When a guinea pig is inoculated with plague material the animal should be placed in a galvanized iron garbage can or other similar container and the opening covered with cheese cloth to prevent the fleas or other vermin which it might harbor from escaping. Again a 6-inch band of tanglefoot fly paper should be attached around the interior of the upper part of the can to further prevent escape of fleas. It must be remembered that every precaution must be taken in the laboratory to prevent the escape of plague-infected fleas. The guinea pig usually dies in from two to five days and shows glandular enlargements, and marked congestion of viscera with a swollen spleen, smears from which may show a profusion of plague bacilli.
The subcutaneous tissues about the site of inoculation show a haemorrhagic oedema.
If guinea pigs are not available one may use white mice which die within forty-eight hours or white rats which live about as long as guinea pigs.
One of the most important points in fighting plague is the detection of plague in the rats and, from noting the locality in which such plague-infected rats were caught, to direct our rat destruction efforts to that particular section of the city. These dead rats should be dropped into a bichloride solution or petroleum preparation in order to destroy the rat fleas. In the laboratory they are dissected and plague-infected ones most easily recognized by the marked subcutaneous injection of the widely reflected skin flap. Oedematous or haemorrhagic glandular swellings are characteristic. The liver shows a yellowish appearance, and as if sprinkled with small whitish dots and the spleen is swollen and congested. There may be effusion into the pleural cavities. Material from the swollen glands or spleen should be stained, cultured and inoculated into animals as for a human case.
If glandular, splenic or other material from human or rat autopsies has to be sent to a distant laboratory the specimen should be placed in a strong salt mouth bottle containing 20% glycerine in water with 2% calcium carbonate.
Agglutination is not very practical owing to the frequent absence of agglutinins from the serum of plague patients. Then, too, there is a marked tendency to spontaneous agglutination on the part of the plague bacilli. Strong states that culturing at 37°C. lessens this tendency to spontaneous agglutination. Again, even when present, the titre of plague-agglutinating sera is usually quite low so that one must work with dilutions of from 1 to 10 or 1 to 20.
Prognosis
Pneumonic and septicaemic plague give an almost absolutely unfavorable prognosis, many stating that every such case dies.
As regards bubonic plague the mortality averages 75%. The Egyptian epidemic of 1900 gave an average mortality of 50%. The mortality in natives is much higher than that among Europeans, these latter often showing death rates under 25% while in the same epidemic natives show from 75% to 95% mortality. Plague pneumonia, however, is absolutely fatal for Europeans as well as natives.