The rickettsias and rickettsia-like organisms are members of two families: the Rickettsiaceae (Rickettsia and Orientia tsutsugamushi) and the Anaplasmataceae (Ehrlichia, Anaplasma, and Neorickettsia). Orientia tsutsugamushi (formerly called Rickettsia tsutsugamushi) was placed into its own genus primarily based on the lack of LPS, the presence of a 54-58 kDa major surface protein, and the lack of a 17 kDa lipoprotein, all of which make it different from species of Rickettsia.

Coxiella and Bartonella, two other genera of intracellular bacteria causing human disease, were at one time included in the Rickettsiaceae family. However, based on phylogenetic differences, these two genera were removed from the Rickettsiaceae family and separated into two families, Coxiellaceae and Bartonellaceae. Bartonella spp. can be cultured on standard bacteriologic media; therefore. Because Coxiella burnetii can survive extracellularly, unlike the rickettsiae, yet requires cultivation in cell culture similar to the rickettsiae.

GENERAL CHARACTERISTICS

 Rickettsiae are fastidious bacteria that are obligate, intra cellular parasites. These bacterial agents survive only briefly outside of a host (reservoir or vector) and multi ply only intracellularly. Organisms are small (0.3 µm × 1 to 2 µm), pleomorphic, gram-negative bacilli that multi ply by binary fission in the cytoplasm of host cells; the release of mature rickettsiae results in the lysis of the host cell.

 EPIDEMIOLOGY AND PATHOGENESIS

This group of organisms infects wild animals, with humans acting as accidental hosts in most cases. Most of these organisms are passed between animals by an insect vector. Similarly, humans become infected following the bite of an infected arthropod vector or by inhalation of infectious aerosols. Characteristics, including the respective arthropod vector of the prominent species of Rickettsia, Orientia, Anaplasma, and Ehrlichia, are summarized in Table 1.

Table1. Characteristics of Prominent Rickettsia* Orientia, Anaplasma, and Ehrlichia spp.

Organisms belonging to the genus Rickettsia do not undergo any type of intracellular developmental cycle. Different species of Rickettsia share some antigenic properties, are genetically similar, and share a similar mechanism of pathogenesis. After being deposited directly into the bloodstream through the bite of an arthropod vector, these organisms induce the endothelial cells of the host’s blood vessels to engulf them and are carried into the cell’s cytoplasm within a vacuole. Following infection, organisms escape the vacuole, becoming free in the cytoplasm. Rickettsia spp. then multiply, causing cell injury and death. Subsequent vascular lesions caused by Rickettsia-induced damage to endothelial cells account for the changes that occur throughout the body, particularly in the skin, heart, brain, lung, and muscle. Rickettsiae also have numerous ways to evade human host defenses such as cell-to-cell spread, escaping from the phagosome, and entering into a latent state (primarily R. prowazekii).

In contrast to Rickettsia and Orientia spp., organisms belonging to the genus Ehrlichia undergo an intracellular developmental cycle following infection of circulating leukocytes. Similar to chlamydiae, A. phagocytophilum, Ehrlichia spp., and N. sennetsu cannot survive outside host cells and, once released, must rapidly induce signals for their own uptake into another host cell that is unique to each genus. How these organisms accomplish this entry, replicate in the host milieu and then exit is largely unknown. E. chaffeensis primarily infects monocytes and causes human monocytic ehrlichiosis (HME), whereas A. phagocytophilum infects bone marrow–derived cells, primarily infecting neutrophils, causing human granulocytic anaplasmosis (HGA).

SPECTRUM OF DISEASE

 Species in the genus Rickettsia are divided into three groups: the spotted fever group, the typhus group, and the scrub typhus group (O. tsutsugamushi), based on the arthropod mode of transmission, clinical manifestations, rate of intracellular growth, rate of intracellular burden, and extent of intracellular growth (see Table 1). Rickettsias are suspected when the triad of fever, headache, and rash is the primary clinical manifestation in patients with an exposure to insect vectors. Infections caused by these organisms may be severe and are sometimes fatal.

Although HME and HGA cause distinct infections, their clinical findings are similar. In general, patients with ehrlichial infections present with nonspecific symptoms such as fever, headache, and myalgias; rashes occur only rarely. The illness can range from asymptomatic to mild to severe.

LABORATORY DIAGNOSIS

 Because rickettsial and ehrlichial infections can be severe or even fatal, a timely diagnosis is essential.

Direct Detection Methods

 Immunohistology and conventional and real-time PCR have been used to diagnose rickettsial and ehrlichial infections. Biopsy of skin tissue from the rash caused by the spotted fever group rickettsiae is the preferred specimen. Organisms are identified using polyclonal antibodies and are detected with the use of fluorescein-labeled antibodies or enzyme-labeled indirect procedures. The sensitivity of these techniques is about 70% and depends on correct tissue sampling, examination of multiple tissue levels, and biopsy before or during the first 24 hours of therapy (see Table 1).

Direct detection of Ehrlichia and Anaplasma from peripheral blood or cerebrospinal fluid (CSF) includes PCR amplification, direct microscopic examination of Giemsa-stained or Wright’s stained specimens, or immunocytologic or immunohistologic stains with E. chaffeensis or Anaplasma species antibodies. Direct microscopic examination of Giemsa-stained or Diff-Quik–stained peripheral blood buffy-coat smears can detect morulae (cytoplasmic vacuoles containing enriched organisms) during the febrile stage of infection in ehrlichiosis; morulae-like structures also can be observed in CSF cells and tissues. Finally, recent reports have described the development of rapid, species-specific real-time PCR assays to detect single or co-infections with Anaplasma species or Ehrlichia species in peripheral blood specimens.

Cultivation

Although the rickettsiae can be cultured in embryonated eggs and in tissue culture, the risk of laboratory-acquired infection is extremely high, limiting the availability of culture to a few specialized laboratories. Blood should be collected as early as possible in the course of disease in a sterile, heparin-containing vial. Similarly, punch biopsies of skin or eschars (slough or dead skin) are also acceptable but must be collected early in the course of disease. These same specimens are also acceptable for PCR.

To date, culture of Ehrlichia and Anaplasma is limited and culture conditions are still being optimized. Currently, the preferred specimen for culture is peripheral blood obtained in a sterile, EDTA- or acid-citrate dextrose-anti-coagulated blood tube; if specimens must be moved, they should be transported overnight at approximately 4° C.

Serodiagnosis

 Although it is not fast, the diagnosis of rickettsial disease and ehrlichiosis is primarily accomplished serologically. Serologic assays for the diagnosis of rickettsial infections include the indirect immunofluorescence assay (IFA), enzyme immunoassay (EIA), Proteus vulgaris OX-19 and OX-2 and Proteus mirabilis OX-K strain agglutination (the Weil-Felix reaction), line blot, and Western immunoblot ting. The Weil-Felix reaction, the fortuitous agglutination of certain strains of P. vulgaris by serum from patients with rickettsial disease, may still be performed in developing countries, but because false-positive and false-negative tests are a continuing problem, these tests have been replaced by more accurate serologic methods such as IFA.

Except for latex agglutination, IFA, and DFA testing for diagnosing Rocky Mountain spotted fever, none of the serologic tests is useful for diagnosing disease in time to influence therapy. This lack of utility for serology is because antibodies to rickettsiae other than R. rickettsii cannot be reliably detected until at least 2 weeks after the patient has become ill. With newer immunologic recombinant reagents under development, the potential exists for new tests for all the rickettsial diseases.

To date, the sensitivity and specificity of serologic assays for ehrlichiosis is unknown but is presumed to be relatively high; indirect immunofluorescent antibody testing is available for E. chaffeensis or A. phagocytophilum. A fourfold or greater rise in antibody titer during the course of disease is considered significant.

ANTIBIOTIC SUSCEPTIBILITY TESTING AND THERAPY

 Tetracyclines, especially doxycycline, are the primary drugs of choice for treatment of most infections caused by Rickettsia, Ehrlichia, or Anaplasma species. Depending on the specific species of Rickettsia, some fluoroquinolones may be used, as may chloramphenicol.

PREVENTION

The best means of preventing rickettsial and ehrlichial infection is to avoid contact with the respective vectors.

اخر الاخبار

اخبار العتبة العباسية المقدسة

وفد العتبة العباسية المقدسة يجري جولة في أروقة معرض بغداد الدولي للكتاب

شعبة التوجيه الديني النسوي تطلق برنامجًا قرآنيًّا لزائرات مرقد أبي الفضل العباس (عليه السلام)

جامعة الكفيل تطلق دورة تدريبية عن تقييم الأداء الجامعي لعدد من ملاكاتها

المجمع العلمي يقيم محفلًا قرآنيًا بذكرى ولادة الصادقين (صلوات الله عليهما)

المزيد

الآخبار الصحية

هل يتنبأ الذكاء الاصطناعي بأمراضنا قبل حدوثها؟

وداعا للنظارات.. قطرة عين واحدة قد تغير كل شيء ؟

دراسة تكشف تأثير "تيك توك" وتطبيقات الفيديو على سلوك الأطفال

"أسرار خفية" في الشاي الأخضر.. تغير قواعد الصحة والوزن

المزيد

الآخبار التكنلوجية

أول قطار يعمل بالهيدروجين في أميركا يدخل الخدمة رسميًا

أجهزة أبل القادمة.. "حصن منيع" في عصر الهجمات الرقمية !

روسيا: تطلق قمرين اصطناعيين بواسطة الصاروخ سويوز 2.1

ميزة ثورية.. أبل تدخل عالم قياس ضغط الدم

المزيد

مواضيع ذات صلة

Coxiella

Chlamydia pneumonia

Urinary Tract Infections

Actinomyces

Neisseria meningitidis

The Brucellae

Diagnostic Laboratory Tests of Bordetella pertussis

Chlamydia psittaci

Laboratory Diagnosis of Chlamydia trachomatis

Spectrum of Disease of Chlamydia trachomatis

Epidemiology and Pathogenesis of Chlamydia trachomatis

Antigenic Structure, Pathogenesis, and Pathology of Bordetella pertussis