Blackleg cases on the rise

By Gary Sliworsky
Ag rep, Emo

Blackleg is a disease of cattle, and less frequently of sheep, caused by the soil-borne bacterium, Clostridium chauvei.
The disease develops rapidly in affected animals and often death occurs before the owner has noticed any sickness in the herd.
Last year, there were more cases of blackleg reported in the province than normal.
Blackleg occurs on all continents. Most parts of Ontario are affected, but new outbreaks are found almost yearly in areas where the disease has not previously been reported.
For this reason, livestock owners should not assume they will not have blackleg losses simply because they never have before.
Because the causative bacteria are soil-borne, the disease may be introduced to new areas in several ways, including windstorms, waterways, and wild animals.
Blackleg commonly occurs in the warmer months of the year in young animals on pasture. Calves and yearlings most often are affected.
Often no symptoms are observed; the animals are found dead on the pasture with no previous signs of illness.
At other times, one or more of the young calves show signs of illness by a high fever, lack of appetite, depression, lameness, and swellings that appear in the muscles on various parts of the body.
Sometimes the leg muscles are involved, or the muscles in the region of the back, hip, flank, chest, or shoulder. In the latter stage of the disease, these swellings spread and become quite mushy, producing a characteristic crackling sound when pressed with the hand.
This sound is due to the gas under the skin which is produced by the growing bacteria.
Putrefaction occurs rapidly in the carcass of an animal infected with blackleg and results in a typical bloated appearance of the carcass soon after death. The legs are extended stiffly and a frothy, bloody discharge often is apparent at the anus and the nostrils.
The skin over the swelling is usually normal, but in the centre it may have undergone dry gangrene.
When cut open and examined, the swellings usually are found to contain discoloured serum and gas. When affected muscles are cut open, they usually are found swollen and either black or darker in colour than normal, with gas present.
It is unwise to cut open a swelling unless necessary for a diagnosis as this increases the contamination of the soil.
Next week’s article will look at the diagnosis, treatment, and prevention of this disease.
Dates to remember
•July 27–Soil and crop tour (watch for details).

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