Foley JE, Lerche NW, Dumler JS, Madigan JE.
A simian model of human granulocytic ehrlichiosis.
Am J Trop Med Hyg 1999 Jun;60(6):987-93
Abstract: Following intravenous inoculation with horse blood-infected with the agent of human granulocytic ehrlichiosis (HGE) from a human fatality, two rhesus macaques (Macaca mulatta) exhibited pyrexia and lethargy on days 4-12 postinfection (PI). Hematology revealed neutropenia, thrombocytopenia, and anemia, with ehrlichial morulae in monocytes and neutrophils on days 4-12. Blood was polymerase chain reaction (PCR)-positive on days 4-12 and bone marrow was PCR-positive on day 11. There was a minor increase in gamma-glutamyl transpeptidase on day 12 and serum interferon-gamma levels increased by day 18. Seroconversion occurred on day 20 PI to a titer of 100 by day 22. Western blot bands characteristic of HGE included 25-, 44-, 80-, 94-, 105-, and 125-kD bands. There was generalized lymphohistiocytic infiltration in the liver, spleen, lymph nodes, and other tissues. The liver had focal hepatocyte apoptosis. There was HGE DNA (by PCR) only in the spleen. Comparable findings were not observed in a monkey that received uninfected horse blood as a control. This animal model of human disease is important for further studies of HGE diagnosis, management, and pathogenesis.
Joon-seok Chae, Janet Foley, J. Stephen Dumler, John
Genetic comparison among naturally occurring Ehrlchia equi and Human Granulocytic Ehrlichiosis (HGE) agent isolates from northern California based on the nucleotide sequences of 16S rRNA, 444 Ep-ank and groESL Heat Shock Operon Genes.
International Conference on Rickettsiae and Rickettsial Disease & American Society for Rickettsiology 14th sesquiannual joint meeting at Marseille on June 13 - June 16, 1999.
Abstract: We examined the DNA sequences of three genes to determine the diversity among 11 naturally occurring isolates of Ehrlichia equi in horses (n=11) and granulocytic ehrlichiosis agent (n=2) in humans acquired from clinical cases in California, USA. Ehrlichia equi isolates were obtained from Sierra (n=6), Mendocino (n=3), Sonoma (n=1) and Marin (n=1) counties in California and human granulocytic ehrlichiosis (HGE) agent isolates were obtained from Humboldt county (n=2) in California, USA. These isolates were analyzed by determining the sequences following polymerase chain reaction using specific primers for 16S rRNA, 444 Ep-ank and groESL heat shock operon genes. All three genes were amplified from 13 clinical samples. The nucleotide sequences of 16S rRNA genes were identical among two human HGE isolates (CAHU-HGE1 and CAHU-HGE2) from Humboldt county and two horse E. equi isolates (CAMEBS and CAMAWI) from Mendocino and Marin counties. The 16S rRNA gene sequence differed in a single nucleotide, positions 34 and 180, in the E. equi isolates from Sierra (CASITL) and Sonoma (CASOLJ) counties respectively, and the other E. equi isolates. The sequences of 444 Ep-ank fragment which included the region encoding the four ankyrin repeats were identical among all E. equi and two HGE agent isolates. These sequences when compared to the same gene sequences of a HGE agent isolate from the eastern United States (BDS and USG3 strains) had two and sixteen differences at nucleotide positions, respectively. When the groESL heat shock operon gene sequences were compared, one HGE agent isolate (CAHU-HGE1) and three E. equi isolates (CASITL, CAMEBS; Mendocino and CAMAWI; Marin counties) had identical sequences. The other isolates had differences in two and four nucleotide positions, 1,483 and 1,546 between CAHU-HGE1 and CAHU-HGE2 isolates and 347, 1,303, 1,448 and 1,496 between CAHU-HGE1 and CASOLJ isolates, respectively. However, the translated amino acid sequences of groESL gene fragments were identical among E. equi, HGE agent and E. phagocytophila except that from the CASOLJ isolate. The results of translated amino acid sequence of 444 Ep-ank from CA isolates were different from the eastern United States (BDS and USG3 strains). These results suggest that E. equi and the HGE agent found in California are similar or identical, but may differ from the isolates of equine and human origin found in the eastern US.
Nicola Pusterla, Jon B. Huder, Christian M. Leutenegger, Ueli Braun,
E. Madigan, and Hans Lutz
Quantitative Real-Time PCR for Detection of Members of the Ehrlichia phagocytophila Genogroup in Host Animals and Ixodes ricinus Ticks.
Journal of Clinical Microbiology, May 1999, p. 1329-1331, Vol. 37, No. 5
Janet E. Foley, Jeffrey E. Barlough, Robert B. Kimsey, John
E. Madigan, Elfriede DeRock, and Amy Poland.
Ehrlichia spp. in Cervids from Clifornia.
Journal of Wildlife Disease. 1998. 34 (4):731-737.
Abstract: Blood samples from six mule deer (Odocoileus hemionus hemionus), 15 black-tailed deer (O. h. columbianus), and 29 elk (Cervus elaphus nannodes) were assayed for human monocytic and human granulocytic ehrlichiosis (HGE) by polymerase chain reaction (PCR), DNA sequencing, and serology to determine whether cervids are involved in the maintenance of these potential human pathogens in California, USA. The deer were sampled in August to October. The 29 tule elk from Point Reyes National Seashore were sampled in August. All deer were seronegative for antibodies to HGE/Ehrlichia equi, while the E. equi
seroprevalence among elk was 17%. The 16S rDNA PCR prevalence in deer was 38% (in mule deer and black-tailed deer) for Ehrlichia-like sp. of white-tailed deer, 5% (one black-tailed deer only) for E. equi, and 0% for E. chaffeensis. The PCR prevalence in elk was 0% for Ehrlichia-like sp. of white-tailed deer, 31% for E. equi, and 0% for E. chaffeensis. The E. equi from two positive elk samples was successfully propagated in HL-60 cell cultures. DNA sequencing confirmed that the Ehrlichia-like sp. sequences from California deer were closely related to sequences reported from white-tailed deer from Oklahoma and Georgia. The E. equi strain from deer and elk resembled other E. equi strains from California. These results suggest that cervids may be important in the natural maintenance of E. equi in California.
Pusterla, N; Weber, R; Wolfensberger, C; Schär, G; Zbinden, R;
Fierz, W; Madigan, JE; Dumler, JS; Lutz, H.
Serological evidence of human granulocytic ehrlichiosis in Switzerland.
European Journal of Clinical Microbiology and Infectious Diseases, 1998 Mar, 17(3):207-209.
Abstract: To investigate whether human granulocytic ehrlichiosis (HGE) is prevalent in Switzerland, 1515 human serum samples from individuals with different risks for tick exposure were tested for antibodies to Ehrlichia phagocytophila, a surrogate marker of the agent of HGE. The distribution of titres showed marked differences between sera of individuals with no or low risk for tick exposure and those with a high risk. The results of serological testing provided evidence of HGE in Switzerland as well as evidence of two types of coinfections: those with the agent of HGE and Borrelia burgdorferi, and those with the agent of HGE and the central European tickborne encephalitis virus.
Jeffrey E. Barlough, Gerhard H. Reubel, John E.
Madigan, Larisa K. Vredevoe, Paul E. Miller, and Yasuko Rikihisa.
Detection of Ehrlichia risticii, the Agent of Potomac Horse Fever, in Freshwater Stream Snails (Pleuroceridae: Juga spp.) from Northern California.
Appl. Environ. Microbiol. 1998. 64:2888-2893.[Abstract][Full Text]
Gerhard H. Reubel, Robert B. Kimsey, Jeffrey E. Barlough, and John
Experimental Transmission of Ehrlichia equi to Horses through Naturally Infected Ticks (Ixodes pacificus) from Northern California.
Journal of Clinical Microbiology, July 1998, p. 2131-2134.
Abstract: We report the experimental transmission of Ehrlichia equi from naturally infected Ixodes pacificus ticks to horses. Three weeks after exposure to ticks, two of three horses developed clinical signs compatible with E. equi infection, while one horse remained asymptomatic. 16S rRNA gene PCR of blood leukocyte lysates was positive for all horses at various time points; two horses seroconverted. The 16S rRNA gene sequences amplified from tick-exposed horses showed more than 99% homology to corresponding fragments of the 16S rRNA genes of E. equi, Ehrlichia phagocytophila, and the human granulocytic ehrlichiosis agent.
Gerhard H. Reubel, Jeffrey E. Barlough, and John
Production and Characterization of Ehrlichia risticii, the Agent of Potomac Horse Fever, from Snails (Pleuroceridae: Juga spp.) in Aquarium Culture and Genetic Comparison to Equine Strains.
Journal of Clinical Microbiology, June 1998, p. 1501-1511.
Magnarelli, LA; Ijdo, JW; Anderson, JF; Madigan,
JE; Dumler, JS; Fikrig, E.
Antibodies to Ehrlichia equi in dogs from the northeastern United States.
Journal of the American Veterinary Medical Association, 1997 Nov 1, 211(9):1134-7.
Astract: OBJECTIVE: To determine whether dogs living in tickinfested areas of the northeastern United States had been exposed to Ehrlichia equi, an etiologic agent of granulocytic ehrlichiosis. DESIGN: Analyses of dog sera. ANIMALS: 106 ill dogs and 12 clinically normal dogs. PROCEDURE: Antibodies to E equi were detected by indirect fluorescent antibody taining methods and western blot analyses. RESULTS: 10 of 106 (9.4%) sera tested from ill, privately owned dogs living in tick-infested areas of Connecticut and New York state had antibodies to E equi, a member of the E phagocytophila genogroup. Titration end points ranged from 1:80 to 1:1,280. Immunoblots revealed antibodies to proteins of E equi having molecular masses of predominantly 29, 40, 44, 105, 120, and 160 kd. There was good agreement between results of serologic testing methods, but use of the human isolate (NCH-1 strain) in western blot analyses detected 2 additional seropositive dogs found to be negative by indirect fluorescent antibody staining methods with the MRK strain. CLINICAL IMPLICATIONS: Dogs living in areas where ixodes scapularis is abundant may be exposed to multiple pathogens, such as E equi or Borrelia burgdorferi. Although mild or subclinical infections with E equi may develop, dogs with marked leukopenia, thrombocytopenia, or anemia should be viewed as possibly having ehrlichiosis. Laboratory diagnosis should include examinations for morulae in granulocytes or monocytes in addition to serologic analyses.
Asanovich, KM; Bakken, JS; Madigan, JE;
Aguero-Rosenfeld, M; Wormser, GP; Dumler, JS.
Antigenic diversity of granulocytic Ehrlichia isolates from humans in Wisconsin and New York and a horse in California.
Journal of Infectious Diseases, 1997 Oct, 176(4):1029-34.
Abstract: The agent of human granulocytic ehrlichiosis (HGE), Ehrlichia phagocytophila, and Ehrlichia equi are very similar. HGE is of variable severity. Genetic and antigenic differences among 3 human isolates (Webster, Spooner, and NY-8) and 1 horse isolate (MRK) were evaluated. The 16S rRNA gene sequences were identical in all human isolates. By use of 5 homologous antisera from these 3 humans and 1 horse and an additional 5 antisera in heterologous reactions, the immunodominant antigens of each isolate were noted to differ in molecular size: 43 kDa in the Webster (Wisconsin) isolate, 46 kDa in the Spooner (Wisconsin) isolate, 42 and 45 kDa in the NY-8 (New York State) isolate, and a 42 kDa doublet in the E. equi MRK isolate from California. Two sera from a Wisconsin patient reacted weakly or not at all with the NY-8 isolate. Antigenic structural diversity exists among otherwise indistinguishable granulocytic ehrlichial isolates.
Barlough, JE; Madigan, JE; Kramer, VL;
Clover, JR; Hui, LT; Webb, JP; Vredevoe, LK.
Ehrlichia phagocytophila genogroup rickettsiae in ixodid ticks from California collected in 1995 and 1996.
Journal of Clinical Microbiology, 1997 Aug, 35(8):2018-21.
Barlough, JE; Madigan, JE; Turoff, DR;
Clover, JR; Shelly, SM; Dumler, JS.
An Ehrlichia strain from a llama (Lama glama) and Llama-associated ticks (Ixodes pacificus).
Journal of Clinical Microbiology, 1997 Apr, 35(4):1005-7.
Barlough, JE; Rikihisa, Y; Madigan, JE.
Nested polymerase chain reaction for detection of Ehrlichia risticii genomic DNA in infected horses.
Veterinary Parasitology, 1997 Mar, 68(4):367-73.
Abstract: A nested polymerase chain reaction was developed for amplifying a 529-bp segment of the 16S ribosomal RNA gene of Ehrlichia risticii from equine buffy coat cells. Confirmation of identity of the amplified bands was accomplished by Southern hybridization and DNA sequencing. The study indicated a detection limit of > 10 copies of the target gene, and specificity for E. risticii as based on a panel of test rickettsiae. Ticks (Ixodes pacificus) collected in an area of northern California enzootic for equine monocytic ehrlichiosis were found to be negative for E. risticii DNA.
Barlough, JE; Madigan, JE; DeRock, E; Bigornia,
Nested polymerase chain reaction for detection of Ehrlichia equi genomic DNA in horses and ticks (Ixodes pacificus).
Veterinary Parasitology, 1996 Jun, 63(3-4):319-29.
Abstract: A nested polymerase chain reaction for detecting Ehrlichia equi in horses and ticks (Ixodes pacificus) was developed. A major second-round PCR product of 928 bp could be readily visualized in ethidium bromide-stained agarose minigels. An internal probe was used to verify the identity of the amplified product by non-radioactive (digoxigenin-based) Southern blotting; additional confirmation was provided by DNA sequence analysis. A dilution study testing the sensitivity of the PCR indicated that DNA derived from < = 7.6 but > 3 infected neutrophils was sufficient to generate a PCR signal. The specificity of the PCR was examined using a panel of rickettsiae, of which only E. equi and the closely-related human granulocytotropic ehrlichia produced PCR bands. In an in vivo infection study, E. equi DNA was detected in blood buffy-coat cells of an experimentally-infected horse on days three through 14 post-inoculation. In a separate study, three of six adult I. pacificus that as nymphs had been fed on an experimentally infected horse were found to be PCR-positive for E. equi.
Munderloh, UG; Madigan, JE; Dumler, JS;
Goodman, JL; Hayes, SF; Barlough, JE; Nelson, CM; Kurtti, TJ.
Isolation of the equine granulocytic ehrlichiosis agent, Ehrlichia equi, in tick cell culture.
Journal of Clinical Microbiology, 1996 Mar, 34(3):664-70.
Abstract: The equine granulocytic ehrlichiosis agent, Ehrlichia equi, is closely related or identical to the human granulocytic ehrlichiosis (HGE) agent. Both are suspected of being transmitted by ticks. We have successfully isolated E. equi in a cell line, IDE8, derived from a putative vector, the tick Ixodes scapularis. Peripheral blood leukocytes from an experimentally infected horse were inoculated onto IDE8 monolayers. Cultures were incubated in a candle jar at 34 degrees C in tick cell culture medium with NaHCO3 and an organic buffer [3-(N-morpholino)-propanesulfonic acid] (MOPS). Within 2 weeks, infected cells were detected in Giemsa-stained culture samples, and the organisms subsequently spread to uninfected cells in the cultures. E. equi was passaged serially by transferring a portion of an infected culture to new cell layers every 2 to 3 weeks. The identity of the organisms was confirmed by PCR using oligonucleotide primers specific for E. equi and the HGE agent and by immunocytology. Homologous equine antibodies and human anti-HGE convalescent serum recognized E. equi grown in tick cell culture. Electron microscopy revealed electron-lucent and -dense ehrlichia-like forms developing within host cell endosomes. E. equi passaged twice in tick cell culture retained infectivity and pathogenicity for the equine host, as demonstrated by intravenous inoculation of a suspension of infected tick cells and subsequent reisolation from peripheral blood, in fulfillment of Koch's postulates.
The horse developed severe clinical signs, i.e., fever, inappetence, thrombocytopenia, icterus, and limb edema, typical of granulocytic equine ehrlichiosis, within 1 week.
Madigan, JE; Barlough, JE; Dumler, JS;
Schankman, NS; DeRock, E.
Equine granulocytic ehrlichiosis in Connecticut caused by an agent resembling the human granulocytotropic ehrlichia.
Journal of Clinical Microbiology, 1996 Feb, 34(2):434-5.
Abstract: The first recognized cases of equine granulocytic ehrlichiosis in New England are described. The DNA sequence of the 16S rRNA gene of the causative ehrlichia was found to be identical to that of the human granulocytotropic ehrlichia, the agent of human granulocytic ehrlichiosis.
Goodman, JL; Nelson, C; Vitale, B; Madigan, JE;
Dumler, JS; Kurtti, TJ; Munderloh, UG.
Direct cultivation of the causative agent of human granulocytic ehrlichiosis
[see comments] [published erratum appears in N Engl J Med 1996 Aug 1;335(5):361]
New England Journal of Medicine, 1996 Jan 25, 334(4):209-15.
Abstract: BACKGROUND. Human granulocytic ehrlichiosis is a potentially fatal tick-borne infection that has recently been described. This acute febrile illness is characterized by myalgias, headache, thrombocytopenia, and elevated serum aminotransferase levels. The disease is difficult to diagnose because the symptoms are non-specific, intraleukocytic inclusions (morulae) may not be seen, and the serologic results are often initially negative. Little is known about the causative agent because it has never been cultivated. METHODS. We studied three patients with symptoms and laboratory findings suggestive of human granulocytic ehrlichiosis, including unexplained fever after probable exposure to ticks, granulocytopenia, and thrombocytopenia. Peripheral blood was examined for ehrlichia microscopically and with use of the polymerase chain reaction (PCR). Blood was inoculated into cultures of HL60 cells (a line of human promyelocytic leukemia cells), and the cultures were monitored for infection by Giemsa staining and PCR. RESULTS. Blood from the three patients, only one of whom had inclusions suggestive of ehrlichia in neutrophils, was positive for human granulocytic ehrlichiosis on PCR. Blood from all three patients was inoculated into HL60 cell cultures and caused infection, with intracellular organisms visualized as early as 5 days after inoculation and cell lysis occurring within 12 to 14 days. The identity of the cultured organisms was confirmed by immunofluorescence microscopy, PCR analysis, and DNA sequencing. DNA from the infected cells was sequenced in regions of the 16S ribosomal gene reported to differ between the agent of human granulocytic ehrlichiosis and closely related species, including Ehrlichia equi and E. phagocytophila which cause infection in animals. The sequences from all three human isolates were identical and differed from the strain of E. equi studied in having guanine rather than adenine at nucleotide 84. CONCLUSIONS. We describe the cultivation of the agent of human granulocytic ehrlichiosis in cell culture. The ability to isolate this organism should lead to a better understanding of the biology, treatment, and epidemiology of this emerging infection.
Richter, PJ Jr; Kimsey, RB; Madigan, JE;
Barlough, JE; Dumler, JS; Brooks, DL.
Ixodes pacificus (Acari: Ixodidae) as a vector of Ehrlichia equi (Rickettsiales: Ehrlichieae).
Journal of Medical Entomology, 1996 Jan, 33(1):1-5.
Abstract: Ehrlichia equi, a rickettsia described from horses in California 30 yr ago, causes equine granulocytic ehrlichiosis throughout the Americas and possibly Europe. Here, we report experimental transmission of E. equi from infected to susceptible horses through bites of western blacklegged ticks, Ixodes pacificus (Cooley & Kohls). In preliminary field studies, only I. pacificus consistently infested horses and vegetation at 3 locations with contemporary cases of equine ehrlichosis, and in particular, I. pacificus was the only species found attached to all of the infected horses. Exposure to bites of ticks in the genus Ixodes poses previously unrecognized and serious health risks to humans and animals.
Madigan, JE; Rikihisa, Y; Palmer, JE; DeRock,
E; Mott, J.
Evidence for a high rate of false-positive results with the indirect fluorescent antibody test for Ehrlichia risticii antibody in horses.
Journal of the American Veterinary Medical Association, 1995 Dec 1, 207(11):1448-53.
Abstract: OBJECTIVE--The original objective was to determine seroprevalence of Ehrlichia risticii antibody among horses in California. On the basis of the unexpected results of the survey, an investigation into the accuracy and reproducibility of results of the indirect fluorescent antibody (IFA) test for E risticii was carried out. DESIGN--Prospective, seroprevalence study. ANIMALS--Healthy horses (n = 655) and horses with clinical signs of equine monocytic ehrlichiosis (EME; n = 514) from various regions of California. PROCEDURE--The IFA test was performed. Results were compared with results of an ELISA and with results of western immunoblot analysis. RESULTS--Overall, 104 of 655 (15.9%) healthy horses had evidence of an antibody response. However, 84 of 514 (16.3%) horses with clinical signs of EME also had positive test results, and of the 8 seropositive diseased horses for which paired (acute and convalescent) samples had been submitted, only 1 had a rise in antibody titers between the acute and convalescent samples. Comparison of results for the IFA test, ELISA, and western immunoblot analysis revealed a high rate of false-positive results for the IFA test. Subsequent studies suggested that routine vaccination of horses with non-E risticii vaccines may have contributed to the false-positive reactions. CLINICAL IMPLICATIONS--The data failed to provide conclusive evidence of E risticii infection among California horses. Owing to the high percentage of false-positive test results, caution is advised when using the IFA test to diagnose EME in horses or to determine the necessity for E risticii vaccination.
Barlough, JE; Madigan, JE; DeRock, E; Dumler,
JS; Bakken, JS. Protection against
Ehrlichia equi is conferred by prior infection with the human granulocytotropic Ehrlichia (HGE agent).
Journal of Clinical Microbiology, 1995 Dec, 33(12):3333-4.
Abstract: A Thoroughbred filly that developed clinical signs of equine granulocytic ehrlichiosis following inoculation with the human granulocytotropic ehrlichia was shown to be resistant to challenge with Ehrlichia equi, a closely related agent. This result further substantiates the close and potentially conspecific relationship between these two granulocytotropic ehrlichiae.
Madigan, JE; Richter, PJ Jr; Kimsey, RB;
Barlough, JE; Bakken, JS; Dumler, JS.
Transmission and passage in horses of the agent of human granulocyticehrlichiosis.
Journal of Infectious Diseases, 1995 Oct, 172(4):1141-4.
Abstract: The human granulocytotropic ehrlichia and Ehrlichia equi produce similar diseases in their respective host species (humans, horses). Currently, the phylogenetic and biologic relationships of these 2 uncultured pathogens remain unclear. Previous studies have revealed nucleotide sequence similarity approaching identity at the level of the 16S ribosomal RNA gene. To investigate the biologic similarities of these 2 ehrlichiae, the susceptibility of horses to the human agent was tested by intravenous inoculation of infected human blood. The results demonstrate that the human granulocytotropic ehrlichia produces a disease in the horse indistinguishable from that caused by E. equi, providing biologic evidence that these 2 organisms are highly related and potentially conspecific. It is possible that cases of human illness now attributed to human granulocytotropic ehrlichia may in fact be caused by 1 or more strains of an ehrlichia known chiefly as an equine pathogen.
Dumler, JS; Asanovich, KM; Bakken, JS; Richter, P; Kimsey, R; Madigan,
Serologic cross-reactions among Ehrlichia equi, Ehrlichia phagocytophila, and human granulocytic Ehrlichia.
Journal of Clinical Microbiology, 1995 May, 33(5):1098-103.
Abstract: Homology in the 16S rDNAs shows that the agent of human granulocytic ehrlichiosis (HGE) is closely related to the veterinary pathogens Erlichia equi and Erlichia phagocytophila. After HGE, patients develop antibodies reactive with E. equi and E. phagocytophila; thus, we hypothesized that these species are closely related and share significant antigenicity. Antisera from humans, horses, dogs, and cattle were tested by indirect fluorescent-antibody assay (IFA) for antibodies reactive with E. equi and other ehrlichiae and tested by immunoblot to identify the specific reactions with E. equi. All convalescent-phase sera from human patients with HGE and from animals infected or immunized with E. equi or E. phagocytophila had antibodies reactive with E. equi by IFA; no reactions with Ehrlichia chaffeensis occurred with these sera, and only one horse naturally infected with E. equi had a serologic reaction against Ehrlichia sennetsu. Human and animal sera obtained after infection or immunization with other Ehrlichia, Rickettsia, and Bartonella species did not react with E. equi by IFA. E. equi immunoblots revealed as many as 19 bands with equine anti-E. equi serum. All HGE agent, E. equi, and E. phagocytophila antisera tested reacted with a 44-kDa antigen of E. equi, while other anti-Ehrlichia spp. sera reacted with this antigen rarely or not at all. HGE agent, E. equi, and E. phagocytophila antisera but not other sera also reacted occasionally with 25-, 42-, and 100-kDa antigens. Most sera reacted with antigens between approximately 56 and 75 kDa, probably heat shock proteins. The HGE agent, E. equi, and E. phagocytophila share significant antigenicity by IFA and immunoblot.