(e) In this class come certain diseases for which particular insects are necessary for the parasite in question, so that certain stages in its life history may be passed therein. The surest means for eradicating such diseases is the destruction of the insects concerned. Up to the present no bacterial disease is known in which this condition exists, unless Rocky Mountain spotted fever and typhus fever shall prove to be due to bacteria. Such diseases are all due to protozoa. Among them are Texas fever, due to Piroplasma bigeminum in this country which has been eradicated in entire districts by destruction of the cattle tick (Margaropus annulatus).
Piroplasmoses in South Africa among cattle and horses, and in other countries are transmitted in similar ways. Probably many of the diseases due to spirochetes and trypanosomes are likewise transmitted by necessary insect intermediaries. In human medicine the eradication of yellow fever from Panama and Cuba is due to successful warfare against, a certain mosquito (Stegomyia). So the freeing of large areas in different parts of the world from malaria follows the destruction of the mosquitoes. The prevention of typhus fever and of trench fever by “delousing” methods is familiar from recent army experience though for typhus this method has been practiced in Russia for more than ten years to the author’s personal knowledge. The campaign against disease in animals and man from insect sources must be considered as still in its infancy. The full utilization of tropical lands depends largely on the solution of this problem.
CHAPTER XXIV.
PATHS OF ENTRANCE OF PATHOGENIC ORGANISMS, OR CHANNELS OF INFECTION.
A. The Skin.—If the skin is healthy there is no opportunity for bacteria to penetrate it. It is protected not only by the stratified epithelium, but also in various animals, by coats of hair, wool, feathers, etc. The secretion pressure of the healthy sweat and oil glands acts as an effective bar even to motile bacteria. Nevertheless a very slight injury only is sufficient to give normal surface parasites and other pathogenics, accidentally or purposely brought in contact with it, an opportunity for more rapid growth and even entrance for general infection. Certain diseases due to higher fungi are characteristically “skin diseases” and rarely become general—various forms of favus, trichophyton infections, etc. A few disease organisms, tetanus, malignant edema, usually get in through the skin; others, black-leg, anthrax, quite commonly; and those diseases transmitted by biting and blood-sucking insects, piroplasmoses, trypanosomiases, spirilloses, scarcely in any other way. Defective secretion in the skin glands from other causes, may permit lodgment and growth of bacteria in them or in the hair follicles. “Pimples” and boils in man and local abscesses occasionally in animals are illustrations. Sharp-edged and freely bleeding wounds are less liable to be infected than contusions, ragged wounds, burns, etc. The flowing blood washes out the wound and the clotting seals it, while there is less material to be repaired by the leukocytes and they are free to care for invading organisms (phagocytosis). Pathogenic organisms, especially pus cocci, frequently gain lodgment in the milk glands and cause local (mastitis) or general infection.
B.
Mucosæ directly continuous with the skin and lined with stratified epithelium are commonly well protected thereby and by the secretions.
(a) The external auditory meatus is rarely the seat even of local infection. The tympanic cavity is normally sterile, though it may become infected by extension through the Eustachian tube from the pharynx (otitis media).
(b) The conjunctiva is frequently the seat of localized, very rarely the point of entrance for a generalized infection, except after severe injury. Those diseases whose path of entrance is generally assumed to be the respiratory tract (see “[Lungs]” below) might also be admitted through the eye. Material containing such organisms might get on the conjunctiva and be washed down through the lachrymal canal into the nose. Experiment has shown that bacteria may pass in this way in a few minutes. In case masks are worn to avoid infection from patients suffering with these diseases, the eyes should therefore be protected as well as the nose and mouth.
(c) The nasal cavity on account of its anatomical structure retains pathogenic organisms which give rise to local infections more frequently than other mucosæ of its character. These may extend from here to middle ear, neighboring sinuses, or along the lymph spaces of the olfactory nerve into the cranial cavity (meningitis). Acute coryza (“colds” in man) is characteristic. Glanders, occasionally, is primary in the nose, as is probably roup in chickens, leprosy in man. The meningococcus and the virus of poliomyelitis pass from the nose into the cranial cavity without local lesions in the former.