In most cases the primary infection is viral, though the causal virus is generally not demonstrated, and there is often concomitant carriage or secondary infection with one of the potential bacterial pathogens commonly present in the nasopharynx, e.g. Pneumococcus, Haemophilus infiuenzae (H. infiuenzae). Staphylococcus aureus and Streptococcus pyogenes (S.pyogenes). Drug-resistant Coliform bacilli or yeasts may come to dominate the throat flora in patients receiving antibiotics, but their presence is generally of little pathological significance.
Streptococcal pharyngitis
The only common primary bacterial cause of sore throat is S. pyogenes, which is found in about 30% of cases of pharyngitis, with or without tonsillitis. Its detection is the main purpose of the bacteriological examination of throat swabs, for it is the only common throat pathogen for which antibiotic therapy is clearly indicated. When it is thought or shown to be present, benzyipenicillin and procaine penicillin should be given by intramuscular injection and followed by phenoxymethyl penicillin given orally for 7-10 days; commonly, the injections are omitted. Erythromycin should be given to patients allergic to the penicillins. Effective therapy should cause rapid amelioration of symptoms, e.g. within 24-48 h, and prevent serious complications such as otitis media and rheumatic fever. Streptococcal pharyngitis cannot be distinguished clinically from viral pharyngitis, so whenever practicable throat swab examinations should be made on patients with sore throat.
In only a few cases are there clinical indications requiring the examination of throat swabs for other pathogens, such as the diphtheria bacillus , Vincent's organisms. Candida or Gonococcus and in the absence of such indications swabs from pharyngitis should be examined only for S.pyogenes. If the presence of commensal nasopharyngeal residents like H. infiuenzae. Pneumococcus, S. aureus and coli-form bacilli is reported to the physician, he may be induced to give inappropriate antibiotic therapy.
Method of Throat swabs. A plain, albumen-coated or charcoal-coated cotton-wool swab should be used to collect as much exudates as possible from the tonsils. posterior pharyngeal wall and any other area that is inflamed or bears exudates. If the patient permits, the swab should be rubbed with rotation over one tonsillar area, then the arch of the soft palate and uvula, the other tonsillar area, and finally the posterior pharyngeal wall. An adequate view of the throat should be ensured by good lighting and the use of a disposable wooden spatula to pull outwards and so depress the tongue. The swab should be replaced in its tube with care not to soil the rim. If it cannot be delivered to the laboratory within about 1 h, it should be placed in a refrigerator at 4°C until delivery or, preferably, it should be submitted in a tube of transport medium.
Culture of Throat swab. In the laboratory the swab should be rubbed, while being rotated, over large ‘well’ areas, about one-third of the surface on each of two blood agar plates, and the wells should be streaked out with a loop over the remainder of the plate. The plates should be incubated at 37°C for 18-24 h, one in air plus 5-10% CO2. the other anaerobically in nitrogen or hydrogen plus 5-10% CO2. It is advantageous, before incubation, to place a 6 mm disk containing 1 unit of benzylpenicillin on the well area of one plate and a disk containing 0.1 units of bacitracin on that of the other.
Next day, colonies of S. pyogenes are recognized by their zones of β-haemolysis, larger and clearer on the anaerobic than the aerobic plate, and their sensitivity to both penicillin (zone diameter >16 mm) and bacitracin (zone >12 mm). Haemolytic Haemophili. which have streptococcus like, colonies, give stronger haemolysis on the aerobic than the anaerobic plate and are resistant to penicillin. The results of these primary sensitivity tests make possible the provisional identification of S. pyogenes, and its immediate reporting to the physician. even when the streptococci and their haemolysis are confined to the confluent mixed growth in the well of the plate and separate β- haemolysis colonies are not available for testing.
The bacitracin test fails to identify rare strains of S. pyogenes that are bacitracin-resistant and misidentifies rare strains of other streptococci that are bacitracin-sensitive. If, therefore well separated β-haemolytic colonies are present on the streaked-out area of the plate. they should be picked and their Lancefield group determined by a rapid co-agglutination or precipitation test . When separate colonies are not present. it is necessary to repeat the confluent (β-haemolytic growth to obtain a pure culture for grouping, but a report to the physician should not be delayed until the results of this confirmatory test are available.
Quantitative Culture. Note and report the relative abundance of S.pyogenes colonies in the primary plate culture, for the organism is more likely to have a pathogenic role when it is numerous (e.g. >100 colonies/plate) than when, it is scanty. An appreciable proportion of healthy persons, e.g. 1-10% of adults and up to 20 or 30% of children, carry the streptococcus in the throat, apparently without harm, and the organism will be detected in a throat swab when a carrier develops a sore throat due to some other pathogen, such as an undetected virus. Such a finding could lead to a misdiagnosis, and if the streptococci are scanty the possibility that they are not the cause of the pharyngitis should be borne in mind.
In a study of the value of standardized quantitative culture, it was found that large numbers of S. pyogenes in throat swabs from 71% of children with streptococcal pharyngitis, but in swabs from only 10% of well children who were throat carriers of the organism. But the numbers of streptococcal colonies on the culture plate are greatly influenced by the efficacy of the procedures of swabbing the throat and transfer to the plate, so that they may not fairly reflect the number of the organisms in vivo . As these procedures cannot be rigidly standardized. account should also be taken of the number of S. pyogenes colonies relative to the number of colonies of throat commensal bacteria in the culture. Although a scanty growth of the streptococcus is likely to be due to harmless throat carriage, the uncertainty of the quantitative distinction makes it advisable that in all cases the organism’s presence should be reported and antibiotics given to eradicate it.
Repots may be given as ‘Many’, Few’ or No S. pyogenes found in culture’. When none is found. it is advisable to add to the report a statemert that ‘Other pathogens, including viruses were not sought’. If a negative result is reported only as ‘No pathogens isolated’, the physician may think that the swab has been examined for viruses, Mycoplasmas, Chlamydias, Diphtheria bacilli, Vincent’s organisms, Gonococci and other rarer throat pathogens.
Anti-streptolySin-O (ASO) titer. In cases of suspected streptococcal infection, e.g. acute rheumatic fever, where throat and other cultures have failed to reveal the organism, the patient’s blood serum should be tested for its content of antibodies to streptolysin-O. In such infections there is usually a steep rise of ASO titer to values well in excess of 200 Todd unit/ml after 2-4 weeks. If the titer is not raised, the patient’s illness is unlikely to be rheumatic fever. A commercial kit (Rapitex ASL) is available from Behring for rapid testing with a suspension al latex particles sensitized with streptolysin-O.
Other throat infections
Haemolytic streptococci other than S. pyogenes are often present in the throat as harmless commensals, but those of groups C and G occasionally, and B rarely, cause pharyngitis. If their presence in large numbers suggests they may have a pathogenic in the patient, their presence and antibiotic sensitivities should be reported to the physician.
Haemophilus influenzae. As haemophili are carried as commensals in the throat in a large proportion of adults and children, their finding in a throat swab should generally be ignored. There are, however, certain circumstances in which a search should be made for them and their presence regarded as possibly significant. Thus, H. influenzae of capsule serotype b is a fairly common pathogen in young children, especially under the age of 4, in whom it may cause pharyngitis,tracheo-laryngo-epiglottitis (croup). bronchopneumonia, bacteraemia and meningitis, and in such cases it should be sought in throat swab cultures. Moreover, in children with suspected bronchitis or pneumonia, it is often difficult to obtain a satisfactory specimen of sputum, and it may then be helpful to examine a throat swab for the presence of Haemophilus and other potential lung pathogens and determine their drug sensitivities. The physician should, however, be warned that though the pathogen in the throat may also be present in the lower respiratory tract, it is more probably confined to carriage in the throat. It may be difficult or hazardous to collect a throat swab from a child with croup, in which case an attempt should be made to demonstrate the pathogen in blood culture, as should also be done in suspected pneumonia and meningitis.
When Haemophilus is to be sought, the throat swab should be inoculated on to a heated-blood agar or Fildes agar plate as well as on to blood agar, and that plate incubated aerobically. A 2µg amoxycillin or ampicillin disk may be placed on the well of the plate so that both the presence and sensitivity of the haemophilus may be reported next day. Until the ampicillin sensitivity is known, the drug of choice for the initial treatment of severe haemophilus infections is chloramphenicol, but its prolonged administration may be dangerous.
Diphtheria. In countries where diphtheria is even moderately common, all swabs from sore throats should be cultured on a selective tellurite indium for Corynebacterium diphtheriae and C. Ulcerans as well as on blood agar S. pyogenes.
Eleks test is used to detect exotoxin production from the isolated bacteria. It is composed of filter paper impregnated in antitoxin and put against the susbected culture.Precipitation lines will be formed around the filter paper
In communities where artificial immunization has made diphtheria rare, the chance of making a positive finding may be too low to justify the large expenditure of labor and materials in routinely setting up the extra cultures. In that case, reliance must be placed on the physician to indicate the few cases in which the possibility of diphtheria has been suggested by the presence of membrane in the throat, extreme constitutional upset or nerve paralyses, and which therefore require examination of the swab by the these methods.
Vincent’s infection. A foetid, ulcerative inflammation of the throat (Vincent’s angina) or gums (gingivitis) is occasionally caused by a combined infection with Vincent’s spirochaetes and anaerobic fusiform bacilli. When the clinical findings suggest the condition, a swab from the affected areas should be examined in a Gram smear. The presence of many Gram-negative Spirochaetes and Fusiform bacilli, e.g. at least two of each per field well filled with pus cells and debris, may be reported as ‘Many Vincent’s organisms in film’. Small numbers of such organisms may be present in the healthy mouth and throat, and should be ignored.
Gonococcal pharyngitis. This condition should be suspected in promiscuous persons who engage in oral intercourse and a swab should be examined for gonococcus by culture on selective medium.
Candidiasis (thrush). In newborn babies and debilitated elderly persons, infection with Candida albicans may cause an acute inflammation with plaques of soft white exudates in the mouth and throat. in patients with these manifestations, a swab taken from the lesions should be examined for the fungus. An aerobic blood agar plate may show the small opaque white colonies of Candida, typically with short pointed ‘rootlets’ projecting from their margins, hut their growth may be slow and incubation at 35-37°C may have to be continued for 48h before they become recognizable. When an examination for candida is indicated. it is best to inoculate the swab on to a plate of Sabouraud agar as well as on to blood agar. A 50 unit nystatin disk and a 20µg amphotericin disk should be placed on the ‘well’ of the Sabouraud plate. Growth of candida sensitive to the antifungal drugs may then be observed and reported after 24 or 48h. and later be subjected to confirmatory tests. The presence of small numbers of yeasts in material from the mouth or throat may reflect selection and opportunistic colonization during antibiotic therapy. In such a case, further antibiotic therapy may be contraindicated.
Viral infections. Several viruses may cause an exudative pharyngitis resembling that caused by Streptococcus pyogenes. One that commonly. does so is the virus of infectious mononucleosis, a condition which may be diagnosed by the demonstration of a lymphocytosis and atypical lymphocytes in a blood film and that of heterophile antibodies in a Paul-Bunnell test on the patient’s serum. Diagnostic tests are usually not attempted for other viral infections of the throat unless the identification is required for epidemiological purposes, when viral culture and serological diagnosis may be attempted.
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