Investigation and Results
Housing and Prophylaxis for Trainees on Arrival at Joint Base San Antonio-Lackland
U.S. Air Force basic training takes place at Joint Base San Antonio-Lackland in San Antonio, Texas. Trainees are organized into rolling military units averaging 900 entering and graduating trainees per week, with each unit further divided into groups of approximately 52 trainees. Trainees are assigned to specific dormitories, which typically house 50–60 persons, sleeping in beds that alternate head directions, creating a foot-to-head orientation in the bay. Trainees arrive weekly and undergo a standardized 7.5-week basic military training (BMT) curriculum organized by week of training, resulting in training activities and associated exposures that are typically consistent for each class.
To prevent invasive meningococcal and streptococcal disease outbreaks, all trainees receive quadrivalent meningococcal (Groups A, C, Y, and W) (Menveo) vaccine within 72 hours of arrival and a single dose of penicillin G benzathine injectable suspension, respectively, within 7 days of arrival (1). Penicillin-allergic trainees receive weekly oral azithromycin to prevent streptococcal disease during BMT.
Identification of First Two Conjunctivitis Cases
On February 5, 2025, a case of N. meningitidis bacterial conjunctivitis was identified in an otherwise healthy BMT trainee who had experienced 2 days of mucopurulent ocular discharge; the trainee had no known exposure to N. meningitidis. Although the symptoms initially suggested viral conjunctivitis (2), the copious unilateral discharge led the health care provider to culture the exudate, which was positive for N. meningitidis 6 days later. Microbial isolates were tested and identified by Vitek 2 (bioMerieux). On February 21, a second trainee (from a different training unit) was also evaluated for unilateral copious mucopurulent ocular discharge associated with periocular edema without corneal involvement. The culture of the exudate from this trainee’s eye was positive for N. meningitidis, raising concern about potential cases of invasive meningococcal disease. An investigation was initiated to identify additional cases, determine the risk for invasive meningococcal disease, and describe the patients and evaluate their response to treatment. The local human research protection program, Defense Health Agency San Antonio Market Office of Research Protocol Support, determined these activities not to be research.*
Surveillance for Conjunctivitis
The first two cases, which occurred among trainees who started training 2 weeks apart, both occurred during both patients’ fourth week of BMT. On February 23, the day that the second patient’s culture result was received, the Trainee Health Surveillance team (a group of epidemiologists and preventive medicine physicians responsible for active and passive disease surveillance of the BMT population) established a registry and began active surveillance to identify cases of mucopurulent conjunctivitis and ensure that a sample was obtained for culture for each case. A confirmed case was defined as a positive N. meningitidis culture result from ocular discharge collected from a person with conjunctivitis. A probable case was defined as symptomatic conjunctivitis with exudate in a patient who had contact with a person with a confirmed case of meningococcal conjunctivitis but without laboratory confirmation. A suspected case was defined as symptomatic conjunctivitis with exudate in a person with no known contact with a confirmed case. Clinicians who worked in emergency departments or primary care on the base were requested to assist in active surveillance by submitting samples of ocular discharge from patients with conjunctivitis to the microbiology laboratory for culture and reporting suspected or probable cases to the Trainee Health Surveillance registry rather than providing standard empiric treatment without culture. Patients with confirmed meningococcal conjunctivitis received topical ocular erythromycin, ciprofloxacin, or moxifloxacin and were referred for an ophthalmologic assessment of corneal involvement.
Identification of Additional Cases
During February 23–May 9, 2025, a total of 79 cases of mucopurulent conjunctivitis were identified among 11,797 trainees who started BMT in San Antonio (6.7 per 1,000); cultures from 41 (52%) patients were positive for N. meningitidis, and 32 (41%) were positive for Haemophilus species. Four (5%) patients received negative culture results, one patient’s ocular culture was positive for Corynebacterium macginleyi, a known cause of conjunctivitis (3), and for one patient, no specimen was collected for culture. Among the 41 laboratory-confirmed cases of N. meningitidis conjunctivitis, 23 (56%) occurred within 1 month of onset of the second case (Figure 1). Among the 32 Haemophilus species conjunctivitis cases, 29 (91%) were identified during the first 3 weeks of training, whereas 36 (87.8%) of the positive N. meningitidis ocular cultures were identified during or after the fourth week of training (Figure 2).
Clinical Characteristics of Patients with N. meningitidis Conjunctivitis
The 41 confirmed cases of N. meningitidis conjunctivitis occurred in trainees in 37 unique BMT groups. During this period, men constituted 78% of the BMT population but accounted for 90% of the N. meningitidis cases. Among trainees with confirmed N. meningitidis conjunctivitis, 33 (80%) reported an antecedent upper respiratory infection (Table). Overall, 35 (85%) patients had unilateral eye involvement. All patients improved within 24 hours of starting treatment with topical moxifloxacin, ciprofloxacin, or erythromycin. One patient was hospitalized after a delay in initiating topical moxifloxacin that led to progression of infection to periorbital cellulitis, requiring a short course of intravenous antibiotics. No patients developed invasive corneal ulceration or orbital cellulitis. Contact tracing, including prophylactic antibiotics for close contacts, was deferred, because prophylaxis is currently recommended only for close contacts of persons with invasive disease (bacteremia or meningitis) (4). Whereas cases of Haemophilus species conjunctivitis occurred in patients who received either penicillin or azithromycin prophylaxis, N. meningitidis infections only occurred in patients who received penicillin.
Whole Genome Sequencing
While serogrouping and sequencing are commonly performed by local, state, and federal public health laboratories for N. meningitidis isolates from cases of invasive meningococcal disease, neither is typically performed for isolates from noninvasive disease cases. However, after diagnosis of the second case, and to guide the public health response, whole genome sequencing of isolates from the first two ocular cultures was performed to determine whether they were related, predict antimicrobial resistance, and ascertain whether virulence factors associated with invasive disease were present. As has been reported for other cases of meningococcal conjunctivitis (5), both isolates were nongroupable, without the presence of csaB, csb, csc, csw, and csy genes associated with encapsulation, suggesting low risk for development of invasive disease. Sequencing demonstrated that the two isolates were both sequence type (ST) 32 and were closely related. ST-32 has previously been associated with meningococcal disease outbreaks caused by the encapsulated serogroup B N. meningitidis; however, because this strain was not encapsulated, it was not expected to cause invasive disease in otherwise healthy patients. The sequenced isolates indicated decreased susceptibility to penicillin based on a mutation in the penA gene, otherwise no other genetic correlates of antimicrobial resistance were identified.
Environmental Investigation and Training Activity Evaluation
The Trainee Health Surveillance team evaluated dormitory cleanliness, including the showers and common areas, and reviewed established cleaning protocols. No environmental specimens were collected for testing. Various field training activities during the fourth week of training were also evaluated to ascertain their risk as a source of transmission and to confirm adherence to cleaning protocols. The health team who observed gas mask cleaning noted that staff members followed recommended cleaning and sanitizing protocols, using liquid sodium hypochlorite disinfecting solution (bleach) at recommended concentrations. As the outbreak continued, other potential common sources of transmission were investigated, including cardiopulmonary resuscitation training. However, because trainees did not practice rescue breathing on the mannequin, this activity posed a low risk for transmission. Evaluation of the military shooting range also did not identify any potential common source of transmission; safety goggles were not shared and were cleaned with hypochlorite disinfectant wipes at the end of each training session.