A defined outbreak of plague in Vietnam resulted in 21 suspected cases. These ranged from acute, fulminating infections to very mild illnesses (pestis minor). The mild cases occurred later in the outbreak. Significant plague hemagglutinating antibody was found in initial or second serum samples or in both collected 9 to 14 days apart in nearly 50% of unvaccinated, asymptomatic contacts of those with plague, and in most of the cases of pestis minor, but in few of the clinically severe cases. This suggests that the diminution in clinical severity during the outbreak may have been due to active immunization, most probably by fleas, with subinfective doses of Pasteurella pestis. Cultures revealed the persistence of P. pestis in buboes after 7 to 14 days of treatment with streptomycin (one case) and streptomycin and chloramphenicol (two cases). P. pestis was found in throat cultures from two patients. and in one non-hospitalized contact, on treatment with antibiotics, after contact with a person thought to have pulmonary plague. Two patients had plague carbuncles near the buboes; these lesions, during the later stages of their evolution, resembled the malignant pustule of anthrax.
Chief, Preventive Medicine Division and Deputy Post Surgeon, U. S. Army Hospital, Fort Ord, California 93941.
Resident in Ophthalmology, Walter Reed General Hospital, Washington, D. C. 20012.
Laboratory Sciences Consultant, Directorate of Professional Services, Office of The Surgeon General, Headquarters, Department of the Army.