Ординатура / Офтальмология / Английские материалы / Basic Sciences in Ophthalmology_Velayutham_2009
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Quellung Reaction
When a suspension of pneumococci is mixed on a slide with a drop of type specific antigen + methylene blue, the capsule become apparently swollen, sharply delineated and refractile.
CRP
An abnormal protein (beta globulins) that precipitate with the somatic 'C' antigen of Pneumococci and appear in the acute phase - of cases of Pneumococci and disappears during convalescence. It also occurs in some other pathological conditions. This is known as the 'C' Reactive Protein (CRP).
Diagnosis
1.Microscopy (Diplococci in Gram' stain)
2.Capsular stain (India ink stain)
3.Quellung reaction
4.Culture on blood agar (alpha hemolytic colonies )
5.Sensitivity to optochin antibiotic disc
Treatment: Pneumococci is sensitive to penicillin and most of the antibiotics.
Prophylaxis: The current polyvalent vaccine contains 23 different highly purified capsular polysaccharides extracted from pneumococci of the most prevalent types.
CORYNEBACTERIUM DIPHTHERIAE
Diphtheria is a localized infection of mucous membranes or skin caused by Corynebacterium diphtheriae. Acharacteristic pseudomembrane may be present at the site of infection. Some strains of C. diphtheriae produce diphtheria toxin, a protein that can cause myocarditis, polyneuritis, and other systemic toxic effects. Respiratory diphtheria is usually caused by toxinogenic C. diphtheriae, but cutaneous diphtheria is frequently caused by non-toxinogenic strains.
Morphology and Characters
C. diphtheriae is an aerobic, nonmotile, nonsporulating, irregularly staining, gram-positive rod. The bacteria are club-shaped and are often arranged in clusters (Chinese letters) or parallel arrays (palisades). C. diphtheriae forms gray to black colonies on selective media containing tellurite. The gravis, mitis, and intermedius biotypes are distinguished by colonial morphology and laboratory tests.
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Toxoid
Treatment of diphtheria toxin with formaldehyde converts it to a non-toxic but immunogenic product (diphtheria toxoid). Immunization with toxoid elicits antibody (antitoxin) that neutralizes the toxin and prevents diphtheria.
Epidemiology
Humans are the principal reservoir for C. diphtheriae. Transmission occurs primarily by close personal contact. The risk is greater that C. diphtheriae will be transmitted to susceptible individuals from patients with diphtheria than from carriers. The incubation period for respiratory diphtheria is typically 2 to 5 days and rarely up to 8 days. Cutaneous diphtheria is usually a secondary infection whose signs develop an average of 7 days (range, 1 to >21 days) after the appearance of primary dermatologic lesions of other etiologies.
In temperate climates, diphtheria primarily involves the respiratory tract; occurs throughout the year, with a peak incidence in colder months. Treatment with antitoxin reduced the case-fatality rate to 5 to 10%.
In the tropics, cutaneous diphtheria is more common than respiratory diphtheria, occurs throughout the year, and often develops as a secondary infection complicating other dermatoses.
Pathology and Pathogenesis
C. diphtheriae infects mucous membranes, most commonly in the respiratory tract, and also invades open skin lesions resulting from insect bites or trauma. In infections caused by C. diphtheriae, initial edema and hyperemia are often followed by epithelial necrosis and acute inflammation. Coagulation of the dense fibrinopurulent exudate produces a pseudomembrane, and the inflammatory reaction accompanied by vascular congestion extends into the underlying tissues. The pseudomembrane contains large numbers of C. diphtheriae organisms, but the bacteria are rarely isolated from the blood or internal organs.
Diphtheria toxin acts both locally and systemically, and the lethal dose for humans is ~0.1 ug/kg. Toxin contributes locally to pseudomembrane formation; systemically, it can cause myocarditis, neuritis, and focal necrosis in various organs, including the kidneys, liver, and adrenal glands. Changes in the myocardium include cloudy swelling of muscle fibers and interstitial edema. These changes are followed within weeks by hyaline and granular degeneration (sometimes with fatty degeneration), progressing to myolysis and finally to the replacement of lost muscle by fibrosis. Thus, diphtheria can cause permanent cardiac damage. In diphtheritic polyneuritis, pathologic changes include patchy breakdown of myelin sheaths in peripheral and autonomic nerves, but recovery of nerve damage is the rule if the patient survives.
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Diphtheria toxin is produced by C. diphtheriae as an extracellular polypeptide. Proteolytic cleavage forms nicked toxin consisting of fragments A and B. Fragment B binds to a plasma-membrane receptor (a precursor of a heparin-binding growth factor resembling epidermal growth factor), and the bound toxin is internalized by receptor-mediated endocytosis. Fragment A is translocated across the endosomal membrane and released into the cytoplasm, where it catalyzes the transfer of the adenosine diphosphate ribose moiety from nicotinamide adenine dinucleotide (NAD) to a modified histidine residue (diphthamide) on elongation factor 2 (EF-2), thereby inactivating EF-2 and inhibiting protein synthesis. One molecule of fragment A in the cytoplasm can kill a cell. Other metabolic alterations are secondary to inhibition of protein synthesis.
Clinical Manifestations
Patients with C. diphtheriae in the respiratory tract are classified as diphtheria cases if pseudomembranes are present and as diphtheria carriers if pseudomembranes are absent. The disease is graded as tonsillar if pseudomembranes are localized to the tonsils, as combined types or delayed diagnosis if more extensive pseudomembranes are present, and as severe if cervical adenopathy or cervical edema is also present. Onset is often gradual, but most patients seek medical care within a few days of becoming ill. Fever of 37.8° to 38.9°C (100° to 102°F), sore throat, and weakness are the most common symptoms, while dysphagia, headache, and change of voice occur in fewer than half of patients. Neck edema and difficulty breathing are noted in <10% of patients and are associated with an increased risk of death. Systemic manifestations are due primarily to toxic effects of diphtheria toxin. Patients without toxicity exhibit discomfort and malaise associated with local infection, whereas severely toxic patients may develop listlessness, pallor, and tachycardia that can progress rapidly to vascular collapse.
Primary infection in the respiratory tract is most often tonsillopharyngeal but may also be (in decreasing order) laryngeal, nasal, and tracheobronchial. Multiple sites are frequently involved, and secondary spread of pharyngeal infection upward to the nasal mucosa or downward to the larynx and tracheobronchial tree is much more common than primary infection at those sites. Systemic toxicity is usually most severe when extensive pseudomembrane extends from the tonsils and pharynx into contiguous regions. A small percentage of patients present with malignant or "bull-neck" diphtheria, with extensive pseudomembrane formation, foul breath, massive swelling of the tonsils and uvula, thick speech, cervical lymphadenopathy, striking edematous swelling of the submandibular region and anterior neck, and severe toxicity.
In tonsillopharyngeal diphtheria, isolated spots of gray or white exudate may appear first. These spots often extend and coalesce within a day to form a
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confluent, sharply demarcated pseudomembrane that becomes progressively thicker, more tightly adherent to the underlying tissue, and darker gray in color. Unlike the exudate in streptococcal pharyngitis, the diphtheritic pseudomembrane often extends beyond the margin of the tonsils onto the tonsillar pillars, palate, or uvula. Dislodging the membrane is likely to cause bleeding. Laryngeal diphtheria often presents as hoarseness and cough. Demonstration of laryngeal pseudomembrane by laryngoscopy helps distinguish diphtheria from other infectious forms of laryngitis. Patients with nasal diphtheria may present with unilateral or bilateral serosanguineous nasal discharge associated with irritation of the nares or lip. Primary or secondary diphtheritic infection occasionally involves other mucous membranes, including the conjunctiva and the membranes of the genitourinary and gastrointestinal tracts.
Cutaneous diphtheria usually presents as an infection by C. diphtheriae of preexisting dermatoses involving the lower extremities, upper extremities, head, or trunk. The clinical features are similar to those of other secondary cutaneous bacterial infections. In the tropics, cutaneous diphtheria may present as a primary cutaneous lesion, typically with morphologically distinct "punchedout" ulcers that are covered by necrotic slough or membrane and have welldemarcated edges.
C. diphtheriae is an occasional cause of invasive infections, including endocarditis and septic arthritis. Risk factors for such infections include preexisting cardiac abnormalities, abuse of intravenous drugs, and alcoholic cirrhosis.
Complications
Obstruction of the respiratory tract can be caused by extensive pseudomembrane formation and swelling early in the disease or by sloughed pseudomembrane that becomes lodged in the airways later in the disease. The risk is greater when infection involves the larynx or the tracheobronchial tree and in children because of the small size of the airways. Myocarditis and polyneuritis are the most prominent toxic manifestations of diphtheria.
Bulbar dysfunction in diphtheritic neuritis typically develops during the first 2 weeks. Palatal and pharyngeal paralysis usually develops first. Swallowing is difficult, the voice is nasal, and ingested fluids may be regurgitated through the nose.Additional bulbar signs may develop over several weeks, with oculomotor and ciliary paralysis more common than facial or laryngeal paralysis. Peripheral polyneuritis typically begins from 1 to 3 months after the onset of diphtheria with proximal weakness of the extremities, which spreads distally. Paresthesia may occur, most often in a glove and stocking distribution. Polyneuritis usually resolves completely, with the time needed for improvement approximately equal to that elapsing from exposure to the development of symptoms.
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Pneumonia occurs in more than one-half of fatal cases of diphtheria. Less common complications include renal failure, encephalitis, cerebral infarction, pulmonary embolism, and bacteremia or endocarditis due to invasive infection by C. diphtheriae. Serum sickness may result from antitoxin therapy.
Laboratory Diagnosis
1.Direct smear examination: Smears stained by Albert's stain show beaded slender rods in typical Chinese-letter pattern.
2.Culture: Swab is inoculated onto Loeffler's serum slope and Tellurite blood agar medium.
3.Animal inoculation in guinea pigs.
4.Elek's agar gel precipitation tests.
Treatment
The decision to administer diphtheria antitoxin must be based on the clinical diagnosis of diphtheria without definitive laboratory confirmation, since each day of delay in treatment is associated with increased mortality. Because diphtheria antitoxin is produced in horses, it is necessary to inquire about possible allergy to horse serum and to perform a conjunctival or intracutaneous test with diluted antitoxin for immediate hypersensitivity. Epinephrine must be available for immediate administration to patients with severe allergic reactions. Patients with immediate hypersensitivity should be desensitized before a full therapeutic dose of antitoxin is given. The dose of diphtheria antitoxin currently recommended by the Committee on Infectious Diseases of the American Academy of Pediatrics is based on the site of the primary infection and the duration and severity of disease: 20,000 to 40,000 units for disease that has been present for < 48 h and involves the pharynx or larynx; 40,000 to 60,000 units for nasopharyngeal infections; and 80,000 to 100,000 units for disease that is extensive, has been present for > 3 days, or is accompanied by diffuse swelling of the neck.Antitoxin is administered intravenously by infusion in saline over 60 min to neutralize unbound toxin rapidly. The ~10% risk of serum sickness is acceptable because of the established therapeutic value of antitoxin in decreasing mortality from respiratory diphtheria. The risk of systemic toxicity is lower in cutaneous diphtheria than in respiratory diphtheria and must be weighed against the potential adverse effects of antitoxin treatment; authorities are not unanimous in recommending antitoxin therapy for cutaneous diphtheria.
Antibiotics have little demonstrated effect on the healing of local infection in diphtheria patients treated with antitoxin. The primary goal of antibiotic therapy for patients or carriers is therefore to eradicate C. diphtheriae and prevent its transmission from the patient to susceptible contacts. Erythromycin, penicillin G, rifampin, or clindamycin is recommended by most authorities.
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Prevention
Vaccines available in the United States for immunization against diphtheria include diphtheria and tetanus toxoids and pertussis vaccine adsorbed (DTP), diphtheria and tetanus toxoids and acellular pertussis vaccine adsorbed (DTaP), diphtheria and tetanus toxoids adsorbed (DT; for pediatric use).
OTHER CORYNEBACTERIA
Medically important coryneform bacteria (formerly called diphtheroids) include members of the normal flora that cause opportunistic infections, human pathogens of relatively low virulence, and animal pathogens that cause occasional zoonotic infections. Reported infections caused by coryneform bacteria have increased substantially in number over the past two decades. Isolates of C. jeikeium and C. urealyticum are often resistant to multiple antibiotics.
Laboratory Diagnosis
Because coryneform bacteria are potential pathogens, it is important not to dismiss them as constituents of the normal flora or as contaminants when they are found in clinical specimens. Laboratory differentiation of coryneform bacteria is important when they are isolated repeatedly, when they are recovered in pure culture or in large numbers, or when they form pigmented or hemolytic colonies.
The coryneform bacteria are a large, heterogeneous group of gram-positive, pleomorphic, irregularly staining bacilli or coccobacilli that superficially resemble C. diphtheriae and are difficult to identify and classify. The genus Corynebacterium is currently divided into three groups of species: the nonlipophilic, fermentative corynebacteria (including C. diphtheriae, C. xerosis, C. striatum, C. minutissimum, and others); the nonlipophilic, nonfermentative corynebacteria (including C. pseudodiphtheriticum and others); and the lipophilic corynebacteria (including C. jeikeium, C. urealyticum, and others). Coryneform bacteria also belong to many other genera (including Actinomyces, Arcanobacterium, and Rhodococcus).
Pathogenesis
C. jeikeium was recognized in 1976 as a cause of infections in immunocompromised hosts. This organism also causes infections in immunocompetent hosts, but severe infections continue to be most frequent in patients with hematologic malignancies and neutropenia. Skin colonization precedes clinical infection. Additional risk factors for nosocomial C. jeikeium sepsis include prolonged hospitalization.
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C. urealyticum was identified as a significant cause of nosocomial urinary tract infections, including acute and chronic cystitis and pyelonephritis. The organism closely resembles C. jeikeium but differs from the latter by producing urease and failing to convert glucose to acidic metabolites. Hydrolysis of urea by urease causes alkalinization of the urine and formation of ammonium magnesium phosphate (struvite) stones.
A. haemolyticum causes pharyngitis and chronic skin ulcers; less frequently, it causes a variety of deep tissue infections, septicemia, and endocarditis.
C. minutissimum is frequently isolated from the lesions of erythrasma, a common superficial skin infection characterized by the presence in intertriginous areas of reddish-brown, scaly, pruritic, macular patches that exhibit coral-red fluorescence under a wood's light. The etiology of erythrasma appears to be polymicrobial; infection of the skin by C. minutissimum has been shown to follow the onset of maceration and scaling. Deep infections caused by C. minutissimum are rare and include abscesses, bacteremia, endocarditis, peritonitis, pyelonephritis, and infection of central venous catheters.
C. ulcerans infections in humans usually present as pharyngitis and can mimic respiratory diphtheria, whereas infections caused by C. pseudotuberculosis typically present as suppurative granulomatous lymphadenitis. Some strains of C. ulcerans and C. pseudotuberculosis produce diphtheria toxin.
C. pseudodiphtheriticum, a commensal of low virulence, is an uncommon cause of pneumonia in men with AIDS and of endocarditis, necrotizing tracheitis, tracheobronchitis, and urinary tract infection in patients without known immune deficiencies. Likewise, C. xerosis and C. striatum only occasionally cause human infections.
Diagnosis
1.Microscopy
2.Culture
Treatment
Most of the strains are susceptible to penicillin, azithromycin, quinolones and clindamycin.
GONOCOCCUS
Introduction
Gonorrhea is a sexually transmitted infection of epithelium and commonly manifests as cervicitis, urethritis, proctitis, and conjunctivitis. If untreated, infections at these sites can lead to local complications such as endometritis,
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salpingitis, tuboovarian abscess, bartholinitis, peritonitis, and perihepatitis in the female; periurethritis and epididymitis in the male; and ophthalmia neonatorum in the newborn. Disseminated gonococcemia is an uncommon event whose manifestations include skin lesions, tenosynovitis, arthritis, and (in rare cases) endocarditis or meningitis.
Neisseria gonorrhea is a gram-negative, nonmotile, non-spore-forming organism that grows in pairs (diplococci). Each individual organism is shaped like a coffee bean, with adjacent concave sides seen on Gram's stain (Fig. 33.6). Gonococci, like all other Neisseria spp., are oxidase positive. They are distinguished from other Neisseria by their ability to grow on selective media and to utilize glucose but not maltose, sucrose, or lactose.
Modes of Transmission
Gonorrhea is transmitted from males to females more efficiently than in the opposite direction. The rate of transmission to a woman following a single unprotected sexual encounter with an infected man is on the order of 40 to 60%. Oropharyngeal gonorrhea occurs in ~20% of women who practice fellatio with infected partners. Transmission in either direction by cunnilingus is rare.
Clinical Manifestations
Gonococcal Infection in Males
Acute urethritis is the most common clinical manifestation of gonorrhea in males. The usual incubation period following exposure is 2 to 7 days, although the interval can be longer and some men remain asymptomatic. Urethral discharge and dysuria, usually without urinary frequency or urgency, are the
Fig. 33.6: Gram-negative diplococci (gonococci) in urethral discharge
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major symptoms. The discharge initially is scant and mucoid but becomes profuse and purulent within a day or two. The clinical manifestations of gonococcal urethritis are usually more severe and overt than those of nongonococcal urethritis, including urethritis caused by Chlamydia trachomatis; however, exceptions are common, and it is often impossible to differentiate the causes of urethritis on clinical grounds alone. Most symptomatic males seek treatment and cease to be infectious. The remaining men, who are largely asymptomatic, accumulate in number over time and constitute about two-thirds of all infected men at any point in time. Together with men incubating the organism (who shed the organism but are asymptomatic), they serve as the source of spread of infection. Prior to the antibiotic era, symptoms of urethritis persisted for about 8 weeks. Epididymitis is now an uncommon complication, and gonococcal prostatitis occurs rarely, if at all. Other unusual local complications of gonococcal urethritis include edema of the penis due to dorsal lymphangitis or thrombophlebitis, submucous inflammatory "soft" infiltration of the urethral wall, periurethral abscess or fistulae, inflammation or abscess of Cowper's gland, and seminal vesiculitis. Balanitis may develop in uncircumcised men. After a decline in gonococcal infections among homosexual men early in the era of AIDS, a disturbing increase in gonorrhea was observed among young homosexual men in the 1990s, probably related to decreased condom use. The clinical features of anorectal and pharyngeal gonorrhea are discussed below.
Gonococcal Infections in Females
Women infected with N. gonorrhoeae usually develop symptoms. However, the women who either remain asymptomatic or have only minor symptoms may delay in seeking medical attention. Increased vaginal discharge and dysuria (often without urgency or frequency) are the most common symptoms.Although the incubation period of gonorrhea is less well defined in women than in men, symptoms usually develop within 10 days of infection and are more acute and intense than those of chlamydial cervicitis.
The physical examination may reveal a mucopurulent discharge (mucopus) issuing from the cervical os. The examiner may check for mucopurulent discharge by swabbing a sample of mucus from the endocervix and observing its color against the white background of the swab; yellow or green mucus suggests mucopus. However, only 35% of women with gonococcal cervicitis actually have a mucopurulent discharge defined by these criteria. Since Gram's stain is not sensitive for the diagnosis of gonorrhea in women, specimens should be submitted for culture or a nonculture assay (see below). Edematous and friable cervical ectopy as well as endocervical bleeding induced by gentle swabbing are more often seen in chlamydial infection.
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Ocular Gonorrhea in Adults
Ocular gonorrhea in an adult usually results from autoinoculation from an infected genital site. As in genital infection, the manifestations range from severe to occasionally mild or asymptomatic disease. The variability in clinical manifestations may result from differences in the ability of the infecting strain to elicit an inflammatory response.
Infection may result in a markedly swollen eyelid, severe hyperemia and chemosis, and a profuse purulent discharge. The massively inflamed conjunctiva may be draped over the cornea and limbus. Lytic enzymes from the infiltrating PMNs occasionally cause corneal ulceration and rarely cause perforation.
Prompt recognition and treatment of this condition are of paramount importance. Gram's stain and culture of the purulent discharge establish the diagnosis. Genital cultures should also be performed.
Laboratory Diagnosis
A rapid diagnosis of gonococcal infection in men may be obtained by Gram's staining of urethral exudates. The detection of gram-negative intracellular diplococci (GNID) is usually highly specific and sensitive in diagnosing gonococcal urethritis in symptomatic males but is only ~50% sensitive in diagnosing gonococcal cervicitis. Samples should be collected with Dacron or rayon swabs. Part of the sample should be inoculated onto a plate of modified Thayer-Martin or other gonococcal selective medium for culture. It is important to process all samples immediately because gonococci do not tolerate drying. If plates cannot be incubated immediately, they can be held safely for several hours at room temperature in candle extinction jars prior to incubation. If processing is to occur within 6 h, transport of specimens may be facilitated by the use of nonnutritive swab transport systems such as Stuart or Amies medium. For longer holding periods (e.g. when specimens for culture are to be mailed), culture media with self-contained CO2-generating systems (such as the JEMBEC or Gono-Pak systems) may be used. Specimens should also be obtained for the diagnosis of chlamydial infection.
Nucleic acid probe tests and PCR are now widely used for the direct detection of N. gonorrhoeae in urogenital specimens.
Treatment
Penicillin, Doxycycline, Ciprofloxacin are effective.
MOREXELLA
Moraxella Catarrhalis
The gram negative coccus now known as Moraxella catarrhalis has undergone three changes of name in as many decades. Originally called Micrococcus