Page:Tropical Diseases.djvu/401

XX] the serum. The progress of the disease, which would otherwise have certainly proved fatal, was at once arrested, and. the animal recovered. They then immunized a horse by intravenous injections of living virulent cultures. After several injections made at intervals (the second after twenty days), they found that reaction, from being intense, became shorter and less pronounced, and that the serum of the animal was now both preventive and curative of inoculated plague in rabbits, guineapigs, and mice. At the Lister Institute an antimicrobic serum is prepared by injection of dead and afterwards of living bacilli into horses, and in which antitoxic bodies are produced as well by immunization against the nucl co-proteins of B. pestis. Accounts had led us to infer that the value of Yersin's discovery was practically established for man. Of 26 cases of plague in China treated with his antipest serum, 24 were reported to have recovered. Further experience in India, Hong Kong, and elsewhere has not confirmed these brilliant results, the serum treatment of plague both by Yersin's and by a number of other sera having so far proved a complete failure, and at the best but modifying the course of the disease and prolonging life.*

These minute ectoparasites are now of considerable importance to the student of Tropical Medicine in view of the part they play in the spread of plague, and possibly of other diseases. They are active parasites of mammals and birds; some suck blood indiscriminately, but the majority restrict themselves to one definite host.

In one family (Sarcopysllidœ) the females eventually attach themselves to heir host as fixed parasites, embedding themselves in its skin when pregnant; to this category belongs the "chigger."

The flea in its external structure is composed of a head, thorax, and abdomen.