An outbreak of pneumococcal pneumonia among military personnel at high risk: Control by low-dose azithromycin postexposure chemoprophylaxis

Military Medicine, Jan 2003 by Sanchez, Jose L

In the winter of 1998-1999 an outbreak of pneumococcal pneumonia occurred among Ranger students undergoing high-intensity training. Thirty pneumonia cases (attack rate = 12.6%) were identified among a group of 239 students. Eighteen students were hospitalized; Streptococcus pneumoniae-- positive cultures were detected in 11 (61.1% of these 18 hospitalized cases. Pneumococci were also identified in throat swabs of 30 (13.6% of 221 nonhospitalized students surveyed. Serum antipneumolysin seroconversions were detected in 30 (18.3% of 164 students tested. An association between development of serum antipneumolysin antibody and pneumococcal pharyngeal carriage/colonization was found. Of 30 seroconverters, eight (26.7%) had S. pneumoniae-positive cultures compared with only 17 (12.7% of 134 nonseroconverters (relative risks = 2.02, 95% confidence interval = 1.02-4.02, p = 0.05). The outbreak was controlled by administrating low-- dose, oral azithromycin prophylaxis (250 mg weekly for 2 weeks) and was associated with a 69% reduction in pneumococcal carriage and a 94% reduction in pneumonia rates.

Introduction

Acute respiratory diseases (ARD) cause significant morbidity in military populations. Training and mobilization centers have traditionally been the foci of respiratory disease epidemics due to close living conditions, the physically and psychologically stressful activities, and a multitude of pathogens brought together by these troops.1,2 For more than five decades, mass antibiotic (penicillin or erythromycin) prophylaxis has been used with success to interrupt outbreaks, to prevent serious clinical sequelae of streptococcal and meningococcal infections, to reduce infection and nasopharyngeal carriage,3-6 and to minimize acute respiratory morbidity in general.7-9 Still, many military medical officers are reluctant to use mass antibiotic prophyla)ds, mainly due to concerns regarding side effects (e.g., allergic reactions), unintended consequences (e.g., antibiotic resistance), and costs. While these concerns are important, they must be weighed against the medical and military operational costs associated with recurrent outbreaks.7-10

In March 1991, in the aftermath of outbreaks of pneumococcal pneumonia and streptococcal pharyngitis, Ranger students at a U.S. military installation began to receive two doses of benzathine penicillin G (bicillin), 4 weeks apart, during their 9-week training cycle. In September 1997, the prophylaxis regimen was reduced to a single dose, and in March 1998, routine prophylaxis was discontinued altogether. Discontinuance of prophylaxis was made by unit medical authorities and was based on a perceived absence of cases and reduced risk of infection. Pneumonia quickly re-emerged as a problem among Ranger students, first in the spring of 1998 and then in the winter of 1998-1999 (Fig. 1). In this report, we describe the winter epidemic of 1998-1999 and discuss further recommendations to preclude future outbreaks.

Materials and Methods

General Description

The investigation consisted of: (1) an initial review of all medical histories and laboratory work of hospitalized cases, (2) review of all class (4-99) medical records since the beginning of training, (3) administration of a questionnaire to obtain demographic, training, and risk factor data, (4) collection of throat swabs for pathogen isolation, and (5) collection of initial (week 5) serum samples for antipneumolysin antibody determinations. At follow-up 4 weeks later at the end of training (week 9), the following were obtained: (1) subsequent case histories of upper and lower respiratory illness among class 4-99 graduates, (2) a second questionnaire, throat swab, and serum sample on all dropouts and recycles in the class, and (3) a questionnaire, throat swab, and serum sample for determination of end-of-- training cumulative illness rates, bacteriological pharyngeal carriage/colonization rates, and serum antipneumolysin titers.

Case Definitions

A case of ARD was defined as a respiratory illness presenting with an oral temperature of >= 100.5 deg F accompanied by one or more of the following signs or symptoms: sore throat, cough, rhinorrhea, nasal congestion, sinus tenderness, rates, rhonchi, or wheezing on auscultation. A pneumonia case was defined as any student who met one or more of the following criteria: (1) an oral temperature equal to or greater than 100.5 deg F with a positive sputum or blood culture or a positive chest X-ray diagnostic of a pneumonic process; (2) symptoms of productive cough and adventitious lung sounds on auscultation with or without a temperature equal to or greater than 100.5 deg F and with a white blood cell count (WBC) of 10,000 cells/mm^sup 3^ or greater; or (3) a throat or sputum culture positive for Streptococcus pneumoniae with at least one of the above two criteria. A carrier or colonized person was defined as an asymptomatic student with a positive throat culture for S. pneumoniae.

Throat Swab Cultures and Polymerase Chain Reaction Testing

Throat swabs for detection of Mycoplasma pneumoniae (by culture and polymerase chain reaction), Chlamydia pneumoniae (by polymerase chain reaction), and Streptococcus pyogenes/S. pneumoniae (by culture) were obtained on all new cases that had not received treatment. All throat swabs were sent by overnight courier to the Navy Respiratory Disease Laboratory, Naval Health Research Center, San Diego, California.