SWINE FEVER, the name applied in Great Britain to a dis ease which occurs only in swine; in America, the synonym, hog cholera, is used; in Germany, Schweinepest.
The disease has a wide distribution throughout Europe, Amer ica and Africa, and is not uncommon in the British Isles. Al though in the past the cause of the disease has been ascribed to various bacilli, it has been recognized for a number of years that the condition is due to a filter-passing virus (q.v.).
The filtrates from the blood of pigs affected with swine fever are free of visible organisms and yet convey the disease when ad ministered to healthy swine. Methods of cultivating the causal virus outside the body are not known. Differences of opinion have existed as to the part played by the visible bacteria in the causation of the disease, but the view is generally held that the virus of swine fever so lowers the resistance of its host that bac teria, normally non-pathogenic to healthy pigs, are capable of becoming pathogenic and so to assume the role of secondary in vaders. The bacillus suipestifer is one of the most important of these and has been shown to be present in the intestines of healthy pigs. Morphologically it occurs as a short, motile rod, and bio logically it is related to the paratyphoid group; it stains readily with the basic aniline dyes and is easily culturable.
Natural infection results from immediate or mediate contagion. The disease may be introduced into healthy herds by the intro duction of infected pigs; exposure of swine in markets in in fected areas is a fertile source of infection; so are vehicles and trucks in which pigs are conveyed from place to place; dealers, castrators who journey from farm to farm, and attendants may serve as agencies whereby contagion is conveyed.
Outbreaks are liable to recur in premises where the disease has existed previously, as the virus appears to retain its vitality for fairly long periods outside the animal body. Within a space of time which rarely extends to three weeks after exposure to infec tion, and may not be more than four days, the disease is ushered in by fever, the subsequent symptoms varying somewhat with the form the disease assumes. The animal has no appetite, is de pressed, ill and separates itself from its companions ; the con junctivae are reddened and congested and may secrete a muco , purulent discharge ; vomition is common, and constipation and diarrhoea are both met with; sometimes the faeces become dys ' enteric and the odour is peculiar and disagreeable.
In a number of cases a skin rash develops, the parts principally involved being the region of the ears, the axilla, groin and the skin covering the abdominal area. The mucous membrane lining the mouth, fauces and pharynx may become inflamed, later ulcers form and become covered with a diphtheritic exudation. As a re sult the animal is unable to feed and respiration is rendered diffi cult. The lungs may be the seat of pneumonic changes and the animal has then a short, dry and paroxysmal cough, a nasal dis charge, difficulty in respiration, and the other symptoms charac teristic of acute lung infection. Gastritis and enteritis are com
mon. The animal lies about and if compelled to move does so reluctantly and sometimes with a staggering gait and an arched back; later it is unable to rise and becomes comatose.
As in other acute septicaemic diseases, a hyperacute form may develop and death supervenes within a few days, or a less acute form may set in where one or more groups of organs become in volved ; a chronic type is also encountered, life drags on over a considerable period and the animal becomes more debilitated and emaciated and remains a possible source of infection to other pigs. Recovery occurs in some cases, the virulence of, and the mortality from, the disease varying widely in different outbreaks.
Methods of prevention consist in keeping of swine in properly constructed piggeries and observing the laws of modern hygiene. Present-day open-air methods of pig raising are of value. Fresh pigs introduced into premises must be submitted to isolation for a sufficient space of time, in order to allow the disease to de velop, if present : a period of rather more than three weeks is desirable. In infected areas every effort must be made to prevent the introduction of the contagion from outside sources. When stud boars are kept special precautions are necessary and the law must be carefully observed.
Since pigs which have recovered completely from an attack of swine fever possess a high degree of immunity, attempts have been made to perfect a hyperimmune serum against the virus of the disease. This serum has been extensively used in various countries with good results and it possesses the power of protect ing swine against the disease, provided it is administered before exposure to infection or in the early days after infection; ob viously it can be of little service after the onset of the disease as it is not potent against the organismal secondary invaders. In herds where the disease exists it is desirable to use the serum upon those animals which are not showing symptoms of the disease and to slaughter the animals affected.
Since the protective power of the hyperimmune serum is ex pended after a few weeks, attempts have been made to confer an active immunity by mixing the immunized pigs with those affected with swine fever, in order that a natural attack of the disease may be developed in a very modified form. This method has been proved to be less dangerous than an alternative method which consists in the simultaneous injection of hyperimmune serum and virulent blood taken from an animal suffering from swine fever.
The former method, as stated, is preferable and it has given good results in practice, but it is not free from danger as some animals develop and die from the disease. (A. R. S.)