The virus is very resistant to cold, having survived a temperature of −60° C. (−76° F.) for several hours (Roux), −10° C. (−14° F.) for ten months (Jobert). Virulence is destroyed however by a temperature of 48° C. (118.4° F.) in five to twenty minutes (Galtier). Light destroys the virulence in 14 hours at 30° to 35° C. (Celli), but encreased pressure has little effect on it (16 atmospheres, Nocard and Roux).
Glycerine at room temperature preserves the medulla in its full potency for four weeks, but destruction ensues if it is heated (Protopopoff). An aqueous solution of iodine (6:100) destroys the virulence (Galtier), as does also citric acid, bromine, chlorine, sulphurous acid, the mineral acids and cupric sulphate.
Ratio of successful inoculation to bites of rabid animals. As rabies is usually transmitted by the bite, it is well to note that not all the bites of rabid animals are effective. Of 183 dogs confined with and bitten by a rabid dog 91 contracted rabies; of 73 cattle bitten 45 became rabid; out of 121 sheep bitten 51 succumbed and of 890 persons bitten, 428 took hydrophobia. Of 440 persons bitten by rabid wolves 291 contracted the disease. The escape of such a large proportion is variously accounted for. Wolves naturally attack the face, throat or hands where there is no protection by clothing, and inoculation is therefore much more certain. Dogs, especially in cold weather usually bite man through the clothes which wipe off the virus from the smooth conical teeth before they reach the skin. Long haired animals are often protected in the same way. In other cases the bite is sustained on a very vascular part and the free flow of blood washes out the poison. In still other cases the rabid animal making a number of snaps in rapid succession comes to the last with the teeth wiped clean and harmless. Again the prompt washing or cauterizing of the wound tends often to protect against infection.
Under favorable circumstances however every bite infects, and the writer has seen six animals, bitten in the same stable, all contract rabies, while a man bitten through the coat sleeve by the same dog, and cauterized an hour later entirely escaped. If the bites are multiple, deep and irregular, the danger is greater.
The licking of wounds is an occasional mode of infection, the rabid dog in the early stages of the disease sometimes showing an unusual disposition to fawn upon his owner.
Again particles of saliva may be projected by sneezing or otherwise and lodge on sores, or on the mucosa of the nose, eye or lip so as to cause infection. Galtier has conveyed the disease experimentally to rabbits in 11 cases out of 75 by making them breathe the atomized infecting liquid, or by dropping it into the nose. In the same way he infected Guinea pigs and sheep.
Galtier and Bujwid have conveyed the disease, exceptionally, to rabbits and rats by feeding infecting emulsions. Galtier has also produced rabies in ⅒th to ⅓d of the cases operated on by instilling the virus into the eye.
Incubation. The duration of incubation varies with the species, individual, the seat and character of the bite, the amount of virus instilled, the potency of the virus, the age, size and weight of the subject, the excitement of rutting, climatic or weather vicissitudes, fatigue, and nervous or febrile disorders.
In the dog incubation ranges from 15 to 60 days, and perhaps 4 to 6 months. It is claimed to have lasted a year but this is somewhat doubtful. In cats it has varied from 15 to 60 days.
In solipeds it ranges usually from 20 to 45 days. The extremes stated are 15 days and 20 months.