Novel Naked Pneumococcal Naval Eye Infection: An Outbreak of Conjunctivitis

Abstract & Commentary

Synopsis: An outbreak of conjunctivitis caused by a macrolide resistant unencapsulated pneumococcus occurred among naval trainees who were receiving azithromycin as prophylaxis against atypical pneumonia.

Source: Crum NF, et al. An Outbreak of Conjunctivitis Due to a Novel Unencapsulated Strain of Streptococcus pneumoniae. Clin Infect Dis. 2004; 39:1148-1154.

In early December 2003, an increase in the number of cases of conjunctivitis occurred at a military training facility in San Diego, California. The base housed 3500 recruits. All had received polyvalent pneumococcal vaccine on arrival. Recruits typically received injections of benzathine penicillin once a month for prophylaxis against group A streptococcal infections. However, in response to an increase in respiratory infections several months earlier, the prophylactic regimen had been changed to weekly doses of azithromycin.

Ninety-two cases of conjunctivitis were identified among the 3500 recruits during a 6-week period; the attack rate was 1.75 cases per 100 person-months. In the most affected unit on the base, the attack rate was 4.3 cases per 100 person-months. Of the 92 cases, 45 were confirmed by culture as being due to S. pneumoniae. All isolates were unencapsulated, and therefore non-typeable. Most were susceptible to penicillin. All were resistant to trimepthoprim-sulfamethoxazole, erythromycin, and azithromycin. Genetic sequencing of 11 isolates by multilocus sequence typing showed that all had an identical allelic profile. This sequence profile had not been previously reported in the multilocus sequence typing database of pneumococcal isolates.

A survey of 151 recruits in the most affected unit showed a pharyngeal carriage rate of 9.9%. The pharyngeal isolates were genetically identical to the conjunctival isolates.

Affected recruits were treated with a variety of topical antimicrobials. Good hygienic practices were stressed, and the recruits were provided with alcohol-based hand disinfectant. The outbreak ended by the end of December; the contribution of control measures is difficult to assess, since the end of the outbreak coincided with the Christmas holidays, at which time the recruits were granted leave.

Comment by Robert Muder, MD

Outbreaks of conjunctivitis are common in school and military populations; most are due to viral agents. Although the pneumococcus is an established cause of purulent conjunctivitis, outbreaks of pneumococcal conjunctivitis are uncommon. Previous outbreaks of pneumococcal conjunctivitis have been reported from colleges and military bases.1,2 In contrast to pneumococcal isolates from other infected sites, which are nearly always encapsulated, several outbreaks of pneumococcal conjunctivitis have involved unencapsulated strains. Unencapsulated strains are non-typeable, and the absence of a capsule may cause some confusion with other streptococcal species in the clinical microbiology laboratory. However, genetic analysis of strains from several outbreaks of conjunctivitis confirms that these strains are indeed pneumococci.1

The relationship between absence of a capsule and the ability of strains to cause conjunctivitis is unexplained. The capsule is an important virulence factor, and nearly all S. pneumoniae isolates from invasive infections are encapsulated. Crum and colleagues note that since conjunctivitis is a superficial infection rather than an invasive one, a capsule may not be necessary for pathogenesis.

This outbreak is notable for several reasons. It was caused by a previously unidentified strain of pneumococcus. It occurred in the setting of universal prophylaxis with azithromycin, and all isolates were highly resistant to macrolides. It is tempting to speculate that prior administration of azithromycin had at least a permissive role in the outbreak. All recruits had received prior immunization with pneumococcal polysaccharide vaccine. The fact that this was not protective is not surprising, since the out break strain did not have a polysaccharide capsule, and the vaccine has not been shown to be protective against upper respiratory tract infections.

Although most cases of conjunctivitis encountered in the outpatient setting are treated without bacterial culture, this outbreak demonstrates the importance of performing cultures in an outbreak situation.

References

1. Carvalho MGS, et al. Confirmation of Nontypeable Streptococcus pneumoniae-Like Organisms Isolated From Outbreaks of Epidemic Conjunctivitis as Streptococcus pneumoniae. J Clin Microbiol. 2003;41: 4415-4417.

2. Martin M, et al. An Outbreak of Conjunctivitis Due to Atypical Streptococcus pneumoniae. N Engl J Med. 2003; 348:1112-1121.

Robert Muder, MD, Hospital Epidemiologist, Pittsburgh VA Medical Center, Pittsburgh, Section Editor, Hospital Epidemiology, is Associate Editor for Infectious Disease Alert.