Accessory Causes. Birds sent to poultry shows will often contract the disease and introduce it into the home flock on their return. It may also be imported in newly purchased birds, or on eggs obtained for hatching. It is even alleged that it has been propagated by feeding healthy hens on the eggs of diseased ones. When chickens run at large it passes easily from flock to flock in the immediate vicinity. The infected manure is, however, the most common channel of infection. Carried on the feet or bill this contaminates the food and drinking water, and washed into streams and ponds, it finally in any case reaches the alimentary canal of the susceptible bird. Or drying up and raised as dust it is inhaled into the lungs. Or finally from any such source it infects any open sore. As granivorous birds, wild and tame, suffer from fowl cholera, it is often introduced by the wild, especially coming from infected poultry yards. Predatory birds, like hawks and buzzards, but the latter especially, are common bearers of infection. Rabbits, which contract the affection so readily, transmit it equally with birds, but man or beast, soiled by the manure will convey it. Insects are among the most prolific bearers, hence, as noted by Salmon, the infection may fail to overstep a close fence in winter, but is subject to no such limitation during the fly season. In this respect chicken cholera agrees with Asiatic cholera, typhoid fever and other affections in which the virus abounds in the alvine discharges. The sale and transportation of the guano from the infected poultry yard is a direct cause of new outbreaks. Feeding on the carcasses or offal of the infected birds is a further cause. It must not be forgotten that the microbe is largely saprophytic, living indefinitely in the organic matter in soils, and determining new outbreaks when brought in contact with susceptible animals. Thus a period of immunity may be followed by infection when new birds are brought in or when young and susceptible ones grow up.
Susceptible Animals. Fowl cholera is preëminently a disease of chickens, but the microbe is successfully transferred to pigeons, peafowl, pheasants, parrots, ducks, canaries, sparrows and other small birds, also to Guinea pigs, rabbits, white and gray mice. Guinea pigs have abscesses in the seats of inoculation (Pasteur); the same is alleged of sheep and horses (Kitt), and man (Marchiafava, Celli). Injection into a cow’s teat caused chronic catarrhal mammitis in which the microbe persisted for a long while (Kitt). Like other members of the group of microbes causing septicæmia hæmorrhagica, the pathogeny and even the morphology are liable to material modification as grown in different environment (genera). Some of the forms of cholera occurring among domesticated birds and held to be distinct diseases may find in this an explanation. Rabieaux claims that under favorable conditions it has been transmitted to the frog.
Incubation. This varies from 18 to 48 hours, the usual being 24 hours.
Symptoms. In some fulminant cases the animal is found dead a few hours after apparently blooming health; it may even have died on the nest or fallen dead from the roost. Cadeac speaks of transient symptoms even in such cases—extreme dulness, prostration, somnolence, seclusion in a cool, dark place, ruffling of feathers, sinking of the head between the wings, drooping, trailing wings and tail, violet comb, gaping, discharge of glairy mucus from the bill, convulsions and death. These symptoms last from two to five hours.
In acute but less fulminant forms there is loss of appetite, depression, debility, apathy, erection of the feathers, sinking of the head, swaying when made to walk, drooping wings and tail, sitting on the breast, convulsive tremblings, discharge of filmy or frothy mucus by the nose or mouth, vomiting, hyperthermia (108° to 111° F. ), sighing, breathing, inflation of the crop, violet colored comb, wattles and mucosæ, great thirst and diarrhœa, at first pultaceous and light yellow, later glairy, green and fetid. The feathers round the anus become soaked and matted with the discharge. Temperature becomes subnormal, the patient falls and is unable to rise, and finally dies in a stupor or convulsions, the illness having lasted 1 to 3 days.
Milder cases occurring chiefly towards the end of an outbreak when the less susceptible animals only are left, or when the microbe has become less virulent, show a larger ratio of recoveries. These show a lack of spirit and vigor, impaired appetite, diarrhœa, emaciation, dulness, prostration, moping, ruffling of the plumes, dark discoloration of the comb, and often swelling of one or more important joints (femoro-tibial, etc.). These may burst and discharge a reddish pus, or simply form dark or grayish swellings. These cases may drag along for a week or more and finally die in marasmus. The minute bacillus is not obtainable from these (Lignieres).
Cases inoculated in the pectoral muscles with only one or two microbes usually have only a circumscribed slough, with loss of condition, and after the elimination of the slough and the healing of the sore the bird proves immune.
Lesions. The alimentary canal is the main seat of morbid changes. The intestinal walls, and especially the mucosa, have points and patches of blood extravasation, extensive areas of congestion with ramified redness, exudation and thickening. The intestinal contents are watery, frothy, browned or blackened by effused blood, and swarming with ferments including the specific bacterium. The mucous surface is brownish or blackish, and epithelial degeneration and desquamation with abrasions are not uncommon especially on the summits of the duodenal folds and villi. Croupous exudates and swelling or ulceration of the follicles are met with. The lymph glands at the base of the cæcum are often enlarged and congested. The crop is full of watery, pulpy, frothy or slimy contents, and its mucosa and that of the pharynx may be deeply congested.
Elsewhere the lesions suggest rather the action of destructive toxins and the profound changes in the blood. Pericardium and endocardium are usually studded with dark petechiæ, and congestion and even slight exudation may be present. The spleen is enlarged, soft, and gorged with blood. The liver is swollen, congested, extremely friable, and mottled, grayish white from degenerations. The kidneys are dark red, and friable. The lungs may show slight hyperæmia only, or a blood engorgement and consolidation, and are then easily reduced to a dark red pulp. Friedberger and Fröhner say that respiratory changes are most frequent in land birds; and intestinal and cardiac in water fowl.
The blood is diffluent coagulating loosely if at all, of a brownish red color, reddening slowly and imperfectly in contact with air, and like the tissues contains an abundance of the characteristic bacterium staining deeply at the poles and clear in the center.