The specific cause of foot-and-mouth disease is a filterable virus that is present in the serum from the vesicles, the saliva, milk, and various body secretions and excretions from the sick animal. In the early stage of the disease it is present in the blood. None of the many investigators have been able to discover the microorganism that produces the disease.
Two of the outbreaks of foot-and-mouth disease in the United States originated from an infected vaccine used for the inoculation of vaccine heifers. The origin of the 1914-'15 outbreak has not been discovered. When introduced into a country, the disease spreads rapidly, through the movement of live-stock affected by the disease. Animals recently recovered may infect other animals. Dogs, birds, people, vehicles, milk, roughage, grains and other material from an infected farm may spread the disease.
The period of incubation is short. Symptoms of disease may be manifested in from one to six days following exposure.
[Illustration: FIG. 106.—A cow affected with foot-and-mouth disease. Note the accumulation of saliva about the lips. (From report of the Bureau of Animal Industry.)]
The first symptoms are fever, dulness, trembling and loss of appetite. This is followed by vesicles or blisters forming on the mucous membrane of the mouth, lips, between and above the claws and the region of the udder. The inflammation of the mouth and feet may be very painful. Long strings of saliva may dribble from the mouth and collect about the lips (Fig. 106). A smacking or "clucking" sound is produced when the animal moves its jaws and lips. The severe pain resulting from the inflammation of the mouth and feet, and the difficulty in moving about and eating and drinking, cause the animal to lose flesh and become emaciated. Milk cows may go dry.
The death-rate is not heavy. Some writers place it as low as two or three per cent. Because of the erosions and sloughing of the tissues of the mouth, feet and udder it becomes necessary to kill many of the animals. Young animals frequently die of inflammation of the digestive tract. The immunity conferred by an attack of the disease is not permanent.
[Illustration: FIG. 107.—Slaughtering a herd of cattle affected with foot-and-mouth disease. (Photographed by S. J. Craig, County Agricultural Agent, Crown Point, Indiana.)]
The most economical measures of prevention and control are to buy and slaughter all diseased and exposed animals, bury the carcasses in quicklime, disinfect the premises (Figs. 107, 108 and 109) and enforce a district, county and State quarantine, until after the infection has died out. This statement may not hold true of methods of control in countries where foot-and-mouth disease is widely distributed.
TETANUS. LOCKJAW.—This is an acute infectious disease that is characterized by spasmodic contractions of voluntary muscles. The specific germ remains at the point of infection, and produces toxins that cause tetanic contractions of the muscles. It commonly affects horses, mules, cattle, sheep and swine. The disease is most common in warm, temperate climates.
[Illustration: FIG. 108.—Disinfecting boots and coats before leaving a
farm where cattle have been inspected for foot-and-mouth disease.
(Photographed by S. J. Craig, County Agricultural Agent, Crown Point,
Indiana.)]