Other alleged causes are based on secondary factors that favor inoculation. The alleged evil influence of the hot season (dog days) is accounted for by the period of rutting of the dog which occurs in spring, and brings together troops of jealous dogs following individual bitches and fighting for their favors. This gives a great impetus to the propagation of the disease which accordingly becomes more prevalent during succeeding months. The statistics of Bouley show, however, that the season of its greatest prevalence in France is March, April and May, that it subsides slowly up to midwinter and encreases again in spring. Boudin’s statistics show the greatest number of cases in man toward midsummer which is explained by the long period of inoculation after bites sustained in the spring, but also because clothing is lighter in summer and infection more likely to reach the skin, and because the heat of the weather tends to cause hyperthermia and encrease susceptibility.
The large preponderance of male dogs among the victims of rabies (7:1) has led to the theory that sex predisposes, but the explanation is rather that the males bite each other in their jealousy while they respect the female, the object of their sexual passion. In inoculated cases, males, females, and castrated contract the affection indifferently and with equal readiness.
The skunk is found to be infected in certain localities, and it has been claimed that the affection is native to this animal, and that the form differs from that which prevails in the dog, but the restriction of the disease to sharply circumscribed areas in Michigan and Kansas, while skunks elsewhere show no such malignant quality, demonstrates that this is but an accidental infection of the family of the Mephitis in a given locality.
Men and children suffer in far greater numbers than women, the difference being mainly due to the protection furnished the female by the flowing skirt, though also in part because of the more frequent contact of men with dogs.
Virulent Matters. In 1813 Gruner and Count Salm demonstrated the virulent properties of the saliva of the rabid dog. This has been many times repeated, though as shown by Pasteur, Raynaud, and others, inoculations with saliva often kill in one or two days by reason of the presence of other infective germs, or suppuration ensues and destroys the rabific virus so that oftentimes not more than one in four develops rabies. Galtier succeeded in inoculating 3 rabbits, 1 dog and 1 sheep with the juice expressed from the salivary glands, and Nocard, Leclainche and Peuch have found this to be very virulent. Paul Bert found the bronchial mucus virulent. Rabies has been frequently conveyed by consuming the bodies of rabid animals, yet Peuch and Galtier have failed to convey infection by the flesh. Eckel and Lafosse successfully inoculated the blood of goat to sheep, of man to dog and of dog to dog. Yet many of the older observers agree with Peuch and Galtier in pronouncing the blood non-virulent. Pasteur, Roux, Bujwid, and Helman failed to obtain virulent cultures from blood. Peuch and Nocard have found the milk virulent, and Bardach found the same to be the case in a woman for two days before her death of rabies. In some partially refractory animals like the sheep, the blood seems to destroy the virus, as intravenous inoculations have been made with impunity. The probability is that the blood is habitually non-virulent in the early stages and in mild cases, but becomes virulent in violent and advanced cases. Pasteur first showed that the virus has a special election for the brain and nervous matter and that the central nervous organs are constantly infecting. Rossi inoculated with a piece of nerve trunk, and Doubouc and Babes advanced the hypothesis that the virus advances along the nerve to reach the brain.
It was formerly supposed that the saliva of omnivora and herbivora was non-virulent but repeated experiment has proved it to be infecting. The blunder was due to the fact that these animals did not often transmit the disease by biting. Some writers even deny that rabies exists at all as an infectious disease. One alleges that the danger of rabies is “less than the gallows.” Experiment shows most conclusively that it is much more certain when the conditions are fulfilled. One may live a long lifetime without seeing a genuine case of rabies, but so he may without seeing leprosy or plague, and he is no more entitled to doubt their existence than he is the existence of Manila, or of the other side of the moon.
Morbific Agent. The actual factor which produces rabies is not certainty known so that we cannot speak of its bacteriology. That it is due to a specific particulate germ is now indisputable. Paul Bert and Nocard filtered the virulent fluid through plaster and found the clear fluid that had passed through non-virulent. Rivolta, Babes, Bouchard and Peuch severally passed a solution of the rabific brain matter through a Chamberland filter and found the clear filtrate harmless. The virulent agent is therefore not a body in solution but a solid (organism) which is held back by the filter. Hallier, Klebs, Galtier, Gibier, Pasteur, Fol, Babes and Dowdeswell have respectively attempted the cultivation and inoculation of organisms found in rabific liquids but none has stood the test of further experiment. Memmo found a blastomyces in the brains of six rabid rabbits and one hydrophobic child. It stains with the aniline dyes but not by Gram’s method; when thrown into the peritoneum of Guinea pigs it produced clonic spasms and death in 24 hours, and in the dog, after an incubation of 8 days caused emaciation, salivation, a disposition to bite, paralysis of the hind limbs and death in 48 hours. Bruschettini using agar or bouillon, containing lecithin or cerebrin and defibrinated dog’s blood, inoculated with pieces of the brain of an inoculated rabbit, obtained in 24 to 36 hours a group of small transparent drop-like colonies at first microscopic, but becoming larger with each new culture on fresh media. These colonies contained a very small, short, thick bacillus, which stains readily in Ziehl’s carbol-fuchsin, and then presents a central clear band giving the appearance of a diploccus. In fluid media spherical forms are produced, but in fresh cultures the diploccus aspect reappears. Injected subdurally in rabbits it gave rise to what appeared to be paralytic rabies and could be inoculated from animal to animal, with similar results. It failed to grow in the usual culture media from which the brain products are absent, thus fulfilling the conditions, of a microbe the point of election of which was the nerve cells. Marx who has sought for Bruschettini’s organism in 60 cases of rabies has only once found anything resembling it and concludes that it was merely a contaminating organism, which caused a paralysis (Centr. f. Bacter. 1896.)
Viability of the poison. Galtier found that the virulent saliva remained potent for 11 days if preserved from drying. It persists for 3 weeks in the brain and medulla kept at 0° to 12° C., for a month in sealed tubes, or for several months if protected against septic microbes and in contact with carbon dioxide.
In water it is preserved for 20 to 38 days so that water soiled with saliva may easily become a means of infection. In graves it has been found virulent as long as 44 days after burial so that in medico-legal cases results may be obtained by exhuming a suspected animal (Galtier).
Haubner found that drying the saliva in thin layers rendered it non-virulent and Pasteur has shown that the rabbit’s medulla loses its virulence in 14 or 15 days when dried in contact with air, and apart from putrefaction. Such laboratory results must however be qualified by the facts recorded by Blaine, Youatt and others in which hound after hound died of rabies from living in a dry kennel in which a rabid animal had preceded them. Until we know something of the living germ itself of rabies, it is unwise to infer too positive results from experiments on that microbe, in what may be but one stage of its existence.