Blackleg

Also called black quarter, quarter evil, or quarter ill  

Description

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 (spores) when exposed to adverse conditions such as heat or drying. These spores can remain alive almost indefinitely. Some live in soil for many years and infect animals later when ingested with feed, or when 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 from animal to animal but produce deadly toxins that may kill the animal if they get into the bloodstream. The toxins of different types of Clostridia vary in their effects and the way they gain access to the bloodstream. These bacteria multiply in the absence of oxygen and release deadly toxins faster than the body can mount a defense (unless the animal was previously vaccinated), causing sudden death from toxemia.

Many of these bacteria are found in the intestinal contents of normal animals (and also in humans) as part of the resident GI tract flora. They also exist in soil that contains manure. They cause disease in certain situations, as when diet or management changes produce an environment more favorable for swift multiplication. When that 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 of disease. Since most of these bacteria are ever-present in the environment, the only way to protect cattle is by vaccination.

True blackleg is caused by Clostridium 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 other animals (such as sheep). False blackleg can be caused by two other Clostridial bacteria (Cl. Septicum and Cl. Novyi) but these are classified as a different disease, called malignant edema.

In an unvaccinated animal that develops blackleg, symptoms appear suddenly and the stockman may just find the animal dead.

In an unvaccinated herd, cattle of all ages are susceptible, but this disease often crops up most frequently in the fastest growing calves. 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 has 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 the affected muscles can often be felt under the skin, with a crackling sensation when you touch it—especially over the 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 is diminished.

At first, the swelling is small and hot but as the disease rapidly progresses, the swelling enlarges and becomes cold and insensitive with decreased blood supply to affected areas.

The skin is discolored, cracked, and dry. The animal usually dies within 12 to 36 hours after first signs appear, often so quickly that you just find it dead without knowing it was sick.

In some instances the swellings occur only in the heart and diaphragm, with no outward evidence. In most cases, however, postmortem examination reveals black, necrotic tissue in the infected areas of the larger muscles, containing pockets of gas bubbles.

Signs

Depression (dull), loss of appetite, lameness, high pulse rate, high fever in early stage (up to 106 degrees F.), subnormal temperature in later stages, hot painful (then later cold) swelling of the upper part of the affected leg or legs caused by gas bubbles under skin, gas bubbles give crackly feeling when touched, skin is dry, cracked and discolored, death within 12 to 48 hours.

Causes

Blackleg is caused by C. chavoei. This spore-forming bacterium can live in soil for long periods of time and mainly affects cattle younger than 2 years of age. 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 digestive 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 the summer.

When there’s an outbreak in unvaccinated young cattle, there is often a history of flooding and then dry conditions, or recent excavation (disturbing the soil) that exposed buried spores.

When ingested, the spores pass through the gut wall into the bloodstream and end up in muscles and other tissues. They may live in the gut, spleen and liver without causing problems, and can lie dormant in muscles for a long time until conditions are right for multiplication.

These bacteria start to grow and produce deadly toxins whenever there is injury or bruising of muscles, or any other condition that reduces oxygen level in the tissues where the bacteria lie dormant. Muscle trauma associated with exertion, transport, herding and handling may trigger multiplication of these bacteria.  

Blackleg occurs most commonly in young animals (6 months to 2 years of age) and tends to affect heavily muscled animals that are on a high plane of nutrition and growing fast—some of the best calves.  

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 then the necrotic material provides ideal habitat for them. It’s a self-perpetuating vicious cycle.

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 (such as in intramuscular injection) but can also spread to the muscle via the bloodstream.

Prevention

It’s not always possible to avoid bruising and muscle damage.  Animals housed together, fighting, jostling around, being shipped, may cause enough bruising for the bacteria (that are already in the tissues) to multiply.  

This disease can be easily prevented with vaccination, however. Many cattle died of blackleg before the advent of vaccination; this was one of the first cattle vaccines developed. Today the only blackleg vaccine available is a combination that includes protection against some of the other clostridial diseases as well.

There are 7 and 8-way vaccines combining protection against most of the clostridial diseases, including blackleg, redwater, malignant edema, black disease, enteroxemia (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 for blackleg consists of vaccinating calves at 2 to 4 months of age, with a booster at weaning time. This 2-dose schedule usually gives lifelong immunity against blackleg but most producers give an annual booster to protect the animal from various clostridial diseases. Some of the other clostridial diseases, 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

Treatment is usually futile unless begun very quickly at first sign of illness. Large doses of penicillin may save the animal in the early stages (initially given intravenously and followed by longer-acting preparations in the muscle—preferably directly into the affected tissue).  If the animal is already debilitated, however, it may not recover. If a herd is experiencing an outbreak, it may help to vaccinate all the 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.
    
Treatment, if the animal is found alive, in time to treat, may also involve fasciotomy (slicing the skin open) to allow oxygen to enter the tissues and get rid of the gas build-up under the skin. The bacteria are multiplying rapidly in this anaerobic environment. Making holes in the skin to open it up, and slicing down deep into the muscle allows oxygen to get in there. In this process the necrotic tissue and pus can also be removed, providing drainage. Opening the affected tissue to the air is one of the more beneficial things that can be done.