– Dr. Michelle Arnold, UK Veterinary Diagnostic Laboratory
1. What is “blackleg”? This is a rapidly fatal disease of cattle, typically calves 6-12 months of age, caused by the bacterium Clostridium chauvoei. Sheep may also be affected. In a majority of cases, affected calves are simply found dead in the pasture with no symptoms of disease. It usually affects calves in good nutritional condition (the “fattest and slickest”) within a group. As the bacterium grows, it emits a toxin (poison) that kills the muscle cells, typically in the hindquarters or thigh muscles. Most animals will die within 12-24 hours of the onset of disease so clinical signs of lethargy, severe lameness, and muscle swelling are often missed. The swollen muscle starts out hot and painful but quickly becomes cold and insensitive as the muscle dies. The bacterium also produces gas that builds up under the skin, causing the skin to feel similar to “bubble wrap” and makes a crackling, rattling sound known as “crepitation” when pushing the skin down over the affected area.
2. Where is the blackleg organism found? The organism that causes blackleg, the bacterium Clostridium chauvoei, is characterized as a “Gram-positive, anaerobic, spore-forming rod”. This description is important because it describes why and how the bacterium survives for long periods in the soil and the trigger that causes it to be deadly. Clostridial organisms are anaerobes which means they like to live and grow where there is no oxygen. In order to survive where there is oxygen, they change to a spore form. A “spore” is a protective form of the bacteria that allows it to survive unfavorable conditions and also enables it to spread. The spore form is found in both soil and water as well as in the digestive tract of living and dead animals. Spores are very resistant to environmental factors and disinfectants. Older studies confirmed the organism can survive as a spore at least 11 years in better soils. Spores can only germinate and grow, known as changing to the “vegetative state”, when there is little oxygen present. This vegetative bacterial form produces the toxin (poison) called Toxin A, considered to be responsible for the destruction of both skeletal and cardiac (heart) muscle cells and death.
3. How do cattle get infected? Although blackleg is one of the oldest and most widely recognized causes of death, the way the disease works is still not fully understood. It is believed that cattle ingest the spores which are then absorbed through the intestines and into the bloodstream where they get distributed to multiple tissues, including skeletal and heart muscles. Once there, the white blood cells called “macrophages” engulf them and the organism can survive months to years within these cells without affecting the animal. However, when the oxygen level drops within the muscle cell, for example due to injury and bruising, the spores germinate, and the vegetative bacteria grow and produce the deadly toxin.
4. Does it only affect the thigh musculature? No, the disease affects both skeletal muscle and cardiac (heart) muscle and is frequently found in both the leg and the heart. Of the skeletal muscles, the thigh (hindquarters) is most common but it may be found in the front legs, along the back (lumbar area), brisket, neck, diaphragm, and tongue or in multiple muscles.
5. Why the name “blackleg”? The toxin produced by the bacteria causes muscle necrosis (death) resulting in dark red to black discoloration of the affected muscle (see Figure 1). When cut, the affected muscle has a characteristic odor of rancid butter. Diagnosis is easily made at necropsy and the bacteria type can be confirmed with several different tests.
6. Does it only occur in young calves? Blackleg is commonly thought of as a disease of calves 6-12 months of age on pasture. However, it can occur in very young calves (1-2 months of age) or even in adult cattle if non-vaccinated or if adults were only vaccinated once as calves.
7. Does it only occur in cattle on pasture? It most commonly affects pastured cattle because they consume spores present in the soil. However, it can occur in housed cattle when feed is contaminated with soil that contains the spores. A very large outbreak in Norway where 72 housed cattle died within 12 days was traced to round bale silage with heavy soil contamination.
8. If there is no history of blackleg on the farm, does that mean there will never be a case there? Blackleg is, in a word, unpredictable. It may suddenly appear on farms where it was never known to exist or has been absent for decades. On the flip side, areas or places where blackleg is common may go many years between cases even without adequate vaccination.
9. Is blackleg associated with a certain weather pattern or season? Most cases occur during or after periods of high rainfall or after movement of soil such as building or cleaning out a pond. Flooding may spread the spores and water saturated soil is thought to have less oxygen which may induce germination of the spores and multiplication. Soil excavation is believed to bring more spores to the soil surface. Blackleg cases are seen throughout the year in Kentucky.
10. Are the available vaccines effective? Vaccination against C. chauvoei is cheap, easy and close to 100% effective in preventing blackleg if given prior to exposure and according to label directions. One dose of blackleg vaccine given to a newborn calf is not enough to be protective because they cannot mount an effective immune response. However, good quality colostrum from the dam will protect the calf from disease up to 3-4 months of age until vaccination is effective. Most blackleg vaccines require a twoshot series administered 3-4 weeks apart. One product, Alpha 7 (Boehringer Ingelheim) only requires one dose but calves vaccinated under 3 months of age must be re-vaccinated at weaning or 4-6 months of age to be protective. All blackleg vaccines recommend an annual booster.
11. Do adult cattle need a blackleg vaccine? This is a common question with no definitive answer. Certainly the incidence of blackleg decreases dramatically after 24 months of age but it still occurs. Since the spores may lie dormant in an animal for years, the potential remains for disease in poorly vaccinated adults. One thing is certain, annual vaccination of adult cows will improve the antibodies produced and delivered in colostrum so longer, stronger protection is provided to her calves at birth.
12. Is blackleg vaccine considered a killed or live vaccine? Neither! Clostridial vaccines are usually labeled as “toxoids” or “bacterintoxoids” because the antibodies produced by the vaccine actually neutralize the deadly toxin produced by the growing bacteria rather than the bacteria alone.
13. Why are blackleg vaccines called 7- or 8-way vaccines and is one better than the other? There are several diseases in cattle caused by different Clostridial species so the antigens are all typically included in the blackleg vaccine. Clostridium chauvoei is the causative agent for blackleg while Cl. novyi is the agent in Black Disease (infectious hepatitis), Cl. septicum in malignant edema, Cl. sordelli in gas gangrene and Cl. perfringens Types C and D in various types of enterotoxemia. An 8-way vaccine contains an additional agent, Cl. haemolyticum, that causes “Red Water” disease in cattle but is not found in KY and considered unnecessary for cattle here. One important Clostridial disease, tetanus, is due to Cl. tetani but it is found in very few blackleg vaccines. Covexin 8 and Calvary 9 (Merck) are examples of blackleg vaccines that contain a tetanus toxoid.
14. What about the blackleg vaccines combined with other agents such as pinkeye? Blackleg vaccines are found in many different combination products including with pinkeye, Histophilus somni (commonly known as “somnus”), and Zoetis makes a combination blackleg and “pasteurella” vaccine called “One Shot Ultra 7”. All these vaccines are considered effective if used according to label directions, meaning a booster is given if indicated.