A Veterinarian’s Perspective
Among many sheep (and goat) producers, the initials “CL” are a dreaded diagnosis. CL stands for Caseous Lymphadenitis. CL is a disease characterized by contagious abscesses of the skin and lymph nodes. It is caused by a bacteria: Corynebacterium pseudotuberculosis.
C.pseudotuberculosis is a highly infectious and hardy bacteria. It is spread into the environment from draining abscesses of infected animals. In one experimental study, draining abscesses continued to contaminate the environment for as long as 37 days after rupture. Once in the environment, the bacteria can live in dark areas, damp areas, soil, and manure for extended periods of time. It is extremely difficult to disinfect a farm environment once C.pseudotuberculosis has become established.
C.pseudotuberculosis typically infects an animal through wounds in the skin. It can be rapidly spread through a flock at the time of shearing, dipping, tail docking, or castration due to contaminated equipment. It is also possible for an animal to be infected with C.pseudotuberculosis via inhalation, and lung abscesses may result.
The classic signs of CL are swelling and abscessing of superficial lymph nodes. Typically these lymph node abscesses occur below the jaw, on the shoulder, in the “armpit” or on the back of the hind leg. Generalized skin abscesses can also occur. Some animals develop internal abscesses in the lungs and/or chest cavity: these animals will show respiratory symptoms, difficulty breathing, and often chronic weight loss. Occasionally an animal will have abscessing in the abdomen and viscera causing chronic weight loss and a decrease in overall production. It is also possible for C.pseudotuberculosis to spread to the central nervous system and cause neurologic signs, or to spread to the mammary gland and cause mastitis.
Definitive diagnosis of CL can only be made by culture of abscesses. There is a blood test available for CL, but it is not 100% accurate. The blood test is called HI and it detects antibodies to exotoxins produced by C.pseudotuberculosis. Most infected animals will produce these antibodies within one month of infection with C.pseudotuberculosis.
Of course, not every abscess on a sheep (or goat) is CL. A typical non-contagious abscess will have a liquid bloody, brown, or white to yellow discharge. Abscesses caused by C.pseudotuberculosis typically have a classic thick, greenish, paste-like discharge. However, any animal with an abscess or swelling that may be an abscess should be isolated far from the flock for treatment.
The best treatment for CL is culling an animal that is confirmed positive. If treatment is to be undertaken, the flock veterinarian should definitely be involved in the plan. The best course of treatment for a special individual animal is complete surgical removal of an abscessed lymph node, if it has not yet ruptured. This surgery is done under general anesthesia and usually at the veterinary clinic. Ruptured abscesses can be flushed out with antiseptic solution such as Betadine or Nolvasan, and the animals can be treated with systemic antibiotics. There is a procedure for “auto-vaccinating” infected animals with vaccine made from their own abscesses, but it is somewhat controversial.
The best prevention for CL is the quarantine and examination of all new flock additions. Again, it is extremely important to know the farm of origin of all new flock additions- the disease history and sanitation procedures practiced. Housing for the sheep should be free of objects that can cause skin injury. Proper disinfection of sheep shears, surgical equipment, needles, and handling equipment should be practiced at all times.
If a CL outbreak occurs within a flock, aggressive control procedures should be put in to place. All animals with open draining lesions should be culled or quarantined. Animals with chronic poor body condition should be culled. HI blood tests can be used to help identify infected animals. Vaccination may be helpful in reducing the number of abscesses in animals and the severity of the disease. Vaccination should only be used in an infected flock, and will not usually result in disease eradication. Again, it is extremely important that the flock veterinarian be involved in a CL control program.
©Dr. Heather Ludlam DVM