of E. muris. Twenty (1%) of 1803 humans from metropolitan Tokyo were found to be seropositive for E. muris antibodies. A serological survey revealed that exposure to E. muris or organisms antigenically cross-reactive to E. muris occurred among dogs, wild mice, monkeys, bears, deer, and wild boars in Gifu Prefecture, nearby prefectures, and Nagoya City, central Japan. However, human beings and Rattus norvegicus rats in this area were seronegative. These results indicate broader geographic distribution of and human and animal species exposure to E. muris or related Ehrlichia spp. in Japan.

Emerging and resurging vector-borne diseases

Over the last four decades, a number of arthropod-borne infections have been recognized for the first time. Some have become of considerable public health importance, such as dengue hemorrhagic fever (DHF), and others are spreading geographically and their incidence is increasing. There has been an important recrudescence of several long-known vector-borne diseases. Malaria, leishmaniasis, dengue, and plague have resurged in numerous foci, in some cases where they were thought to be under effective control. In most instances, the appearance of new diseases and syndromes and the resurgence of old can be associated with ecological changes that have favored increased vector densities. Dam construction, irrigation and other development projects, urbanization, and deforestation have all resulted in changes in vector population densities that appear to have enabled the emergence of new diseases and the resurgence of old diseases. Greatly increased human travel has spread infectious agents, introducing them into areas in which they had been hitherto absent. It is essential to understand the factors that caused increased vector densities and hence the transmission of disease to prevent the emergence and resurgence of more diseases, as well as to serve as a basis for effective control.

Sequential evaluation of dogs naturally infected with Ehrlichia canis, Ehrlichia chaffeensis, Ehrlichia equi, Ehrlichia ewingii, or Bartonella vinsonii

Historically, disease manifestations in dogs seroreactive to Ehrlichia canis antigens by indirect immunofluorescent antibody testing have been attributed to infection with either E. canis or Ehrlichia ewingii. A 1996 study by Dawson and colleagues provided PCR evidence that healthy dogs from southeastern Virginia could be naturally infected with Ehrlichia chaffeensis. This observation stimulated us to determine which Ehrlichia spp. infected sick dogs that were referred to our hospital from the same region. Based upon PCR amplification with species-specific primers, sick dogs seroreactive to E. canis antigens were determined to be infected with four Ehrlichia species: E. canis, E. chaffeensis, E. equi, and E. ewingii. Coinfection with three Ehrlichia species (E. canis, E. ewingii, and E. equi) was documented for one dog. An additional canine pathogen presumed to be tick transmitted, Bartonella vinsonii subsp. berkhoffii, was identified in 7 of 12 dogs. Importantly, our results indicate that in naturally infected dogs, E. chaffeensis can cause severe disease manifestations that are clinically and serologically indistinguishable from disease manifestations of E. canis or E. ewingii. In addition, our findings support the efficacy of doxycycline for treatment of E. canis, E. equi, and E. ewingii infections but indicate that, based upon the persistence of E. chaffeensis DNA for 1 year following treatment, E. chaffeensis infection in dogs may be more refractory to doxycycline treatment. Undetected coinfection with Bartonella may also complicate the evaluation of treatment efficacy while resulting in disease manifestations that mimic ehrlichiosis.

Human ehrlichioses: newly recognized infections transmitted by ticks

Human ehrlichioses are tick-borne infections caused by bacteria in the genus Ehrlichia. Human monocytic ehrlichiosis is caused by Ehrlichia chaffeensis and human granulocytic ehrlichiosis is caused by an agent similar to Ehrlichia equi. E. chaffeensis infects mononuclear phagocytes and is transmitted by Lone Star ticks (Amblyomma americanum) found in the south central and eastern United States. The agent of human granulocytic ehrlichiosis infects mostly neutrophils, it transmitted by Ixodes species ticks, and occurs mostly in the upper midwest and northeast United States. Despite the undifferentiated presentation of both ehrlichioses with fever, headache, myalgias, leukopenia, thrombocytopenia, and elevated liver enzyme activities, the diagnostic methods are distinct. Occasional severe complications include meningoencephalitis, adult respiratory distress syndrome, shock, and opportunistic infections. Immunocompromised patients are at high risk for death. An adverse outcome is associated with delayed diagnosis and therapy; thus, empirical treatment is advocated. Treatment with doxycycline usually results in prompt defervescence and cure.

Serologic evidence of infection with Ehrlichia spp. in wild rodents (Muridae: Sigmodontinae) in the United States

Rodent (Muridae: Sigmodontinae) blood and sera collected from 14 states were tested for seroreactivity to a cultured isolate of the human granulocytic ehrlichiosis (HGE) agent by using an indirect immunofluorescence assay. Of the 1,240 samples tested, 136 (11%) were found to be reactive at titers of > or = 32. Rodents with HGE agent-specific antibodies were found in New York (23% of 491 samples; geometric mean endpoint titer [GMT] = 441), Connecticut (11% of 100 samples; GMT = 481), California (9% of 32 samples; GMT = 323), Colorado (2% of 212 samples; GMT = 256), Florida (7% of 27 samples; GMT = 362), Maryland (7% of 15 samples; titer = 64), New Jersey (4% of 76 samples; titer = 256), and Wisconsin (13% of 8 samples; titer = 128). Samples from Georgia (n = 16), Illinois (n = 27), Nevada (n = 27), North Carolina (n = 52), Ohio (n = 57), and Utah (n = 100) were not reactive. The earliest seroreactive sample was from a Peromyscus leucopus mouse collected in June 1986 in Connecticut, and the majority of the seroreactive samples (68%) were from this species. Samples from other Peromyscus species (P. boylii, P. maniculatus, and P. gossypinus) were also found to be reactive, with a GMT for the genus of 410. Several species of Neotoma woodrats (N. fuscipes, N. lepida, N. albigula, and N. mexicana) from California and Colorado had antibodies that reacted with the HGE agent (genus GMT = 194), suggesting that enzootic cycles of Ehrlichia spp. exist outside of the areas of confirmed human disease. Attempts to amplify and detect ehrlichial DNA from the limited tissues available (n = 40 animals) were unsuccessful. Further studies are needed to determine the identity of the organisms inducing antibody production in these rodent species and to elucidate the epidemiology and public health importance of these agents.

Experimental transmission of Ehrlichia canis (Rickettsiales: Ehrlichieae) by Dermacentor variabilis (Acari: Ixodidae)

Four trials were conducted in which laboratory-reared Dermacentor variabilis nymphs were exposed to Ehrlichia canis by feeding on experimentally infected dogs as soon as classical morulae were detected in peripheral blood monocytes. After molting 25, 50 or 90 adult tick pairs were permitted to feed on 7 Ehrlichia-naive dogs. Transmission occurred in trials 1 (1/1 dog), 3 (1/1 dog) and 4 (2/2 dogs) but not in trial 2 (0/3 dogs), with 4 of 7 dogs becoming infected. Successful transstadial transmission was demonstrated by detection of morulae in peripheral blood lymphocytes and by seroconversion to Ehrlichia canis 30 d post-exposure. Incubation periods ranged between 17 and 22 days (mean = 19). Clinical signs, typical of ehrlichiosis, included mucopurulent ocular discharge, lymphadenopathy and malaise with accompanying pyrexia, leukopenia and thrombocytopenia. Pyrexia, thrombocytopenia and erythrophagocytosis and vacuolization of the cytoplasm of monocytic cells were observed 1-4 d prior to detection of morulae. This is the first demonstration that a tick other than Rhipicephalus sanguineus is capable of transstadial transmission of this important pathogen of dogs.

Immunodominant major outer membrane proteins of Ehrlichia chaffeensis are encoded by a polymorphic multigene family

Several immunodominant major proteins ranging from 23 to 30 kDa were identified in the outer membrane fractions of Ehrlichia chaffeensis and Ehrlichia canis. The N-terminal amino acid sequence of a 28-kDa protein of E. chaffeensis (one of the major proteins) was determined. The gene (p28), almost full length, encoding the 28-kDa protein was cloned by PCR with primers designed based on the N-terminal sequence of the E. chaffeensis 28-kDa protein and the consensus sequence between the C termini of the Cowdria ruminantium MAP-1 and Anaplasma marginale MSP-4 proteins. The p28 gene was overexpressed, and antibody to the recombinant protein was raised in a rabbit. The antibody and serum from a patient infected with E. chaffeensis reacted with the recombinant protein, three proteins (29, 28, and 25 kDa) of E. chaffeensis, and a 30-kDa protein of E. canis. Immunoelectron microscopy with the rabbit antibody revealed that the antigenic epitope of the 28-kDa protein was exposed on the surface of E. chaffeensis. Southern blot analysis with a 32P-labeled p28 gene probe revealed multiple copies of genes homologous to p28 in the E. chaffeensis genome. Six copies of the p28 gene were cloned and sequenced from the genomic DNA by using the same probe. The open reading frames of these gene copies were tandemly arranged with intergenic spaces. They were nonidentical genes and contained a semivariable region and three hypervariable regions in the predicted protein molecules. One of the gene copies encoded a protein with an internal amino acid sequence identical to the chemically determined N-terminal amino acid sequence of a 23-kDa protein of E. chaffeensis. Immunization with the recombinant P28 protein protected mice from infection with E. chaffeensis. These findings suggest that the 30-kDa-range proteins of E. chaffeensis represent a family of antigenically related homologous proteins encoded by a single gene family.

Western immunoblotting analysis of the antibody responses of patients with human monocytotropic ehrlichiosis to different strains of Ehrlichia chaffeensis and Ehrlichia canis

In order to evaluate the relative sensitivity of the detection of antibodies against various antigenic proteins of Ehrlichia chaffeensis for the diagnosis of the emerging infectious disease human monocytotropic ehrlichiosis, Western immunoblotting was performed with 27 serum samples from convalescent patients with antibodies, as demonstrated by indirect immunofluorescence assay. Among 22 patients with antibodies reactive with the 120-kDa protein, 15 showed reactivity with the 29/28-kDa protein(s) and the proteins in the 44- to 88-kDa range. Two of the serum samples with this pattern reacted with the 29/28-kDa protein(s) of only the 91HE17 strain, and one sample reacted with only that of the Arkansas strain, indicating that the antibodies were stimulated by strain-specific epitopes. Overall, antibodies to the 29/28-kDa protein(s) were detected in only 16 patients' sera, suggesting that this protein is less sensitive than the 120-kDa protein. Two of 12 serum samples from healthy blood donors had antibodies reactive with the 120-kDa protein; one of these samples reacted also with the 29/28-kDa protein(s) of Ehrlichia canis, suggesting that unrecognized ehrlichial infection might have occurred, including human infection with E. canis. A high correlation between reactivity with the 120-kDa protein by Western immunoblotting and the recombinant 120-kDa protein by dot blot supports the potential usefulness of this recombinant antigen in diagnostic serology.

Sequence and characterization of an Ehrlichia chaffeensis gene encoding 314 amino acids highly homologous to the NAD A enzyme

DNA sequence analysis of the nadA gene of Ehrlichia chaffeensis revealed a 942 bp open reading frame with the capacity to encode 314 amino acids. The amino acid sequence of the E. chaffeensis quinolinate synthetase A (NAD/A) has 53.6% identity and 82% similarity to the NAD A of the cyanelle of Cyanophora paradoxa. Portions of the homologous genes of E. canis and E. muris were also sequenced. The amino acid sequences of the NAD A of E. canis and E. muris have 89.2% and 93.2% homology, respectively, to the NAD A of E. chaffeensis. We propose that the nadA gene may be an excellent candidate for a genetic tool for the phylogenetic study of ehrlichiae.

PCR amplification and comparison of nucleotide sequences from the groESL heat shock operon of Ehrlichia species

Degenerate PCR primers derived from conserved regions of the eubacterial groESL heat shock operon were used to amplify groESL sequences of Ehrlichia equi, Ehrlichia phagocytophila, the agent of human granulocytic ehrlichiosis (HGE), Ehrlichia canis, Bartonella henselae, and Rickettsia rickettsii. The groESL nucleotide sequences were less conserved than the previously determined 16S rRNA gene sequences of these bacteria. A phylogenetic tree derived from deduced GroEL amino acid sequences was similar to trees based on 16S rRNA gene sequences. Nucleotide sequences obtained from clinical samples containing E. equi, E. phagocytophila, or the HGE agent were very similar (99.9 to 99.0% identity), and the deduced amino acid sequences were identical. Some divergence was evident between nucleotide sequences amplified from samples originating from the United States (E. equi and the HGE agent) and sequences from the European species, E. phagocytophila. A single pair of PCR primers derived from these sequences was used to detect E. chaffeensis and HGE agent DNA in blood samples from human patients with ehrlichiosis.

Comparison of nested PCR with immunofluorescent-antibody assay for detection of Ehrlichia canis infection in dogs treated with doxycycline

A partial 16S rRNA gene was amplified in Ehrlichia canis-infected cells by nested PCR. The assay was specific and did not amplify the closely related Ehrlichia chaffeensis, Ehrlichia muris, Neorickettsia helminthoeca, and SF agent 16S rRNA genes. The assay was as sensitive as Southern hybridization, detecting as little as 0.2 pg of E. canis DNA. By this method, all blood samples from four dogs experimentally infected with E. canis were positive as early as day 4 postinoculation, which was before or at the time of seroconversion. One hundred five blood samples from dogs from Arizona and Texas (areas of E. canis endemicity) and 30 blood samples from dogs from Ohio (area of E. canis nonendemicity) were examined by nested PCR and immunofluorescent-antibody (IFA) test. Approximately 84% of dogs from Arizona and Texas had been treated with doxycycline before submission of blood specimens. Among Arizona and Texas specimens, 46 samples were PCR positive (44%) and 80 were IFA positive (76%). Forty-three of 80 IFA-positive samples (54%) were PCR positive, and 22 of 25 IFA-negative samples (88%) were negative in the nested PCR. None of the Ohio specimens were IFA positive, but 5 specimens were PCR positive (17%). Our results indicate that the nested PCR is highly sensitive and specific for detection of E. canis and may be more useful in assessing the clearance of the organisms after antibiotic therapy than IFA, especially in areas in which E. canis is endemic.