William P. Shulaw, Extension Veterinarian
(Originally Published in Sheep Team Newsletter October 2003)
What is it?
Caseous lymphadenitis is the technical name for the disease of sheep and goats that is often referred to as CLA, CL, contagious abscesses, “cheesy gland” or “boils”. It is an infection primarily of the lymph nodes but it can affect internal organs such as the liver, lung, kidney, and spinal canal. Although abscesses are relatively common in most animal species, they usually occur sporadically, are often confined to individual animals, often occur following a puncture wound or injury, and they can be caused by a variety of bacteria common in all environments such as streptococci and staphylococci. However, the bacterium that causes CLA is Corynebacterium pseudotuberculosis. This organism is present in the drainage from the abscess and is capable of living in the barn environment for a long period of time. In infected flocks, it is common for many of the animals to develop CLA.
What does it look like?
The disease occurs in two forms. The superficial form affects the lymph nodes that are close to the surface of the skin and results in swollen nodes commonly seen under or at the rear of the jaw, in front of the shoulder, in front of the hind leg, or above the udder or scrotum. The internal form usually affects organs such as the lung, liver, and kidney.
In the superficial form, the abscess causes the lymph node to swell up and, usually, to eventually rupture releasing thick, cheesy pus. Reoccurrence is common after the abscess appears to heal. If the abscess involves a solitary node and doesn’t interfere with the animal’s eating or movement, it may not appear to be ill.
If an internal organ is infected, you can’t usually tell that it is infected until function of the organ is affected or until enlargement of the abscess causes systemic signs of weight loss and unthriftiness. This disease is considered part of the “thin ewe syndrome” because of this characteristic. Some Australian studies show that a majority of the abscesses are internal.
How common is it?
This disease is a very common infection in sheep and goats in the USA. Although prevalence data for goats are hard to come by, surveys done in the early 1990s of cull ewes at harvest indicated that CLA caused more losses due to condemnation of the carcass, or portions of the carcass, than all other disease conditions combined. It was also a major cause of losses to the lamb industry.
How is it spread?
Discharges from an opened abscess contain billions of the infectious organisms. C. pseudotuberculosis can survive several weeks to several months outside the animal, and contamination of feeders, gates, bedding, and shearing equipment is common. Animals with draining abscesses contaminate their environment, and the bacteria infect other sheep or goats through minute abrasions of the skin when they are exposed to them. Sheep are often infected during shearing when the skin is broken or cut by contaminated shearing equipment. There is some evidence that the bacteria may be able to penetrate intact skin.
Once the bacteria enter the skin, blood or lymph fluid carries them to the regional lymph nodes. They are usually able to escape the body’s defenses and begin to form a thick-walled abscess that continues to enlarge. If it is a lymph node under the skin, usually we can easily detect it; and sometimes the abscess is cut open during shearing. If it is an internal abscess in a lymph node or organ, its presence may not be suspected until the sheep or goat loses weight or develops organ failure.
Is the condition treatable?
Yes, but often the result is not satisfactory. Large abscesses can be opened and drained (collecting as much of the drainage as possible), but the animal should be isolated in an area that can be easily disinfected until there is no more drainage. Frequently, the abscess will reappear in a few weeks to a few months. Sometimes it is difficult to drain abscesses that are close to vital structures, such as blood vessels and nerves near the curvature of the jaw. Antibiotics are not usually very helpful because the wall of the abscess is thick, and it is difficult to get penetration to the site where the bacteria are. Sometimes surgical removal of the entire lymph gland is possible, and this may prevent drainage of discharges into the environment and may prevent reoccurrence. Treatment of internal abscesses is difficult and usually unsuccessful.
How do we control and prevent CLA in an infected flock or herd?
* Isolate affected animals before the abscess ruptures. Cull the animal unless it MUST be retained. Clean and disinfect any areas that might be contaminated by discharges from a ruptured abscess.
* Remove sharp objects on feeders etc., sharp corners, wires, etc. from the animals’ environment that may become contaminated and serve to inoculate the animals’ skins.
* Provide your own shearing surface for sheep shearers to avoid possible contamination of your sheep with some other flock’s bacteria. Disinfect all shearing gear before starting to shear. Four ounces of bleach in a gallon of water with a 2-minute contact time should be adequate on smooth clean surfaces. Shear affected animals last and shear from youngest to oldest if possible to reduce the risk of opening an abscess in the process of shearing.
* Obtain new sheep or goats from CLA-free flocks/herds if possible.
Can I vaccinate for this disease?
A vaccine is available for CLA, but it is licensed for use only in sheep. Vaccination will not cure abscesses or make them go away. Vaccination will not prevent all new abscesses in exposed animals, but it has been shown to reduce the number of new abscesses substantially. Lambs can be vaccinated as early as 4 weeks of age if exposure in a heavily contaminated flock is likely. However, they should have booster vaccinations at about 4 week intervals until they are 12-16 weeks of age. Older sheep need two doses four weeks apart and annual booster vaccinations.
A combination of vaccination, culling, and management of the environment to reduce the risk of exposure can reduce or eliminate the disease over time. Vaccinated animals may show some lameness or depression following vaccination. The vaccine is not approved for use in goats, and some owners have reported occasional severe reactions. A veterinarian should prescribe its use in goats.