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Outbreak of Community-Acquired Pneumonia Caused by Mycoplasma pneumoniae --- Colorado, 2000
On May 18, 2000, the Colorado Department of Public Health and Environment (CDPHE) was contacted by a family physician in Moffat County, Colorado (1998 population: 12,700), about a large number (>50) of community-acquired pneumonia cases diagnosed by chest radiograph in a group practice over several months. An investigation by state public health officials and CDC implicated Mycoplasma pneumoniae as the cause of illness. This report summarizes the results of the investigation and underscores the importance of investigating outbreaks of severe unexplained respiratory illness to enable implementation of appropriate treatment and control measures.
During January--July 2000, 109 persons were diagnosed with pneumonia by chest radiograph in group practice A (the largest outpatient practice in the county), compared with 21 persons in the same practice during January--June 1999. A case was defined as an acute infiltrate consistent with pneumonia on a chest radiograph in a person aged 2--49 years with illness onset during January--July 2000. Medical records were abstracted to collect demographic and clinical information.
Following recognition of the outbreak, throat and nasopharyngeal swab specimens were collected from acutely ill persons who agreed to be tested. During early June, specimens from seven case-patients underwent polymerase chain reaction (PCR) testing for bacterial pathogens and for viral culture at CDC. Acute and convalescent serum specimens were available from six patients (including five of the seven patients for whom PCR was performed and one patient for whom PCR testing was not performed); these paired serum specimens were tested at CDC for antibodies by the Remel test. The paired serum specimens also were tested for complement fixation (CF) antibody titers to respiratory viruses and M. pneumoniae at the CDPHE laboratory.
Ninety-one patients had illness that met the case definition; 64 (70%) had illness onset during April--July (Figure 1). The median age was 11 years; 59 (65%) were aged 5--14 years, and 52 (57%) were male. Records of 77 (85%) patients were reviewed. Symptoms included cough (77 [100%]), fever (72 [94%]), sputum production (44 [57%]), and abnormal lung auscultation findings (54 [70%]). Three (3%) patients were hospitalized.
All eight patients tested had laboratory evidence of M. pneumoniae infection. Specimens from four patients were positive by PCR and the Remel test and had a fourfold rise in CF titers; two patients were positive by PCR alone (serum not collected); one patient had a positive Remel test and two convalescent-phase CF titers >1:128, consistent with recent infection (PCR not performed); and one patient had a positive Remel test and two convalescent-phase CF titers of 1:32, consistent with recent infection (PCR negative). PCR testing for nucleic acid of Chlamydophila pneumoniae was negative as was viral culture and serologic testing for viral respiratory pathogens, including influenza and respiratory syncytial virus.
In mid-June, CDPHE, in conjunction with the county public health nursing service, notified local health-care providers that M. pneumoniae had been confirmed by laboratory testing and provided information about the illness, including appropriate antibiotic treatment and treatment of symptomatic close contacts. Local media reports provided the community with similar information.
Reported by: KB Mezarina, Univ of Colorado Health Sciences Center, Denver; A Huffmire, MD, J Downing, N Core, Moffat Family Clinic, Craig; K Gershman, MD, R Hoffman, MD, State Epidemiologist, Colorado Dept of Public Health and Environment. Respiratory Diseases Br and Meningitis and Special Pathogens Br, Div of Bacterial and Mycotic Diseases; Respiratory and Enteric Viruses Br, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases, CDC.
M. pneumoniae is a common cause of acute respiratory tract infections (e.g., pharyngitis, tracheobronchitis, and pneumonia), especially in school-aged children. Although some infections can be fatal, most illnesses attributed to M. pneumoniae are relatively mild, and pneumonia caused by Mycoplasma rarely results in hospitalization (1,2). Outbreaks can occur in closed settings (e.g., institutions and summer camps) or can occur as communitywide epidemics (3,4). Communitywide epidemics often may not be recognized (5).
The highest incidence rates of pneumonia caused by Mycoplasma are among children aged 5--9 years followed by children aged 10--14 years (6). Children aged 2--4 years have higher rates than adults, although M. pneumoniae accounts for a low proportion of all pneumonias in this age group for which viral and other bacterial etiologies predominate (6). During outbreaks, the estimated frequency of pneumonia among school-aged children with M. pneumoniae infection has been 10%--19% (4,6). The incubation period for Mycoplasma is approximately 3 weeks (7). High rates of transmission have been documented within families, with a high proportion of secondary cases involving lower respiratory tract infection (7,8). In a study of community spread, transmission of Mycoplasma within schools was relatively low compared with spread within families; clustering of infections also occurred among neighborhood playmates (9).
The findings in this report are subject to at least four limitations. First, case ascertainment was conducted at only one of several medical practices in the affected community. Second, case ascertainment included only cases of pneumonia rather than the broader spectrum of acute respiratory illness that probably was occurring. Third, determination of the beginning of the outbreak was not possible with available data. Fourth, laboratory testing was performed only during a limited portion of the outbreak because acute isolates were available for only a fraction of possible patients following recognition of the outbreak. A portion of the cases, especially those occurring earlier in the outbreak, may have been attributed to other agents such as influenza and respiratory syncytial virus. However, because of the relatively mild nature of the symptoms, the prolonged duration of the outbreak, the occurrence of cases among school-aged children, and the laboratory results, M. pneumoniae was most likely the cause of the outbreak.
Definitive diagnosis of M. pneumoniae traditionally has depended upon isolation of M. pneumoniae or a fourfold rise in CF antibody titers between acute- and convalescent-phase serum specimens collected 4 weeks apart; isolation may require several weeks and acute and convalescent titers often are difficult to collect. Single elevated CF antibody titers are of limited use for clinical diagnosis. Although the CF and Remel tests both indicated Mycoplasma infection on the six paired serum specimens tested, the Remel test is now preferred because of its improved specificity. PCR testing of oropharyngeal or nasopharyngeal swabs offers more sensitive and rapid diagnosis of acute M. pneumoniae infections; however, this test is not widely available (10).
Macrolides and tetracycline are the antimicrobials of choice for Mycoplasma infections. Tetracycline should not be used for children aged <8 years because it may cause permanent dental discoloration. Prophylactic antimicrobial therapy with azithromycin substantially reduces the secondary attack rate in institutional outbreaks (3). No data support routine chemoprophylaxis during community outbreaks of M. pneumoniae.
Evaluation of clusters or outbreaks of acute respiratory illness may be important to determine appropriate treatment of infected persons and appropriate control measures, including use of chemoprophylaxis. The possible etiologic agents depend on the predominant acute respiratory syndrome observed (i.e., prolonged or paroxysmal cough, bronchitis, influenza-like illness, pneumonia, and rapidly progressive pneumonia). As demonstrated in this outbreak, factors such as the population affected, incubation period, and clinical features may suggest a particular agent and help to guide laboratory testing. CDC can assist local, state, and territorial health departments with the investigation of acute respiratory disease outbreaks of unknown etiology.
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