In cattle it ranges from 14 to 50 days (exceptionally 70 days), in sheep and swine from 14 to 60 days. It has been claimed to have lasted 4 months in swine and 5 months in sheep.
In rabbits subdural inoculation has an incubation of 15 days, shortening in 25 removes to 8 days, and finally to 6.
In man incubation is alleged to be even more varied. The rule is from 14 to 64 days, but it is claimed to have been as short as three days and as long as 1 to 12 years (Chabert). In the human being, however, there is always the danger of the disease caused by simple dread (lyssophobia) and until these protracted cases can be verified by successful inoculation on the lower animals, they must be held as extremely doubtful. The Montpellier cadet has been often quoted who left, a few days after he had been bitten, spent ten years in Holland, then returning to the school and learning for the first time that his fellow cadet, who had been bitten by the same dog, had gone mad, he too became rabid without loss of time. Such cases have often been cured by moral suasion and have been seized upon to corroborate the heresy that there is no such thing as genuine rabies in man.
Any incubation, in man or beast which has exceeded 40 days should be considered as doubtful, until certified by the successful inoculation of rabbits or other small animals. For casual inoculations the incubation rarely varies much from the time embraced between 16 and 30 days. It is abridged by a special receptivity; by an overdose of the poison; by the inoculation of a virus of unusual potency; by the youth of the animal inoculated; by great heat of the weather; by all forms of violent excitement; by rutting; and by the inoculation of the virus on the head and above all on the cerebral meninges. By this last method incubation may be reduced to 6 days.
Symptoms in Dogs. In dogs as in other animals rabies is manifested in two great types: the furious and the dumb or paralytic, which, however, usually succeed each other in fully developed cases. Yet the furious phenomena may be entirely omitted, and again the victim may die in the early furious stage so that the paralytic does not appear. The prominence of one form over the other is to some extent determined by the germ derived from a previous case of the same kind or by the family, temperament and habits, bull dogs and hounds being specially subject to the furious type, and house and pet dogs having rather the paralytic form.
The premonitory symptoms are in the main the same for both types, and as these may enable us to recognize the disease before the period of extreme danger, it is especially important that they should be well understood.
Some marked change in the disposition or habits of the animal is the first obvious variation from health, and in a district or country where rabies exists any such change should be the warrant for instant seclusion of the dog before there is any disposition to bite. The unwonted habit may be of almost any kind. The lively, amiable dog may become suddenly dull, apathetic or taciturn; the quiet, unexcitable dog may become unusually affectionate, fawning and demonstrative, licking the owner’s hands and face and perhaps infecting him before any suspicion is aroused. Sudden capricious changes from fawning to apathy or sullenness, or the opposite, should be dreaded. The noisy dog may become suddenly silent; while the silent dog may take to howling without apparent cause. A great restlessness, watchfulness or nervousness, a tendency to start at the slightest sound, and a disposition to move at frequent intervals in search of an easier position or place to lie in, are most dangerous symptoms. A morbid appetite, with a disposition to pick up and swallow all sorts of nonalimentary objects (straw, thread, cord, paper, pins, nails, coal, marbles, pebbles, cloth, earth, dung or urine), in a mature dog is most suggestive. Searching around, scraping, tearing sticks, clothes and other objects to pieces, licking of smooth cold stone or metal, of his penis, or of the generative organs of a bitch, are often early phenomena. The dog may hide in a dark corner, going to sleep and grumbling or growling when disturbed. He may make night hideous with his howls, baying at the moon. He may stand with a dull, melancholy, hopeless expression of countenance, as if beseeching his master for relief from his nameless suffering. But as yet there is no disposition to bite. The dog still responds to the call of the master, but with dulness and apathy, in marked contrast with his usual prompt, alert and loving response. There may be congestion, itching and irritation in the seat of the bite, and it may be licked, scratched or gnawed until raw, tender or bleeding. An early change in the voice may be noticeable. There is at first a certain hoarseness, which gradually develops into the pathognomonic rabid howl which is quite recognizable at a distance. The dog turns his nose upward, and with open mouth, emits a howl which, at first hoarse and low, rises into a shriller and higher note before completion and which may be repeated several times without closing the mouth. It is not an ordinary howl but rather a cry of distress, and, to the educated ear a grave note of warning. Sometimes the dog is dumb from the start.
Close observation will often detect evidence of mental delusions and hallucinations even at this early stage. The apparently sleeping dog suddenly starts up with an air of suspicion and excitement. His eyes may turn after phantom flies or other objects at which he will presently snap. He moves hither and thither with a curious, inquisitive air, searching in dark corners, or under curtains or articles of furniture for some imaginary object. He may stand in attentive attitude listening to fancied sounds, and then bound in that direction in spite of an obstructing wall or utensil. If recalled to reality by his master’s voice his healthy attitude and affection may be completely though temporarily restored. He may compose himself to rest or sleep, and soon again start a victim to further delusions.
The furious stage is ushered in by a more pronounced manifestation of the above symptoms. The rabid howl is more frequent and characteristic. The insomnia and the restless movements, change of place, searching, scratching or tearing are likely to be more prominent. The delusions and the watching or snapping at phantoms are more marked. The causeless sniffing and scraping may imply disorder of the sense of smell. The exalted hyperæsthesia and reflex excitability is usually a most marked feature, frequently modified by a sinister, inimical or malevolent disposition. Some subjects show evidence of intense itching, most marked perhaps in the seat of the wound; others are abnormally sensitive to currents of cold air; they start at the slightest noise, a flash of light, a touch, or the approach of a stranger. A paroxysm of fury may be caused at once by shaking a stick at the patient, but, above all, by the test of presenting a dog before his eyes. He at once rouses himself and, with flashing eyes, dashes himself on his supposed enemy and bites viciously, but without bark or growl. The sudden and mute attack is pathognomonic. But there may be symptoms suggestive of some hypoæsthesia. A blow from the dog’s master is usually borne in silence. The animal shrinks from the blow but does not yelp nor howl. During a paroxysm he will lacerate his gums or loosen his teeth by biting on a stick or iron bar; he will even seize a red hot bar without shrinking. He will often gnaw his limbs or body so as to expose and even lacerate the muscles, driven by the pruritus, or the insatiable disposition to bite. Yet he has still an instinct of self preservation, retiring from a light, or blazing brand, and seeking to evade the tongs used to seize him.
The eyes assume a remarkable expression. They are often fixed, and at times the pupils are widely dilated allowing the reflection of light from the brilliant tapetum lucidum, resembling flashes of fire, and adding greatly to his ferocious appearance, and the sense of terror that he inspires. The conjunctiva is congested, of a dark red and the general expression of the eye is very striking. The soft, trusting, affectionate eyes may still for a moment meet those of the loved master, but there is in them a dull, hopeless, anxious and suffering expression, which appeals for pity and should be viewed with dread. To others there is likely to be less of the affectionate appeal and more of the sinister, suspicious, resentful and malicious expression. To the stranger, therefore, the eye may be a better guide than even to the owner. Squinting of the eyes and closure of the lids for a few seconds are not uncommon. As the disease advances the dilatation of the pupils is more marked, indicating the paralysis of the optic nerve.