Prophylactic doxycycline for travelers' diarrhea in Thailand. Further supportive evidence of Aeromonas hydrophila as an enteric pathogen.

A randomized double-blind study to determine the efficacy of a three-week course of doxycycline (100 mg daily) in preventing travelers' diarrhea was performed in 1980 among 63 United States Peace Corps volunteers during their first five weeks in Thailand, an area where doxycycline-resistant enterotoxigenic Escherichia coli are known to be common. Eight (24%) of 33 volunteers taking placebo and 3 (10%) of 30 taking doxycycline developed travelers' diarrhea for a calculated protection of 59%, but this was not statistically significant (p = 0.12). Aeromonas hydrophila was isolated from 8 to 19 volunteers with either travelers' diarrhea or mild diarrhea in the placebo group, but from only 1 of 12 in the doxycycline group (p less than or equal to 0.05). Furthermore, doxycycline significantly prevented colonization of the gastrointestinal tract with A. hydrophila while it was being taken (p less than or equal to 0.01). Enterotoxigenic E. coli was isolated from only one volunteer with travelers' diarrhea in the placebo group and from none in the doxycycline group. Doxycycline prophylaxis of travelers' diarrhea in this geographic area, though not shown to be significantly protective, further supports the role of A. hydrophila as an enteric pathogen.