Severe ehrlichiosis in an adolescent taking trimethoprim-sulfamethoxazole

Human monocytotropic ehrlichiosis-A systematic review and analysis of the literature

Human monocytotropic ehrlichiosis (HME) is a tick-borne bacterial infection caused by Ehrlichia chaffeensis. Most available data come from case reports, case series and retrospective studies, while prospective studies and clinical trials are largely lacking. To obtain a clearer picture of the currently known epidemiologic distribution, clinical and paraclinical presentation, diagnostic aspects, complications, therapeutic aspects, and outcomes of HME, we systematically reviewed the literature and analyzed and summarized the data. Cases of HME are almost exclusively reported from North America. Human infections due to other (non-chaffeensis) Ehrlichia spp. are rare. HME primarily presents as an unspecific febrile illness (95% of the cases), often accompanied by thrombocytopenia (79.1% of the cases), leukopenia (57.8% of the cases), and abnormal liver function tests (68.1% of the cases). Immunocompromized patients are overrepresented among reviewed HME cases (26.7%), which indicates the role of HME as an opportunistic infection. The incidence of complications is higher in immunocompromized compared to immunocompetent cases, with ARDS (34% vs 19.8%), acute renal failure (34% vs 15.8%), multi organ failure (26% vs 14.9%), and secondary hemophagocytic lymphohistiocytosis (26% vs 14.9%) being the most frequent reported. The overall case fatality is 11.6%, with a significant difference between immunocompetent (9.9%) and immunocompromized (16.3%) cases, and sequelae are rare (4.2% in immunocompetent cases, 2.5% in immunocompromised cases).

Ehrlichia-induced hemophagocytic lymphohistiocytosis in a pediatric kidney transplant recipient

BACKGROUND

Kidney transplant patients are susceptible to a variety of infections in the post-transplant period due to the use of immunosuppressant medications. Ehrlichiosis is a rare infection in solid organ transplant recipients with signs and symptoms that mimic rejection and other viral infections. Hemophagocytic lymphohistiocytosis (HLH) is a potentially fatal hyperinflammatory syndrome that can be triggered by infections.

METHODS

We describe a pediatric kidney transplant recipient who experienced secondary HLH due to ehrlichiosis within the initial post-transplant month.

RESULT

Our patient improved after treatment with doxycycline, corticosteroids, and intravenous immunoglobulin (IVIG).

CONCLUSION

Clinicians should consider infections such as ehrlichiosis as a potential cause of illness in febrile solid organ transplant recipients in immediate post-transplant period, especially when accompanied by a compatible exposure history.

Patient characteristics, treatment patterns, and outcomes of Rickettsial diseases among a commercially insured population in the United States, 2005-2017

Rickettsial diseases (RDs) are transmitted to humans by ectoparasites, including ticks and fleas. Symptoms range from mild febrile illness, to severe disease or death. Doxycycline is the treatment of choice for patients of all ages; early treatment based on clinical diagnosis is critical to prevent severe outcomes. We conducted a descriptive analysis using insurance claims data captured by IBM MarketScan^® research databases to describe demographics, treatment patterns, and outcomes of patients diagnosed with RDs in the United States during 2005–2017. Overall, 14,830 patients had a RD diagnosis during 2005–2017; 7,517 (50.7%) spotted fever rickettsiosis (SFR), 4,571 ( 30.8%) ehrlichiosis, 1,362 (9.2%) typhus group rickettsiosis (TGR), and 1,193 (8.0%) other rickettsial diseases. Among all patients diagnosed, 53.1% received doxycycline. Prescription rates varied by diagnosis and age; 24.1% of TGR and 61.1% of SFR patients received doxycycline; 23.9% of persons < 8 years received doxycycline, compared with 47.7% for 8–17 years, and 55.4% for ≥ 18 years. RDs are frequently diagnosed in the outpatient population; however, providers prescribed the recommended treatment to about half of patients. Continued education of treatment recommendations is critical to prevent severe outcomes.

Assessment of Risk Factors and Outcomes of Severe Ehrlichiosis Infection

IMPORTANCE

Ehrlichiosis cases in the US have increased more than 8-fold since 2000. Up to 57% of patients with ehrlichiosis require hospitalization and 11% develop a life-threatening complication; however, risk factors for serious disease are not well documented.

OBJECTIVE

To examine risk factors associated with severe ehrlichiosis.

DESIGN, SETTING, AND PARTICIPANTS

An analytic cross-sectional study of patients diagnosed with ehrlichiosis by polymerase chain reaction (PCR) between January 1, 2007, and December 31, 2017, was conducted in a single tertiary-care center in a region endemic for ehrlichiosis. Analysis was performed from February 27, 2018, to September 9, 2020. A total of 407 positive Ehrlichia PCR results were identified from 383 unique patients, with 155 unique patients meeting study criteria. Patients hospitalized at other institutions who had a positive Ehrlichia PCR performed as a reference test (n = 222) were excluded as no clinical data were available. Electronic medical record review was performed to collect demographic, clinical, laboratory, treatment, and outcomes data. Cases were excluded when there were insufficient clinical data to assess the severity of illness (n = 3) and when the clinical illness did not meet the case definition for ehrlichiosis (n = 3).

EXPOSURES

Date of presentation, onset of symptoms, date of PCR testing, date of treatment initiation, site of care, age, birth sex, race/ethnicity, Charlson Comorbidity Index, trimethoprim with sulfamethoxazole use within the prior 2 weeks, and immunosuppression.

MAIN OUTCOMES AND MEASURES

Requirement for intensive care unit (ICU) admission.

RESULTS

Of the 155 patients who met inclusion criteria, 99 patients (63.9%) were men, and 145 patients (93.5%) identified as non-Hispanic White; median age was 50 years (interquartile range, 23-64 years). Intensive care unit admission was indicated in 43 patients (27.7%), 94 patients (60.6%) were hospitalized on general medical floors, and 18 patients (11.6%) received care as outpatients. In adjusted analysis, time to treatment initiation was independently associated with an increased risk for ICU admission (adjusted prevalence ratio [aPR], 1.09; 95% CI, 1.04-1.14; P < .001). Documentation of tick exposure was independently associated with a decreased risk for ICU admission (aPR, 0.54; 95% CI, 0.34-0.86; P = .01). There appeared to be a nonsignificant change toward a decreased need for ICU care among immunosuppressed persons (aPR, 0.51; 95% CI, 0.26-1.00; P = .05).

CONCLUSIONS AND RELEVANCE

This study suggests that delay in initiation of doxycycline therapy is a significant factor associated with severe ehrlichiosis. Increased recognition of infection by front-line clinicians to promote early treatment may improve outcomes associated with this increasingly common and life-threatening infection.

Incidence and treatment of hemophagocytic lymphohistiocytosis in hospitalized children with Ehrlichia infection

We report a large cohort of pediatric patients with human monocytic ehrlichiosis (HME), enabling an estimated incidence of secondary hemophagocytic lymphohistiocytosis (HLH) in hospitalized children with HME. Among 49 children with PCR-confirmed Ehrlichia infection, 8 (16%) met current criteria for HLH. Those with HLH had more significant hematologic abnormalities and longer durations from symptom onset to admission and definitive anti-infective therapy. Among these eight, three received chemotherapy plus doxycycline, one of whom died; the other five were treated with doxycycline without chemotherapy, and all survived without HLH recurrence. Our findings demonstrate that antimicrobial therapy alone can successfully resolve Ehrlichia-associated HLH.

Hemophagocytic Lymphohistiocytosis Secondary to Ehrlichia Chaffeensis in Adults: A Case Series From Oklahoma

Hemophagocytic lymphohistiocytosis (HLH) is a syndrome of pathologic immune activation that occurs as either a familial disorder or as a sporadic condition in association with a variety of triggers. Infections are the most common cause of HLH in adults and should be searched for as early treatment usually results in a favorable outcome. Human monocytotropic ehrlichiosis (HME) is a very rare cause of HLH. Failure to consider ehrlichiosis can result in misdiagnosis and an increased length of hospitalization and healthcare cost as described in our report. Treatment for secondary HLH is aimed at reducing hypercytokinemia and eradicating inflammatory and infected cells. It is important to promptly initiate doxycycline when tick-borne diseases are being entertained as a possible trigger, as the antibiotic is effective, safe and inexpensive.

Early diagnosis of Ehrlichia ewingii infection in a lung transplant recipient by peripheral blood smear

Ehrlichiosis in lung transplant (LT) recipients is associated with severe outcomes. Ehrlichia ewingii is a less frequent cause of symptomatic ehrlichiosis, characterized by cytoplasmic inclusions (morulae) within circulating neutrophils. We report a case of E. ewingii infection in an LT recipient diagnosed promptly by blood smear exam and confirmed with molecular studies.

Diagnosis and Management of Tickborne Rickettsial Diseases: Rocky Mountain Spotted Fever and Other Spotted Fever Group Rickettsioses, Ehrlichioses, and Anaplasmosis - United States

Tickborne rickettsial diseases continue to cause severe illness and death in otherwise healthy adults and children, despite the availability of low-cost, effective antibacterial therapy. Recognition early in the clinical course is critical because this is the period when antibacterial therapy is most effective. Early signs and symptoms of these illnesses are nonspecific or mimic other illnesses, which can make diagnosis challenging. Previously undescribed tickborne rickettsial diseases continue to be recognized, and since 2004, three additional agents have been described as causes of human disease in the United States: Rickettsia parkeri, Ehrlichia muris-like agent, and Rickettsia species 364D. This report updates the 2006 CDC recommendations on the diagnosis and management of tickborne rickettsial diseases in the United States and includes information on the practical aspects of epidemiology, clinical assessment, treatment, laboratory diagnosis, and prevention of tickborne rickettsial diseases. The CDC Rickettsial Zoonoses Branch, in consultation with external clinical and academic specialists and public health professionals, developed this report to assist health care providers and public health professionals to 1) recognize key epidemiologic features and clinical manifestations of tickborne rickettsial diseases, 2) recognize that doxycycline is the treatment of choice for suspected tickborne rickettsial diseases in adults and children, 3) understand that early empiric antibacterial therapy can prevent severe disease and death, 4) request the appropriate confirmatory diagnostic tests and understand their usefulness and limitations, and 5) report probable and confirmed cases of tickborne rickettsial diseases to public health authorities.

First reported case of Ehrlichia ewingii involving human bone marrow

A 65-year-old female with a history of multiple tick bites presented with fever and pancytopenia. Intracytoplasmic rickettsial morulae were detected on peripheral smear and bone marrow biopsy specimens, and PCR amplified Ehrlichia ewingii DNA from both specimens. To our knowledge, this is the first report of E. ewingii infection of human bone marrow.

Myocarditis after trimethoprim/sulfamethoxazole treatment for ehrlichiosis

Myocarditis after Treatment for Ehrlichiosis

The manifestations of human monocytic ehrlichiosis range from a mild febrile syndrome to a severe multisystem illness. Myocardial involvement is uncommon. We report a woman, 78 years of age, who was treated with trimethoprim/sulfamethoxazole after a tick bite, in whom myocarditis was subsequently diagnosed. She recovered completely after doxycycline therapy.

Macrophage activation syndrome secondary to human monocytic ehrlichiosis

OBJECTIVES

To present a case of human monocytic ehrlichiosis (HME) that was complicated by macrophage activation syndrome (MAS), also known as secondary hemophagocytic lymphohistiocytosis (sHLH).

METHODS

Data was collected from patient's electronic medical records at the University of Alabama at Birmingham. The patient is a part of a larger cohort of patients with all-cause MAS treated at our center.

CASE

A 63 year old renal transplant recipient male on maintenance immunosuppressive therapy presented with high grade fever, leukopenia, thrombocytopenia and elevated transaminases and initially met clinical criteria for severe sepsis. On further investigation, clinical and laboratory criteria for MAS were met. He was treated with a combination of doxycycline for HME and a novel combination of anakinra (interleukin-1 receptor antagonist), and high dose corticosteroids. The discussion focuses on clinical presentation, pathogenesis and treatment of MAS with an emphasis on MAS secondary to HME.

CONCLUSION

Macrophage activation syndrome or sHLH is a dysfunctional, hyperactive and potentially fatal immune system response that results in multi-organ dysfunction. With increasing incidence of Ehrlichia chaffeensis as an emerging pathogen, clinicians should be aware of this fulminant and potentially fatal complication of HME.

Human ehrlichiosis and anaplasmosis

Human ehrlichiosis and anaplasmosis are acute febrile tick-borne diseases caused by various members of the genera Ehrlichia and Anaplasma (Anaplasmataceae). Human monocytotropic ehrlichiosis has become one of the most prevalent life-threatening tick-borne disease in the United States. Ehrlichiosis and anaplasmosis are becoming more frequently diagnosed as the cause of human infections, as animal reservoirs and tick vectors have increased in number and humans have inhabited areas where reservoir and tick populations are high. Ehrlichia chaffeensis, the etiologic agent of human monocytotropic ehrlichiosis (HME), is an emerging zoonosis that causes clinical manifestations ranging from a mild febrile illness to a fulminant disease characterized by multiorgan system failure. Anaplasma phagocytophilum causes human granulocytotropic anaplasmosis (HGA), previously known as human granulocytotropic ehrlichiosis. This article reviews recent advances in the understanding of ehrlichial diseases related to microbiology, epidemiology, diagnosis, pathogenesis, immunity, and treatment of the 2 prevalent tick-borne diseases found in the United States, HME and HGA.

Severe ehrlichia infection in pediatric oncology and stem cell transplant patients

Ehrlichiosis, a tickborne illness transmitted by tick vectors Amblyomma americanum and Ixodes scapularis, can be acquired in endemic areas. Clinical manifestations range from asymptomatic to fulminant in nature. We report three cases of ehrlichiosis in pediatric oncology patients, one of whom was a stem cell transplant recipient. Early symptoms included fever, malaise, and vague gastrointestinal symptoms. Laboratory abnormalities were initially attributed to chemotherapy toxicity. Illness was severe in all three patients and one patient died even after initiation of doxycycline. These cases emphasize the need for a high index of suspicion for tickborne illness in oncology patients, and the importance of a low threshold for starting empiric treatment before confirming the diagnosis.

Rickettsial and ehrlichial infections

Rickettsial diseases represent a clinically homogeneous group of infections characterized by fever, headache myalgias, variable presence of a rash, and a broad spectrum of neurological manifestations. Epidemiological information including time of year, geography, history of arthropod exposure, and animal contact gives important clues to the diagnosis, and should be actively elicited. Abnormalities in hematological indices of liver function tests should also increase suspicion for illness. Delay in initiation of doxycycline therapy while awaiting laboratory confirmation of infection has been associated with progressive neurological impairment and death. Clinicians should maintain a low threshold to initiate empiric therapy for rickettsial diseases in any patient with neurological findings and compatible exposures, signs, or laboratories, as these syndromes represent readily treatable causes of neurological dysfunction.

Clinical outcomes of solid organ transplant recipients with ehrlichiosis

Because of our experience with severe Ehrlichia infections in lung transplant recipients, we reviewed all cases of ehrlichiosis in solid organ transplant recipients at Barnes-Jewish Hospital in St. Louis, Missouri. Between 1996 and 2007, 25 cases of ehrlichiosis were identified. We retrospectively collected demographic, clinical, laboratory, and outcomes data, and we compared the 5 cases in lung transplant recipients with 20 cases in other solid organ transplant recipients (heart, 2; kidney, 13; liver, 5). The presenting symptoms in the majority of both groups consisted of fever and headache. Clinical outcomes were worse in the lung transplant group and included a greater need for intensive care unit treatment (80% vs. 20%, P=0.02), longer length of hospital stay (21 vs. 5 days, P=0.02), and propensity to develop acute lung injury or acute respiratory distress syndrome (60% vs. 10%, P=0.04). No mortalities occurred in either group of patients. In an endemic area, ehrlichiosis is not unusual in solid organ transplant recipients, and lung transplant recipients tend to have a more severe illness.

Human monocytic ehrlichiosis in children

BACKGROUND

Human monocytic ehrlichiosis (HME) is a tick-borne illness caused by Ehrlichia chaffeensis. Data about disease in children have been largely derived from case reports or small case series.

METHODS

A retrospective review of all medical and laboratory records from 6 sites located in the "tick belt" of the Southeastern United States was carried out. Demographic, history and laboratory data were abstracted from the identified medical records of patients. Bivariate statistical comparisons were performed using Fisher exact test or Wilcoxon rank sum tests.

RESULTS

Common clinical signs and symptoms of patients with HME (n = 32) included fever (100%), headache (69%), myalgia (69%), rash (66%), nausea/vomiting (56%), altered mental status (50%) and lymphadenopathy (47%). Only 48% had a complaint of fever, headache and rash. Common laboratory abnormalities included thrombocytopenia (94%), elevated aspartate aminotransferase (90%), elevated alanine aminotransferase (74%), hypoalbuminemia (65%), lymphopenia (57%), leukopenia (56%) and hyponatremia (55%). The median number of days of illness before the initiation of antirickettsial therapy was 6. Patients who received sulfonamides before starting doxycycline therapy developed a rash, were admitted to the hospital, and started doxycycline at a later date. Twenty-two percent of patients were admitted to the intensive care unit with 12.5% of patients requiring ventilatory and blood pressure support.

CONCLUSIONS

Although HME has been recognized among children for almost 20 years, there is only a limited knowledge about its clinical course. Even among physicians practicing in endemic regions, few cases are diagnosed each year. More work is needed to understand the true burden of disease and the natural history among asymptomatically and symptomatically infected children.

Human ehrlichiosis in transplant recipients

To characterize the impact of immunosuppression on human ehrlichiosis, we reviewed cases of ehrlichiosis occurring in transplant recipients and immunocompetent patients at three hospitals in Nashville, Tennessee. Between 1998 and 2006, 15 transplant patients were identified as having ehrlichiosis, diagnosed either by whole blood polymerase chain reaction (PCR) (n = 14) or serology (n = 1). They were compared with 43 immunocompetent patients diagnosed by whole blood PCR. We retrospectively collected demographic and clinical information. The species of Ehrlichia (E. ewingii or E. chaffeensis) was determined for patients diagnosed by PCR. The 15 transplant recipients with ehrlichiosis included 7 kidney recipients, 6 heart recipients, 1 liver recipient and 1 lung recipient. Transplant recipients had more infections with E. ewingii than immunocompetent patients (23% vs. 5%, p = 0.08). Transplant recipients experienced less rash (0% vs. 36%, p = 0.006) and presented with significantly lower hepatic enzymes, but more leukopenia and renal dysfunction than immunocompetent patients. Doxycycline therapy was started within 48 h of presentation in 73% of transplant recipients and 78% of immunocompetent patients (p = 0.7). No patient died in either group. Ehrlichia infections can occur in transplant recipients who live in an endemic area. With prompt treatment, the infected transplant recipients in our study had similar, favorable outcomes compared to immunocompetent patients.

[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.

Tick-borne infections in children: epidemiology, clinical manifestations, and optimal management strategies

Ticks can transmit bacterial, protozoal, and viral infections to humans. Specific therapy is available for several of these infections. Doxycycline is the antimicrobial treatment of choice for all patients, regardless of age, with Rocky Mountain spotted fever, human monocytic ehrlichiosis, or human granulocytic ehrlichiosis. Chloramphenicol has been used to treat these infections in children but is demonstrably inferior to doxycycline. In patients with Mediterranean spotted fever, doxycycline, chloramphenicol, and newer macrolides all appear to be effective therapies. Therapy of Lyme disease depends on the age of the child and stage of the disease. For early localized disease, amoxicillin (for those aged <8 years) or doxycycline (for those aged >/=8 years) is effective. Doxycycline, penicillin V (phenoxymethylpenicillin) or penicillin G (benzylpenicillin) preparations, and erythromycin are all effective treatments for tick-borne relapsing fever. Hospitalized patients with tularemia should receive gentamicin or streptomycin. Doxycycline and ciprofloxacin have each been investigated for the treatment of tularemia in outpatients; however, these agents do not yet have established roles in the treatment of this disease in children. Combination therapy with clindamycin and quinine is preferred for children with babesiosis; the combination of azithromycin and atovaquone also appears promising. Ribavirin has been recently shown to markedly improve survival in patients with Crimean-Congo hemorrhagic fever. The role of antiviral therapy in the treatment of other tick-borne viral infections, including other hemorrhagic fevers and tick-borne encephalitis, is not yet defined.

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.

Renal, hepatic, and marrow dysfunction in a patient with chronic renal insufficiency

Severe illness with acute renal failure, leukopenia, thrombocytopenia, and coagulopathy frequently occurs with hemolytic uremic syndrome/thrombotic thrombocytopenic purpura, hematological malignancies, sepsis, and collagen-vascular diseases. We present a 16-year-old male fast-food worker with underlying chronic renal insufficiency who manifested these abnormalities as a result of Ehrlichia chaffeensis sepsis. Doxycycline therapy and aggressive supportive care led to complete recovery.

Ehrlichia chaffeensis: a prototypical emerging pathogen

Ehrlichia chaffeensis is an obligately intracellular, tick-transmitted bacterium that is maintained in nature in a cycle involving at least one and perhaps several vertebrate reservoir hosts. The moderate to severe disease caused by E. chaffeensis in humans, first identified in 1986 and reported for more than 1,000 patients through 2000, represents a prototypical "emerging infection." Knowledge of the biology and natural history of E. chaffeensis, and of the epidemiology, clinical features, and laboratory diagnosis of the zoonotic disease it causes (commonly referred to as human monocytic ehrlichiosis [HME]) has expanded considerably in the period since its discovery. In this review, we summarize briefly the current understanding of the microbiology, pathogenesis, and clinical manifestations associated with this pathogen but focus primarily on discussing various ecological factors responsible for the recent recognition of this important and potentially life-threatening tick-borne disease. Perhaps the most pivotal element in the emergence of HME has been the staggering increases in white-tailed deer populations in the eastern United States during the 20th century. This animal serves as a keystone host for all life stages of the principal tick vector (Amblyomma americanum) and is perhaps the most important vertebrate reservoir host for E. chaffeensis. The contributions of other components, including expansion of susceptible human populations, growth and broadening geographical distributions of other potential reservoir species and A. americanum, and improvements in confirmatory diagnostic methods, are also explored.

Infections with Ehrlichia chaffeensis and Ehrlichia ewingii in persons coinfected with human immunodeficiency virus

The clinical course and laboratory evaluation of 21 patients coinfected with human immunodeficiency virus (HIV) and Ehrlichia chaffeensis or Ehrlichia ewingii are reviewed and summarized, including 13 cases of ehrlichiosis caused by E. chaffeensis, 4 caused by E. ewingii, and 4 caused by either E. chaffeensis or E. ewingii. Twenty patients were male, and the median CD4(+) T lymphocyte count was 137 cells/microL. Exposures to infecting ticks were linked to recreational pursuits, occupations, and peridomestic activities. For 8 patients, a diagnosis of ehrlichiosis was not considered until > or =4 days after presentation. Severe manifestations occurred more frequently among patients infected with E. chaffeensis than they did among patients infected with E. ewingii, and all 6 deaths were caused by E. chaffeensis. Ehrlichiosis may be a life-threatening illness in HIV-infected persons, and the influence of multiple factors, including recent changes in the epidemiology and medical management of HIV infection, may increase the frequency with which ehrlichioses occur in this patient cohort.