Comparison of serological and bacteriological methods in the confirmation of plague infections.

A recent, defined outbreak of bubonic plague in a remote area of the central highlands of Viet-Nam provided an opportunity to undertake studies of antibody production during the course of infections with Pasteurella pestis (Yersinia pestis). The haemagglutination (HA) test of Chen & Meyer, modified to a microtechnique, was used in studies of patients with clinical plague, and in studies of unvaccinated, asymptomatic contacts of plague patients.HA antibody was demonstrated in the sera of nearly 45% of the unvaccinated, asymptomatic contacts of plague patients, and a large proportion of these exhibited 4-fold or greater rises in titre during the outbreak. In 6 of the 16 clinical plague cases, the diagnosis could not be confirmed by bacteriological methods; however, 5 of the 6 exhibited HA antibody in sera collected on admission or in second sera collected 9-14 days later or in both. Of the 5 with positive sera, 4 had very mild clinical illnesses (pestis minor). These findings were in sharp contrast to those among the 8 surviving, bacteriologically confirmed cases, most of whom had clinically very severe illnesses. Only 1 of the 8 had demonstrable plague HA antibody in acute and early convalescent sera; however, all of the latter on whom follow-up studies could be obtained exhibited HA antibody in sera collected 1-1(1/2) months later.These findings indicate that natural active immunization with P. pestis was occurring during the outbreak, and further, that developing immunity from earlier subclinical infections may have accounted for amelioration of clinical signs and symptoms, as well as for failure to isolate P. pestis in patients with pestis minor.HA tests may be particularly useful in confirming the diagnosis of plague if employed with sera collected 3-6 weeks after the onset of illness. The HA test would appear also to provide a highly reliable tool in surveys for plague in areas of unknown endemicity.