Source of Soldiers' Resistant Infections Still Mysterious

FORT SAM HOUSTON, Tex., May 16 -- The source of hard-to-treat infections in U.S. soldiers injured in Iraq, Kuwait, and Afghanistan remains a mystery, according to military researchers.

Investigators had thought that the continuing outbreak of Acinetobacter calcoaceticus-baumannii complex among soldiers at military hospitals in Germany and the U.S. might be derived from skin carriage of the pathogen, said Matthew Griffith, M.D., of Brooke Army Medical Center here.

But tests on healthy soldiers stationed in Iraq seem to rule that out, he and colleagues reported online today in Infection Control and Hospital Epidemiology. The article is slated for the June issue.

Many of the A. baumannii infections seen in injured soldiers are resistant to most of the medications used against them, CDC investigators reported in November 2004.

"We need to know where these infections are coming from," Dr. Griffith said.

To test the idea that A. calcoaceticus-baumannii was on the soldiers' skin before injury and simply traveled to the wound site to cause the infection, Dr. Griffith and colleagues tested 102 soldiers at a single U.S. military base in Iraq from August through October 2006. The researchers took skin swabs from the forehead, the fingers webs, and the toe webs.

The cultured skin swabs yielded a range of bacteria, including a related species, Acinetobacter lwoffii, found on seven soldiers, Dr. Griffith and colleagues reported. However, none of the samples yielded A. baumannii.

The army base at which the samples were obtained is representative of the Iraqi environment, surrounded by desert, irrigated farmland, and urban areas, the researchers said. Also, the soldiers tested were drawn from a variety of specializations, including combat and non-combat personnel.

There is some evidence that the rate of skin carriage of the bacteria varies depending on climatic conditions, Dr. Griffith and colleagues said, and it's possible that the extremely dry conditions in Iraq during the testing affected the findings.

Nonetheless, Dr. Griffith said, the finding seems to point at a nosocomial source for the infections.

"This observation refutes the concept that the bacterium is acquired prior to injury among soldiers deployed to Iraq," he said. "If skin carriage is not the source of A. calcoaceticus-baumannii complex infection, then the other possibility is that the bacteria contaminates the wounds after injury."

That could happen, he said, "while an injured soldier is awaiting treatment or in the hospital during or after receiving medical care."

The authors pointed out several limitations of the study. The main limitation, they said, was that "we were unable to completely rule out the possibility of false-negative findings. The long delay between sample collection and processing could have decreased the yield of subsequent cultures."

They also suggested that it was possible that they failed to detect some colonization by not sampling other body sites.

Antimicrobial susceptibility testing of isolates from two major military hospitals was reported by the CDC and indicated widespread resistance to agents commonly used to treat /A. calcoaceticus-baumannii/ infections.

The best performance was seen with imipenem (Primaxin), to which more than 80% of isolates were susceptible. But resistance ranged from:

• 20% to 52% for amikacin (Amikin).
• 65% to 92% for ampicillin/sulbactam (Unasyn)
• 73% to 100% for piperacillin/tazobactam (Zosyn)
• 78% to 100% for cefepime (Maxipime)
• And 80% to 97% for ciprofloxacin (Cipro).

In some cases, the CDC said, the only effective agent is colistin (polymyxin E), but susceptibility testing for colistin was not performed.