Blackleg disease: Vets analyze common but preventable infection affecting livestock
Clostridial diseases are rapidly fatal because of the toxins created when these bacteria multiply. Blackleg, traditionally also called black quarter, quarter evil or quarter ill, killed a lot of cattle before the advent of vaccination. The blackleg vaccine was one of the first cattle vaccines created.
Several highly-fatal livestock diseases, including blackleg, are caused by a group of bacteria called Clostridia. These bacteria can form a protective shell-like covering and go into a dormant stage – as spores – when exposed to heat or drying. These spores can remain viable almost indefinitely. Some live in soil for many years and infect animals later when ingested with feed or introduced into a wound.
Spores can also exist within the bodies of animals in a latent, hidden, dormant state without causing disease, then suddenly come to life and multiply when conditions become favorable.
Clostridial diseases are not contagious animal-to-animal, but produce deadly toxins that may kill the animal if the bacteria enters into the bloodstream. Toxins of different types of Clostridia vary in their effects and the way they gain access to the bloodstream. Unless the animal was vaccinated, toxins multiply in the absence of oxygen and release deadly toxins faster than the body can mount a defense, causing sudden death from toxemia.
Many of these bacteria are found in the intestinal contents of normal animals and humans as part of the gastrointestinal (GI) tract. They also exist in soil that contains manure. Contamination of soil by blackleg spores may occur from infected feces or from decomposed carcasses of animals that died of the disease.
Spores enter the body via the GI tract if the animal ingests contaminated feed, water or soil. Spores may be picked up when grazing or when dirt is present in feed or baled hay. Blackleg is primarily a disease of pastured cattle with most cases occurring during summer.
These bacteria cause disease in certain situations, as when diet or management changes produce an environment more favorable for swift multiplication. When this happens, the resulting disease, such as blackleg, is acute and generally fatal within a few hours unless the animal is treated at the first signs. Since most of these bacteria are present in the environment, the best way to prevent disease is by administering vaccinations.
True blackleg is caused by C. chauvoei – a gram-positive, spore-forming, rod-shaped bacterium – and characterized by acute inflammation of the muscles, severe toxemia and sudden death. True blackleg is common only in cattle, but infection induced by trauma occurs occasionally in sheep. False blackleg can be caused by two other Clostridial bacteria – C. septicum and C. novyi – but is classified as a different disease called malignant edema.
An unvaccinated animal that develops blackleg may present symptoms suddenly and the stockman may simply find the animal dead. In an unvaccinated herd, cattle of all ages are susceptible, but this disease often occurs most frequently in the fastest-growing young animals.
First signs are depression and lameness – the animal is very dull. Inflammation in the muscles causes swelling of the upper part of the affected leg. The animal may have a high fever up to 106 degrees, but by the time symptoms become obvious, the temperature may have dropped.
Swelling caused by gas bubbles in affected muscles can often be felt under the skin, with a crackling sensation when touched – especially over hips and shoulders. The swollen leg is hot and painful, but soon becomes cold and painless as the swelling enlarges and blood supply to the area diminishes. The skin is discolored, cracked and dry. The animal usually dies within 12 to 36 hours after first signs appear.
In some instances, the swelling occurs only in the heart and diaphragm, with no outward evidence presented. In most cases, however, postmortem examination reveals black, necrotic tissue in infected areas of the larger muscles, containing pockets of gas bubbles.
When there’s an outbreak in unvaccinated young cattle, there is often a history of flooding followed by dry conditions or recent soil disturbance which exposed buried spores.
When ingested, spores pass through the gut wall into the bloodstream. Eventually, spores are deposited in muscles and other tissues. They may live in the gut, spleen and liver without causing problems and can lie dormant in muscles a long time until conditions are right for multiplication.
These bacteria start to grow and produce deadly toxins whenever there is an injury or bruising of muscles, or any other condition that reduces oxygen level in tissues where the bacteria lie dormant. Muscle trauma associated with exertion, transport, herding and handling may trigger multiplication of bacteria. Blackleg infection often begins with some type of bruise or trauma, creating damaged tissue which starts the anaerobic process. These bacteria can enter the tissue via a direct poke or puncture – even an intramuscular injection – but can also spread to the muscle via the bloodstream.
Blackleg occurs most commonly in young animals six months to two years of age and tends to affect heavily muscled animals on a high plane of nutrition and growing fast.
These organisms are gas producers when they multiply, and the gas accumulates under the skin. All Clostridial organisms are anaerobes, thriving in environments devoid of oxygen. They not only cause muscle damage and necrosis, but necrotic material provides ideal habitat for them. The disease is a self-perpetuating vicious cycle.
It’s not always possible to avoid bruising and muscle damage. Animals penned together, fighting, jostling around and being shipped may cause enough bruising for bacteria – already present in the tissues – to multiply.
Today, the blackleg vaccine is a combination including protection against a few Costridial diseases. There are seven- and eight-way vaccines combining protection against most of the clostridial diseases: blackleg, redwater, malignant edema, black disease, enterotoxemia – a gut infection caused by C. perfringens types C and D – and sometimes tetanus. All Clostridial diseases can be acutely deadly, but they are also unique in that they can be very effectively prevented by vaccination.
Prevention consists of vaccinating calves at two to four months of age, with a booster at weaning time. This two-dose schedule usually gives lifelong immunity against blackleg, but most producers give an annual booster to protect against other Clostridial diseases. Some of them, such as redwater and black disease, can be a threat at any time during the life of the animal, so the combination vaccine is often given annually, or even more often if redwater is a concern.
If animals die of blackleg, the carcasses should be destroyed by burning or buried deeply in a fenced-off area to limit heavy spore contamination of the soil to prevent future cases.
Treatment is usually futile unless begun quickly at first sign of illness. Large doses of penicillin may initially save the animal if given intravenously and followed by longer-acting preparations given directly into the affected muscle tissue.
If the animal is already debilitated, it may not recover. If a herd is experiencing an outbreak, it may help to vaccinate all living animals and administer penicillin at the same time. The penicillin will halt proliferation of bacteria in exposed animals and give them time to develop immunity from the vaccine.
If the animal is found alive in time to treat, treatment may involve slicing the skin open to allow oxygen into the tissues and get rid of gas build up under the skin as bacteria are multiplying rapidly in the anaerobic environment. Necrotic tissue and pus can also be removed, providing drainage. Opening affected tissue to the air is one of the more beneficial things that can be done.
Information for this article was obtained from several veterinarians including: Salmon, Idaho Veterinarian Dr. Robert Cope; Associate Professor of Farm Animal Surgery at Ohio State University Dr. Andrew Niehaus; Western College of Veterinary Medicine Large Animal Clinical Sciences Department Head Dr. John Campbell; and Clinical Assistant Professor of Livestock Medicine and Field Service in the College of Veterinary Medicine at Kansas State University Dr. Manuel F. Chamorro.
Heather Smith Thomas is a corresponding writer for the Wyoming Livestock Roundup. Send comments on this article to firstname.lastname@example.org.