SAN ANTONIO – The largest ever military investigation of rabies exposure involved more than 900 Air Force personnel interviewed and 200 receiving post-exposure prophylaxis (PEP) at a cost of about $400,000.
In a recent issue of the Morbidity and Mortality Weekly Report, the national Centers for Disease Control and Prevention reported the January 2014 incident, which involved rabid Mexican free-tailed bats (Tadarida brasiliensis) sighted in an older building used to house an Air Force basic training squadron.
Preventive medicine and public health staff from Joint Base San Antonio-Lackland, TX, interviewed 922 Air Force personnel, both trainees and instructors, and determined that PEP, consisting of human rabies immune globulin and the four-dose vaccination series given over 14 days, was indicated for 22% of them.
In response to the incident, the investigators called for more stringent inspections of older structures for possible infestation with rabid bats.
“Careful adherence to current building codes likely is more than adequate to prevent bat infestations in new buildings,” report authors said. “For older structures, which might have been built under less-stringent codes and have become porous over time, routine inspections by animal control specialists for bat and other animal infestations are recommended.”
PEP is used when humans are bitten or scratched by a bat and that bat is either unavailable for testing or tests positive for rabies. It’s also recommended for those who might have had an exposure but were unaware, for example waking in a room with a bat or being near a bat while having a condition that might result in a lack of awareness of a bat bite, according to the report.
“Military trainees, who have multiple reasons for altered sleep, might be at increased risk for an undetected bat bite while sleeping,” the authors added.
No one among the 866 trainees and 56 instructors reported bats in the training building, although the creatures had been spotted numerous times, according to the report. The investigation began after health personnel responding to a different concern found out about bat sightings in the building, which included sleeping bays.
“This supports previous reports that document an under-appreciation of the health risks associated with indoor bat exposures,” according to the investigators from the CDC’s Epidemic Intelligence Service, the Texas Department of State Health Services and the Air Force. “The degree of risk was conveyed at subsequent meetings with military leaders and to all incoming trainees.”
The report noted that, over the past several decades, most indigenous human rabies infections in the United States have resulted from the bite of an infected bat, although the bite was not reported in more than half of the cases. The rabies virus is common in Mexican free-tailed bats in Texas, meanwhile, with 18% carrying the virus among 8,904 tested from 2001-2010.
The report pointed out that no known rabies infections have ever been acquired during Air Force basic training. The death of a recent Lackland graduate in 2011 was attributed to raccoon-variant rabies not found in Texas, and it was determined the trainee likely was infected while animal-trapping in North Carolina before enlisting in the military.
Previous Mass Exposures
To guide the investigation, military and public health officials used risk assessments of previous mass bat exposures. “Because at least four such mass exposure episodes have been reported since 2010,” the authors wrote, “it might be beneficial to have a formal, national guideline outlining the appropriate strategy in these situations.”
During the inspection of an older building for an unrelated health concern on January 17, the Lackland preventive medicine and public health teams responded quickly when a few trainees mentioned seeing bats in their sleeping bays on multiple nights while their colleagues were asleep. It was determined that the 45 trainees currently living in the affected dormitory would require rabies PEP, and the recruits got their first dose of vaccine that evening, receiving human rabies immune globulin three days later, after sufficient local supply was established.
Upon further investigation, the team found that bats might have been spotted in other dormitories within the same building from Dec. 22, 2013, to Jan. 16, 2014, and rabies PEP then was recommended for all trainees meeting criteria for moderate risk or high risk of exposure, including those who had recently moved out of the building.
PEP was not indicated for any instructors.
Adverse reactions included two cases of urticaria and six of pruritus, which were in line with the respective incidence rates of 1.6% and 3.3% previously reported, according to the report.
Working with a commercial bat control specialist, Lackland’s civil engineering unit determined, based on guano urine, and grease markings, that between 400 and 600 bats had been nesting inside wall crevices of the building for several years. The report suggested that the bats, whose usual southward migration was delayed by an unseasonably warm period, entered the dormitory for warmth when cold temperatures arrived.
The other occupied older buildings were inspected and found to be free of bats.