C. D. STEIN.
BLACKLEG is known also as black quarter, quarter ill, and symptomatic anthrax. It is an acute, infectious disease that affects mainly young cattle and sometimes sheep.
Blackleg occurs in nearly all sections of the United States and the world. It is more prevalent and causes greatest losses in cattle-raising and cattle-feeding areas in the Central West and Far West. Most cattlemen in the districts where the soil is heavily infested with the blackleg organism are familiar with this rapidly fatal disease.
Most susceptible to blackleg are cattle 6 to 18 months old. Suckling calves under 4 months and cattle past 2 years are rarely attacked. Cattle of improved breeding and fat, thrifty calves and yearlings usually develop the disease first. Common stock and thin, scrubby, or stunted calves seldom become infected. Sheep, goats, and hogs may contract the disease occasionally. Man, horses, dogs, cats, and fowl appear to be immune. Guinea pigs, which are used in laboratory diagnostic tests, are highly susceptible.
Spring and the fall are the seasons most favorable for the development of blackleg, but outbreaks may occur at any time of the year. The infection in some areas appears to be more prevalent on wet bottom land than in hilly areas, but it may occur on any type of terrain.
THE CAUSE of blackleg is a rod-shaped, gas-producing micro-organism, Clostridium chauvoei (blackleg bacilli). It forms spores, which are very resistant to destruction by heat, cold, drying, and disinfectants. The spores survive in the soil of a pasture and retain their ability to germinate and produce the disease for many years.
The blackleg germ is one of the anaerobic bacteria, which grow only in the absence of oxygen. Minute puncture wounds that exclude the air therefore favor the development of blackleg.
HOW THE INFECTION enters the body tissues is not definitely known. It is generally believed that animals usually acquire the disease when food, water, and soil contaminated with the blackleg germs gain entrance to the body through small punctures of the mucous membrane of the digestive tract or skin. Penetrating wounds of this nature may be produced by thorns, briers, stubble, burs, and barbed wire. The organism may also gain entrance to the membranes of the mouth during eruption of the teeth.
Because the infection in sheep usually occurs through new wounds, the site of blackleg infection in freshly docked lambs frequently occurs in the tailhead or hip and in the area of skin cuts in freshly sheared animals.
The infection is not directly communicable from affected to healthy animals by mere contact.
BLACKLEG is marked by high fever (105 to 107 F.) and the formation of gaseous swellings under the skin, especially on the hind quarter or shoulder, and also on the neck, chest, flank, rump, or other parts of the body. The swellings usually cause stiffness or lameness and sometimes complete paralysis of the affected part.
The swellings at first are small, hot, and painful. They grow in size and later become cold and painless. In sheep the skin over the swellings has a purplish cast. Other general symptoms, such as loss of appetite, suspension of rumination, rapid breathing, and great depression develops as the disease progresses. The temperature falls, and violent convulsions may occur just before death. Some animals recover, but most affected animals die in 12 to 36 hours after the first symptoms appear. The onset of the disease is so sudden that in new outbreaks the first animals to be affected usually are found dead in the pasture without showing any previous symptoms.
DIAGNOSIS is based on history, clinical symptoms, postmortem examination, and the laboratory examination of specimens from suspected cases.
Blackleg infection should be suspected when calves and yearlings in known blackleg districts die suddenly, showing gassy swelling of the muscles while on pasture, or when sheep develop symptoms after shearing or docking.
Further evidence of blackleg are found if the carcass is examined blackleg few hours after death. It will usually show rapid and excessive bloating, which causes the legs on the upper side of the carcass to extend out straight and which forces a bloody-colored foam to escape from the natural body openings. One or more gaseous swellings located on the region of the hip, shoulder, neck, jaw, brisket, or loin may be observed. They emit a crackling sound on pressure.
If the swellings are lanced, a frothy, dark-red fluid with a characteristic sweetish-sour, rancid odor like that of butyric acid is discharged. Long, deep cuts made in the affected muscles at the site of the swellings expose tissues that are blackened and streaked with dark-red areas, in marked contrast to the unaffected muscle tissue hence the name "blackleg." The body lymph glands draining the affected areas may be enlarged and inflamed. The body cavities may contain a bloody exudate, but the internal organs usually show little or no alterations.
But in contrast to anthrax, which is sometimes confused with blackleg, the spleen is not enlarged, and the coagulability and color of the blood are normal.
Since blackleg may be mistaken for malignant edema, anthrax, hemorrhagic septicemia, sweetclover poisoning, and other abnormal conditions, a laboratory examination should be made of tissue specimens from suspected cases if any doubt exists as to exact nature of the trouble.
MEDICAL TREATMENT is of little value in an established case of blackleg with advanced symptoms. Veterinarians have reported that antibiotics, such as Aureomycin, Terramycin, and penicillin, have given good results in the treatment of cases showing early symptoms of the disease.
The incidence of infection in sheep can be greatly reduced by observing cleanliness in all routine operations and keeping the animals in uninfected pastures.
VACCINATION is the only effective and reliable means known for protecting animals against blackleg. Before the first vaccine was developed in 1883 in France, it has been estimated that 20 percent of the calves in badly infected districts died of blackleg. The vaccine has since been greatly improved. Some of the older types of vaccines in the form of powder, pills, filtrate, and aggressin have remained in use, but blackleg bacterin, a highly effective immunizing agent, is the product used most extensively now.
Between 1897 to 1922, the Department of Agriculture prepared and distributed about 47 million doses of powdered vaccine. Losses from blackleg were reduced from 10 percent to less than 0.5 percent in those 25 years. In compliance with an act of Congress, the free distribution of the vaccine was discontinued on July 1, 1922.
Blackleg bacterin and other types of vaccine, used in accordance with directions, usually produce in 10 or 12 days a high degree of immunity, which lasts 9 to 12 months or longer. As some young calves may not develop and retain lasting immunity, all animals younger than 4 months when vaccinated in the spring should be revaccinated in the fall to insure protection during the fall and winter. Some cattle owners in badly infected areas follow the practice of vaccinating every 6 months all animals between the ages of 2 and 10 months.
A product known as antiblackleg serum is sometimes used for immunizing valuable calves exposed in outbreaks. This product immediately increases the animal's resistance to blackleg, but the type of immunity it confers ceases after about 2 weeks.
In some blackleg districts where the soil is also believed to be infected with the organism that causes malignant edema infection in animals, a bivalent or mixed bacterin containing both organisms and known as Clostridium chauvoei-septicus bacterin has been used for the prevention of both blackleg and malignant edema.
RECOGNIZED CONTROL measures in outbreaks of blackleg include:
The isolation and treatment of all animals showing early symptoms of the disease;
The vaccination of apparently well but exposed animals and, if feasible, their immediate removal to a new pasture on higher ground;
The prompt disposition of dead animals by complete burning or deep burial in quicklime;
The destruction of manure, bedding, and other contaminated material by burning;
The cleaning and disinfection of contaminated stables; and,
The rigid enforcement of restrictions against skinning dead animals, feeding the carcass to other animals on the farm, and removing the carcass from the premises to a rendering plant.
When a suspected outbreak of blackleg occurs, a veterinarian or the State livestock sanitary official should be promptly notified.
C. D. STEIN, a graduate of the University of Pennsylvania, is a veterinarian in the Animal Disease and Parasite Research Branch of the Agricultural Research Service. He has been with the Department of Agriculture since 1911, having served in three divisions of the former Bureau of Animal Industry. For many years he has done research on anthrax, equine infectious anemia, and other infectious diseases of livestock.