limited in their sphere of influence, and, as only a very small proportion of those who receive the microorganism are susceptible to it, the perpetuation and spread of meningitis must depend on the healthy carriers who pass the meningococcus on from one to another until a susceptible individual is infected and develops meningitis. The virulence of the organism is also a determining factor. Prevention. From our present knowledge preventive measures are clearly indicated, though very difficult to carry out. Epidemic cerebrospinal meningitis is a good example of a group of diseases in which a more precise knowledge of the modes of transmission of the disease makes it obvious that prevention is a matter of extreme practical difficulty. Flügge estimates that healthy carriers of this disease are ten times more numerous than recognized cases, and, therefore, are more than ten times as prolific a source of infection. While the isolation of the known cases will prevent a certain number of secondary cases, this measure alone cannot hope to control the disease. It is obviously impractical to undertake to make bacteriological examinations sufficient to discover all the carriers in a community of any considerable size; moreover, the control of so many carriers when discovered would require military rule. We must frankly admit that when cerebrospinal meningitis has once become epidemic it cannot be stamped out by any known means of practical application. This does not mean that we should assume a supine attitude, for, even though the disease cannot be satisfactorily controlled, a certain number of secondary cases can be prevented. Every case and every suspected case should at once be reported to the health authorities and the patient isolated. The virus is contained especially in the discharges of the mouth and nose, and these secretions should be disinfected. house should be placarded, visiting prohibited, and isolation practiced. These measures will help diminish the number of carriers. The Personal prophylaxis consists in avoiding the infection so far as possible, and in the use of antiseptic gargles and nasal douches. When the disease is epidemic people should keep away from large public gatherings, crowded street cars, avoid the use of public drinking cups, and the like. They should be advised to exercise more than the usual care as to personal cleanliness. The closing of the schools may, under certain circumstances, be justified. Urotropin in moderate doses has been suggested as a possible, though quite unproven, prophylactic. While rigid quarantine is not, as a rule, effective in controlling this disease, localized outbreaks in institutions, military camps, or small towns may be kept from spreading by a strict system of isolation, even with a military cordon. Antimeningitis serum is useful in the treatment of the disease; it is not practical as a preventive. It must be introduced into the subdural space by lumbar puncture. The serum should be provided free of cost or at a minimum price by health authorities. Further, boards of health should provide laboratory facilities for the bacteriological diagnosis of the disease, and the recognition of carriers. Sophian and Black recommend an active immunization induced by inoculating killed cultures of the meningococcus. The cultures are grown on 2 per cent. glucose agar, and after 18 hours' growth are washed off in distilled water, shaken for 20 minutes, heated at 50° C. for 1 hour, and tested for sterility. One million bacteria are injected at the first dose, 7 days later the same number, and 7 days later 2,000,000. The injection of the dead meningococcus confers a considerable immunity, and may prove to be a valuable measure for personal prophylaxis. Chronic carriers should be inoculated with the killed cultures, and their sphere of activity should be restricted. Furthermore, they should be impressed with the danger to their fellowmen, and given careful instructions concerning spitting, sneezing, coughing; the care of fomites, such as handkerchiefs, spoons, cups, etc.; and the importance of cleanliness of the teeth, mouth, nose, and throat. 1J. A. M. A., Aug. 17, 1912, LIX, 7, p. 527. CHAPTER IV INSECT-BORNE DISEASES GENERAL CONSIDERATIONS The fact that disease may be transmitted through the bites of insects was suspected for years, but it was not until 1893 that it was demonstrated as a new principle by Theobald Smith in the case of Texas fever of cattle and the tick.1 Since then many diseases have been added to the list, which is constantly growing. We now know that some diseases are always transmitted through insects and others occasionally. A thorough comprehension of the subject is necessary for sanitarians and others in the fight against disease in all climates and in all places. It may be stated as a general law that, if a period of incubation in the insect is necessary, it indicates that the parasite probably belongs to the animal kingdom and passes part of its life cycle within the insect. This constitutes the so-called extrinsic period of incubation. Malaria and yellow fever are examples of this class, which is spoken of as biological transmission. If, on the other hand, insects convey infection at once without a period of incubation in the insect, the transfer is a mechanical one; in this case the insect does not play the part of an intermediate host in the true biological sense, and there is no cycle of development of the parasite within the insect. These cases are almost all bacterial infections. It may be stated as a general rule that the insect hosts are not harmed by the parasites which they harbor and which are pathogenic for man. Thus, the malarial protozoon is pathogenic for man, but a saprophyte for the mosquito. The same is true of yellow fever and the Stegomyia, Texas fever and the tick, plague and the flea, sleeping sickness and the tsetse fly, typhoid and the house fly, typhus fever and the louse, etc. The intermediate host in the zoological sense is that animal which harbors the asexual phase of the life cycle of the parasite; the definitive 1 The other names associated with the early work upon insects and their relation to disease are: Manson, Finlay, Ross, Grassi, and the U. S. Army Commission-Reed, Carroll, Lazear, and Agramonte. host is the animal which harbors the sexual phase. Thus, in malaria man is the intermediate host, the mosquito the definitive host. In popular parlance, the insects are spoken of as the intermediate hosts in all cases. Insects transfer infections mechanically in a variety of ways. The mouth parts, legs, or outer surfaces of the body may be smeared with the virus, which is thus simply carried to the lips, fingers or food, and thus enter the susceptible individual; or the virus may remain attached to the proboscis of a biting insect, thus transferring the infection very much as a hypodermic syringe would; or the virus may be contained in the dejecta of the insect and be scratched or rubbed into the wound made by the bite; or the virus may be contained in the digestive tube or the body cavity and be released when the insect is crushed. Insect-borne infections are types of true endemic diseases, as they are necessarily limited in geographical distribution to the habitat of the insect host. As a rule, only one species, or at most a single genus, acts the part of a host for any particular infection, excepting in the mechanical transference of infection by insects. Malaria is confined to Anopheles, yellow fever to Stegomyia, Texas fever to the Margaropus annulatus, sleeping sickness to the Glossina palpalis, etc. This is a question of specificity. The specific nature of some of these diseases may be due to the fact that the parasite is not pathogenic for other hosts. Thus, yellow fever and malaria cannot be given to any other animal than man, even though large amounts of the infected blood be inoculated. The disease may be specific, in the sense that it is confined. to one species, because the insect conveying the infection refuses to bite other than its own host. True specificity is found in all the cases of biological transference, whereas mechanical transference of infection may take place through widely separated genera. In some instances the virus is transmitted hereditarily through the insect from one molt to another, and even from one generation to the next. So far as known, however, hereditary transmission takes place only in those "insects" having an incomplete metamorphosis, such as the ticks. Brues suggests that the hereditary transmission of a virus is practically impossible in insects passing through complete metamorphosis, owing to the active phagocytosis during the pupal stage. Protozoa, bacteria, and even parasitic worms may be transferred by insects. The character of the disease cannot be predicated from the nature of the insect host. Thus, ticks convey Pirosoma and alsc spirochetes; flies convey trypanosomes, bacteria, the eggs of worms, and a variety of other infections; mosquitoes are concerned in the transmission of the plasmodium, a protozoon, filaria, a round worm, and a filterable virus (yellow fever). Insect-borne diseases may occur in great epidemics, as yellow fever, malaria, dengue, plague, relapsing fever, etc. When this occurs it means that the particular insect involved prevails in enormous numbers in the epidemic area. Ticks and mites belong to the lower class of the Arachnida and are not, strictly speaking, insects (insecta), but are here considered in the same group for practical convenience. All the parasitic animals which live upon man and the higher animals may act as go-betweens in the transportation of the microorganisms of disease. Parasites which live upon the skin FIG. 17.-A SOUTH AFRICAN BLOOD-SUCKING FLY (PANGONIA), ILLUSTRATING LONG PROBOSCIS TO PIERCE HEAVY FUR OF CERTAIN ANIMALS. (Brues.) permanent parasites. Many of the insect-borne diseases were formerly known as "place diseases." Thus, in yellow fever it was realized that the infection was not conveyed directly from man to man, but it was believed that the house or place became infected, and it was thought that the virus lived in the soil, upon the bedding, or on the clothing. This led to the notion that fomites or inanimate objects played an important rôle in the transference of disease. The early studies in bacteriology gave countenance to this view until our knowledge of the part played by insects and the importance of "contacts" has placed fomites in a subordinate and oftentimes negligible position. The prevention of the class of infections belonging to the insectborne diseases depends upon a knowledge and thorough comprehension of three factors: (1) the disease, (2) the parasite, and (3) the insect. The suppression or control of the insect depends upon a thorough knowledge of its biology. Entomology, therefore, has become a vitally important subject so far as preventive medicine is concerned. Without an acquaintance with the life history and habits of the insect host there |