Microbiological diagnosis of streptococcal pharyngitis: Lacunae and their implications

1. Microbiological diagnosis of streptococcal pharyngitis: Lacunae and their implications • Presented by • Dr. Arifur Rahman • MPhil Student • Journal: IJMM, Year: 2006;

2. Abstract • Post-streptococcal sequelae, especially acute rheumatic fever(RF)/rheumatic heart disease(RHD) continue to occur in significant proportions in many parts of the world. • Isolation of beta hemolytic streptococci from throat cultures and their identification as GAS in the laboratory is necessary.

3. INTRODUCTION • RF and RHD are major concern in most developing countries which are the post sequelae of acute streptococcal infections. • Hence microbiological diagnosis of clinically suspected pharyngitis is essential for the confirmation of a group A streptococcal (GAS) infection. • Differentiating streptococcal pharyngitis from that of a viral etiology will help the clinician to prescribe the patient.

4. Clinical Features • In general, the clinical features of GAS pharyngitis are not specific and cannot be easily differentiated from that of non-streptococcal pharyngitis. • Patients with GAS pharyngitis usually complain of pain while swallowing, fever, enlarged cervical lymph nodes and fatigue. • Headache, nausea, vomiting and abdominal pain may be seen especially in children. Tonsils are reddened and swollen.

5. Clinical Specimens • The success in isolating GAS in culture or getting a positive result with a rapid antigen detection test lies in collection of a well-taken throat swab. • Two swabs may be rubbed well over both the tonsils and posterior pharyngeal wall. • Specimens should be collected prior to any antibiotic treatment and processed in the laboratory without any delay. Blood sample may be collected for antibody tests.

6. Isolation and identification of GAS • Laboratory diagnosis of streptococcal pharyngitis depends upon- • The successful isolation of β -hemolytic streptococci (BHS) and • Its identification as GAS. • Sheep blood agar plates (BA) is used for culture. Pre-sterilized quality BA plates are either not easily available in the market or they are expensive.

7. Identification of GAS • Bacitracin susceptibility In many laboratories bacitracin susceptibility test is the method of choice to identify GAS. This test has a sensitivity of >95%, is only a presumptive test and is not recommended since group G and C streptococci can give false positive results. If this is done regularly, one can resort to GAS identification by this method.

8. Group identification • The recommended method of GAS identification is by testing β -hemolytic colonies on BA for group A specific carbohydrate antigen. • Identification of GAS by grouping is a specific method although there are isolated cases where group A has been identified as group G or vice-versa, due to antigenic cross-reactions. Grouping can be done on the organism isolated from throat cultures or extracts prepared directly from throat cultures.

9. Numerous methods are available in the laboratory for this, of which Lancefield's hot-acid extraction technique and Fuller's formamide extraction method are the most widely used. • In general GAS can be identified on the same day if BHS are well isolated on the BA plate. Otherwise it takes an additional 24 hours for the final identification. • The rapidity with which GAS is identified in a throat culture isolate is important for treatment and this could unnecessarily predispose the child to post-streptococcal sequelae.

10. Rapid Antigen Detection Tests • RADTs have a basis of extracting carbohydrate antigen from BHS and identify them by immunological methods. • They can provide a result within a few hours even when done directly on throat cultures. • Although tests such as latex agglutination, co-agglutination, enzyme immunoassays, liposomal and optical immunoassays have been employed for this purpose, most of them have sensitivity between 70% and 90%, but with a specificity of >95% with culture as a gold standard.

11. Serological Diagnosis • Demonstration of a significant or four-fold rise in titer on paired serum samples taken at an interval of 7 to 14 days apart will indicate an ongoing or an acute infection. On the other hand, presence of GAS in throat in the absence of a significant rise in antibodies indicates a carrier state and no GAS infection. Practical difficulties in getting two serum samples from children and the time taken to demonstrate a four-fold rise in titer make this unfeasible on a routine basis.

12. Therefore, isolation of BHS and its identification as GAS in the presence of clinical symptoms of pharyngitis confirms its streptococcal etiology and needs no further confirmation by antibody tests. ASO and ADNB are often used to: (1) confirm GAS infections where facilities for culture do not exist (2) to confirm doubtful positive rapid antigen detection test or (3) confirm a diagnosis of post-streptococcal etiology. Thus, culture techniques, which give direct evidence of GAS etiology and takes less time than antibody detection, are recommended in a clinical setting. Often this can be achieved within 24 hours of processing throat culture which will help the clinician to institute penicillin prophylaxis at the earliest.

13. Interpretation of Cultures • In countries where streptococcal infections are endemic, pharyngeal carriage of GAS is a common event. • Therefore isolation of GAS from throat cultures should be interpreted with caution, especially in a child with viral pharyngitis. • In such instances, determination of four-fold rise in titers of anti-streptococcal antibodies can differentiate bona-fide GAS pharyngitis from GAS carrier state.

14.   Epidemiological Typing • Typing of GAS strains based on T and M proteins are of epidemiological interest.With a GAS vaccine becoming a reality now than ever before, data on the prevalence of M types in a community has acquired great significance.With the advent of gene sequencing technology, M typing technique has been revolutionized and almost 100% typing can be done with the modern gene sequencing method .

15.   Conclusions • Microbiological diagnosis of GAS pharyngitis can be a challenging task at every stage of its execution. • Emphasis on basic bacteriological techniques, quality of the reagents and techniques as well as proper interpretation of results based on local conditions is the secret of its successful implementation. • For this, we have to upgrade our expertise to diagnose GAS infections in the laboratory. Until such times, RF/RHD will continue to cause much morbidity and mortality because microbiological techniques form an integral part of their diagnosis.